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42 South

Masts - keel stepped vs deck stepped on race boats

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Currently looking at race boat where the mast has been converted from keel stepped to deck stepped.

There is this thread back in 2005, but some of the discussion centres around cruising boats. The pluses and minuses are pretty well discussed there so I won't go into detail again.

The boat in question is a 2010 model between 35 to 40 feet, and it seems that the local fleet is confined to sailing areas inside a bridge - to get to race in the open ocean they need to drop and re-hoist the rig to pass under the bridge. I will be asking for any engineering documentation for the change.

I seem to recall that there is either some insurance, or race entry conditions imposed on boats with deck stepped masts???? What's the current thinking on this?

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I've always been a fan of keel stepped masts especially on larger boats. Sounds like the boat you are looking at was modified to get under a bridge.

I would think there are a lot of variables to make this a safe conversion. Is this a conversion done on other boats of the same or similar type in this area? I think you are on the right track about engineering.

Racing? One design or PHRF... would it still be a one design? Did the conversion make the mast longer or shorter? Could change a PHRF rating.

Insurance - I would make sure the company is throughly advised of the modifications. You would not want a claim denied due to undisclosed modifications. Or faulty this or that.

Also, think about resale. Will this change the price of the boat? It could even be a positive modification, if all is done properly and this boat can now access areas others can not.

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In Perth they do this on just about everything... Ask @Swanno how it works on Syrenka (60 footer)

There are a couple of places the locals use, a friend is about to add this option to a Dufor 36, the budget is about 15K for the job.

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5 hours ago, Rushman said:

In Perth they do this on just about everything..

Bingo, yes from Perth. So would this affect value for a yacht outside of this area?

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2 minutes ago, 42 South said:

Bingo, yes from Perth. So would this affect value for a yacht outside of this area?

So you are looking at a boat that has this lowering ability?

Handy feature to have if you ever have to drop the mast.  If your club has a hoist that can handle the weight of the mast and enough room to swing the mast onto land, there is no need to get a crane in.

 

 

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5 hours ago, Rushman said:

In Perth they do this on just about everything... Ask @Swanno how it works on Syrenka (60 footer)

There are a couple of places the locals use, a friend is about to add this option to a Dufor 36, the budget is about 15K for the job.

In the early days in Perth and certainly well before the 1987 AC in Fremantle (that introduced recreational vessel infrastructure adjacent the ocean) every blue water boat in town had a mast sitting on a deck mounted tabernacle lowering system, irrespective of size.

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Milt Vogel's Peterson 39 Apogee had a mast hinged at deck level to clear the Huntington Harbor bridge. They won all kinds of pickle dishes with that boat.

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7 hours ago, 42 South said:

Bingo, yes from Perth. So would this affect value for a yacht outside of this area?

42 South, 

 

The problem is outside of Perth most owners put their heads in the sand and don’t understand the benefits of a hinged mast. When you look at a lot of the bigger boats in Perth they all can lower their masts so they can race in the river or outside. The interesting thing is they are not all deck stepped. A lot of boats still have the mast keel stepped and the hinge is about 1- 1 1/2 feet above the deck. The masts normally are hinged at the back and the forward side of the mast is bolted together with 1 or 2 bolts. 

 

The mast bend does change a little, but not as must as most would think. What you will find is the sail makers in Perth understand this and cut the sails to suit. 

 

The intresting thing is the crews in perth  lower the mast and put them back up very quickly. From the harbour entrance to the bridge is only about 1 mile or so and the crews lower the mast, go under the bridge and have it back in place before they are back at the mooring ready to race. 

 

Perth has a lot of great boats for sale at great prices it’s a pity that the transport costs are so high either way across the country. 

 

Pulpit

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Very few boats over here in WA are converted from keel stepped to deck stepped.

Most have a hinge built into the rig just above deck level.

When the rig is standing there are minimal to no noticeable differences with the same rig without the hinge. The hinges generally have bolts which are in place when the rig is standing such that if the forestay goes the rig isn’t going to immediately lower itself.

While the rig is being raised or lowered its a different story, it depends on how well the hinge has been constructed and the systems which go with the raising/lowering process.

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Thanks Pulpit for your advice.

From the photos it is not clear what system it has but I suspect it has what Sandgroper describes. My only concern is that it will keep pointing towards the sky :rolleyes:

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14 hours ago, hoppy said:

I doubt it was mentioned in the linked thread, but keel stepped masts can have a thinner profile and less stays than a deck stepped mast. I noticed this when my Sun Odyssey 40 was moored next to a Sun Fast 40.

Hoppy I noticed that too once when I pulled up beside a Chevy Corvette I saw its radio aerial was fatter than mine.

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30 minutes ago, 42 South said:

Thanks Pulpit for your advice.

From the photos it is not clear what system it has but I suspect it has what Sandgroper describes. My only concern is that it will keep pointing towards the sky :rolleyes:

As long as the forestay is attached it should

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16 hours ago, hoppy said:

I doubt it was mentioned in the linked thread, but keel stepped masts can have a thinner profile and less stays than a deck stepped mast. I noticed this when my Sun Odyssey 40 was moored next to a Sun Fast 40. The taller keel stepped SF mast was thinner with less stays and most probably much lighter than my rig.

I've noticed that a few Perth yachts for sale have a hinge to allow the mast to be dropped. 

Why do I feel just a little dumber for having read that?

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That's Hoppy for you !!

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20 hours ago, hoppy said:

. I noticed this when my Sun Odyssey 40 was moored next to a Sun Fast 40. The taller keel stepped SF mast was thinner with less stays and most probably much lighter than my rig.

 

3 hours ago, Monkey said:

Why do I feel just a little dumber for having read that?

1 hour ago, hoppy said:

can you get any dumber? did not think it was possible.

Hoppy strange how you missed the SF 40 having a 3 spreader fractional versus your 2 spreader masthead with baby stay.

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On ‎2‎/‎10‎/‎2019 at 6:10 AM, Rushman said:

In Perth they do this on just about everything... Ask @Swanno how it works on Syrenka (60 footer)

There are a couple of places the locals use, a friend is about to add this option to a Dufor 36, the budget is about 15K for the job.

Ours was 'keel' stepped though hinged at the deck as was Wild Side.

The keel step allowed a bit of movement at deck level though having not sailed on a similar sized deck stepped boat, I am not sure on any advantage/disadvantage.

The keep step did save us once when the forestay broke though.

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This is how the traditional boats do it in our area..

The man in the middle is standing in front of a wooden tabernacle  that goes right down to the hull. Often the boom is hinged from the tabernacle, not the mast, so you don't have to disconnect the boom to lower the mast..

Although this boat being shown is a hire boat (hence the staged photo), it and the privately owned Broads cruiser fleet are raced. (even in the river in shot)

Less traditional boats tend to have a deck mounted  stainless steel tabernacle,  often with an A frame to help keep it all in line. (rear pivot points in line with mast, point of A attached to the forestay at deck level..

 

maxresdefault[1].jpg

Oh if you come to Norfolk, UK,  and wish to hire it https://www.marthamboats.com/hire/yachts

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The only concern would be if a keel stepped mast was just chopped at deck level and a tabernacle added.

Deck stepped masts have to somewhat higher moments of inertia because a deck end is considered "pinned" and not "fixed" like keel stepped. So the tube has to be a bit bigger (typically) or at least have thicker walls.

If the mast was on a cruising boat it might be ok with relatively high safety factors for the tube already. For a higher end racing boat you might be flirting with sadness.

 

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1 hour ago, Zonker said:

The only concern would be if a keel stepped mast was just chopped at deck level and a tabernacle added.

Deck stepped masts have to somewhat higher moments of inertia because a deck end is considered "pinned" and not "fixed" like keel stepped. So the tube has to be a bit bigger (typically) or at least have thicker walls.

If the mast was on a cruising boat it might be ok with relatively high safety factors for the tube already. For a higher end racing boat you might be flirting with sadness.

 

Or the mast can be sleeved near the step.  My IC has the step above the gooseneck.  It's very convenient to leave the boom and associated rigging after de-stepping the mast.  The mast is sleeved, as is the mast stub below it.  The mast stub is made from the same carbon section as the actual mast.

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   No, the entire mast tube needs to have higher moments for a deck stepped mast. 

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 No, the entire mast tube needs to have higher moments for a deck stepped mast. 

And the difference is not small.

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Here is how ours worked on a 70ft stick

IMG_1559.PNG

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4 hours ago, Moonduster said:

 No, the entire mast tube needs to have higher moments for a deck stepped mast. 

And the difference is not small.

I think increase in required I is about 15% (without checking my books).

If you think about buckling failure it takes place in the middle of the panel between spreader and deck or between spreader and spreader. That is a tricky place to locally increase the section modulus. Not saying impossible. Just a little challenging. Just doing it near the deck is not correct.

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10 hours ago, longy said:

   No, the entire mast tube needs to have higher moments for a deck stepped mast. 

In principle only to somewhere below the (lower) spreaders.

In practice for alloy masts that would mean the whole section.

What also changes is the positioning of the spreader(s), and possibly standing rigging dimensions.

In short, not a good idea for racing spars or offshore/ocean going boats, though carbon spars will negate the disadvantages ( I guess :blink: )

Talking convential rigged sailing yachts, of course.

 

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Wasn't Speedboat/Rambler 100/Loyal  deck stepped? 

I think also that Pogo 50s are deck stepped. Might be wrong though.

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I thought the vor65 is deck stepped. I remember something about it when they dropped the mast off vestas. 

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21 minutes ago, Alcatraz5768 said:

I thought the vor65 is deck stepped. I remember something about it when they dropped the mast off vestas. 

Correct

Is Comanche deck stepped?

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Just so that some of the confused realise that a folding mast does not have to be deck stepped.

It can remain as a keel stepped rig with a folding flange arrangement ,

so all the benefits of a lighter mast section etc can be kept when one changes to a folding mast, I have posted exhibit "A" below.

Yer I know its difficult to see ,but the addition of a folding system to a keel steeped mast does not have to be large and heavy. 

The flange is visible  about 30cm up from the cabin top. The pivot pin is on the aft face. The mast folds aft. The flange is angled upwards towards the back of the mast.

Normally 2 bolts are tightened through the front of the flange to maintain the mast tube integrity.

System works fine and have done many river and offshore miles out of Freo with this type of arrangement. 

As has been pointed out , a lota boats in WA have similar arrangements.

 

image.thumb.png.59dd4df793f58113a66a76a51920d619.png

 

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Most, but not all, canting keel boats have deck-stepped masts. It's a little tricky to build a decent step in the middle of the canting mechanism.

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20 hours ago, Zonker said:

The only concern would be if a keel stepped mast was just chopped at deck level and a tabernacle added.

Deck stepped masts have to somewhat higher moments of inertia because a deck end is considered "pinned" and not "fixed" like keel stepped. So the tube has to be a bit bigger (typically) or at least have thicker walls.

If the mast was on a cruising boat it might be ok with relatively high safety factors for the tube already. For a higher end racing boat you might be flirting with sadness.

 

Hi Zonker,

I believe what you wrote is entirely correct, but I'm not convinced that the mast simply being keel stepped makes it "fixed".

It's not like someone poured a cement block around the mast from the cabin top to the keel step.

More often, it is an oversize hole in the deck to insert the spar covered with a neoprene/canvas mast boot. We used to put itsy bitsy wooden wedges around the hole (lotta good that did).

 

 

 

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It's the two connections (at keel and at deck) that make it fixed. It's engineering and nothing in the real world exactly matches theory i.e. your keel stepped mast probably has a bit of flexibility because the wedges aren't made of steel, but it's close enough.

Old science/engineering joke:  "Assume a cow is a perfect sphere..."

Diagram of different types of end fixing

masts1.gif

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5 hours ago, t.rex said:

Hi Zonker,

I believe what you wrote is entirely correct, but I'm not convinced that the mast simply being keel stepped makes it "fixed".

It's not like someone poured a cement block around the mast from the cabin top to the keel step.

More often, it is an oversize hole in the deck to insert the spar covered with a neoprene/canvas mast boot. We used to put itsy bitsy wooden wedges around the hole (lotta good that did).

 

 

 

Not sure if you are looking at this correctly.  In the context used by Zonker "Fixed" means a "Fixed End" - as in the mast butt to mast step connection - not the deck level connection 

However, a few things to note:

The mast compression loads are a step function which increases incrementally at each spreader, or rather each shroud tang and as Fiji Bitter noted, the mast section below the first spreader experiences the greatest compression loading.  In that section, you want a greater “I”, or alternatively, shorten mast panel length.

On a keel stepped mast, the butt/step connection is seldom truly fixed. After all you slip the butt onto the step so there has to be some play even if it is only a little and once all the rigging is loosened, the mast can lift vertically.  So that connection in the real world is closer to a cross somewhere in between a "Fixed End" and roller connection, usually tending more towards a Fixed End depending on the friction between the butt and step, if any.  The deck connection has little to do with whether the butt/step connection is considered fixed – it is the end condition only.

The deck connection more closely models a roller connection, but with mast chocks in place, it begins to model a pin connection, which is an improvement.

IMO, while the “Fixed End” of a keel stepped mast assists in increasing buckling resistance, so too does the deck connection, which creates another mast panel and “effectively” adds another set of spreaders.

In the illustrations they show how the ”Fixed End” of a keel stepped mast enhances buckling resistance, and this is true if the butt/step connection is truly fixed (i.e. welded or fully bolted together).

My pet theory is that it may be closer to a “Pin” connection at the mast step and that it is the additional panel created between the first spreader and mast butt (in the section of the mast with the greatest compression loading) that mainly increases the buckling resistance.  Of course, this may be entirely incorrect, but I’ve never fully bought in to the “Fixed End” explanation

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