Plywood question

Hello all,
I am looking at building a 22 to 26 foot sailing cat to take out in the rivers and offshore only 25 miles or less(offshore fishing with my son). The question I have is what kind of plywood should I use? Could I use an exterior grade pine plywood then cover it with either epoxy paint, gelcoat, or a layer of fiberglass? I assume the pine plywood used with bulkheads is strong enough considering I am river sailing and going offshore a little ways and in good weather. What should the skin thickness be at minimum? I know that these questions depend on a lot of variables but I am just looking for some basic rule of thumb to stay safe. I am building this cat for several reasons 1. The wife hated it when I rolled the 27' sailboat over to about 48 degrees she didnt like the leaning (so the boat was sold). 2. Project for my son and I. 3. I figure I can build a plywood cat with out all the luxuries for a low price (I have lots of tools and lots of wood working experience as well as FG experience and welding experience and I have all the tools.) and I already own a small outboard and generator.

 

Well, you can use regular exterior ply and encapsulate it, but look into the cost diffential between that and marine ply. For your sized boat it may only amount to a few hundred bucks. When you consider the amount of time you're going to put into this project, the cost of the plywood will become a lot less significant.

Something else to consider - a catamaran is a lot more complex structurally than a monohull. Significant forces on the bridgedeck connections to the two hulls - that deck needs to keep the two hulls acting as one even when one is on a wave crest and the other in a trough. Likewise, the loads on masts and sails is higher since the cat doesn't heel to relieve pressure. Bottom line - it would be wise to either buy a proven design, or hire a designer to assist you. You're going to invest a significant chunk of time and money in this project - you want to have a high degree of confidence that the end product will work.

Check out the Boat Design Directory on this site for a bunch of plans and designers.

 

If you ever want to sell your boat, the difference in resale value will be many times higher than the extra cost of marine grade plywood. You will also save some weight if you use Okoume plywood. Look for BS1088 and Lloyds approval and 15 years warranty or more.

 

I agree entirely with the last two posts!
I am currently building a small, stitch and glue, hunting skiff.
The cost of the epoxy and glass was over $1000 and the cost of the plywood was about $500 for good quality marine ply. I could have saved about 200 bucks by using x-grade, but then I would be coating cheap plywood with expensive epoxy, reducing the value of my boat by much more than the 200 bucks I had saved.
With the marine ply I KNOW that there are no voids large enough to compromise the strength of the material. With exterior grade you can be assured that there will be some voids and this is where the trouble begins.
My firm belief is that you simply can not save enough money by using x-grade to justify the risk that your boat will have a drasticly shortened lifespan.
Hope this helps!

unhippy

 

I do not care about resale I am just wanting to build the cheapest safest boat to play with for a little while.

 

Hour many hours are you estimating this boat will take you to build?

 

You can have cheapest or you can have safest, but you can't have both.

unhippy

 

Bearwen. I sure as hell hope you are building it from and following a plan that is from a reputable plan company. You are responsible for their safety.

 

If you use cheap plywood, please add enough flotation, like foam, to make the boat sail all right even with one hull fully flooded. I think we all are concerned about your sons safety 25 miles ashore.

 

If you use ext.grade ply, at least use fir, pine ply is junk even for home building.

 

Traditionally, the planking stock in wooden craft is the best stuff on the boat and for good reasons. Primarily, the planking supplies a large percentage of the longitudinal strength in the hulls, particularly true in the long, narrow forms found in cats. Other issues such as voids preventing fair bends, overlapping inner plys creating hard spots, inner layer plys not having species as resistant to rot as the outer layer, pockets formed with the large void count found in exterior grades will promote rot to form, etc., etc., etc.

You can use a lesser of grade sheet goods, if you like, but it will cost you across several elements of the structure, many unseen, by the amateur builder/designer. Which brings me to my next concern, of taking people you love and care about farther from shore then THEY can swim back to, if you've screwed up a critical calculation in your design process. Cats, by their very nature, require higher strength to weight ratios then conventional hull forms. This asks much more in the engineering of the vessel, if reasonable performance and safety is to be expected. How healthy are your engineering skills? Have you increased scantling dimensions to offset strength issues in the lesser grade materials?

 

never never NUNCA never use exterior plywood in any kind of boat. forgo thr expence of marine plywood. i worked for a boatbilder once that told me doug fir except epoxy best. he build epoxy boats since the 50's and he held 7 world records from the miami to new york run in a sportfish. i do believe that he last record still stands

 

Maybe strip planking, if you make the strips yourself, is a safe low cost alternative.

 

Some years back I went from a 30 foot monohull sailboat down to a 26 foot cat-cruiser and lost her in the gulf about TEN MILES out when the port pontoon parted company with us in a sudden blow that generated very high waves. Floatation saved us (along with the Coast Guard air lift) and I decided then that closer to shore would be the best plan with a cat & a low powered engine.
I firmly believe that the 30 foot mono would have handled that day much better than the cat, given the greater strength of a single hull. So I caution you to follow advice given on here about safe design, safe materials, safe floatation, and second thoughts on sailing far offshore even ocasionally unless you are fully crewed and have excellent emergency equipment.

 

If you use a cheaper plywood, at least buy fir. Pine has a lot of resin and the bond with resin is always poor.