Balsa cored hull?

[Itchy]

I'm new to this forum but I,ve been restoring / rebuilding boats for a while.
My latest project is a 1987 Baja Force 200. Stringers, floor, interior rebuild.

After some exploritory grinding and drilling on the interior of the hull I found that the hull is balsa cored. The 1" by 1" square blocks taped together. I admit I don't know much about cored hulls.

1st question: The core doesn't seem to be rotten but the wood is very soft it can be dug out easily with a screw driver. Is it suppose to be soft?

2nd question: I have heard horror stories about soaked, rotten cored hulls. I only checked one spot, is there any reason to be concerned about the rest of the hull? Even though the floor and stringers were rotten the transome is in great shape whith no rot.

3rd question: Is there anything wrong with buying a boat with a cored hull?

4th question: How do I tell when looking for future projects if the hull is cored?


Sorry for the long post

Thanks in advanced.

[Itchy]

Anyone there?

[Verytricky]

I am - but I am of no use to you....

[longliner45]

its balsa wood ,,yes its easy to remove,,this is what the new wave generation has to deal with ,,not like real wooden boats,,,not trying to be smart ,,just the facts.... replace balsa with balsa and around through hulls use epoxie longliner..........answer for #1 answer for #4 get a survey,,,,,there is no more old growth wood ,,most of the new boats are plastic and glass with balsa cores

[Itchy]

Longliner- thanks for the reply. The core that I'm talking about is the hull itself, not the stringers or floor. How would this be easy to remove? If the balsa core was wet would'nt the boat be unrepairable? The only thing I can think of would be to cut the entire perimeter of the inside of the hull and peel up the fiberglass to get the core out.

[longliner45]

yes ;the balsa is sandwiched between glass or something else someone else here can help you more then me,,,,,good luck,,,longliner

[mobjack68]

Hey Itchy,
couple of things with the balsa core. The balsa will "wick" water , that is it will transfer water away from the hull breach. About the only way for the balsa to get wet is from a hull injury, inside or outside. If the balsa is wet, it will be next to impossible to dry it out without opening the laminant, that is stripping away the inside or outside layer of glass. If you are talking about a salt water boat, the closer you can get to the injury? the balsa will show up with dark streaks/marks (decomp of microscopic marine animals) if it is a freshwater boat, may not be able to trace the location of the injury other than physical inspection. We hooked a vacuum pump to a crack in a sailboat, sealed the inlet around the crack, drilled a series of 1/8 inch holes around the injury about 16" out. We kinda figured that if the air could be drawn in thru the small holes and be "sucked" towards the pump, the water would be drawn with it. It worked, we sucked almost a gallon of water out of the hull, the boat was only in the water about 4 hours after it was struck (regatta, you know how those guys are). If you are dealing with balsa core that is wet all the way to the gunnels, your boat has a major problem, it was exposed to a lot of water for a long time. Quick check...the manufacturer will be able to give you an idea of the "dry" weight of the boat. You can get pretty close by adding the features, etc of your boat....calculate the difference, that may give you an idea of how much water the balsa core may be holding. Couple of xtra pounds may not matter....so long as the reason for the absorption is cared for...
good luck
mobjack68

[fiberglass jack]

first things first you need the tools, get your hands on a mositure meter will cost a few hundred dollars, this way u can check the whole hull and with a marker write down the readings on the hull , then take a photo so you will have a record of the hull, you could get one of the local boat repair shops to do this for you just pay them for thier time , too many boat owners try to get a free check up, be honest with them tell them u are doing it your self, most of the guys out there will steer u right and give u some good addvice. yes to your first ?, but it should be dry not wet, only after u have run a meter over the hull will u know, the balsa is soft and is easy cut with a knife u can even chip away with ur thumb nail, the core acts like a I beem , 2 layers of glass sandwiched the balsa very strong and light weight, and a lot cheaper than a soild fiberglass hull , but water may get into the core and u will get rot , nowadays they use foam most of the time, if you do have to remove the core on the hull do this by sections say 1 foot wide from top to bottom this way you will keep the shape when u have the first set of sections done , go to the next set, always have the boat checked out for moisture before u buy, alot of times people will drill holes and try to suck the water out the only proplem is that its imposable to dry will take a lot of time and the balsa will have dry rot after sometime the balsa will turn into dust and u no longer have a cored hull , if u have wet core remove it there is no bandaid sulution , if someone came to my shop and told me drill some holes to dry it out i tell them take there boat away not doing a 1/2 arse job

[roy marine]

sorry about your luck..cored hull bottoms are a big problem.
1/ You can remove the local balsa, however the water will migrate throughout the structure. The previous responses are good, use a moisture meter to locate the troubled areas and tap with a plactic hammer to find softness.
2/Sounds like your boat had rainwater sitting in it. This situation is typical because the wood at the low positions has absorbed water, the transom is seperate from the bottom core and the stringers, therefore no migration.
3/yes
4/Get information, call the manufacturer & get facts
Generaly speaking you may open the local damaged area then open a second area to allow air flow. Use a shopvac at one end a heater at the inlet. This is the best way to 'dry out' on a small scale. Once dry use a epoxy repair and seal the area as best you can. Remember you must replace the skin in the area inorder to retain the strength as the core system is design for the skin to carry the loads. After this find the inlet for the water and seal it. Many 'sins' are comitted in both production & repair, no protrusion into the core should be in contact with the wood. By this I mean wood screws are not allowed.

[Itchy]

Thanks for the replies, I don't think I have a problem whith the core in this boat, I just don't want to get burned on the next one.
Do you guys have any links for a good moisture meter?

I've also done some research since this post and it seams that almost all the manufacturers since the early 80s use core laminates in there hulls. Now what do I do, Start building my own boats without cores? or is the balsa core hull an ok design if is properly done?

[tgwhite]

Itchy:

Your research may be only half right. Most reputable manfacturesrs today do not core below the waterline. If they do, it would be for racing applications to lessen weight.

[yokebutt]

Yes, end-grain balsa is one of the best core materials available, provided it is installed correctly.

Yoke.

[mobjack68]

I don't know Yoke....Vacuum bagged Klegecell core gets some awful high marks...AND no rot.
mobjack68

[yokebutt]

Mob,

Yes and no.

Balsa has a much higher compressive strength than klegecell/divinycell, good when you're bolting on loaded hardware. (yes, I'm aware of the irony)

Properly installed balsa has corebond adhesive or resin in all the gaps between blocks, thus ensuring that water won't spread too far if the skin is compromised. Compared to plywood coring, the balsa is much stiffer, lighter and less prone to rot.

But, balsa is also about twice as dense as "regular" foam core. (H-80/K75)

Yoke.

[dougfrolich]

Balsa scores the highest in shear strength too!

[Itchy]

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?

[rturbett]

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

[Itchy]

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

[rturbett]

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

[Karsten]

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.

[yokebutt]

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.

[Karsten]

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.

[rturbett]

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

[roob76]

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.

[Itchy]

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

[rturbett]

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

[roob76]

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.

[jimslade]

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.

[dougfrolich]

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.

[adamfocht]

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

[buckknekkid]

They still using it at SKATER?:?:

[adamfocht]

I haven't heard, nor have I seen otherwise

[Itchy]

adamfocht- thanks for the info. Where in MI are you, do you work in the marine business?

[buckknekkid]

Adam ask Joe if hes still getting Road Head?,, hahahaha :D

[adamfocht]

haha I'll have to ask him that

itchy - I work for Sunsation boats out of Michigan

you can check out our site at www.sunsationboats.com

[Itchy]

Sunsation builds some nice boats. Do you guys use balsa in the sun hulls?
I talked to your sales department a while ago about a tour of the facility. Going to try to make if up there this winter. I'm in Goodrich (near Grand Blanc) so I'm a couple of hours away.
What part of production do you work in?

[adamfocht]

We build fully cored hull and decks and use alot of balsa in our secondary parts as well. We like the properties balsa offers and have had no problems using balsa after 25 years in our boats.

I run the lamination shop for Wayne and Joe at Sunsation.
We'd be glad to give you a tour and show you our facility.

adam