Balsa-Cored Bottoms

[matauwhi]

G'day, mates. No cored hull here. No worries here. Cheers.

[daddle]

Santa Cruz 50 here. Bobbing happily since 1982. Balsa cored everything. Very light. Zero issues. Recent survey passed with perfect score.

[Dockhead]

Originally Posted by DeepFrz

You folks with plywood cores, be afraid, be very afraid. If water gets in anywhere it will soon wick through the whole sheet of plywood and probably much more. Water will wick through plywood much easier than end grain balsa core that is properly installed.

The key to keeping any core deck healthy is to make sure that no water can reach the core material. Even foam will eventually soak up water and rot.

Certainly true. Encapsulated balsa blocks are far superior in terms of resistance to delamination or rot.

But balsa blocks add no tensile strength -- only compressive. So you have to use plywood wherever you have through-bolted hardware or big loads. Plywood has considerable tensile strength and rigidity, so a GRP-plywood sandwich is very, very strong.

[GordMay]

Originally Posted by Dockhead

Certainly true. Encapsulated balsa blocks are far superior in terms of resistance to delamination or rot.

But balsa blocks add no tensile strength -- only compressive. So you have to use plywood wherever you have through-bolted hardware or big loads. Plywood has considerable tensile strength and rigidity, so a GRP-plywood sandwich is very, very strong.

Isn’t delamination somewhat analogous to a tensile failure?
Under what circumstances is a cored structure placed under tension, especially at through bolted load areas?
Why doesn’t a balsa core contribute to tensile strength?

➥ Core Failure Modes - Fig.2 - Cruisers & Sailing Photo Gallery

➥ Cored Laminates - Fig. 1 - I-Beam Analogy - 1. I-Beam Analogy Cruisers & Sailing Photo Gallery

[Dockhead]

Originally Posted by GordMay

Isn’t delamination somewhat analogous to a tensile failure?
Under what circumstances is a cored structure placed under tension, especially at through bolted load areas?
Why doesn’t a balsa core contribute to tensile strength?

➥ Core Failure Modes - Fig.2 - Cruisers & Sailing Photo Gallery

➥ Cored Laminates - Fig. 1 - I-Beam Analogy - 1. I-Beam Analogy Cruisers & Sailing Photo Gallery

Delamination, as the name suggest, is a failure of the bond between the core and the outer skins. I don't see how it's analogous to tensile failure. IF the bond is weak, then a shear force between the core and skin can break it, ruining the structural properties of the whole system even though no individual elements have suffered any failure.

The fact that a structure is cored will of course contribute to tensile strength of the whole system, as shown in your excellent drawings. That is because the core moves the outer skin out to a point where geometrically its tensile properties can be used. But that doesn't mean that any tensile properties of the core are necessarily at work. A field of unconnected balsa blocks doesn't have any such properties, obviously.

A cored structure bearing some kind of a point load will experience compressive loads on one side and tensile loads on the other, just like a bridge, say, or a beam. Balsa blocks will not bear any part of the tensile loads themselves, although as you correctly point out their effect of spreading the skins apart from each other will allow the whole structure to bear such loads much better, like an I-beam. As you said, a cored structure is already stronger than a non-cored structure of the same mass merely because it is thicker.

But if you need still more strength in tension, then a plywood core will give that. Plywood has fibers running in both directions and so has its own structural properties, which a collection of balsa blocks not connected to each other does not. That's why good boat builders use plywood, and not balsa cores wherever you have a through-bolted deck fitting under load. The sandwich of GRP-ply-GRP is quite a bit stronger in tension than GRP-balsa-GRP. Some parts of my decks have 20mm thick plywood cores and can bear some tons of force applied in a relatively small area.

GRP-balsa-GRP is fine structurally for boat bottoms because balsa blocks do have a lot of compressive strength. Unlike tension, they don't need to be connected together to have compressive strength. So this type of sandwich will have high impact resistance.

[David_Old_Jersey]

I am on solid glass hull and decks - a price to be paid for that in weight

But have come accross cored decks with water ingress. and I would never ever want that mess in my hull - even if localised. alledgedly.
(not saying that my solid glass hull does not have it's own risks, especially on the Osmosis - but was a choice by me).

I suspect however that cored hull (and deck) design and build quality have come on in leaps and bounds over the last 25 years - including through trial and error. But nonetheless seems a bit too much faith in a PO or the builder for my tastes. Kinda like Ferrocement?

[daddle]

Originally Posted by David_Old_Jersey

Kinda like Ferrocement?

Kinda like anything. Quality is quality. And some brands of boat attract maintenance, some don't.

[zeehag]

and i am still very glad i have a 2 1/2 inch thick hull and a goood solid ply cored deck.

[bewitched]

A cored hull, whether it be balsa, ply or foam is a great way to get a strong, stiff hull. But sure it can be compromised either through poor construction, maintenance or if it experiences forces it was never designed to take - i.e pounding on a reef.

Cored hulls are strong and stiff because they utilise the appropriate material at each level of the structure: Materials that are strong in tension and compression at the outer skins (glass fibre, carbon fibre) and materials that are sufficiently strong in shear for the core (ply, balsa, foam).

If the objective is to achieve a light hull, clearly the lighter core material is preferred.

I know I'm going against the opinions of others here, but I don't believe that the core material needs to be particularly strong in either tension or compression for a hull. It may be the case that the cores of some hulls do take compressive and tensile loads, but in my opinion it's not good design (if the design intent is for a light hull) because these loads would resolved for less total weight in the outer skins.

Having said that, if one has determined to use say ply for the core, there is no reason why the inherent properties of that material should not be utilised. i.e. carry some of the tensile or compressive loads at the extremes.

A pretty good overview of core construction can be found here: Cores

[daddle]

Originally Posted by bewitched

I know I'm going against the opinions of others here, but I don't believe that the core material needs to be particularly strong in either tension or compression for a hull.

Resistance to compression is exactly why balsa or honeycomb is used rather than structural foams. At least in the past - perhaps more modern foams are suitable. Foam crushes then delaminates, ruining the whole system.

Pounding on a reef? You'll want steel for that.

[bewitched]

So why does pretty much every modern race boat these days have a foam core?

[Dockhead]

Originally Posted by bewitched

A cored hull, whether it be balsa, ply or foam is a great way to get a strong, stiff hull. But sure it can be compromised either through poor construction, maintenance or if it experiences forces it was never designed to take - i.e pounding on a reef.

Cored hulls are strong and stiff because they utilise the appropriate material at each level of the structure: Materials that are strong in tension and compression at the outer skins (glass fibre, carbon fibre) and materials that are sufficiently strong in shear for the core (ply, balsa, foam).

If the objective is to achieve a light hull, clearly the lighter core material is preferred.

I know I'm going against the opinions of others here, but I don't believe that the core material needs to be particularly strong in either tension or compression for a hull. It may be the case that the cores of some hulls do take compressive and tensile loads, but in my opinion it's not good design (if the design intent is for a light hull) because these loads would resolved for less total weight in the outer skins.

Having said that, if one has determined to use say ply for the core, there is no reason why the inherent properties of that material should not be utilised. i.e. carry some of the tensile or compressive loads at the extremes.

A pretty good overview of core construction can be found here: Cores

The article you link has some good information, including this table:



which illustrates the very large structural advantages of sandwich construction.

And this table:



shows which part of a sandwich works in tension (someone was puzzled by this I think).

As to the question about whether or not the core needs to have any structural properties in tension: well, it depends. For a hull, not so much. The skin is going to be thicker and there won't be point loads. In most cases you don't need the core to contribute to tensile strength so balsa blocks, which have different good qualities, are fine.

In other cases, you need all the tensile strength you can get, like where a davit is mounted to a transom, say. There a plywood core will be stronger because the core contributes to the transom's resistance to bending over under the load from the davit.

As to the question about compressive strength: You want this in a hull. A core which can't be crushed helps the whole system resist impacts. No, it won't be as impact resistant as a heavy steel hull, but a thick sandwhich of GRP and balsa will be damned strong. The outer skin of my boat is Kevlar forward of the keel, to add even more resistence to impact. .

[bewitched]

You definately want compressive strength in a hull, but it's not necessary in the core. Because the core doesn't take compressive loads.

Take these examples:

Foam core = Compressive strength 1.3 MPa

Balsa core = Compressive strength 6.3MPa

Now slap a skin of glass fibre each side for a composite construction and the strength is 293.8MPa.

The core isn't where the compressive strength comes from. As long as it can take the shear, that's all it needs to do.

Build it out cardboard if you want - just make sure you don't get it wet

[Tropic Cat]

Originally Posted by bewitched

....Build it out cardboard if you want - just make sure you don't get it wet

As David Pascoe points out, this is exactly what Sea Ray and some other prominent boat builders did.

[Mule]

Originally Posted by phillw

The article was written in 2002 and I note that more and more builders are going to cored hulls whether it be foam, balsa or even polypropylene honeycomb. Schionning's have this to say comparing balsa to foam;

Balsa can absorb water. It needs extreme neglect to rot (very unusual). Water soaks along the end grain quickly and travels very slowly across the grain. We use balsa under the waterline especially because of its high compression strength for beaching etc, any core types must be sealed. Damages to all cores result in the same sort of repair. Notice a damp spot when drying out to antifoul.... simply grind back the surface glass exposing the core, dry it out and re-glass - its that simple.

After redoing the flybridge and the cockpit decks on my 1982 Mainship Mk 1 and now moving on to the bow decks I can say with certainty that that is not what I found when I peeled up the decks. The re-forcing plywood was fine, wet but OK and could have been dried out, not a pithy mess resembling peat moss as the balsa.

If I only see balsa in model airplanes it will be more than I want. This Balsa nightmare I describe is the #1 problem covered on the Mainship Forum.