Lightweight medium-large composite canoe

Hi there boat builders.

This site is very informative, yet... I hunger for more wisdom.

I'm in the design stage of planning a moderately large composite canoe.

It will likely be about 21'-24' long, with a beam of about 3-4 feet, and a midship height of about 16"-18". It will be human-powered with an anticipated load of up to about 900 lbs.

A light boat is key. My current 17'6" canoe weighs about 45 lbs. Many sea kayaks of similar length are even lighter. Building a boat three or four feet longer ideally shouldn't add more than 10-15 lbs. I'd like to keep the new boat as light as possible, while remaining seaworthy.

With these points in mind, I'm planning on using epoxy with a foam core composite in layers of Kevlar and surface layers of S-glass.

I know that commercial manufacturers often use foam cores in the bottom, and embedded foam ribs in their canoes.

I'd be willing to put up with a couple of pounds of extra weight if a continuous foam core throughout the hull provided substantial benefits in hull strength compared with a hull design with embedded foam ribs.

My questions then are:
1. Is it possible to safely meet my weight goals for a canoe hull with the above dimensions? If not, what is reasonable?

2. What core and thickness should I use, and what density of core? I'm thinking Corecell M foam 1/8"-1/4", but wonder about alternatives. Perhaps a higher density/thickness on the bottom, and lighter on the sides? Or stay with low-density foam and add more Kevlar to the bottom???

2. How many layers of cloth on the inside vs. the outside. Opinions vary depending on who I talk to. I'd like more opinions.

I welcome all thoughts and comments! (And hope I hear lots)

Cheers.

 

The safety will depend on where you want to take it. It will not be suitable for whitewater rafting with that load and a hull of 60lbs.

Carbon fibre will give you the best strength for weigh but is expensive.

Whatever you do it needs some knowledge of composite construction to get the best strength for weight. The location of bulkheads is a consideration. The stiffening around any opening in the deck is important detail. I expect the optimum Corecell thickness will be more than 1/4" for the hull but could be suitable for deck.

The thickness of the layup (skin) needs to be able to handle local loads anticipated such as any abrasion and point loading; even things like standing on the panels. Of course you can add a little extra strength to where you know there will be point loads. Then you need to consider the thickness of core to get the panel stiffness so it does not flex unduly under normal load. This depends on shape and spacing between supports.

There is some very good construction detail on a boat of similar displacement shown here:
http://picasaweb.google.com/adventuresofgreg/ExpeditionBoatBuilding#5165504938502216706
The hull shell on this was 54kg before any fitout but is all carbon fibre and is designed for safe ocean travel. So maybe greater strength than you need.

Rick W

 

Someone with the smarts can give you the particulars, but boats are exponential, as in twice as big = four times the materials. So 10-15# is probably not right, and if twice as big = four times the stresses, (I don't know if that is true) then the weight will be even more, with more structure being needed.

 

Thanks for the tips Rick and SamSam.

A little further information:
-The boat will not have bulkheads or a deck - it is an open boat, not a kayak.
-I intend to use it for coastal crusing in sheltered waters. Whitewater is not in the picture!
- I considered using carbon fibre, but didn't want something so brittle, especially in light of local waters where there lots of submerged rocks and shoals. That's why I thought Kevlar might be more appropriate, especially if its underlain by a core like Corecell.

Thanks for the pointer about exponential rise in materials. Going for a boat of 45 lbs at 17.5' to 21 feet yields a ratio of about 1.25. Squaring this -> 1.56. This would move the weight estimate to 45*1.56 -> 70 lbs. This is still light for a 21 foot boat. Is the estimate reasonable do you think?

Thanks!

 

It seems pretty light. If you could do it I think it would be fairly fragile. You might google 'war canoes' and follow that and also look up canoe specific sites and forums. Get all the info from canoe mfgrs on what they offer to get an idea of what's possible as far as weights and capacities, but you won't learn the layups, that will be sort of a learning experience. A lot depends on the shape and end use.