Cored yacht construction

[smithy09 50]

OK. Here is where I display my total ignorance.

 

I have been looking at quite a number of yachts that are either balsa cored, cedar cored or foam cored. This seems to be a classic way of building yachts in enzud.

 

How are these yachts constructed? Are they built like a wooden yacht, over frames, then glassed outside and in? Or is there another way? As they don't seem to have internal frames and stringers as a wooden yacht wood have, what gives them the strength? Are the bulkheads usually glassed in to give the strength around the chainplates? Or is there usually some kind of ring frame from the mast foot that is glassed in at build time?

I am sure you guys can educate me here!! Thanks!

[MrWolf 0]

Take a look at the Timberwolf thread in multi talk it has a very good picture demo of how to build a Foam cored hull in a female batten mould.

 

Cedar core is built over temporary frames. Each plank is glued to the next and runs lengthways.

Once the cedar is strip planked, the outside of the cedar is faired (planed and sanded) and then glassed over.

The hull is then flipped, faired inside, then glassed.

The Cedar is strong in itself, but the Glass either side makes it much stronger again.

with this method you do not need stringers.

The bulkheads are then glassed into the hull.

 

Most balsa cored hulls are laid up in a female mould.

A gelcoat is applied to the mould, then the outside laminate.

The Balsa core is then bedded in a slurry of core bonding adhesive.

After that the Inside skin.

Once again no stringers are required.

Here the balsa core serves the purpose of separating the two skins which makes the whole structure a lot stiffer (than just a solid laminate would).

The balsa core will require a good deal more laminate on it then is required over the cedar core because the cedar is longitudinally a much stiffer core than balsa.

[smithy09 50]

Thanks Mr. Wolf. That just about explains it to me! I guess once you have got that far you just bond structural items like ring frames and chainplate supports to the inside of the hull just like a normal female moulded glass hull??

[MrWolf 0]

Thanks Mr. Wolf. That just about explains it to me! I guess once you have got that far you just bond structural items like ring frames and chainplate supports to the inside of the hull just like a normal female moulded glass hull??

 

A lot of cored boats require longitudinal girders beside the keel floors as well.

Then that along with the bulkheads and ring frames are all glassed in.

 

Keep an eye on the Timberwolf thread as all the frames are going in as we speak !

[Guest]

After a little cruising on Future Perfect I LOVE cedar core. Easy to keep clean inside as it is a clean smooth surface, no condensation coz the cedar insulates better than glass, and light and stiff enough to be bloody fast for a cruiser.

[Guest]

What is the difference in strength between cedar and kauri.

With the price of Fiji Kauri, I was wondering if it would stack up as an alternative.

If it is stronger the strip can be thinner which would offset the weight difference.

The better impact resistance would make it tougher in service, and it is more rot resistant as a bonus.

In the back of my head I'm thinking the downside may be that the gap between the two glass layers is where the strength of system lies.

[Atom Ant 0]

Slacko I'm no expert but I believe the difference would be weight. When building with a core there isn't the need for such density & strength in the wood and therefore Kauri would be overkill in this instance. Cedar was popular due to it's strength relative to its weight.

[smithy09 50]

That Timberwolf thread is brilliant!! Really interesting. How toi build a foam cored boat from scratch... Thanks heaps Mr. Wolf. Great thread!

[SloopJohnB 323]

Hey smithy09

I have a booklet on the "Bead & Cove Foam System" if you are interest.

 

It uses planks of foam over a male mould, very similar to timber planking.

[smithy09 50]

Thanks sjb. Just an interested bystander at the moment, but you never know. Hey what do you do with these foam/timber cored boats when you want to put a skin fitting through the hull? Lots of epoxy in the hole before you push the fitting through?? I would have thought with balsa a cedar there would be a tendency to get water into the cross grain quite badly..

[Gappy 9]

The core is used to keep the two skins apart which are doing all the work the core only really needs to be reasonably good in shear as this is the main load that it will see as things bend twist etc. Using kauri over cedar would just be a heavy option nothing gained here. In the Shaw boats they have used alot of wood call Polonia lately this is lighter than cedar about the same price only problem been is there isn;t to much info on its strengths etc so the boats been built aren't huge with big loads more just the sports boat size. On the karma Police she is a pretty light Carbon over foam laminate the deck for example I am pretty sure is 200Grams carbon on the inside and 300 on the outside over 15mm foam.

 

Cheers

 

Gappy

[benny14 6]

Thanks sjb. Just an interested bystander at the moment, but you never know. Hey what do you do with these foam/timber cored boats when you want to put a skin fitting through the hull? Lots of epoxy in the hole before you push the fitting through?? I would have thought with balsa a cedar there would be a tendency to get water into the cross grain quite badly..

 

Just put a couple tubes of Sika in there Smithy, she'll be right!!

[smithy09 50]

Hmmmm. Yeah I guess it's just seal it and hope huh?

[Absolution 7]

After you cut the hole for the skin fitting you should rebate the core back 5-10mm, fill the rebate with epoxy and leave it to cure before installing the fitting.

[smithy09 50]

Thanks Fineline. Rebate? You mean increase the diameter of the hole, or like countersink/counterbore from the inside? I guess you would have to re-drill after that yes?

[Absolution 7]

By rebate I mean cutting back the core but leaving the GRP intact on the interior and exterior.

[smithy09 50]

OK. I get it now. Yes that makes perfect sense. Thanks!

[Guest]

With a smaller put hole I usually an allen key on a slow drill to get out the foam, block off the bottom, fill with epoxy and redrill later. Also stops the 2 skins crushing together when tightening the fitting.

[smithy09 50]

With a smaller put hole I usually an allen key on a slow drill to get out the foam, block off the bottom, fill with epoxy and redrill later. Also stops the 2 skins crushing together when tightening the fitting.

 

You guys have all been there before obviously!! Yeah, great with foam. Cedar or balsa would reqire a little more work I guess, but same deal.. Thanks guys.

[col j 0]

drilling and glueing in some glass tube from the inside works well too. thats what we did when we re built pahi but that was done full and proper the whole way. even the bolts holding the deck fittings on have been done like that. certainly helps with the loads being pulled across the deck compared to wood slowly compressing making the hole bigger as well as any leaks not letting water into the deck.

 

timber boat thats glassed opposed to a 'core construction' but ply decks can squish up like foam sometimes and kauri can soak up a bit of water.