How would you redo this Mast Base?

I can see that there's been a bit of compromising going on here. It looks as if the mast has ended up forwards of the position that the designer would have wanted - which would have been on top of the bulkhead, where the wooden post is. I'm wondering if someone added the metalwork more recently to take some of the load that's caused the deck to sink? The post doesn't look like original equipment to me, but if the post is fully in touch with the deck and substantially bolted to the bulkhead I think I'd say the it looks like a reasonable effort, go sailing (in our dreams) and monitor it.
 
anoccasionalyachtsman said:
I can see that there's been a bit of compromising going on here. It looks as if the mast has ended up forwards of the position that the designer would have wanted - which would have been on top of the bulkhead, where the wooden post is. I'm wondering if someone added the metalwork more recently to take some of the load that's caused the deck to sink? The post doesn't look like original equipment to me, but if the post is fully in touch with the deck and substantially bolted to the bulkhead I think I'd say the it looks like a reasonable effort, go sailing (in our dreams) and monitor it.
Click to expand...
Regarding the 'flat' on the deck, if you are reasonably certain that the core is sound then I wouldn't get too sweaty about it...it looks to be that it is just the take up of the camber of the original fibre glass deck that has compressed to the flat top of the internal compression post. As for the mast base, If its just the 'T' section base you are replacing then have you enquired about having one cast using the original (suitably 'repaired' with car body filler) as a mold? Failing that £400 sounds damned expensive for what is essentially a simple fabrication. I would look further afield for a fabricator with more realistic expectations!
 
anoccasionalyachtsman said:
I can see that there's been a bit of compromising going on here. It looks as if the mast has ended up forwards of the position that the designer would have wanted - which would have been on top of the bulkhead, where the wooden post is. I'm wondering if someone added the metalwork more recently to take some of the load that's caused the deck to sink? The post doesn't look like original equipment to me, but if the post is fully in touch with the deck and substantially bolted to the bulkhead I think I'd say the it looks like a reasonable effort, go sailing (in our dreams) and monitor it.
Click to expand...

Hi. The post fwd of the bulkhead is original,, but no obvious fitting.
 
Swenns Dad said:
Although I haven’t physically inspected the core, it is solid (as tested with tap hammer... prodded in vicinity of bolt holes). I’ve also asked a surveyor what he thinks.... he’s on the fence (ie, signs are that it’s solid... but clearly depressed). He suggested it could be as a consequence of the post under the foot settling... but that’s almost impossible to verify.

Need to be realistic about how much of the cherry I bite off... I was considering cutting out the deck under the foot and setting in a Tufnell base... see attached... but again, how far to I go!
Click to expand...
Endgrain balsa core sustains a compression load of 1850 psi (Gougeon Brothers). If the core is intact and the mast is properly supported, then you need a larger support base. To get a rough handle on compression load, add up the breaking strengths of your shrouds, upper & lower, etc, on one side and multiply with centreline - chain plate distance.

I would make an larger aluminium plate 12-15mm with tapered wedge in the same material. Any other attachments you might need can be easily bolted, tapped or welded to it.

Aluminium can be easily worked with regular wood working tools, saws, sanders etc. and well within the abilities of an interested amateur
 
The wedge shaped block is vital to mast strength given that above the base the mast is a thin walled tube with enormous downward pressure such that we need all the pressure on the thin walls to be equal around the circumference. Excess pressure on one side (back) would tend to cause buckleing of that area of wall. So a mast sitting on a sloping cabin top even via a base will not do. You need that wedge to bring the flat base so mast is vertical to the base. On my boat the ali wedge disintegrated and I replaced it with multiple layers of CSM mat and polyester resin to make a wedge about 15mm high at the front down to near zero at the back. Wood would do again but might not last for ever. ol'will
 
I'd bI think the imperative would be to increase the area of the base plate to spread the compression load, I look to increase the area by say a factor of around 10 ( increasing each side, multiplying by a little over 3). I wouldn't use timber or plywood, Tufnol or a glass epoxy sheet would work.
I'd be very tempted to get a new based plate made starting with a sheet of aluminium 4mm or 5mm thick bent to the shape of the deck, to spread the compression load of the mast. Get the fabricator to weld the tennon which locates the mast onto the base plate, then add some SS hoops onto the base plate, just in case. I'd anodise the fabrication if you can, otherwise use a good aluminium primer and paint.
Then bed it in with your favorite sealant.
You could use SS but you'd need to electrically isolate it from the heel of the mast.

I wouldn't duplicate the original, you need to spread the compression load over more deck area.
 
Ian_Edwards said:
I'd bI think the imperative would be to increase the area of the base plate to spread the compression load, I look to increase the area by say a factor of around 10 ( increasing each side, multiplying by a little over 3).....

I wouldn't duplicate the original, you need to spread the compression load over more deck area.
Click to expand...
No.
The compression needs to go straight through the deck material to the post (and maybe bulkhead), down to the keel and spread by the bulkhead into the body of the hull. The areas of deck with no post or bulkhead under them bring next to nothing to the party.

The imperative is to make sure the deck material between the mast step and the post/bulkhead does not compress. Also whatever the wedge is made of needs not to compress.

The top and bottom skins won't be rigid enough to spread the load over a significant area of deck core.
 
If it was me I would cut out a bit of the deck under the mast to inspect the core. If the ply core is shot you can scoop it out with a spoon and build up glass and epoxy layers to the height you want for new mast base. Why go to all the trouble only to find out later the core is knackered? It very easy to inspect the core at this stage and the glass/ epoxy work would be an easy job
 
William_H said:
The wedge shaped block is vital to mast strength given that above the base the mast is a thin walled tube with enormous downward pressure such that we need all the pressure on the thin walls to be equal around the circumference. Excess pressure on one side (back) would tend to cause buckleing of that area of wall. So a mast sitting on a sloping cabin top even via a base will not do. You need that wedge to bring the flat base so mast is vertical to the base. On my boat the ali wedge disintegrated and I replaced it with multiple layers of CSM mat and polyester resin to make a wedge about 15mm high at the front down to near zero at the back. Wood would do again but might not last for ever. ol'will
Click to expand...
What boat do you have?
 
Ian_Edwards said:
I'd bI think the imperative would be to increase the area of the base plate to spread the compression load, I look to increase the area by say a factor of around 10 ( increasing each side, multiplying by a little over 3). I wouldn't use timber or plywood, Tufnol or a glass epoxy sheet would work.
I'd be very tempted to get a new based plate made starting with a sheet of aluminium 4mm or 5mm thick bent to the shape of the deck, to spread the compression load of the mast. Get the fabricator to weld the tennon which locates the mast onto the base plate, then add some SS hoops onto the base plate, just in case. I'd anodise the fabrication if you can, otherwise use a good aluminium primer and paint.
Then bed it in with your favorite sealant.
You could use SS but you'd need to electrically isolate it from the heel of the mast.

I wouldn't duplicate the original, you need to spread the compression load over more deck area.
Click to expand...

Hi. I don’t think making the base spread over the deck will add any value... for the purposes of compression (although enlarging it to encompass fittings is probably an idea to pursue).

Thanks for your input though ?
 
geem said:
If it was me I would cut out a bit of the deck under the mast to inspect the core. If the ply core is shot you can scoop it out with a spoon and build up glass and epoxy layers to the height you want for new mast base. Why go to all the trouble only to find out later the core is knackered? It very easy to inspect the core at this stage and the glass/ epoxy work would be an easy job
Click to expand...
No need to cut it out yet, just drill a hole and watch what the drill brings up.
 
geem said:
If it was me I would cut out a bit of the deck under the mast to inspect the core. If the ply core is shot you can scoop it out with a spoon and build up glass and epoxy layers to the height you want for new mast base. Why go to all the trouble only to find out later the core is knackered? It very easy to inspect the core at this stage and the glass/ epoxy work would be an easy job
Click to expand...
Indeed.
I would try to avoid removing a big area of the top skin of the deck. A few ~8mm holes maybe. Start by enlarging existing screw holes. break up any rotten wood with a sharpened allen key in a power drill. Get it dry. Stuff it with chopped glass. Flood it with epoxy.
If the top skin looks badly abused/splintered etc, add a couple of layers of glass cloth and epoxy over a slightly bigger area. Or maybe as WH suggested, build up the top to replace the wedge in fibreglass.
 
"No.
The compression needs to go straight through the deck material to the post (and maybe bulkhead), down to the keel and spread by the bulkhead into the body of the hull. The areas of deck with no post or bulkhead under them bring next to nothing to the party."

I don't agree with that, look at the basic physics, the compressive force on the deck and balsa core in essence the pressure exerted on the deck. Think of it this way, if you put the compression of the mast and rigging onto a nail, that would give a very high pressure, and the nail would go straight through the deck, if you spread it over a large enough area, the balsa core will easily take it.
What happens on the underside of the deck is another matter, that depend on how well the load is managed into the mast compression post. It may well be that also needs to be reinforced. However, the OP doesn't mention any problems on the inside, just that the deck has compressed on the outside, spreading the load will prevent that occurring again.
As an alternative, you could cut out the balsa core under the heel of the mast and replace it with a much stronger material which is capable of directly transferring the load into the compresion, both are equally good solution.
My working assumption was that any correction that's required in the form of a wedge would be built into the fabrication, why would you want a wedge when you could correct any misalignment when fabricating a new base plate?
If you look at it another way, some yachts don't have compression past at all, the have ring beam which reinforces the deck and transfers the mast compression force directly into the keel area, or a catamaran which typically uses a box beam structure to transfer the loads. There are lots of way of designing a structure to take compressive loads, you don't to have transfer it directly in line.
 
Ian_Edwards said:
"No.
The compression needs to go straight through the deck material to the post (and maybe bulkhead), down to the keel and spread by the bulkhead into the body of the hull. The areas of deck with no post or bulkhead under them bring next to nothing to the party."

I don't agree with that, look at the basic physics, the compressive force on the deck and balsa core in essence the pressure exerted on the deck. Think of it this way, if you put the compression of the mast and rigging onto a nail, that would give a very high pressure, and the nail would go straight through the deck, if you spread it over a large enough area, the balsa core will easily take it.
....
Click to expand...
but the balsa core will only take it if it's supported over the whole area, so the pressure is evenly applied. If the structures above or below don't put an even pressure on the core, then you have to work out if the peak pressure on the core is too high. .

If you spread the force of your nail over a nice big area using a thick piece of hardwood on top of the deck, the core will still fail if it's supported on a bed of nails.

And once the balsa has got damp because the fixings leak, it's all over.

Competently built boats have ply or hardwood or solid laminate under the mast step.
 
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