Use of wood as core in polyester

I'm working on a boatbuilding project and I think about use of wood as core in polyester fiberglass laminate. I hear something about that, but I can get THE good answer.

Many peoples said that polyester resin not bond on wood but others said that polyester not bond on rotten wood but bond correctly on new wood if resin is dilute (with stryene) and use on a resin rich coat. I understand that epoxy will be the best but it is to costly for my project.

I think about using White Cedar as core. Somes said that end grain is the only good installation, others said that plank (longitudinal orientation) is better because end-core give a very rigid laminate but it cant absorb shock and will destroy easily. Because soft wood are more elastic in longitudinal and because my laminate will be 1/4 inch thick (Gerr's formula), it is better the put 1/8 inch FRP each side of a longitudinal wood core. If I go with longitudinal installation, it is better to take rough plank or planned plank.

What do you think about a 2 X 6 lumber laminate in the keel?

What about use of 1/4 inch Fir plywood as core?

 

delamination

The problem with poly-resin bonding with wood is moisture in the wood. Moisture will cause delamination. Bonding will be inhibited on some degraded wood surfaces because of the woods condition. Rotten wood may very well cause a problem for you. But there are many spectrums of rot in wood so it depends how bad it is.

A good rough surface is best, 60 to 80 grit sanded surface clean of mold/mildew, grease, wax, and any moisture. Test with a moisture meter if your uncertain.

Of course you understand that laminated wood is typically a heavy alternative to foam.

 

Epoxy costs 4 or 5 times as much as polyester, right? But then you have a much better bond, and it's friendlier to work with. How much of your total costs is in the resin?

 

I just have to add that "friendlier" here means, less explosive. But there is still a great risk with epoxy of creating allergies!!!!!
Use good breathing protection, wear gloves and avoid direct contact.

 

Absolutly guys, be safe always. Know your chemicals. For instance did you know:

Styrene is a vasoconstrictor. Meaning that as you inhale it, one of the many ways that your body reacts to it is by reducing the size of your blood vessels. This reduction in size can lead to sudden heart failure for those with plaque and cholesterol buildup.

It also damages the kidneys and liver. The liver damage is typically reversable but the kidney damage is not.

I wear a Positive Flow Air-Line Respirator with a full face mask. The air is supplied from a pump far away from the work area. If you wear a cartridge mask be aware that styrene is absorbed through your eyes while your working with it. It's only partial protection.

Be safe. Thanks.

 

Thanks for answer

About the weight and the price of wood core vs foam core. For my project I need about 16 m2 (172 sq ft) of 1/2" core. A box of 200 sq ft of Balsa 1/2" cost 480$US and weight 79 lbs (36 Kg). For foam the weight (8 lbs/ft3) 66.5 lbs (30Kg). I dont have the price for the moment. A 1/2" core of cedar cost me about 40$US and weight (at 20 lbs/ft3) 65 Kg (145 lbs). So for all the boat the difference is 30-35 Kg. I think that the 1/2" cedar core will be stronger of 1/2" foam core.

About moisture of cedar wood core. If I use this core, I put it a 8% humidity (it is the equilibrum point for 40% relative humidity in air at 70 F ( 20 C). So if I understand, the problem with polyester is when using on old boat where is it impossible to dry the boat enough to have a low humidity wood. Yesterday, I visite a small repair shipyard. they actually work on a 60 feet 25 years old fishing boat. The boat is in Oak and they received the boat very damp. They wait 2 months (boat inside a polyethylen frame with few heat (here we have very cold winter)) and next month they will put fiberglass in the hold. They have no choice to use Epoxy. My second passion is woodworking and I use hardwood. I need 2 years in a well ventiled and heating place to take a plank of hardwood adrop humidity from 40% to 8% without to much defect on wood. So drying a old boat up to 8% is technically impossible (with many close area in a boat). Today I will buy some stryren and I try some test (poly FRP on wet plank, wet plank but styren dilute, dry rough plank, dry planned and sanded plank, wiht and without styren dilution, end-grain vs longitudinal). I will put some test plank outside to snow and rain to check what happend.

About the cost of Epoxy vs Polyester resin. For all my boat, I need about 400Kg of resin. My price is 1500$US for polyester and about 6000$US for Epoxy. I will build a 23 feet open boat without cabin and any other pricing gadget. So I focus on a 9000$US total cost. I know that is very small but I try to reach it. I will put a old 2 strokes motor and I work on everything (electricity, motor mechanic, holstry, welding). For the moment I'm at 5000$US (wood femal mold, polyester resin and fiberglass, fuel tanks,plywood as bulkhead an floor frame, Old 150HP 2 strokes motor). I already have electronics (GPS with charts, deep sounder, VHF, compass). So $4500 more for resin is to much for me.

About safety, I understand that I will need a good ventilation and I dont work all the year with that. At the beginning, i would build the boat in the winter but I decide to build it a the end of the spring so it will be easier to ventilate the shop.

Those somes one have information about use of end-grain wood vs longitudinal (plank) installation for core?

 

This is why it's such a BAD, BAD idea to use poly or epoxy on excisting vessels.

 

"The opinion of the majority is not necessarily correct” – Yi Qing Cui

I never saw this before. I love it

Do you think same thing on new boat (with humidity controled)?

 

Thank, got that quote from my agenda!

No, I believe on new boats wood and poly/epoxy can be very well mixed. Humidity control is much better and all areas can be reached. Which is impossible in excisting vessels.

 

I understand your cost issues very well. 6000CAD for 400kg of epoxy seems to be a fair price. If you use bidirectional glass fabrics instead of chopped straned matte, you will use less resin and have a lighter , stiffer laminate, but you probably know.

One problem with polyester on wood is that it will travel water or humidity through the skin, so the wood will start to swell. That's why old polyester boats with osmosis problems is trated with epoxy. If your boat lives on a trailer under a roof maybe it's ok? Epoxy is a better water barrier than polyester.

 

Sorry about the wrong dollar.
I think you should be able to get the epoxy for 4500USD.
400kg is quite alot! 15CAD/kg or 12USD/kg ?

 

My boat will effectivly live on a trailer and I dont use Gelcoat but Polyurethane paint for the bottom. My price for Epoxy is 3800 $CND (3000$US) for 45 Imp gal (205 liters) and I need two. The from shipyard near me take is Epoxy at same place (because transport regulation and price) and pay same price. He probably help me to find best price on fiberglass.

I will use 32 Kg of Mat only and 220 Kg of Combo (bi-axe + mat) of different kind
1708-1808-2410 (I take the roll that I can find at good price) for a 10 mm (0.400") skin (or two 5mm skin with core). I think that I cant go lighter

 

I think you will be all right
There is a lot of old sailing polyester boats with wooden stringers for example.
To be a little patriotic; take a look at www.amt.no for very good bidirectional glass...

 

Wood as stiffener

Jfblouin,

I dont want to start and arguement here because there has been a long arguement about this in a boatbuilder magazine before.

Wood in general is structural if used as a stiffener. By itself there is no need to cover it with fiberglass laminate. Coating it with a waterproofing epoxy or paint will suffice. Solid wood is heavy.

Wood, unless of very good marine quality, will eventually rot. Fberglass will not. In the future, you will be hunting down for weakened structure while the glass is as healthy as ever.

When used as a "top hat" configuration to reduce weight, some people swear by marine plywood with a thicker "cap" than the web. The principle is correct because the "cap" or "crown" gains its strength by the cross sectional area of the crown. Usually the crown is of unidirectional grain, the web is of plywood material. You have to laminate this with fiberglass to obtain a homogenous structure.

When balsa is used as a "core material" (foam can be used) the core acts in compression, therefore end grain balsa is used. balsa is good in compression.

Polyester does not stick well (due to moisture content) to wood nor does it penetrate well. Epoxy penetrates the wood better, hence, a better bond.

My favorite, in terms of cost, is single skin laminate, cored stiffener design.

Rx

 

Evaluating cost

If cost seems to be of major consideration, i suggest you start getting your surface areas now.

The keel, bottom, deck, cabin have all different laminate schedules. Break it down then run a spread sheet.

Run several design method, single skin, sandwich laminate, wood stiffeners, cored stiffeners, using polyester, using epoxy,ect.

Summarize the quantity of each material and multiply the unit cost of the materials by the quantity. add all costs.

You will be surprised at the cost variance of the MATERIALS.

Another factor that comes in is cost of construction. One method may be easy to build, another is not. Give yourself an hourly cost no matter if it is free. Dont forget, there may be time you will have to hire a skilled hand. Labor cost (or hours) runs from 30% of the cost of the boat for the unskilled but could go down to 18% for the skilled.