Mast step & coachroof construction.

Gordonmc

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Advice please.
I am helping a friend sort the mast step on his boat and have found an odd sort of set-up (to me) for the coachroof. The boat is balsa cored with a heavy GRP upper skin on the roof and a less heavy GRP mat inner. The whole lot has suffered a sag under the mast step.
I found the compression post was not under compression with the mast off. It has been removed and the upper skin has undergone surgery for a section around the tabernacle mounting to be lifted, approx a foot by 18 inches.
As expected some of the balsa was rotten and soaking... especially around cable gland holes. A piece of ply which had been glassed in under the tabernacle was completely delaminated and rotten.
I am getting to the question.
The upper and lower skins have been bonded together around the ply. In other words the balsa core did not butt the ply. The bond was not even in thickness.
Is this for some structural reason, eg to allow the mast step to flex?
If not the plan will be to cut back more of the balsa, infill with West and micro balloons, then glass/epoxy in new ply before replacing the compression post with an interference fit. It will mean the upper and lower skins will be seperate, surely not a problem with all that epoxy, nes pas?
Comments welcome.
 

Viking

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Most step masts have a s/s or some form of post support directly under the mast down to the keel. Just a bonded fibreglass sandwich I dont think will ever be strong enough to support the mast, let alone its loadings.???
 

bigwow

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Why not fill the area created by the removal of the rotten balsa with a larger pad of ply between the two layers of fibreglass. I think Viking has missed the fact you have a compression post n'est pas?
 

oldsaltoz

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G'day Gordon,

Have you established why the compression was not under any pressure? is the boat on the hard? If so depending on type it may be she has shifted her shape.

This is best done from below deck, preserves the original exterior finish and location of existing bolt and cable holes.

If you plan to replace the ply backing may I suggest you first fully shape it and add a radius to all sides, edges and corners; drill out the bolt holes for the tabernacle oversize by 20 to 30 mm, then drill 6 mm holes on a grid pattern about 30 mm apart.

Next coat it with an epoxy resin mixed with hardener then add up to 40% by weight of methylated spirits to thin it, put lots around the edges, end grain, it will soak in and need re coating.

When ready to install, mix more epoxy resin and add Micro-fibres (harder and stronger than balloons) to the mix, spread this on the top-side of the ply and push up to the underside of the deck, the excess will pass thru the larger holes drilled for the tabernacle bolts and the grid, this should reduce the amount of air trapped.

Use the compression post to support it till cured, put a layer of plastic or cling wrap on top of the compression post to prevent it sticking and to close off any holes.

The area between the new backing pad and balsa should be filled with a mix of micro-fibres close to the mast base and add more micro balloons to the mix as you move away, this will allow the glass to flex without crushing.

Hope this helps

Avagooodweekend......
 

davey

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Coachroof Problems

Advice please.
I am helping a friend sort the mast step on his boat and have found an odd sort of set-up (to me) for the coachroof. The boat is balsa cored with a heavy GRP upper skin on the roof and a less heavy GRP mat inner. The whole lot has suffered a sag under the mast step.
I found the compression post was not under compression with the mast off. It has been removed and the upper skin has undergone surgery for a section around the tabernacle mounting to be lifted, approx a foot by 18 inches.
As expected some of the balsa was rotten and soaking... especially around cable gland holes. A piece of ply which had been glassed in under the tabernacle was completely delaminated and rotten.
I am getting to the question.
The upper and lower skins have been bonded together around the ply. In other words the balsa core did not butt the ply. The bond was not even in thickness.
Is this for some structural reason, eg to allow the mast step to flex?
If not the plan will be to cut back more of the balsa, infill with West and micro balloons, then glass/epoxy in new ply before replacing the compression post with an interference fit. It will mean the upper and lower skins will be seperate, surely not a problem with all that epoxy, nes pas?
Comments welcome.

I love the bit about the marina skip! Anyhow I will forward this info to a friend who has similar problems although the real problem is an annoying water leak. I suspect that the GRP underneath the tabernacle is cracked hence the mast will have to be taken down. The boat has a compression post inside the cabin, however this is not liked and he is talking about removing it and glassing-in a beam inside the cabin. Somehow I don't think this is a good idea as boats without compression posts usually have two bulkheads to carry the downthrust. The boat is a Caprice MK5.

Re the plywood idea. In the past I have had great success in pressure cooking wooden parts using Cuprinol 5 Star. If the parts are too big to go into a pressure cooker they can be boiled in Cuprinol. This expels the air but when the wood cools, the preservative is drawn deeply into the wood. I did some tapered bungs this way for another friend who was re-doing his deck drains (the old crazy system was being replaced with a better system) After the unwanted holes were bunged up they were glassed over so the bungs might outlast the pyramids let alone the skipper!
 

William_H

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Mast support

For the OP you might consider lots of layers of chopped strand mat and polyester resin to fill in place of plywood. this assuming you can fit it in from the top. This will take compression easily. If you want to use epoxy get a woven roving or CSM that is suitable for epoxy but polyester is cheaper and will be fine.
Re post by davy who has a friend who wants to redesign to do away with compression post. As you have said this will be a major redesign project. I presume he doesn't like the post in the centre an area of most convenient access to fore cabin. The usual thing is to have massive bulkhead (s) that extend well in from the sides and have a deep (top to bottom) beam across the top. (ideally a solid bulkhead with a hole cut for access.) This support needs to thick (fore and aft) enough to stop any tendency to flex or buckle and it needs to transfer the forces down to the keel or at least the hull sides but also transfer the forces to the chain plates.
Tell him the loads can be huge when the boat is hard pressed. Add to that the inertia of the weight of the mast in rough water. Many well designed boats with a post and bulkhead will still complain the loo door jams when the pressure is on from the mast.
I would be thinking 2 layers of plywood spaced by a layer of foam perhaps 25mm thick. The plywood then would be faced with very thick fibreglass or better layers of carbon fibre. Cut an oval hole in the bulkhead which goes about 75mm short of the roof and is as wide as is needed. But wider it is the weaker it is. Just a few thoughts Yes I do support redesign of boats but you have to be conservative and make it strong olewill
 

davey

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Major Redesign Of Mast Step

Many thanks for the advice and I just hope that matey learns to live with the compression post. A bulkhead isn't really feasible for the Caprice as the boat is just under 6 metres long. If a bulkhead was to be constructed to support the mast thrust it would divide the two forward berths into two thus making the boat only suitable for dwarfs. OK another friend does have a slightly larger boat which has no post there. The boat is a Robert Ives Four-21. These boats are a bit rare as allegedly only forty seven were ever built. One can see that it was an expensive boat in its day because it is a proper pocket cruiser that's built like a brick-outhouse. The coachroof on the Ives has a massive GRP U beam incorporated into it but it was built like that. Incidentally when the boat was craned-in about a year ago the crane meter said 3.5 ton! That's quite a bit for a twenty one footer although the weight did include food and water. Anyhow the Ives has made it from Plymouth to the Caribbean without a water-maker which is pretty good going. The route was the old triangular run as done by the sailing ships of long ago. The skipper reckons that taking the compression post out of a Caprice will ruin the boat for ever more and I have to agree.

Some people do funny things with boats, for example I have what used to be a splendid unsinkable aluminium launch "project boat" which was bought principally for its Volvo model 100 stern drive. Several owners ago the boat fell into the hands of a so-called carpenter who butchered it by trying to make it into a cabin cruiser. Now this is as absurd as trying to make a fine sports-car into a camper van however this didn't deter the carpenter at all. Bulkheads and seat benches have been cut out by drilling hundreds of holes and joining up the holes with a chisel. A horrible wooden cabin (now all rotten) was then built on top. I have come close to scrapping the boat several times but possibly it could still have another life as a fishing boat or work-boat. God help anyone who has work done by that so-called carpenter as he must be a person with 100% enthusiasm but 10% skill.
 

William_H

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Many thanks for the advice and I just hope that matey learns to live with the compression post. A bulkhead isn't really feasible for the Caprice as the boat is just under 6 metres long. If a bulkhead was to be constructed to support the mast thrust it would divide the two forward berths into two thus making the boat only suitable for dwarfs. OK another friend does have a slightly larger boat which has no post there. The boat is a Robert Ives Four-21. These boats are a bit rare as allegedly only forty seven were ever built. One can see that it was an expensive boat in its day because it is a proper pocket cruiser that's built like a brick-outhouse. The coachroof on the Ives has a massive GRP U beam incorporated into it but it was built like that. Incidentally when the boat was craned-in about a year ago the crane meter said 3.5 ton! That's quite a bit for a twenty one footer although the weight did include food and water. Anyhow the Ives has made it from Plymouth to the Caribbean without a water-maker which is pretty good going. The route was the old triangular run as done by the sailing ships of long ago. The skipper reckons that taking the compression post out of a Caprice will ruin the boat for ever more and I have to agree.

Yes you can not fit a bulkhead and beam in lieu of compression post if you can't cope with dividing the bunks. ie if the bunk area forward is not long enough with bulkhead under the mast. If however you abandon forward bunks then it could be a good storage area forward.
A 21ft boat at 3.5 tonne is pretty heavy. My 21ft class min weight is 760kg that is ready to sail but no motor. That is about 1/5 of your friends boat weight. good luck olewill
 

kingfisher

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Dear all,

I'm currently dealing with the same problem. Over the years, the area around the mast was becoming more and more soggy, untill last season, you could push the deck inwards with your thumb. Also, inside the boat, there was a gap between the bulkhead and the deck.
Because of the renovation in the house, I didn't have time to deal with the boat. Until this weekend, when I opened up the deck. This is what I found:
soggydeck.jpg


The square in the middle is the compression post top. On this should go the mast step block, the the rest of the balsa replaced with closed cell foam, then the whole thing gets glassed over. The one square meter bit which was replaced will be covered with tekdek.

QUESTION: what do I glass in as mast step? It is a block 250mmx150mmx30mm. Iroko? Teck? Plywood with loads of epoxy?
 

Topcat47

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I know it's of no help to anyone, but if ever I needed a reason not to buy a "sandwich" construction boat, this is it. Thankfully Snark is solid glass.
 

kingfisher

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I know it's of no help to anyone, but if ever I needed a reason not to buy a "sandwich" construction boat, this is it. Thankfully Snark is solid glass.

Unfortunately, anything bigger than Snark will be sandwich structure. Otherwise it would become too heavy. In my case the rot was caused by deck fittings without backing, drilled straight through the balsa core. Then again, I don't think my half-tonner was built with a 40-year lifespan in mind.
 
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