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What benefits does a well designed one-off aluminum build have?


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#1 reis123

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Posted 24 February 2012 - 04:40 PM

For the sake of any comments, assume three things: a boat built by a respected yard using the medium; a boat size greater than 54 foot loa; and serious offshore racing, or weight savings gained by building hull and deck with newer composite technology, definitely not a priority...

#2 New Morning

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Posted 24 February 2012 - 04:51 PM

Huge topic. How about narrowing it down a bit more. Are you concerned about:

- build cost?
- maintenance cost?
- resale value?
- where will it be used and for what purpose?

#3 reis123

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Posted 24 February 2012 - 04:58 PM

Replies in Bold

Huge topic. How about narrowing it down a bit more. Are you concerned about:

- build cost? Not overly concerned with cost of build, however, comparatively speaking, value is always important...
- maintenance cost? Not any more than other types of builds, given certain parameters...
- resale value? Not concerned with resale value...
- where will it be used and for what purpose? Offshore duty from time to time, coastal cruising, day sailing, like any other medium, if I own it, so to speak...




#4 kimbottles

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Posted 24 February 2012 - 04:59 PM

I have owned traditional wood, contemporary wood (cold molded), fiberglass, cored fiberglass and aluminum boats. (I am currently having an eGlass sheathed strip planked wood boat built.)

They all have advantages and disadvantages.

Aluminum is fairly easy to build one off, it is easy to repair (relatively speaking) and it is fairly light (compared to steel.) It is somewhat hard to paint, I left my aluminum boat bare. An epoxy barrier coat is necessary for bottom paint.

It does need to be monitored for galvanic action. I would not have an aluminum boat without a permanently mounted electrolysis meter which could be monitored continuously (I am speaking from experience on this.)

There is no "best" boat building material, they all have their pluses and their minuses.......

#5 Bob Perry

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Posted 24 February 2012 - 05:12 PM

The ability to have integral tanks is big. The ability to tie all the structure together relatively easy is big. I like to design aluminum boats.

#6 IrieMon

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Posted 24 February 2012 - 05:15 PM

estar should be along momentarily, but check his site: http://www.bethandevans.com/hawk.htm

#7 reis123

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Posted 24 February 2012 - 05:29 PM

The ability to have integral tanks is big. The ability to tie all the structure together relatively easy is big. I like to design aluminum boats.


Why would an aluminum build require more attention to integral tanks? Similarly, why would aluminum imply to you that the structure be tied together relatively easily when compared to other mediums? Do you have less requests for aluminum, other words is the medium more a departure creating interest for you, or, is the medium somehow inherently fascinating or a challenge? Thanks for any more thoughts you might have here!



#8 Jose Carumba

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Posted 24 February 2012 - 07:17 PM

We build large motor yachts in fiberglass, steel, and aluminum. I think what Bob means is that it is easier to design and build tanks and structure in metal boats. In metal all you need to do is weld plates or shapes (Ts, Ls etc) to one another with the proper welds and sequencing. In most fiberglass boats the tanks need to be built of metal and fit into the boat be secured. We do it much more complexly. Our tanks are integral with the hull, like a metal boat, but its not so easy. Integral frp tanks need to be sealed very well against leakage. You do not want diesel fuel to get into the fiberglass. Fiberglass even when compressed via vacuum bagging or resin infusion will wick diesel fuel. I recall a case where a large 75-100 ft vessel (not one of ours) with integral tanks had diesel fuel wicking through the figerglass and along the stringers from fwd of the engine room to the transom. It took a lot of time and money to fix, and I am not convinced they got all the fuel out. We have a process which seals the tanks very well but we are able to use it because the tanks are so big we can get people inside them to do the work. Hard to do on smaller vessels so tanks are generally built separately and mounted in the boat.

Tying structure together in a metal boat requires the proper type of welding and sequencing whereas in a custom frp boat the structural members are bonded into the hull separately requiring secondary bonding to the existing fiberglass which is a weaker bond. newer resins and techniques reduce this problem, and if the structure is infused with the hull laminate the problem is virtually eliminated. So generally there are many more steps and considerations when building an frp hull vs a metal one.

I ddn't explain this too well but maybe you get the general idea.

#9 Estar

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Posted 24 February 2012 - 07:22 PM

Bob is the expert . . . listen to him . . . perhaps ND should reconsider an aluminum hull :)

My less informed opinion is that all the material have their pros and cons. If cost is absolutely no objective, I think you simply can't beat a sophisticated composite hull (carbon/kevlar pre-preg). But with cost/value as a factor aluminum has some advantages - the main three being ( A ) you do not need to build a mold. You just cut frames and use them as your form. and ( B ) you do not need to (and should not) fair or paint the hull, and ( C ) the engineering is simpler and easier to understand than with most other methods - aluminum is a uniform isotropic material so you know exactly what you have by looking at its thickness and don't have to worry about fiber orientation of layer bonding.

#10 Cruisin Loser

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Posted 24 February 2012 - 07:28 PM

If all the deck hardware is welded on, you should have no deck penetrations, hence no annoying drips, and no possibility of wet deck core from poorly bedded fittings.

#11 Hiracer

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Posted 24 February 2012 - 07:34 PM

The ability to have integral tanks is big.




+1



I have a 36.5' sailboat and carry 99 gallons of fuel. Pretty cool, IMO. And my B/D ratio goes up when I fuel up.

105 gallons of water and none of that under the settees.

Find that in a glass boat same size!

#12 New Morning

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Posted 24 February 2012 - 08:20 PM

Here is what I wrote a few years ago after investigating aluminum boats and eventually deciding to go with fiberglass: Why Fiberglass?

The only thing I'll add to that is that after three years of cruising, essentially all of it outside the US, I think the biggest advantage of aluminum, or disadvantage of fiberglass, is maintenance. The fiberglass hull requires maintenance because we all expect it to look good. A bare aluminum hull like Hawk, many of the Dashew boats and a lot of French cruisers, is expected to have a pretty industrial look. While I'm sure the owners are not abusive, it's just less worry when coming up to an ugly dock or when a local boat rafts up to sell fuel, fish or food. There is just less worry about potential damage than with a fiberglass boat.

#13 Jose Carumba

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Posted 24 February 2012 - 08:47 PM

Here is what I wrote a few years ago after investigating aluminum boats and eventually deciding to go with fiberglass: Why Fiberglass?

The only thing I'll add to that is that after three years of cruising, essentially all of it outside the US, I think the biggest advantage of aluminum, or disadvantage of fiberglass, is maintenance. The fiberglass hull requires maintenance because we all expect it to look good. A bare aluminum hull like Hawk, many of the Dashew boats and a lot of French cruisers, is expected to have a pretty industrial look. While I'm sure the owners are not abusive, it's just less worry when coming up to an ugly dock or when a local boat rafts up to sell fuel, fish or food. There is just less worry about potential damage than with a fiberglass boat.


It's funny how that works both ways. In my world the aluminum is the harder material to maintain because our boats are painted, and if the paint skin gets penetrated it bubbles and peels.

#14 reis123

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Posted 24 February 2012 - 08:56 PM

Here is what I wrote a few years ago after investigating aluminum boats and eventually deciding to go with fiberglass: Why Fiberglass?

The only thing I'll add to that is that after three years of cruising, essentially all of it outside the US, I think the biggest advantage of aluminum, or disadvantage of fiberglass, is maintenance. The fiberglass hull requires maintenance because we all expect it to look good. A bare aluminum hull like Hawk, many of the Dashew boats and a lot of French cruisers, is expected to have a pretty industrial look. While I'm sure the owners are not abusive, it's just less worry when coming up to an ugly dock or when a local boat rafts up to sell fuel, fish or food. There is just less worry about potential damage than with a fiberglass boat.



Thanks. I read the piece you linked to your web page. Very straight forward in assessment. I cannot help but sense you see aluminum as having properties more rugged in its application, but, perhaps I am generalizing...

#15 Hiracer

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Posted 24 February 2012 - 09:12 PM

It's funny how that works both ways. In my world the aluminum is the harder material to maintain because our boats are painted, and if the paint skin gets penetrated it bubbles and peels.




Here, here. Everybody talks about how steel is tough, but in many ways it's only as tough as its paint, i.e., not very.

#16 cancouper

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Posted 25 February 2012 - 04:25 AM

What about relative impact strength / ductility of Aluminum versus Composite? According to reports, there's going to be an awful lot of tsunami junk in our waters (PNW) over the next number of years - already have a lot of other junk. Better to bend than break?. Had anyone seen any research reports or design studies on this?

#17 New Morning

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Posted 25 February 2012 - 05:29 AM

Thanks. I read the piece you linked to your web page. Very straight forward in assessment. I cannot help but sense you see aluminum as having properties more rugged in its application, but, perhaps I am generalizing...


It's more rugged from a cosmetic standpoint, no question. Which will do better if you smash it onto a reef is a raging debate that I will leave to others. Bend versus break is a problem for engineers and not quite as obvious as it may first appear. As we just saw in Italy, and many times before, steel and aluminum do have limits. I've seen fiberglass boats spend some time on a reef, get heavily gauged, but get dragged off and repaired. I think you really need an engineer to define the boundaries of each material, and then you've got to differentiate between different kinds of fiberglass (different matts and resins) and then again between fiberglass and carbon fiber.

The point of Hawk, many Dashew boats and many European boats (German, Dutch and French) is that they are not painted. The natural "corrosion" happens on the surface, it doesn't bubble under paint. Once you apply paint you've got problems. All the painted aluminum boats I know of are enormously more maintenance than fiberglass; a constant battle. But if you're ok with naked aluminum aesthetics, then it's fine.

Depending on the resin, naked fiberglass can absorb a lot of water which passes into cores and all kinds of nasty problems while other fiberglass hulls built with other resins are OK. Naked aluminum is not going to win any beauty contests, but if you smack into the dock, or the local boat bounces up and own on your hull for 15 minutes while you buy some fresh veggies or fish, no worries. The fresh scratch will quickly look like the rest of the hull.

It really depends on the usage of the vessel. If you park an aluminum boat in a "hot harbor" with a crappy electrical system for 300 days out of the year like a lot of fiberglass sail boats are typically used, you'll need to really be on guard for electrolysis.

The tradeoffs are not simple and thus neither is the choice. So much is dependent on the usage. Hauled all winter, or in the water all year? Sits in a marina for 300 days / yr, or spends 300 days / yr at sea, on the hook or on the hard?

I'd keep this topic alive in SA for a long time before making a choice, but I suggest that it will all come back to your intended usage.

#18 Soņadora

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Posted 25 February 2012 - 06:55 AM

there's no way I could own an aluminum boat. I'm too much of a clutz. I'd unkowingly drop something like a screwdriver or a pair of pliers into the bilge. Mix with a little seawater and next thing you know you've just burned a hole through the hull.

#19 Rohanoz

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Posted 25 February 2012 - 10:08 AM

there's no way I could own an aluminum boat. I'm too much of a clutz. I'd unkowingly drop something like a screwdriver or a pair of pliers into the bilge. Mix with a little seawater and next thing you know you've just burned a hole through the hull.


It's funny, I've heard his before. And, a mate with a magnificent Alu cruising yacht has warned me about it whenever I am on his boat.
But, my aluminum dinghy has had sinkers, brass swivels, steel boat hooks gal anchor, etc all in the bottom of the boat for about 15 years without an issue other than gunk and salt encrustations!
What changes this on a cruising sailboat?

#20 reis123

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Posted 25 February 2012 - 12:33 PM


Thanks. I read the piece you linked to your web page. Very straight forward in assessment. I cannot help but sense you see aluminum as having properties more rugged in its application, but, perhaps I am generalizing...


It's more rugged from a cosmetic standpoint, no question. Which will do better if you smash it onto a reef is a raging debate that I will leave to others. Bend versus break is a problem for engineers and not quite as obvious as it may first appear. As we just saw in Italy, and many times before, steel and aluminum do have limits. I've seen fiberglass boats spend some time on a reef, get heavily gauged, but get dragged off and repaired. I think you really need an engineer to define the boundaries of each material, and then you've got to differentiate between different kinds of fiberglass (different matts and resins) and then again between fiberglass and carbon fiber.

The point of Hawk, many Dashew boats and many European boats (German, Dutch and French) is that they are not painted. The natural "corrosion" happens on the surface, it doesn't bubble under paint. Once you apply paint you've got problems. All the painted aluminum boats I know of are enormously more maintenance than fiberglass; a constant battle. But if you're ok with naked aluminum aesthetics, then it's fine.

Depending on the resin, naked fiberglass can absorb a lot of water which passes into cores and all kinds of nasty problems while other fiberglass hulls built with other resins are OK. Naked aluminum is not going to win any beauty contests, but if you smack into the dock, or the local boat bounces up and own on your hull for 15 minutes while you buy some fresh veggies or fish, no worries. The fresh scratch will quickly look like the rest of the hull.

It really depends on the usage of the vessel. If you park an aluminum boat in a "hot harbor" with a crappy electrical system for 300 days out of the year like a lot of fiberglass sail boats are typically used, you'll need to really be on guard for electrolysis.

The tradeoffs are not simple and thus neither is the choice. So much is dependent on the usage. Hauled all winter, or in the water all year? Sits in a marina for 300 days / yr, or spends 300 days / yr at sea, on the hook or on the hard?

I'd keep this topic alive in SA for a long time before making a choice, but I suggest that it will all come back to your intended usage.


While there is a certain beauty associated with clarity of purpose gained through directness or plainness of build, casting aside any assumptions of what comprises aesthetics by certain measures, intended usage is what seems to be the umbrella in which so many design decisions rest.

I keep a copy of a paper prepared on the properties of strength written by engineering experts having been prepared on the specific build qualities of a Transpac 52. The paper is probably all but indecipherable to anyone without having spent at least three to four years in study of mathematics, mechanical engineering, chemical engineering, and naval architecture, preferably at a leading university near you. I would tend to agree with you that any serious discussion of a boat's inherent strength tested by the elements or challenged by unforeseen difficulties provided is too easy to generalize upon. We probably tend to over simplify these things, because we must; one can easily imagine aluminum's strength though when compared to the other mediums under consideration - mainly composite technology in the form of wood or some form of reinforced plastics. But this is why I ask, I am not an engineer by training.

Then one must ask themselves how will the boat be used, where, and quickly enter the discussion budget for maintenance. I imagine this boat would receive generous care by skilled and professional yards. She'll be sailed around the east coast of the U.S. and Canada, with time too around the islands off the Florida coast. Sailing to Europe in time. But what is a guide here is to keep her in top condition is more dictated by need than constraints of budgetary consequence. She'll be used year round regardless of where.

#21 jhiller

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Posted 25 February 2012 - 12:42 PM

Bob is the expert . . . listen to him . . . perhaps ND should reconsider an aluminum hull :)

My less informed opinion is that all the material have their pros and cons. If cost is absolutely no objective, I think you simply can't beat a sophisticated composite hull (carbon/kevlar pre-preg). But with cost/value as a factor aluminum has some advantages - the main three being ( A ) you do not need to build a mold. You just cut frames and use them as your form. and ( B ) you do not need to (and should not) fair or paint the hull, and ( C ) the engineering is simpler and easier to understand than with most other methods - aluminum is a uniform isotropic material so you know exactly what you have by looking at its thickness and don't have to worry about fiber orientation of layer bonding.


For absolute toughness you cannot beat aluminum. As Evans can elucidate the toughness requires vigilence of the electrical and bonding system. If I knew rocks and coral were in my future I go for AL in a New York minute . A complete monococue hull and deck with integral tanks and plate that is literally bulletproof . The Brits even built warships from it until one burned up after a hit from an exocet missle. Even pirates don't have them yet....




#22 Estar

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Posted 25 February 2012 - 01:15 PM

there's no way I could own an aluminum boat. I'm too much of a clutz. I'd unkowingly drop something like a screwdriver or a pair of pliers into the bilge. Mix with a little seawater and next thing you know you've just burned a hole through the hull.



I have heard those stories, but I believe they stem mostly from uber thin racing hulls (palmer Johnson) in stiffer but less corrosion resistance hulls (60601 aluminum?). I have seen no sign of that vulnerability on my own hull (8083 aluminum). I have accidentally dropped various stainless and bronze bits in the hull and only found/recovered them years later and never seen any sign they have marked or damaged the hull in any way.I honestly don't worry about this anymore.

For absolute toughness you cannot beat aluminum.


If you have the money for both engineering and materials, you can do some amazing things with composites - pre-preg kevlar with inserted titanium fibers can be damn 'tough', with pre-pre thin ply carbon for amazing stiffness and fatigue resistance and a slightly elastic core for shock load absorption.. But you need some serious aerospace grade composite engineering to get that all right. Aluminum is easier for the rest of us to get strong and right. It's also easier to make completely leakproof and watertight decks (which can be done in composite but almost no-one does).

#23 kdh

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Posted 25 February 2012 - 02:08 PM

If you have the money for both engineering and materials, you can do some amazing things with composites - pre-preg kevlar with inserted titanium fibers can be damn 'tough', with pre-pre thin ply carbon for amazing stiffness and fatigue resistance and a slightly elastic core for shock load absorption.. But you need some serious aerospace grade composite engineering to get that all right. Aluminum is easier for the rest of us to get strong and right. It's also easier to make completely leakproof and watertight decks (which can be done in composite but almost no-one does).

Part of the Hinckley lore is that the particulars of their e-glass/kevlar/carbon construction were determined in part by Bob Hinckley shooting at samples in his back yard with his hunting gun. The boats have bounced off a lot of rocks successfully in Maine.

#24 Tucky

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Posted 25 February 2012 - 02:39 PM

I sailed to Bermuda once in an aluminum sloop- the boat inspired tremendous confidence for many of the reasons mentioned- simple structure with no deck penetrations or leaks and an absolutely solid feel with no flexing, creaking or movement other than small parts.

Of course, multihulls built of this material haven't been the same-

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#25 Soņadora

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Posted 25 February 2012 - 04:01 PM

I have heard those stories, but I believe they stem mostly from uber thin racing hulls (palmer Johnson) in stiffer but less corrosion resistance hulls (60601 aluminum?). I have seen no sign of that vulnerability on my own hull (8083 aluminum). I have accidentally dropped various stainless and bronze bits in the hull and only found/recovered them years later and never seen any sign they have marked or damaged the hull in any way.I honestly don't worry about this anymore.


I've always wondered if that was just superstition.

#26 Mung Breath

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Posted 25 February 2012 - 04:54 PM

I guess I can pipe in here. Coup D' Etat ( McC&R 46') is 40 years old and my first aluminum boat after a series of 'previously loved' fiberglass boats (J/24, Bristol, Shields, Morris). How one uses a boat has a lot of bearing on what constitutes best construction. Mine lives on a mooring in WLIS away from any marina, has no need for AC power, has a simple but aggressive sailplan with few thru-deck fittings, cruises and races long distances in light or very rough weather, sometimes both. At 32,000# displacement and 15,400# of ballast she's no modern bouy racer but is surprisingly nimble with a CF rig. For longer distances, her sea motion is silk-like which probably says more about her design than construction material.

Downside (20%)
  • Electrolysis: yes, one has to pay attention. But don't we all? In 40 years, my hull has lost <2% of its density and she didn't always enjoy great TLC. When we refit her, we completely rewired her (DC only, eliminated the AC) with proper chafe protection (nothing unusual). She has four (4) hull zinc plates and one on the MaxProp that do their job. When she gets a bath or rarely spends the night at the dock, her 500 Ah of AGM's get charged via a multi-phase/3-bank plugged into a standard 110 GFI outlet. I'm not suggesting electrolysis is to be dismissed but with common sense it's an overblown issue. How hard is it not to drop pennies into the bilge - or check it once in a while?
  • Paint: absolutely...if you're going to paint the hull, expect an accelerated cosmetics program. We repainted last year and already have bubbles in a few spots where salt water collects on deck or where corrosion existed previously. Coming from fiberglass, this is a culture shock. So we'll keep small tubes of paint on board for quick repairs between re-sprays and accept minor imperfections - it's only cosmetic. If she weren't already painted when we bought her, I would give serious thought to keeping her bare. If she were used like HAWK, no question that she'd be bare. For now, it's s a bit like shaving your head before you're at the comb-over stage...I'm not that brave yet.
Benefits: (80%)
  • She's a battleship by day and a wooden boat by night; there's just something very confidence-inspiring about this naval ship-like combination
  • No leaks anywhere - she's dry as a bone
  • No squeaks - not even one from her 'wooden boat' interior in the roughest of seas
  • No oil canning or pounding - she's pure silence
  • No squishy, saturated balsa-cored decks or hull around fittings
  • No rotted bulkheads, chain plates, sole braces, engine or tank mounts
  • No hull blisters
  • No keel to hull attachment failures
  • Easy to permanently repair and patch
  • Easy to modify (weld aluminum bow roller, winch pads, old instruments or deck hardware holes, etc.)
  • Foam insulation above the water line, an extra layer above the headliner, plenty of dorades and opening ports keeps her comfortable.
  • Treadmaster on the deck helps keep her relatively quiet in a rainstorm
Every construction medium has trade-off's. If she weren't painted, 75% of the unique maintenance would disappear and I'd still be left with the above benefits. That said, if I wanted a bouy racer, floating condo plugged into shore power, didn't expect to race or cruise in open water, didn't have a passion for the pedigree itself, didn't know people killed when they were T-boned at night in fog, etc., aluminum wouldn't be as appealing. But I have noticed a pleasant calming effect to no longer worrying about the usual balsa-core issues; this has been replaced with a minor malaise over paint blistering.

#27 reis123

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Posted 25 February 2012 - 05:39 PM

I guess I can pipe in here. Coup D' Etat ( McC&R 46') is 40 years old and my first aluminum boat after a series of 'previously loved' fiberglass boats (J/24, Bristol, Shields, Morris). How one uses a boat has a lot of bearing on what constitutes best construction. Mine lives on a mooring in WLIS away from any marina, has no need for AC power, has a simple but aggressive sailplan with few thru-deck fittings, cruises and races long distances in light or very rough weather, sometimes both. At 32,000# displacement and 15,400# of ballast she's no modern bouy racer but is surprisingly nimble with a CF rig. For longer distances, her sea motion is silk-like which probably says more about her design than construction material.

Downside (20%)

  • Electrolysis: yes, one has to pay attention. But don't we all? In 40 years, my hull has lost <2% of its density and she didn't always enjoy great TLC. When we refit her, we completely rewired her (DC only, eliminated the AC) with proper chafe protection (nothing unusual). She has four (4) hull zinc plates and one on the MaxProp that do their job. When she gets a bath or rarely spends the night at the dock, her 500 Ah of AGM's get charged via a multi-phase/3-bank plugged into a standard 110 GFI outlet. I'm not suggesting electrolysis is to be dismissed but with common sense it's an overblown issue. How hard is it not to drop pennies into the bilge - or check it once in a while?
  • Paint: absolutely...if you're going to paint the hull, expect an accelerated cosmetics program. We repainted last year and already have bubbles in a few spots where salt water collects on deck or where corrosion existed previously. Coming from fiberglass, this is a culture shock. So we'll keep small tubes of paint on board for quick repairs between re-sprays and accept minor imperfections - it's only cosmetic. If she weren't already painted when we bought her, I would give serious thought to keeping her bare. If she were used like HAWK, no question that she'd be bare. For now, it's s a bit like shaving your head before you're at the comb-over stage...I'm not that brave yet.
Benefits: (80%)
  • She's a battleship by day and a wooden boat by night; there's just something very confidence-inspiring about this naval ship-like combination
  • No leaks anywhere - she's dry as a bone
  • No squeaks - not even one from her 'wooden boat' interior in the roughest of seas
  • No oil canning or pounding - she's pure silence
  • No squishy, saturated balsa-cored decks or hull around fittings
  • No rotted bulkheads, chain plates, sole braces, engine or tank mounts
  • No hull blisters
  • No keel to hull attachment failures
  • Easy to permanently repair and patch
  • Easy to modify (weld aluminum bow roller, winch pads, old instruments or deck hardware holes, etc.)
  • Foam insulation above the water line, an extra layer above the headliner, plenty of dorades and opening ports keeps her comfortable.
  • Treadmaster on the deck helps keep her relatively quiet in a rainstorm
Every construction medium has trade-off's. If she weren't painted, 75% of the unique maintenance would disappear and I'd still be left with the above benefits. That said, if I wanted a bouy racer, floating condo plugged into shore power, didn't expect to race or cruise in open water, didn't have a passion for the pedigree itself, didn't know people killed when they were T-boned at night in fog, etc., aluminum wouldn't be as appealing. But I have noticed a pleasant calming effect to no longer worrying about the usual balsa-core issues; this has been replaced with a minor malaise over paint blistering.


Thanks for the thoughtful comments. Purposeful build in some tangible way is maybe a clarification of what properties aluminum might lend its qualities...

#28 islandplanet

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Posted 26 February 2012 - 06:53 PM

Mung,

Gorgeous boat my friend. Was just looking at your blog.

#29 us7070

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Posted 26 February 2012 - 07:18 PM

What about relative impact strength / ductility of Aluminum versus Composite? According to reports, there's going to be an awful lot of tsunami junk in our waters (PNW) over the next number of years - already have a lot of other junk. Better to bend than break?. Had anyone seen any research reports or design studies on this?


pretty hard to beat steel in that regard...

I took this photo in Las Palmas Grand Canary, before the start of the 2007 ARC.

supposedly, the dent is from the bulb of a big freighter...

you can view the full resolution picture in the caption anarchy forum here: http://forums.sailin...showtopic=65917

Posted Image

#30 Bob Perry

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Posted 26 February 2012 - 07:23 PM

No question that steel is the most rugged, durable boat building material and that pic tells the whole story. Alu is next.

#31 reis123

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Posted 27 February 2012 - 12:52 AM

No question that steel is the most rugged, durable boat building material and that pic tells the whole story. Alu is next.

.

Good to know.

James McCurdy wrote 25 years ago in Offshore Yachts "Steel construction cannot be considered as satisfactory for an offshore cruising yacht less than 75 to 100 feet in length......The extra thickness [for using steel] required for corrosion is the same for a small boat or a large ship. This means that the smaller vessel must carry a disproportionate and unacceptable amount of weight that is not required for strength."

I wonder if this is still true today?




#32 Bob Perry

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Posted 27 February 2012 - 02:52 PM

Yes it is. The weight of steel has not changed.

#33 Estar

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Posted 27 February 2012 - 03:34 PM

Good to know.

James McCurdy wrote 25 years ago in Offshore Yachts "Steel construction cannot be considered as satisfactory for an offshore cruising yacht less than 75 to 100 feet in length......The extra thickness [for using steel] required for corrosion is the same for a small boat or a large ship. This means that the smaller vessel must carry a disproportionate and unacceptable amount of weight that is not required for strength."

I wonder if this is still true today?


As stated I don't think it was ever true . . . there have been many quite successful (more than satisfactory) 30-40' steel cruising boat. That's just a plain unarguable fact. I could list dozens of wonderful steel 40'ers - but let me just say one word 'Joshua'.

McCurdy may be saying that aluminum would have been even better for those boats - and that is most probably true from a designer's standpoint, but the owners wanted steel and the steel boats have been successful cruising boats in their owner's eyes.

#34 trenace

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Posted 27 February 2012 - 03:37 PM

As it seems to me you're suggesting in your second paragraph, it depends on how one defines "satisfactory," and from what perspective.

I'd expect there are home builders that are completely satisfied with even their 30-foot-range steel boats.

But from the standpoint of a designer who is thinking about what a pig the boat is compared to what would be the case in say aluminum, the same boat may be thought unsatisfactory.

#35 Anomaly2

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Posted 27 February 2012 - 03:38 PM

Yes it is. The weight of steel has not changed.


there you go again, facts are stubborn things.....Posted Image

But, have "coatings" changed/improved?--- to make it possible to avoid having to "oversize" plate thickness just to deal with corrosion issues? I have no idea, just asking.

#36 Estar

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Posted 27 February 2012 - 04:25 PM

As it seems to me you're suggesting in your second paragraph, it depends on how one defines "satisfactory," and from what perspective.

Absolutely.

A 30' steel boat is likely to be very unsatisfactory If you want a planing racing hull, but rather satisfactory if you want to winter in Baffin island


I'd expect there are home builders that are completely satisfied with even their 30-foot-range steel boats.

Absolutely there are. And there are also completely satisfied owners of professionally dutch built 40' steel boats - we know several. There is a steel yard near Bob (waterline yachts) that makes terrific steel boats - we know 3 owners who all think their boats are just about perfect cruising boats.

But from the standpoint of a designer who is thinking about what a pig the boat is compared to what would be the case in say aluminum, the same boat may be thought unsatisfactory.

Absolutely

But IMHO it is the owner's opinion of whether the boat is 'satisfactory' that in the end is most important.

I might also comment that the actual real world at sea difference in passage times between the same waterline steel and aluminum cruising boats is not going to be very high.


As far as steel coatings, I am no expert, I am a fan of aluminum for this reason, but while epoxy and isocyanate coatings have improved there are some other excellent ones that are less good or harder to get because of health and safety and environment reasons (thinking of coal tar and flame zinc spraying). So, I wonder if the net has been an advance or rather just about holding things equal.

#37 Bob Perry

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Posted 27 February 2012 - 04:26 PM

Anom:
I think you could get by with thinner plating IF you could keep the shell fair and that's hard with thin plating unless you have more frames and there is the weight problem again.

#38 SemiSalt

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Posted 27 February 2012 - 04:35 PM

James McCurdy wrote 25 years ago in Offshore Yachts "Steel construction cannot be considered as satisfactory for an offshore cruising yacht less than 75 to 100 feet in length......The extra thickness [for using steel] required for corrosion is the same for a small boat or a large ship.


This "extra thickness" is not just a safety margin for corrosion, but also for panel stiffness. A panel of steel with the same tensile strength as a panel of fiberglass would be so thin as to be too flexible. [The advantage of cored fiberglass construction is that the thickness makes it stiff.] Of course, you could use much more closely spaced frames, but that takes all the fun out of it. [Edit: Bob dropped in with a similar observation while I was typing.]

Derector, or someone at Derector's yard, once scoffed at comments that aluminum was not tough enough. He suggested the doubters be allowed to hammer at some of the Al with a sledge hammer.

There are plenty of designs that have been spec'ed for construction on both steel and Al, e.g. the Van de Stadt 34. They gave the steel version a bit more displacement, and somewhere I read a comment that it's not as lively in light air.


Attached File  vds34.jpg   36.1K   22 downloads

#39 trenace

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Posted 27 February 2012 - 04:35 PM


But IMHO it is the owner's opinion of whether the boat is 'satisfactory' that in the end is most important.

I might also comment that the actual real world at sea difference in passage times between the same waterline steel and aluminum cruising boats is not going to be very high.


Agreed on both points, or at least down to some length that I don't know but surely far, far less than the 75 foot figure.

It's kind of interesting how worked up we get -- myself included -- about a boat being say 1/3 of one knot slower due to some design detail. Aggh, can't have that.

Obviously that's of massive importance racing, but when not racing, not so much. I guess it does add a day to a voyage of three weeks or so.

#40 Tucky

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Posted 27 February 2012 - 04:48 PM

Mung,

Gorgeous boat my friend. Was just looking at your blog.



Hey Mung, the aluminum boat I sailed on was called Alacrity, I think the was 42 or 43 feet. Same designer and builder. Nice boat, as is yours. I know of a couple of others. You probably know better than me.



#41 Estar

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Posted 27 February 2012 - 05:01 PM

There are plenty of designs that have been spec'ed for construction on both steel and Al, e.g. the Van de Stadt 34.


Here are the numbers for difference hull materials for Hawk's original design (the Van de stadt samoa).

Displacement (steel)18.00 t
Displacement (aluminium)14.60 t
Displacement (wood core)14.60 t

Ballast (steel)5.10 t
Ballast (aluminium)5.50 t
Ballast (wood core)5.50 t

I might comment that the Samoa design (and these numbers) includes teak decks and solid wood interior - so you can 'improve' (lighter displacement with more ballast) on these numbers quite a bit (about 2tons) by using painted decks and core panel interior (as we did on Hawk), and then improve even more by leaving off the genset and other frivolous equipment :)

#42 Hiracer

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Posted 27 February 2012 - 05:34 PM

James McCurdy wrote 25 years ago in Offshore Yachts "Steel construction cannot be considered as satisfactory for an offshore cruising yacht less than 75 to 100 feet in length......The extra thickness [for using steel] required for corrosion is the same for a small boat or a large ship. This means that the smaller vessel must carry a disproportionate and unacceptable amount of weight that is not required for strength."


This is only true if you decide you are going to let your steel yacht corrode. Some of us frown on that kind of practice.

Logically speaking you can make the same argument about aluminum, needing more metal than required for strength, because it also corrodes, albeit differently.

It is more difficult to design a 40' or smaller boat in steel because stability becomes an issue. It's hard to get a decent B/D ratio and at the same time get an acceptable SA/D ratio. More often than not, SA/D is sacrificed on account of stability limitations. The result is a slug. Bob, correct me if I'm wrong here.

#43 Bob Perry

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Posted 27 February 2012 - 05:42 PM

Hi:
No you are pretty much right.
The steel boat will have to displacemen more to carry the weight of steel. You can get the same B/D if you have enough displacement. But you will need a big rig if you want the same SA/D.
With the ballast inside the steel fin you should be ablt to get the lead low enough so that there ample stability.

I've never quite bought into the "thicker becasue it will corrode" theory either. I prefer my plate stiffness and fairness reason for thickness.

#44 Hiracer

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Posted 27 February 2012 - 06:02 PM

prefer my plate stiffness and fairness reason for thickness.


Me too. Welding thin plates of steel tends to make for ripples except in the hands of gifted welders, of which there are few. Plate thickness is a work around for that problem.

#45 Hiracer

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Posted 27 February 2012 - 06:12 PM

With the ballast inside the steel fin you should be ablt to get the lead low enough so that there ample stability.


I think deep draft is even more important for smallish steel vessels. My boat was drawn by Brewer as modified full keel (4'8"), but built with a fin keel (6'1") thereby getting the lead about a foot deeper, which can only help. Also, the rig is much bigger than originally drawn. The net effect is still not a fast boat, but nor is she a slug. The down side is that I have more sail area to contend with compared to other boats my 'size,' and I have to reef earlier. She is not a stiff boat. OTOH, the motion is sweet.

No doubt her character and speed would be different in aluminum.

#46 Mung Breath

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Posted 27 February 2012 - 07:43 PM


Mung,

Gorgeous boat my friend. Was just looking at your blog.



Hey Mung, the aluminum boat I sailed on was called Alacrity, I think the was 42 or 43 feet. Same designer and builder. Nice boat, as is yours. I know of a couple of others. You probably know better than me.



ALACRITY was built in 1982, 40'6"

Here's a link to the Paul Luke' website Owners Registry - good research info: http://www.peluke.co...r_registry.html
Coup D'Etat was originally, 'SITZMARK' (BTW, I actually like that name better but have a Dutch wife and in-laws)

#47 Cherie320

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Posted 27 February 2012 - 09:31 PM

Mung,

Gorgeous boat my friend. Was just looking at your blog.


And that's the advantage of aluminum. Every boat I see that is built from aluminum, pulls at my heart strings. Well, except for the lake variety pontoons, and those junk all over the place workboats. Mung....she looked great in RED too.

#48 rule69

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Posted 27 February 2012 - 10:59 PM

I had a buddy who made a catamaran out of core-ten with a modified space frame that he claimed came out at a weight competitive with AL... Not sure I buy that and weathering steel is also a leap of faith.

And, while tossing out really suspect data I remember a lovely evening spent in Apia aboard a steel boat with the owners of three other steel boats. After a couple of bottles of wine the war stories started and each of them had a horrific sounding story of an isolated corrosion problem (eg, poking a hole in the deck where all the steel around it was okay). Alcohol was involved, they all loved their metal boats and sea stories are often just that. But there may be a grain of truth. Anyway, I'd worry about contamination in the metal from welding or whatever and problematic things like dissimilar thru-hulls as well as the concern with electricity. Mostly I think the lessons if there are any would be to 1) be careful during construction and 2) don't put your boat together so that you can't get at the hull plating from inside if you need to... IMHO, those rules should apply to plastic boats too.

#49 curm

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Posted 28 February 2012 - 01:53 AM


I have heard those stories, but I believe they stem mostly from uber thin racing hulls (palmer Johnson) in stiffer but less corrosion resistance hulls (60601 aluminum?). I have seen no sign of that vulnerability on my own hull (8083 aluminum). I have accidentally dropped various stainless and bronze bits in the hull and only found/recovered them years later and never seen any sign they have marked or damaged the hull in any way.I honestly don't worry about this anymore.


I've always wondered if that was just superstition.



I thought it was galvanic corrosion?

#50 UrbanChicago

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Posted 28 February 2012 - 04:20 AM

A few years ago I bought a brand-new road bicycle made from a fairly new steel alloy called True Temper OX Platinum that, among other things, gets stronger when you weld it and let the weld cool down at room temperature.

It's got all of the fantastic feeling of the old-school steel bikes and all of the strength of steel yet the frame weighs less than an aluminum frame of the same stiffness.

This particular alloy is highly unlikely to be suitable for marine use, but perhaps there are other new alloys that may be.

#51 Presuming Ed

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Posted 28 February 2012 - 06:30 PM

Getting steel right:

http://www.morganscl...nture Cruising)

#52 Hiracer

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Posted 28 February 2012 - 06:56 PM

Getting steel right:

http://www.morganscl...ure+Cruising%29


Five years ago I looked seriously at a 34' Koopmans designed Dutch built steel sailboat from the early 1980s that had nearly pristine steel plates via visual and electronic inspection. The paint looked great, interior and exterior, except for the usual high wear areas on deck. Done correctly, paint over steel can work very well. But it has to be done well.

She had a SA/D ratio of 14 point something, so I passed.

Ironically, the builder of the boat married an American gal and eventually retired from boat building and moved to Idaho where he operated a little steel-art shop. I met him there briefly, but this was before I looked at the boat. It was several years later after looking at the boat that I made the connection between the art shop and the boat. That was a "small world" moment.

#53 Estar

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Posted 28 February 2012 - 07:07 PM


Getting steel right:

http://www.morganscl...ure+Cruising%29


. . . . Done correctly, paint over steel can work very well. But it has to be done well.


One thing the dutch have told us, that is not done correctly in that Morgan's Cloud photo, is on a steel boat to rely mostly on ring frames - and very very few longitudinals. Because the longitudinals trap water while the rig frames let it flow to the bilge.

We learned while building Hawk, that the Dutch like ring frames but the American metal boat builders like longitudinals (apparently makes a fair hull easier). On Hawk we double framed her with both, but if I were going steel I would go the Dutch route. Their steel boats seem to hold up the best of anyone's.

#54 Hiracer

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Posted 28 February 2012 - 07:17 PM



Getting steel right:

http://www.morganscl...ure+Cruising%29


. . . . Done correctly, paint over steel can work very well. But it has to be done well.


One thing the dutch have told us, that is not done correctly in that Morgan's Cloud photo, is on a steel boat to rely mostly on ring frames - and very very few longitudinals. Because the longitudinals trap water while the rig frames let it flow to the bilge.

We learned while building Hawk, that the Dutch like ring frames but the American metal boat builders like longitudinals (apparently makes a fair hull easier). On Hawk we double framed her with both, but if I were going steel I would go the Dutch route. Their steel boats seem to hold up the best of anyone's.



The Dutch boat interior in this case had blown in foam over paint above the water line, with only paint below waterline, FWIW. I know this is a topic of great religious-like dispute in the steel boat community. The point about avoiding longitudinals is interesting because it makes so much sense. I've heard of limber holes in longitudinals also. Just thinking out loud, maybe the best is to do both but lay the longitudinals over the rings. Does that make any sense?

#55 wkd928

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Posted 28 February 2012 - 10:32 PM

How did you arrive at the 2% figure for loss of hull density - presumably to corrosion?

I guess I can pipe in here. Coup D' Etat ( McC&R 46') is 40 years old and my first aluminum boat after a series of 'previously loved' fiberglass boats (J/24, Bristol, Shields, Morris). How one uses a boat has a lot of bearing on what constitutes best construction. Mine lives on a mooring in WLIS away from any marina, has no need for AC power, has a simple but aggressive sailplan with few thru-deck fittings, cruises and races long distances in light or very rough weather, sometimes both. At 32,000# displacement and 15,400# of ballast she's no modern bouy racer but is surprisingly nimble with a CF rig. For longer distances, her sea motion is silk-like which probably says more about her design than construction material.

Downside (20%)

  • Electrolysis: yes, one has to pay attention. But don't we all? In 40 years, my hull has lost <2% of its density and she didn't always enjoy great TLC. When we refit her, we completely rewired her (DC only, eliminated the AC) with proper chafe protection (nothing unusual). She has four (4) hull zinc plates and one on the MaxProp that do their job. When she gets a bath or rarely spends the night at the dock, her 500 Ah of AGM's get charged via a multi-phase/3-bank plugged into a standard 110 GFI outlet. I'm not suggesting electrolysis is to be dismissed but with common sense it's an overblown issue. How hard is it not to drop pennies into the bilge - or check it once in a while?
  • Paint: absolutely...if you're going to paint the hull, expect an accelerated cosmetics program. We repainted last year and already have bubbles in a few spots where salt water collects on deck or where corrosion existed previously. Coming from fiberglass, this is a culture shock. So we'll keep small tubes of paint on board for quick repairs between re-sprays and accept minor imperfections - it's only cosmetic. If she weren't already painted when we bought her, I would give serious thought to keeping her bare. If she were used like HAWK, no question that she'd be bare. For now, it's s a bit like shaving your head before you're at the comb-over stage...I'm not that brave yet.
Benefits: (80%)
  • She's a battleship by day and a wooden boat by night; there's just something very confidence-inspiring about this naval ship-like combination
  • No leaks anywhere - she's dry as a bone
  • No squeaks - not even one from her 'wooden boat' interior in the roughest of seas
  • No oil canning or pounding - she's pure silence
  • No squishy, saturated balsa-cored decks or hull around fittings
  • No rotted bulkheads, chain plates, sole braces, engine or tank mounts
  • No hull blisters
  • No keel to hull attachment failures
  • Easy to permanently repair and patch
  • Easy to modify (weld aluminum bow roller, winch pads, old instruments or deck hardware holes, etc.)
  • Foam insulation above the water line, an extra layer above the headliner, plenty of dorades and opening ports keeps her comfortable.
  • Treadmaster on the deck helps keep her relatively quiet in a rainstorm
Every construction medium has trade-off's. If she weren't painted, 75% of the unique maintenance would disappear and I'd still be left with the above benefits. That said, if I wanted a bouy racer, floating condo plugged into shore power, didn't expect to race or cruise in open water, didn't have a passion for the pedigree itself, didn't know people killed when they were T-boned at night in fog, etc., aluminum wouldn't be as appealing. But I have noticed a pleasant calming effect to no longer worrying about the usual balsa-core issues; this has been replaced with a minor malaise over paint blistering.





#56 Heavy Metal

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Posted 29 February 2012 - 03:30 AM

I cant speak for the pros and cons of a new, custom built alloy boat. However 5 years ago i bought a 1982 custom 42fter (ex racer) in alloy and ive been extremely pleased with the material. Plus points are

1. You neednt worry about hidden issues ie soggy core, delam, rot etc. If there are corrosion type issues, they are very easy to see.

2. Reparing or modifying the structure is very easy. I extended the cabin head 4ft aft to make the cockpit more cruser friendly and create more usable space down below. All up, the total time, including relocating the hatch and adding new windows etc etc, was 20 man hours.

3. During the refit and convertion from racer to cruiser we sandblasted the interior and coated it with some fancy 3 pot and 2 pot epoxy goop, therefore if copper is dropped it shouldnt cause an issue

4. My boat is constructed on ring frames at 400mm intervals, therefore adding a new cruising interior was piece of cake. We simply cut ply to shape and bolted them onto the ringframes.

5. Lightness and therefore performance. At 5500kgs with a criuising interior, not many newer "performance' cruiser/racers of a similar size beat us to holiday spots, or around the bouys.

6. When we race, we get a great deal of respect on the startline

#57 Mung Breath

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Posted 29 February 2012 - 03:39 AM

I cant speak for the pros and cons of a new, custom built alloy boat. However 5 years ago i bought a 1982 custom 42fter (ex racer) in alloy and ive been extremely pleased with the material. Plus points are

1. You neednt worry about hidden issues ie soggy core, delam, rot etc. If there are corrosion type issues, they are very easy to see.

2. Reparing or modifying the structure is very easy. I extended the cabin head 4ft aft to make the cockpit more cruser friendly and create more usable space down below. All up, the total time, including relocating the hatch and adding new windows etc etc, was 20 man hours.

3. During the refit and convertion from racer to cruiser we sandblasted the interior and coated it with some fancy 3 pot and 2 pot epoxy goop, therefore if copper is dropped it shouldnt cause an issue

4. My boat is constructed on ring frames at 400mm intervals, therefore adding a new cruising interior was piece of cake. We simply cut ply to shape and bolted them onto the ringframes.

5. Lightness and therefore performance. At 5500kgs with a criuising interior, not many newer "performance' cruiser/racers of a similar size beat us to holiday spots, or around the bouys.

6. When we race, we get a great deal of respect on the startline


LOL - seriously. The first reaction I had to looking a the mettalic fine entry of my own bow was, "God help the poor SOB that port tacks this thing!"




#58 Mung Breath

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Posted 29 February 2012 - 03:41 AM

How did you arrive at the 2% figure for loss of hull density - presumably to corrosion?


I guess I can pipe in here. Coup D' Etat ( McC&R 46') is 40 years old and my first aluminum boat after a series of 'previously loved' fiberglass boats (J/24, Bristol, Shields, Morris). How one uses a boat has a lot of bearing on what constitutes best construction. Mine lives on a mooring in WLIS away from any marina, has no need for AC power, has a simple but aggressive sailplan with few thru-deck fittings, cruises and races long distances in light or very rough weather, sometimes both. At 32,000# displacement and 15,400# of ballast she's no modern bouy racer but is surprisingly nimble with a CF rig. For longer distances, her sea motion is silk-like which probably says more about her design than construction material.

Downside (20%)

  • Electrolysis: yes, one has to pay attention. But don't we all? In 40 years, my hull has lost <2% of its density and she didn't always enjoy great TLC. When we refit her, we completely rewired her (DC only, eliminated the AC) with proper chafe protection (nothing unusual). She has four (4) hull zinc plates and one on the MaxProp that do their job. When she gets a bath or rarely spends the night at the dock, her 500 Ah of AGM's get charged via a multi-phase/3-bank plugged into a standard 110 GFI outlet. I'm not suggesting electrolysis is to be dismissed but with common sense it's an overblown issue. How hard is it not to drop pennies into the bilge - or check it once in a while?
  • Paint: absolutely...if you're going to paint the hull, expect an accelerated cosmetics program. We repainted last year and already have bubbles in a few spots where salt water collects on deck or where corrosion existed previously. Coming from fiberglass, this is a culture shock. So we'll keep small tubes of paint on board for quick repairs between re-sprays and accept minor imperfections - it's only cosmetic. If she weren't already painted when we bought her, I would give serious thought to keeping her bare. If she were used like HAWK, no question that she'd be bare. For now, it's s a bit like shaving your head before you're at the comb-over stage...I'm not that brave yet.
Benefits: (80%)
  • She's a battleship by day and a wooden boat by night; there's just something very confidence-inspiring about this naval ship-like combination
  • No leaks anywhere - she's dry as a bone
  • No squeaks - not even one from her 'wooden boat' interior in the roughest of seas
  • No oil canning or pounding - she's pure silence
  • No squishy, saturated balsa-cored decks or hull around fittings
  • No rotted bulkheads, chain plates, sole braces, engine or tank mounts
  • No hull blisters
  • No keel to hull attachment failures
  • Easy to permanently repair and patch
  • Easy to modify (weld aluminum bow roller, winch pads, old instruments or deck hardware holes, etc.)
  • Foam insulation above the water line, an extra layer above the headliner, plenty of dorades and opening ports keeps her comfortable.
  • Treadmaster on the deck helps keep her relatively quiet in a rainstorm
Every construction medium has trade-off's. If she weren't painted, 75% of the unique maintenance would disappear and I'd still be left with the above benefits. That said, if I wanted a bouy racer, floating condo plugged into shore power, didn't expect to race or cruise in open water, didn't have a passion for the pedigree itself, didn't know people killed when they were T-boned at night in fog, etc., aluminum wouldn't be as appealing. But I have noticed a pleasant calming effect to no longer worrying about the usual balsa-core issues; this has been replaced with a minor malaise over paint blistering.





With an audio gauge survey of the hull.

#59 wkd928

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Posted 29 February 2012 - 08:50 AM

Hailing boats with "aluminium!!" instead of starboard is priceless..

Mung, it struck me that 2% is such a small number and i wondered if it was a uniform measurement - perhaps the machine testing needed recalibration and there is no actual corrosion? Im struggling to picture uniform alloy corrosion over the entire surface of sheet alloy.

#60 Presuming Ed

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Posted 29 February 2012 - 11:56 AM

Hailing boats with "aluminium!!" instead of starboard is priceless..


How about this one.... "Starboard"...."Granite! With freeboard issues!"




#61 Mung Breath

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Posted 29 February 2012 - 12:40 PM

Hailing boats with "aluminium!!" instead of starboard is priceless..

Mung, it struck me that 2% is such a small number and i wondered if it was a uniform measurement - perhaps the machine testing needed recalibration and there is no actual corrosion? Im struggling to picture uniform alloy corrosion over the entire surface of sheet alloy.


No, good question. The audi gauge process measures precise hull density at dozens of specific points around the hull. Density comparisons are made to it's original plating thickness - and to previous surveys - and hopefully the results are close uniformly. It my case, there was zero corrosion (over 40 years) in some areas and as much as 5% in a couple. I threw out 2% as an non-precise average.

But point is, this was not always a boat that received a lot of TLC. I found a few old coins, tools, fittings, etc, throughout the interior hull with no signs of corrosion. I would never buy a used aluminum boat without a survey because not all aluminum is created equal and/or perhaps boat this lucky.

#62 wkd928

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Posted 29 February 2012 - 08:49 PM

OK that makes much more sense - i can definitely picture spots with some evidence of corrosion and the vast amjority being completely corroseion free - ours seems to be the same. Although I didnt commision an audio gauge survey when we were looking at ours although i did have the bare aluminium exposed at one point on the hull that looked funky and had that tested using dye - looking for something more sinister that turned out not to be there. Subsequently weve stripped the hull back to bare alloy in a few places and there has been no corrosion to speak of. I dont know enough about boat construction to contribute in any kind of authoritative way here but so far i really like aluminium.

Hailing aluminium really happened, when i first came out and looked at the boat we went out and raced in the local Leukemia cup event (I think!). The guys on the boat made a big thing of it as they were not sure of the then owners eyesight, although sailing he was still recovering from a stroke!

#63 Mung Breath

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Posted 01 March 2012 - 11:47 AM

OK that makes much more sense - i can definitely picture spots with some evidence of corrosion and the vast amjority being completely corroseion free - ours seems to be the same. Although I didnt commision an audio gauge survey when we were looking at ours although i did have the bare aluminium exposed at one point on the hull that looked funky and had that tested using dye - looking for something more sinister that turned out not to be there. Subsequently weve stripped the hull back to bare alloy in a few places and there has been no corrosion to speak of. I dont know enough about boat construction to contribute in any kind of authoritative way here but so far i really like aluminium.

Hailing aluminium really happened, when i first came out and looked at the boat we went out and raced in the local Leukemia cup event (I think!). The guys on the boat made a big thing of it as they were not sure of the then owners eyesight, although sailing he was still recovering from a stroke!



The corrosion will typically be on the inside, not visible externally. The test will pick this up but can not be performed through epoxy.

#64 Estar

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Posted 01 March 2012 - 12:25 PM

The corrosion will typically be on the inside, not visible externally. The test will pick this up but can not be performed through epoxy.



I have been involved in a couple aluminum surveys and then rebuilds. The most common place for corrosion seems to be in the stuffing box/stern tube area. That area is also hard to ultrasound. The other most common problem, which is not really 'corrosion' is a small cracked/pin-holed weld into the keel envelope (usually first starts after a grounding shock loads a weld line), allowing salt water into the envelope, then eating the welds from the inside out (And often cracking a few if the water freezes during the winter. We pumped very low viscosity epoxy crack sealer under pressure into the keel envelope after the lead had cooled and shrunk to avoid/minimize this. If the keel envelop top is welded watertight, this is not a major problem but is annoying. The inside of water tanks and sump of diesel tanks and under chain locker are other common areas where there is some pitting. On the positive side, a couple of these boats have had bronze and/or stainless thruhulls (mounted on insulated pads) and I have been surprised how trouble free the aluminum has been around those.




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