20'x48" riversled, what to use for core?

[gotmuddy]

I want to build me a riverboat for the ozark rivers around me. low sides, light weight. Here is a fine example:





This is a supreme boat, made in flippin, AR. Great boats, this is just what I want to model mine after.

I was thinking of using 1/2" treated or marine plywood for a core and fiberglassing over it.

[rwatson]

That's a relatively heavy method for that size boat.

Consider hi-density foam core and fibreglass - sort of like a 20ft surfboard.

[gotmuddy]

that sounds good. Where is a good place to look for foam.

[rwatson]

Most fibreglass shops will either sell or put you onto a foam supplier. Brands like Airex, DIAB etc are common.

Its worth searching around this site for info on foams.

Some people use the expensive hi-density marine foams for the hull, and cheaper foams from hardware shops for the deck and topside fittings

[PAR]

The cheaper foams usually delaminate under repeated load, shearing at the bond line. A plywood boat of this size would be very difficult to beat in a weight to strength ratio, not to mention cost. You need a whole lot more fabric and goo on a foam cored build then a plywood build, so costs and effort go up substantially, compared to a plywood build which technically doesn't even need the 'glass.

If I was building this boat, I'd use a 3/8" to 1/2" bottom (good plywood) and 1/4" to 3/8" sides, depending on the speed/toughness you wanted. If lightly powered and mostly just poled around then keep it light with a 3/8" bottom and 1/4" sides and seating. Tape the seams and hit it with house paint when you done. It'll work fine, last a while, given reasonable care and most importantly will be easy to repair, unlike a fancy, foam cored 'glass version. If you build a light plywood version, as I've suggested, it'll weight less then 150 pounds empty (75 lbs., for the bottom, maybe 40 - 60 lbs. for the topsides and another 40 for the stiffeners, bulkheads, seats, partitions, etc.). The heavier duty version (1/2" bottom, 3/8" topsides) will be in the 180 pound range. An extra heavy duty version would use two layers of 1/4" plywood on the bottom, the outer being sort of a sacrificial layer, that gets replaced every few years as needed.

In short don't over think this thing and try hard to keep it light so it's easy to handle and propel.

[waikikin]

I like to learn new stuff.... River Sled is a new term for me, they look like a pretty useful craft (I googled "supreme river sled"), very similar to our oyster punts & work scows.

[gotmuddy]

Thanks for the reply PAR. Top speed on this boat will be 15-18mph and the boat will be propelled by a 9.9hp mariner outboard. This boat is being purpose-built for the buffalo river. Slow speeds, low water, and lots of big jagged rocks. Maybe if I coat the bottom in epoxy the wood will last.

[gotmuddy]

Originally Posted by waikikin

I like to learn new stuff.... River Sled is a new term for me, they look like a pretty useful craft (I googled "supreme river sled"), very similar to our oyster punts & work scows.

These riverboats float in almost nothing.

[masalai]

gotmuddy,
Rather than just an epoxy coat on the bottom whilst it is upside down a light cloth on the last (bottom layer sacrificial plywood layer) glued with epoxy and a bit of cloth tape to hold inside edges firmly...

[PAR]

If it's expected to bang into rocks and river bottoms fairly regularly, a thin 'glass sheathing isn't going to help much. You'll want Xynole or a sacrificial layer of something or runners or both. Light 'glass cloth is fine for a canoe that bounces of stuff at 2 MPH, but not a boat as envisioned here. Sheath the bottom with Xynole and install hardwood runners.

[gotmuddy]

the epoxy was planned to be the sacrificial layer. I intended to put 1/8-3/16" layer on and repair as it wears off.

[rwatson]

Originally Posted by gotmuddy

the epoxy was planned to be the sacrificial layer. I intended to put 1/8-3/16" layer on and repair as it wears off.

If you mix the final layer with say, West Systems Colloidal Silica or a similar product, the epoxy becomes very hard and very abrasion resistant.

[waikikin]

something like this? http://www.youtube.com/watch?NR=1&fe...&v=_VWZvj6-q4o

[PAR]

Epoxy alone isn't an especially tough coating, even with unique filler materials added. In fact, tests show adding fillers (most) usually decreases resin strength, not increase. This is mostly from bubbles introduced during the mixing process.

Fabrics are what provide notable abrasion resistance, not fillers. Again Xynole is the best in regard to the usual and cost effective choices, having on the order of 6 times better abrasion resistance then regular 'glass cloth. You can step up to high tech fabrics, with their related huge jump in cost, but pound for pound, Xynole (about $9 bucks a yard) will do a fine job in a light, small craft, expected to bash into stuff.

On the other hand, sacrificial runners and/or a double layer plywood bottom, could be an alternative. You could skip all the resin and 'glass work, just taping the seams. Every few years or so, you'd remove and replace the worn out runners and keep on going. Maybe every several years, you might have to remove and replace the outer layer of plywood, but again, fairly easy and no mess, just screw another bottom panel down, install the runners and toss it back in the water.

In the end, you need to decide what you want. If you want bullet proof, then the materials and build effort will be initially high, as will be subsequent repairs. Conversely, if you use the sacrificial layer approach, knowing full well it's going to take a beating, you replace and repair as needed, which keeps initial cost and build effort down, as you're paying for the durability over time, instead of all at once.

[gotmuddy]

What if I cover the bottom with UHMW for the sacrificial layer? Nice slick bottom.