Balsa cored hull?

So if and when I decide to build a custom boat should I use balsa or foam, Which has the best all around properties when used in power boats?

 

trying balsa

I just recieved shipment of end grain balsa sheets- I am going to try one hull on my shark mould. I made a few test panels and am completely impressed. Absolutely stiff - 1/4" endgrain with 10 oz cloth on each side. I'm betting I end up doing the second hull as well.(kind of important because its a cat!) I am going to be religous about forcing thickend epoxy between the blocks. thanks for the recipes for filllers on another thread.
Rob

 

rturbett- What size boat are you building, did you build a mold or are these one off construction. any pics?

 

This is a Shark catamaran- 20 feet long. I purchased the plans, and built the mold.There are a few pics of a Shark I rebuilt, as well as one or two of the mold, on this website- check out rtubett's gallery. I'm sure I will post more as the work progresses.
If you're from Michagan, Gibbs used to build sharks there. You've probably seen some around.
Rob

 

How to find water in balsa:
Put your boat in a nice cool shed, pull it out on a sunny day and look at it with a thermo camera. Because water needs a long time to heat up it will show up as cool areas on the thermo camera.

What core to use on a boat:
Balsa has very good shear strength but it is heavy and gets even heavier when it soaks up water. Shear strength alone is also not the most important value. More important is the toughness of the core. This is the amount of energy it can absorb before it breaks. Balsa is the most brittle core. So if you hit something the core cracks into 1000 pieces and debonds from the skins. With hitting something I'm also talking about waves. So don't use balsa on a fast boat in the slamming area.

Much better are linear PVC cores or Corecell. Cross linked PVC is also quite brittle. The disadvantage of linear PVC is that it softens at very low temperatures. It's great if you want to cruise Alaska but pretty useless on a black hull in the caribean sea. So that leaves only Corecell. That's what you would use on a Volvo 70 in the slamming area. So it's good for crashing over the waves at 30kn or so. All foam cores also have closed cells. So if there is a leaking through hull fitting the water can't "wick" through the whole boat.

 

Karsten,

I like your IR camera trick, a lot cheaper and reliable than many other.

Corecell is really low in shear strength, as far as I know, for really heavy slamming loads, single skin laminates are your only recourse, unpalatable as it might be.

In the end, balsa just shouldn't be used by anyone who can't install it properly

Yoke.

 

It depends on the weight. You can get A grade corecell from 58kg/m3 to 210kg/m3. Balsa is about 150kg/m3 without beeing soaked full of resin and/or water with a shear strength of 2.9Mpa. Corecell with a weight of 150kg/m3 has a shear strength of 1.6Mpa and a shear elongation of 50%.

Yes the static shear strength is less but your core is not going to fail under a static load. It's going to fail due to impact and for impact loads it's more important how tough the material is (how much energy it absorbs before it breaks).

You are also correct that core bond is absolutely important as is filling all the cuts (if you have a cut core) with adhesive. Now this is something where the skill of the builder comes into play.

 

I made a few test panels using saran wrap as a clamp. I was able to wrap it circumfrentially around the mould. Worked like a poor mans vacuum bagging system. The core bond was perfect, but made me realize my epoxy was a little too thick to reach the bottom of all the cuts. (a real vacuum system would probably solve that0

Can you reccomend a specific corecell that 1/4 thick that would best mimic balsa's good properties? The divinnycell I played with needed too many layers of glass to make it stiff enough.
Thanks,
Rob

 

try spherecore from sphere.tex call them up and they will send you a free sample.

copied nfrom their website.

Description sphere.core SBC
sphere.core SBC is a newly developed mat consisting of volumised short glass fibers.

The basic material possesses similar characteristics to sphere.core S. The basic material will be compressed through a special process and kept compressed through a stitchbonding process.

This compression reduces the cavities between the glass fibers and the microspheres by approximately 50% per volume compared with the sphere.core S quality, resulting in a considerable reduction of the resin absorption so that the specific weight is only around 420 –440 kg/m³ after impregnation and complete saturation with resin.

Because of this low specific gravity and the high strength parameters of the material after curing of the impregnated resin, sphere.core SBC can be excellently used as substitute for core materials based on plywood, BALSA, PVC foam etc. Owing to the perfect laminate homogeneity and the extremely high strength parameters, the external top layers can be designed significantly thinner for sandwich designs and yet the physical parameters are higher than those with sandwich designs produced by using plywood, BALSA or PVC foam.

With identical laminate thicknesses, the physical parameters of a laminate produced with sphere.core SBC are far superior to the laminates with core materials made of PVC foam or BALSA.

Laminates produced with sphere.core SBC in the core area are not classic sandwich laminates where “alien” core materials are bonded together with external top layers made of pure GRP, but have the characteristics of a homogenous total laminate produced according to the wet in wet method.

A laminate with sphere.core SBC has particularly good characteristics in terms of extremely low moisture absorption, which is in the area of the values of normal GRP laminates.

 

What works best for transom, stringers, floor, etc?

 

Roob76

Do you have any experience with those materials? I have played with some of them- I got their sample pack last season- but haven't built anything substantial with them.
Rob

 

no i don't i have a sample also but have not gotten around to putting it to use. i think i am just going to take a 2"x12" piece if the 6mm and laminate with 12oz baxial skin and take a piece of 1/4" marine ply and laminate it with the same 12 oz skins and do some testing for stiffness and breaking strength. if it is as strong as the ply piece it will be fine. i will also test both with 17oz skins but not sure when i can get around to it. currently working on another project.

 

Balsa-core should NEVER be used below the waterline. Its old tech. There are much better choices out there today. I use Nidacore, and have had great success with it.

 

Balsa is one of the best core materials around--I am glad for your success with Nidacore but don't slam Balsa--Learn how and when to use it and be pleasantly surprised, use poor quality control, and well you get what you deserve.

 

After having used foam cores, balsa, honeycomb in all forms of layup (hand, vacuum bagged wet preg and pre-preg, and vacuum infused) balsa would be my core of choice provided it was installed properly. Many manufacturers today won't touch balsa and more than a few have had problems with it which I attest to improper scheduling and / or installation.

After repairing boats that were built with balsa from over 20 years ago, even in cases where the boats were built with standard gp PE resins, the amount of rot and "black" core that had to be replaced was mainly dependant on how well the boat was cared for. If you have a good gel coat, backed up by a good skincoat (preferably with a good dcpd or ve resin) and you install the core the right way balsa has some incredible properties. Baltek is now offering a 5-6lb density core called SB-50 which weighs half as much as their standard SB-100 (10-11lb density) and the laminate properties are almost the same.

All in all, if you figure out what thickness you need and where, and you install the core properly, I would say that anytime you can core a laminate schedule instead of building solid you are farther ahead of a solid glass laminate (in terms of weight, cost, labor and strength) so you can't really make a bad choice in using any core, I just prefer using balsa and would stick with it anytime I could.

just my 2 cents