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      A Few Simple Rules   05/22/2017

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      Moderation Team Change   06/16/2017

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hippophagy

Building a lighter plywood/epoxy Windmill more like a Javelin/Cherub

56 posts in this topic

I'm thinking of building a Windmill in my basement in NYC. People rave about them and it seems like a pretty straightforward build; this would be my first build (I'll have the help of an experienced wood boat builder). I'm interested in possibly adapting it a bit so the hull would be a little lighter (I'm not worried about building to class specs). I live in New York City and I sail out of Sebago Canoe Club in Jamaica Bay where we launch from a floating dock so the lighter the better. I'm wondering if a Windmill could be built more like a modern ply/epoxy skiff such as the Javelin or the Cherub to maximize weight savings and possibly increase strength.

 

I read this very thorough Javelin build plan by Luis Pinto and also this great build page on the Cherub by Jim Champ. It seems that you could take the basic forms of the Windmill and build it more like these boats. In his excellent Windmill build article Tom Lathrop recommends 9mm ply for the hull bottom and 6mm for the sides. The Javelin builders seem to be getting away with 3mm ply covered with 4oz S glass with 4mm ply over the false floor. Both the Cherub and the Javelin incorporate a false floor and I'm considering doing the same on mine.

 

To take things a step further (or a step too far some would say) I'm wondering if I can rig this hull a bit more like a skiff with a single trap and a spinnaker pole. The reason I just don't build a modern skiff is I'm hoping for a hull that could be a bit more versatile. I'd like to be able to do short cruises for beach camping (with a cruising rig) and take my wife out once in awhile in a boat that she can sit down in. The more modern skiffs seem a bit trickier to build and less suited to other uses.

 

So in summary I want to keep to the basic plan of the Windmill, build it a bit lighter using S glass and thinner ply and rig it a bit more like a modern skiff but also have the option to rig it for cruising. I'd like it to be fast but it will be classless (or in a class of it's own depending upon how you look at it) so it's not about being faster than anyone else. I'd like to be able to do other things than try to go fast but I understand it's a narrow hull designed for racing so it will never be a big wide heavy day cruiser.

 

Do you think any of this is worth doing? Will I see reasonable weight savings and comparable strength? Am I adding too much and compromising a time tested design? I know one of the selling points of the Windmill is cost savings in materials and rigging. Am I defeating the purpose by adding all this stuff? Will I end up with a lighter faster more versatile boat or a compromise that sucks money and time (and just basically sucks)? Any and all opinions and suggestions are welcomed. Thanks!

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Rather put your cash, time and skills towards building a class legal boat, the class is not that important, but being able to race it and to re sell it later is worth a lot.

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Rather put your cash, time and skills towards building a class legal boat, the class is not that important, but being able to race it and to re sell it later is worth a lot.

As far as racing goes for me I'd just do open races at my club. I have no car so getting the boat to races would be a challenge. I see your point about resale but if I build it and I like it I'll probably hold onto it.

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A Windmill is a great boat. They shorted the dagger board and simplified the rudder planform (recently) to make it even better.

 

It is kind of amazing that a boat designed in 1953 at legal weight is as fast(er) as a V-15.

Windmill WM 89.5 Vanguard 15 VGD15 90.5

http://offshore.ussailing.org/Portsmouth_Yardstick/Current_Tables/Centerboard_Classes.htm

 

With all that freeboard and woven Dacron sails with short battens it still compares well to the Tasar with low freeboard, mylar and full length battens. (... length helps) Tasar TASA 88.2

 

Before they had a minimum weight (or tanks), a neighbor built one to 165 lbs in the conventional manner. Weigh each piece of wood before you buy it.

 

I think the traditional plans are 1/4 inch plywood with stringers. There is no reason (that I know of) that you could not make (vacuum bag) your own (balsa cored) plywood with mahogany face veneers (I think someone did that and it measured and finished well, ask the class). People have also built them out of 3 layer western red cedar. The outside layer grain was on a diagonal, as if cold molded. Each piece of cedar wood was 2 inches wide as if looking at a 2x4 edgewise, except beautifully finished in polyurethane over WEST Epoxy.

 

The best match for your needs might be Tom's stitch and glue way, except make the panels yourself. Core them with balsa. I think what you want is a boat that measures with the addition of corrector weights and maybe floor boards and extra glass.

 

Someone has tried a trapeze, you need a steady breeze.

 

Of the boats I have sailed, the Windmill and J-22 I liked the best.

 

You might want to look into 6 oz bi-directional stitched glass. That is the experimental material of choice for my experimental build - a high freeboard hull, with room for a second person or a dog, under a laser rig that I hope can sail even with traditional lasers.

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I was looking at making a cat that is designed for ply. I then found someone who had gone so far as getting a cut file for cnc cutting. He'd done it for Duflex panels but there are others, core with glass skins. I heard that consistent marine ply is now hard to get.

duflex or polycore seem to be lighter than ply anyway.

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Ed knows what he's talking about. I don't know what the class rules are for corrector weights, but it might be worth looking into if you end up making the boat much lighter. If you built it out of 1/4" ply and changed some of the structure, you could save some weight surely. 3mm you could certainly do (I have in my Frosty...old school Moths do this and obviously the aforementioned Javelins too), but you will pay pretty stiff penalty in the form of fragility. 3mm ply is pretty dainty stuff and needs to be looked after a lot more carefully than 1/4". Also, since you want to limit weight because of your launching setup, consider that most people don't actually have it as good as you! Launching on a dolly is a far bigger pain in the ass than storing and launching from a dedicated dock. Most big sailing programs are pulling heavy ass 420's, FJ's and other boats up on docks. MIT hauled super heavy tech dinghies onto their docks for years, until they shaved a whiole bunch of weight last year by going carbon. The point is, a standard Windmill isn't heavier than these boats that get hauled onto docks by kids all the time, so as others have pointed out, i might be worth your while to at least try to make it class legal so that in the event that you do want to sell it, you will have a somewhat easier time. All of that said, good luck! Post pictures of your progress here, I for one would love to see how you make out. The Windmill is an excellent boat that is underappreciated by most people since they haven't actually sailed one. As Ed pointed out, given when it was designed, its performance is pretty remarkable.

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WindMills are pretty good as is. Be careful about removing the toughness in your quest for a few pounds. There are good places to remove weight and for your application the skin and especially the bottom may not be the place if you like boats that don't have dents in the skin.

 

Interior members are fair game and so forth. Spend some time with the plans or better yet track a WindMill down and sail it to see where the weight is best removed.

 

Impressive boats and very far ahead of their time. Enjoy the building and sailing.

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Also food for thought. If you redesign and change the specs from a Windmill you won't have a Windmill when your done. If your planning on racing Portsmouth and it is not a class legal boat then the rating is wrong and you won't be able to honestly race it with a mixed fleet and you won't be accepted into the Windmill fleet either. The "Mill" is a great boat and it sounds like you have the initiative to build a good fast boat. Build it class legal and then you have something in the end that will have good value. Go rouge and you have a fun to blast around boat that you can't race and will only have value to yourself.

FM

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May not have listened well enough. The question might be, "Could one make the Windmill hull shape double bottomed using thinner ply?"

 

For me at least, double bottom boats wipe out my back. I likely would have more experience in a JY-15 or an Vanguard 15 if not for Sunday night pain, that lasted, after regattas in each boat. I like double handed boats, but literally can't stand double bottom boats.

 

So yes, but try spending a weekend with the ergonomics of each interior layout before you build one. Should have addressed this directly the first time.

 

The shorter DB fixes the ergonomic issue of high speed jibes with the board high enough to not trip over. The DB has to be down far enough to clear the vang. The new rudder profile is deeper, so there is more control off wind in waves.

 

If you switch to a compression vang, or gnav that would solve a lot of issues for the crew during tacks upwind, when you are generally forward in the boat. There are people who know stuff and can help with the early decisions. North Sails has a fast set of sails, but they might need adjustment if you use a compression vang, Ethan can help. And I think I saw Dave Ellis post here. He has first hand knowledge that would be most beneficial to your cause... I don't know if he is working with a sailmaker right now.

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Should look at US-1 dinghy which is IIRC essentially a cut down windmill hull with a una-rig.

US-1 may have some of your details worked out already. But maybe you don't want less freeboard than the windmill.

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Should look at US-1 dinghy which is IIRC essentially a cut down windmill hull with a una-rig.

US-1 may have some of your details worked out already. But maybe you don't want less freeboard than the windmill.

 

The Windmill is a fun boat; if you want to build something that's a good pick. However if you want to sail there are many around in good shape at a price far far less than you can build one for.

 

Having sailed both the Vanguard 15 and the Windmill a bit, it would surprise me if the V-15 was slower in any conditions other than light air. More difficult for a home builder though

;)

 

FB- Doug

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OK, I'll weigh in here. But it may not be what all my Windmill friends expect........

I, like you apparently, like to tinker with my boats. The Windmill really is a good boat as it is.

But, my racing days with crew are over. Single handing henceforth. (Yes, it's me, not any of the numerous pick-up crew I've used over the years.)

I'd use the normal ply on the bottom. (Boat #1 back in the day bottom flexed up and down enough to send a little kid (me) off balance.)

Thinner on the topsides.

Double bottom about four inches up, same "V" profile as the bottom. Foam blocks or the like to hold them apart.

This allows the topsides to be that thin with strength. It negates the need for (heavy) side tanks. It is self-bailing. It holds the dagger board well, negating the need for bracing on the top of it.

And the 'Mill is deep enough that it still will have more "sit room" than most boats.

I once put a Hobie 14 sail on a Windmill. No reason that a more modern rig can't be used.

The US 1 uses just the mainsail. (That boat started as a Windmill that did not pass measurement muster. So they cut down the freboard and made it a single hander. A moose in the genre of a Finn to sail, best by big guys. Not as fast as a Windmill.)

I once bought a Windmill (been sailing them since 1955) that had been converted to a mainsail only. They had moved the mast step forward a foot.

Used the regular mainsail. The class mainsail is low aspect, putting significant sail aft down low. A taller rig with shorter boom would make a new mast step forward unnessesary, if the mast were set up with no rake.

If you put the mast forward it becomes more difficult to use a trapeze, as it pulls you forward; My Contlender had a well-raked mast, so, while it still pulled me forward, not as much. My A-Cat's trap was attached so far up that it didn't matter as much.

A Hobie 14 sail, if you can find one, is about 115 square feet. Main and jib of the Windmill is 119. There are other small cat's sails that would work. I bought one from Masthead Enterprises used sails for my Raider frankenboat that had two reefs in it. Handy. Don't use the catamran moosy mast, however. A multihull generates much more force on the rig with their righting moment. You'd need a lighter mast or you'd flip at the dock. Just use spreaders, and a diamond stay up front if necessary.

As for using a trapeze. I found that upwind and close reaches it helped. On broader reaches I got my butt on the deck.

Sounds to me like a fun project.

And don't worry about Portsmouth handicaps. There are modification numbers to apply for boats with changes.

Dave Ellis

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I wanted to say thanks to everyone who responded. Particularly to Ed and Dave for your lengthy replies. You gave me a lot to think about and I know it's informed by many years of experience. I'll continue to post my ideas and progress in this thread. Thanks again. Matt

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Campfire stories: other people will know if this is true.

 

Some guy named Laviano used to bring 3 hulls to nationals and would select based on the conditions. An easy to build hull allows you experiment with hull shape at full scale. I think he went on to do something for AC boats...

 

The Mooreman fiberglass boats were from the generically fastest of those hulls.

They have a pronounced knuckle at the bow under the chines. If you think you want to push volume below the chines, that is probably a good philosophy working within tolerance of the nominal lines.

 

Those molds were picked up by Mike McLaughlin. The rules allow a 1/4 inch of out of plane radius in the bottom panels. In the Mooreman this may have been centered. I think Mike added more volume below the chines within the rules. If you make your own plywood and want to play with a parameter, getting volume below the chine might be possible.

 

If you are always planing, volume below the chine is not as important. Maybe not even helpful.

 

The Mooreman glass boats seem to have panel stiffness arranged in this order based on a deflection test and patterning of internal structural elements: 1) bottom the thickest, by a lot. 2) interior tank walls (and deck), there are all sorts of people loads there, 3) outside shear panels thinnest by a lot. The transom is double walled, but structural elements pattern.

 

The McLaughlin boats are Kledgecell. A lot more headroom on minimum weight so all the panels seem stiff with no patterning of internal structure.

 

Talk to the class measurer, but I think the selected Laviano/Mooreman hull was the most dart like at the chines with the gunwales max flared between the thwarts. Really don't know about the transom or rocker. Just sort of know about the other stuff.

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So I decided to build to class specs. I'm building the boat in a 200sqft room in my Manhattan apartment basement. I'm following Tom Lathrop's composite build instructions found here. You can follow my build on this Facebook page.

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Thanks for the link. Somehow it never occurred to mark the plywood in the vertical.

 

If you build it like Tom's, it will be fast.

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The latest composite Windmill hulls built by Johannsen Boat Works seem to perform at the top of the fleets being very stiff and at minimum weight. Some durability issues in the the trunk area have also been addressed. Check out the Windmill site.

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A standard Windmill is still pretty light. You shouldn't have a problem pulling it onto a float. And you'll be glad its class legal in case you decide to try class-racing, or to sell the boat.

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About 1960 Homer Luzier of Sarasota, Florida was building plywood Windmills using no nails/screws. He was a craftsman, so all joints were perfect, so the old Resoursenal (sp?) glue (Elmer's, believe it or not) worked fine. And the boats lasted and were like furniture.

But they were significantly lighter than most other Windmills.

Finally, at a Sarasota regatta (As a high school sophomore I was crew for the winner, Dennis Snell) all boats were weighed and the class adopted as a minimum weight the lightest boat.

It weighed 198# and was by Homer L.

Those boats had no air tanks and, obviously, no double bottom.

So, if you can get the hull down to that weight finished, you are doing well.

You can add up to 15# of lead and still be legal. So make it light and add lead later if you want it class legal.

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So I decided to build to class specs. I'm building the boat in a 200sqft room in my Manhattan apartment basement. I'm following Tom Lathrop's composite build instructions found here. You can follow my build on this Facebook page.

Glad to see that logic has prevailed. Enjoy the build and then enjoy pitting your skills against others using similar equipment.

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About 1960 Homer Luzier of Sarasota, Florida was building plywood Windmills using no nails/screws. He was a craftsman, so all joints were perfect, so the old Resoursenal (sp?) glue (Elmer's, believe it or not) worked fine. And the boats lasted and were like furniture.

But they were significantly lighter than most other Windmills.

Finally, at a Sarasota regatta (As a high school sophomore I was crew for the winner, Dennis Snell) all boats were weighed and the class adopted as a minimum weight the lightest boat.

It weighed 198# and was by Homer L.

Those boats had no air tanks and, obviously, no double bottom.

So, if you can get the hull down to that weight finished, you are doing well.

You can add up to 15# of lead and still be legal. So make it light and add lead later if you want it class legal.

When I was sailing prams in the late 60s, the hot set-up was a Luzier hull and a Bremen sail.

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Some good progress on the build here https://www.facebook.com/windmillsailing Any tips on sailing the Windmill solo? I weigh 145lbs but I'm fairly athletic. I intend to put reef points in the sail for cruising and I'm also considering a trap too. I seem to remember people suggesting just going with just the main for soloing. Found this great photo on the class page of someone soloing http://windmillclass.com/photos/243.jpg with both sails. Thoughts? Thanks!

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No problem sailing a Windmill alone.

With a proper jib stay, sailing under mainsail alone works very well if the dagger board is angled aft under the boat. Balances just fine.

(I placed third in a Midwinters in high winds and a light crew sailing upwind with no jib.)

Not quite so balanced with a reef in the main and flying the jib.

This is a relatively low aspect rig. With a reefed mainsail it ends up with a lee helm due to the jib's relative size.

Even if the dagger board is angled as far as it can be toward the bow under water, still there is lee helm.

Unless you sail it "on its ear", inducing weather helm. Not fast or comfortable.

Now, sailing with a reefed main and no jib would be very quick in a blow.

Dave Ellis

National Champ 1957.59.80,85 04,05

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Looks like the OD Windmill class had a resurgence this year at their nationals with 37 entries. :)

No surprise at the winner.

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Hiking strap position and tension help for single handing. And getting the mainsheet block up off the floor. Here is what I remember:

 

A pair of straps anchored at the shock cord location back on the keel under the tiller. Rear thwart holds them up and out exactly where you want them. They are pretty high in the boat as, with no double bottom, there is depth. I remember them as about a foot off the floor - impossible to miss if you step across both on the tacks.

 

If you ever have a crew, it helps to give them a little room. We ran with the mainsheet ratchet on a foot long wire about a foot back from the back of the trunk. A spring at the base and a PVC riser kept the sheet feed to the ratchet above the straps even when fully hiked.

 

Both jib and main sheets cleated in cam cleats on the rail. The aft end of the DB well had a pod that could be sat on with a set of double ended controls for vang and maybe outhaul. The traveler and Cunningham were run to the side tanks just under the sheet cleats. The jib halyard was high on the port side of the DB well.

 

With controls and straps in these positions the boat is easy to single hand for traditional mark rounding courses.

 

The boat loves to be sailed light.

 

Thanks for guiding me to the 37 boat results. It reminds me of how wrong I could be about defining a good start.

 

Sailing against Ethan and Trudy in Charleston for the first time, my thoughts were "This guy is a 505 world champion. And a sailmaker (North). He has great boat speed and he wins a lot of races. But he sure does not know how to get a good start."

 

But then I thought about what the definition of a good start was in my mind: Clear air, favored end...

 

But there is this other thing... favored tack. Being in phase, in clear air, right at the start. Ethan and Trudy, for me, redefined "Good start" in the first regatta we "competed" with them. They don't get trapped out of phase at the start very often.

 

As I looked at the list, I think I learned something positive about sailing from every familiar name on that list.

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As I looked at the list, I think I learned something positive about sailing from every familiar name on that list.

Looking at the list, was there any "woodies" up in front?

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I see that two of the new FRP hulls were 1st & 9th but can't tell which of the others were ply.

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Near as I can tell, the top placing Windmill at the 2014 Nationals placed 11-th.

Fine looking boat it is.

Dave Ellis

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Near as I can tell, the top placing Windmill at the 2014 Nationals placed 11-th.

Fine looking boat it is.

Dave Ellis

I stand corrected. Larry in 5048 looks like this.

 

336.jpg

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Larry was 5th in his beautiful wooden Windmill, and some of the best boats have been wood. But, if you are not inclined to build your own boat, the new Johannsen boats are really nice!

Cheers,

Ethan

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Larry also built a couple of incredible Contenders.

Like fine furniture.

And fast, with Ethen sailing one.

Glad to hear that he has built a 'Mll, too. I'll bet it is spectacular.

Dave Ells

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So nice to see this thread has been active since I last checked it. I'm picking up a wood mast and some sails from Southern NJ this Sat. Still building. Question for those who have built composite Mills. Tom mentioned using 4" 9oz (although in one place he mentions 10oz) fiberglass tape. He also mentions that biaxial tape is stronger (and more expensive) but I can only find 4" biaxial tape in 12oz. Will this work or is it too heavy? I've searched and searched but can't find any 9oz or 10oz biaxial tape. Should I go with the 12 or does anyone know where to find 9 or 10oz?

 

Thanks,

 

Matt

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I get where you are coming from. I was looking for light biaxial glass fabric to go on last over foam and mahogany veneer and discovered that the UK and Italy have that stuff, and maybe some surf board manufacturers as X-Glass. I was looking for 250 gsm and found a person in LA that had imported some and cut the roll in half... but back to tape:

 

This link has 300 grams per square meter http://www.wessex-resins.com/westsystem/reinforcing-fabrics.html

 

and a conversion table.

 

Selcom in Italy http://www.abic.se/pdf/COMPOSITE_GLASS_FABRIC_ABIC.pdf

 

makes what you are looking for. I think you want EBX 300 cut into the tape width you are looking for.

 

http://www.multiaxialfabricselcom.com

 

There is a factory in Alabama that would run material that light if you were buying a lot.

 

Thick seam tape drives weight into the boat. I would use light woven tape before using too thick biaxial.

 

Make friends with the people at WEST epoxy. They know stuff.

 

My thoughts on stretch for rope and fabric is that stretch is always a percentage of a length.

 

This moth sailor uses a 4 to one mainsheet arrangement

http://www.internationalmoth.co.uk/wp-content/uploads/2014/07/Nathan_Outeridge_2-310x310.jpg

 

But the traveler bridle is so long that when fully sheeted, the loaded mainsheet length is maybe 5 feet? If you consider the load difference on each side of the ratchet block - even less. (For what it is worth, this guy does not seem to over sheet. Could not find a picture block to block.)

 

It is the same thing with the tape. Yes, biaxial is better, especially if you are using polyester resin, because there is no crimp. But stretch matters most where length of the fiber is long.

 

Just some thoughts. I am open to correction.

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Anyone had this happen on a Lathrop build? Double checked all measurements and the gap is the same on both sides. Seems like the stern is low.

 

10553759_1532632790292665_60161506807420

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Near as I can tell, the top placing Windmill at the 2014 Nationals placed 11-th.

Fine looking boat it is.

Dave Ellis

I stand corrected. Larry in 5048 looks like this.

 

336.jpg

Hah! Pretty sure that picture is of Larry and me sailing the nationals at Rock Hall. That's one of his glass boats, and it's sooooo uncomfortable. Definitely prefer the woodie. SailWriter, you should know the origin of that picture, since you were the PRO at the event!

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Yea, I think that was the race where I called for a triangle/W/L. The RC helper put the number on the back of the RC boat and I did not check on it.

Well, everybody turned down wind after the first beat. I took a look at the transom sign to find that the W/L/W designation was posted.

Had to quickly call the mark boat to get a ride to the windward mark to take finishes. But don't tell anybody about that goof.

5048 at least had wood thwart seats, eh.

The original plywood Windmills were not stiff on the bottom. I once crewed on Windmill #1 (yes, the first one.) You could see the bottom pumping on each wave. Modern wooden construction may very well be stiffer than glass boats.

Dave Ellis

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After searching far too long I've settled on this 6oz 6" Biaxial non-woven tape from Boat Builder Central.

 

http://boatbuildercentral.com/proddetail.php?prod=E_tape_6oz_biax

 

I ordered a bunch of West resin and 206 and 207 hardner as well as collodial silica for filleting. A few questions:

 

1. How much resin will I need? I ordered 3 gallons.

2. How much hardener? I ordered 2 quarts of 207 and 1 quart of 206.

3. The tape is 6" wide 6oz. Tom specs 3 or 4" wide 9oz. Is it worth it to try to cut down the 6" tape into a narrower tape? It doesn't look like it has much of an edge. I've heard this can be a pain. If I use the whole 6" will I gain anything besides weight? Will 6oz 6" non-woven end up being about the same as 4" 9oz woven?

4. From what I read collodial silica seems to be the right filler for a high density fillet. Is this the right stuff? Should I be adding anything else?

 

Thanks,

 

Matt

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Haven't read this lately, but follow the stitch and glue recommendations in here exactly.

 

http://www.westsystem.com/ss/assets/HowTo-Publications/GougeonBook%20061205.pdf

 

It should have coverage in here. As I recall, adding sand (colloidal silica) makes the epoxy hard to sand. Use sawdust whenever you can, or the low density fairing fillers.

 

On page 298, these guys call out low density filler and two layers of 9 ounce on each side. Colloidal silica is not low density but rather high density. What you are looking for is a thick section with strong surface layers and light interior that supports shear. Balsa saw dust?

 

The biaxial is easy to cut at that weight. The picture shows an offset lap which reduces the amount of cloth on the other side of the bend... I would copy their picture and use a low density filler as much as possible.

 

Hope this helps.

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The joy of tape is no loose fibers. If you are going to make cuts and have glass strings everywhere, just buy the cloth weight you need.

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A note on the use of colloidal silica, whether from a bulk bag or a certain white tube. The use of this is to make the mix thixotropic, that is it only will move under load. It will also by virtue of being made from silica, basically quartz be very very very hard. This can be a good thing but if you are planning on sanding, not so much. But a hand plane is good while the epoxy is still green.

 

For most joints now I prefer a mixture of the microfibers from the white tube, not the milled glass ones but the real cotton linen ones, and then add the wood flour from the non white tube epoxy guys. The joints have been strong enough for my multiple use and have been very easily worked. When I was making custom paddles I used to run the laminated blades through the planer without damaging the blades. Would never be able to do that with the silica. For bulking up a joint some microballons can be used but you'll need to make your own choices there. The fibers do most of the work, the wood flour keeps it in place before cure, and if you want it to sand a bit easier and be a bit lower density add the balloons. Then if you've done your fillet pretty well after the epoxy has gotten grabby go over it with a brush and a bit of balloons mixed into some resin and it will level out the surface.

 

As always try it first on some non boat part. Good luck.

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Biaxial tape... there are two reasons for it. First, in normal fglass cloth, the fibers run 0-90. So when you're taping a seam, only half the fibers are adding strength to the seam. The rest are parallel and contribute no strength. Biaxial tape is 45-45 oriented, so every fiber spans the sean and contributes to its strength. Secondly, biaxial tape bends into a tighter radius easier than cloth... easier to get it to conform, and less filet material needed.

If you're cheap like me, make your own biaxial tapes by unrolling cloth on a large clean table, and cut strips of desired width out of it on the 45 deg diagonal.

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Mr. Yacht, longer pieces of plywood require fewer scarfs. Conversely, transverse frames require precise fitting.

 

Does anyone make 16'- 18' length marine grade ply with the majority of ply grain perpendicular to the length?

 

I suppose it would work to use shorter pieces of ply ( with the majority of grain in the stronger direction )with butt joints and attaching the transverse frames at those points, but that kind of complexity is not really the appeal of plywood, is it?

 

I suppose you could just come out and suggest we all leave it to the professionals, CAD, CAM, and vacuum molds, etc etc....

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If conformability is a big issue, go to a satin weave instead of plain weave. You can also get 90/10 satin and some other 90 degree tapes. The +/- 45 is generally good because it transfers shear efficiently. Ordinary tape (balanced 0/90) is quite poor really.

 

The ëngineering" in most stitch and glue boats is suboptimal at best. Look at all the little plywood stitch and gle motorboats. Built with stringers rather than transvere frames (because it is easy and drains nicely and is "strong"). And yet almost without fail, teh plywood is put with face grain fore-aft. This makes no sense with stringer construction, and then the chines are taped with 0/90 tape. It works but it would be so much better were it built better. But nobody seems to know this.

Sure, but they over spec the ply so it's fine for a motorboat. For a sailboat it's different with the weight and rig tension issues.

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Ned, Amati, Fastyacht:

I've only designed and built 3 plywood dinghies, so I'm no expert, but I will restate the more or less obvious here:

  1. On a boat, whether sail or power, the global strength needed is often greater in the longitudinal direction, so the composite section of the stringers and ply contribute to that strength (and stifness, which is usually more important than strength when it comes to skin panels).
  2. Scarfing plywood across the face grain is easier not only because the scarfs are 8' apart as opposed to 4', but because the feather edge of the scarf is perpendicular to the grain, and is therefore supported.
  3. As long as you are using 5-ply (or more) sheets of plywood, there is still significant stifness and strength in the direction perpendicular to the face grain. You must be much more careful with the 3-ply stuff.

Consequently, I usually do the following: with single layers of plywood, I will run the face grain fore and aft if I have 5-ply (or more) sheets. For three ply sheets, I run the grain athwartships, and live with doing more scarfs, which will also be uglier because they will run parallel to the face grain. If I have double layers of plywood, I can run the grain fore and aft with 3-ply sheets with no problem. In areas with very tightly spaced stringers, I might get away with running the face grain of 3-ply sheets fore and aft, particularly if there is a lot of athwartships curvature. Also, I do an engineering analysis of the composite strength of the panels, to make sure I'm not assuming too much, and compare that with ABS standard for Offshore Racing Yachts, and/or what has worked on the previous dinghy.

By the way, the ABS standard is very conservative when it comes to the strength of plywood panels. That's why I do my own composite analysis. I use the standard for estimating the loading.

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Whaaaat? ABS Rules for dinghies? Haha.

 

I disagree about the scarfs. They work great. You are right though that if you don't carefully support the edge, they will tear out. (Hint: support the edge...)

Yeah, I know using ABS requires some extrapolation beyond the normal range of sizes and displacements to get loading for dinghies, but the resulting scantling requirements I get are in the same ballpark as other similar dinghies. I think it's a good reality check.

 

I've tried supporting the edge passively. I will take your hint and try more aggressive measures, such as using tape backing or even gluing on a wood backing strip to be sanded off later.

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FY,

It has been a while so I'll have to dig around a while for an example, but I use the formula in the front of the book (sorry, I don't have it here with me) for the pressure, and then either, 1. calculate the required section modulus for the type of member (skin, stringer, or frame/bulkhead) based on the ABS formulas, or 2. calculate the required section modulus from first principals as you do, using a safety factor I'm comfortable with. Then, if the size is close to what I see in other dinghy designs, I'm comfortable.

 

For the determination of the allowable stress, I use the values in the USDA Wood Handbook.

 

For plywood section modulus, I first calculate a composite moment of inertia (I) using something like 1/20th for the across-grain stiffness, and ignoring the top layers if they are loaded perpendicular to the face grain. I then determine the Section modulus from that, once again taking into account the orientation of the face grain when selecting the appropriate distance from the centroid.

 

I don't perform an actual extrapolation, but I realize the rule was not designed for 14' planing dinghies, so my use of of the rule is a de-facto extrapolation.

 

My current dinghy I didn't calculate from scratch, I just reduced the spacing of the stringers to reduce the skin thickness as compared to my previous dinghy. This also allowed me to reduce the stringer depth, making sure the section modulus of the T-beam formed by the stringer and skin was proportional to the reduced loading (due to a smaller tributary area). I kept the same width so I could actually hit the stringers with the fasteners, which is a semi-blind operation. In the end, the weight of the extra stringers was almost perfectly matched by the reduced weight of each stringer. The weight savings was in the skin.

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I must state that this is not a blind plug and chug calculation process. I look carefully at other designs, even talk with a designer friend, back-calculate section modulus from other designs, etc.

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How useful would a tabernacle be for the mast (so it can be easily raised and lowered while on the water) like the one found on the Wayfarer? I was reading about Frank Dye's adventures and it seems to be a pretty useful thing. Thoughts?

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Not sure the geometry or the hull stability (static) support this sort of thing. The boat really wants to be under sail. I think most are keel stepped now, but remember some thwart stepped. I would sail a season before doing something like this, as it looks like a secondary modification.

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The urgency may depend on how length challenged you are for storage. Or if you're planning on going through many culverts on the way to your yachting arena.

 

And would be best to build it in early.

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Has anyone tried this ziploc epoxy mix method that Duck Flat-Wooden Boats uses?

 

"The right proprotions of epoxy (both parts) and whatever is being used

for the filleting compound are dropped into a ziplock bag, and the bag
closed with minimum air in it. The actual mixing is done IN the closed
bag (you know, knead, knead? the technical term is palpating, I believe,
but that too easily leads to palpitations and other terrible
puns...sorry,sorry,I'll get back on topic...). Mixing in the bag avoids
a whole lot of mess, unless there's too much air in it, in which case
one can create a very messy local burst. Once mixed, a corner of the bag
is cut off, and voila, a cake-decorating epoxy tool c/w thickened epoxy.
The size of the the snip off the corner depends on how thick, or thin a
bead is wanted."

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I always mix in cup and then dump it in bag. Pretty sure I would misjudge the bog thickness in the bag. Waste probably works out equal. I lose a little to the cup but can get the rest right in the corner of the bag. I imagine bag mixing would get a good waste layer throughout bag.

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I think it would be difficult to get a thorough mixing in the bag. It's not that messy if you use plastic yogurt or cottage cheese containers and tongue depressor or similar round ended stick for the mixing. After mixing, you can always put it in the bag for your cake decorating application method. I just wipe the mixing stick off with a paper towel and it's ready for the next time. The leftover epoxy can be peeled out of the plastic container when it's cured.

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What Shu said. Remember according to the guys who wrote the book, the main failure of epoxy applications is improper mixing. So mix it, mix it good. Epoxy first then add filler to taste.

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