Small Cruising Cat

I know this question has probably been asked before, but I want to make sure I have all the Info I need before I start. I'm panning to build a small cruising cat, something like the Double Shuffle 5.6m catamaran by Ray Kendrick or something similar. The cat is intended for lake cruising, so very calm waters. The question is can I build this cat with exterior Ply since the whole outside will be fiberglassed and it wont spend much time on the water since the cat will be trailerable and mostly parked in my driveway?

http://teamscarab.com.au/5.6cat/design.html

 

Yes you can. I have build several skiffs with exterior plywood. Left outside they lasted about three years. They only got the original two coats of paint and no more maintenance.

 

How long do you think it will last with exterior ply and the whole outer shell fiberglassed?

 

How long's a piece of string?

It all depends how well you build in the first place and how well you maintain the boat.

 

If you fiberglass the boat and it never gets any water intrusion, indefinitely.

 

Thanks guys!!!

 

How much of the entire boat cost do you expect to save by using inferior ply ?

Most projects of that size would be lucky to justify the use of potentially problematic materials. Just wait till you get a sheet of plywood snap in two after you saw halfway through it because of a significant void. With marine ply, most reputable sellers would exchange it for a good piece.

Plywood isn't just about keeping the rot out - its also about not having hollows and defects that degrade and break with pressure and use, that exterior plys are usually never exposed to.

You know the old saying "The Quality shows long after the price is forgotten"

 

So, for a project that big you recomend marine ply even if the boat its for lake sheltered waters?

 

It seems a no brainer to me.

Sheltered waters isn't the whole question - boat hulls are under strain from just daily use. Trailer loading, foot traffic, dock impact , moderate sailing - especially in multihulls.

Also, more boats are severely damaged while sitting in a yard or on the water where water makes ingress over months through damage or building faults, than are damaged by storms. Marine ply is much, much more tolerant of these types of problems because of the quality of construction (much fewer voids, better quality veneers etc)

My guess is that on a boat this size, the ratio would be about

Total Material Cost of the Boat ~ $25,000

Marine Plywood - $6,000
V
Acceptable Standard Construction Grade Plywood - $4000

(You would have to make your own sums on your own project of course.)

Consider the next two major components
1) A quality paint job - this often forms 20% of the overall cost, and adds a huge amount to a boats resale value.

2) Fibreglass and epoxy.

Both of these major cost elements rely totally on the plywood for their integrity.

So, by saving a lousy $2-3K, you are risking the performance of these major components. The sums get even more compelling when you factor your own labour into the project.

Its like cutting down on your houses foundations specs - to see the whole structure fall apart over the years.

If you are shy about the little bit extra cost of marine plywood, I assume that this project is a major investment that you wouldn't want to risk watching your hard earned savings and work slowly deteriorate over ten -twenty years. I also find that having that niggly doubt of the hull quality takes the fun out of using the boat - you find that you start having doubts about the hull integrity, and worry about the safety aspects.

Likewise, when you decide to sell, you want to recoup as much as you can from your efforts. Being able to present an obviously sound hull, with the receipts for quality products will be a big help.

Its very rare that buying quality materials doesn't create 3-4 times the savings over time.

 

Wow, thanks rwatson for taking the time to explain it that well. After all, with all the hours Im going to put into this project I want to make sure that the result its a good quality product.

 

Quality paint and adhesives are usually less expensive in the long run, but I think he fears construction grade wood unnecessarily. It is just a matter of designing to the properties of the materials, which is what I do daily as an engineer. You can build a house, a boat or even an aircraft structure out of wood, composite, aluminum, or steel, all can be equally strong, but each material has certain advantage and disadvantages. The choice on which is used usually revolves around costs and certain other considerations.

If you were to use the lower grade (and lower cost) materials from a lumber yard, if built properly, all it would do is likely add some weight to the finished hull. But it can be just as durable if materials are selected properly and just as much care is put into the construction. What you do not want to do is take plans designed for marine plywood and substitue the same thickness of contruction grade plywood. The substitution has to be done with the various properties of the different materials in mind. THis is also something I am asked to do as an engineer, the builder wants to substitue some other materials and I give him the new thinkness and fastenting schedule to allow the use of the new material.

If you want to keep it light and be durable, than the higher cost materials are required. Otherwise it will not hold up for more than a few seasons of careful use.

I have built a number of small boats, paddle and sail powered, using salvaged or lumber yard grade wood. I hand selected and milled the wood to find the best quality pieces, it adds time but saves lots of cost (and it is part of the creative process as well). Some of the boats I intend to last a while, some were not; just fun single season knock abouts.

So consider your intentions, and long term plans for the boat you want to build, and choose accordingly.

Good luck with your project.

 

Maybe not so unnecessarily .

Here you carefully list all the caveats - "better than average quality, heavier specs, hand selection etc" .... to a complete novice at boatbuilding.

Then, the end result " ... it adds time but saves lots of cost."

Well, assuming his time is not worth anything (a bit of stretch) - would you like to estimate the cost saving on this sort of project ?

How far off on my estimate would I be do you think for a project of this size ? I bet "a lot" is not much at all !

This material civilisation goes to great lengths to test, organise, classify, grade and specify all manner of manufactured goods to avoid time waste and
future failure.

If using substitute materials was such a big saving, boat designers would specify thicker, heavier but cheaper options. But they don't, because it just doesn't.