9/8" Plywood Bottom

[COLD-EH']

I have a boat that is subject to the abuse of hitting rocks and sometimes boulders. The current hull is built out of 0.105" mild steel and I end up with cupping and dents in the hull. I was wondering if the extra density of 3 laminated layers of 3/8" Fir plywood would work better. I Inderstand sheeting the bottom with HDPE would help distribute load however it's expensive and added weight. I would consider installing it however. Not all of the hull would necessaraly need 3 layers as I have found the bow portion needs the most strength for boulder hits and the bottom just needs enough density to prevent cupping, and that shouldn't happen with plywood anyways. It doesnt have to last forever!

this is kinda how I treat it! http://www.youtube.com/user/WildBill.../0/zmIAWEdrgVg

[Gaffers]

Does your metal hull already have epoxy on it?

If so why dont you add a few layers of graphite epoxy to the bottom (using the correct technique of course) it's very strong a resists chips. A lot of wood built kayaks use this technique as they are in rivers full of rocks and are constantly dragging on river beds.

[COLD-EH']

For bottom coat I used a mixture of gelcoat, tooling resin, graphite and silica. The chipping doesn't bother me and the bottom coat worked very well however hitting watermellon sized boulders has a tendency to create dents and going over boulder beds with knife edges creases the steel affecting hull shape and performance.

[gonzo]

Make a composite of plywood inside and metal cladding outside

[PAR]

No amount of epoxy and graphite will help this issue Gaffers. The boat has a 12 gauge, nearly a 1/8" thick steel plate hull. This is getting dented, buckling and oil canning from impacts. What can some goo and slippery filler do?

Okay, maybe a 1/4" thick layer of silica and milled fibers might add a wee bit of stiffness, but it would likely chip off in large chunks when hit by big rocks, not to mention the weight. Toss in a few layers of biax and you might have a chance, but it'll still dent.

HDPE wouldn't be your best choice, unless you feel abrasion is your biggest worry, not impact and load dispersal. The problem here is two fold, your boat isn't designed for this service and adding the reinforcement necessary to accept these types of loads should be preformed internally, not externally.

Yes, you'll still get dents, even with plywood or HDPE. They will not be as big as without your "sheathing" but the impact will transmit through and put a dent in you steel. Again, the only way to prevent this is to decrease frame spacing and reinforce the frame bays, possibly with plywood.

A picture of your boat and an accurate description of the service you're employing the boat would be helpful. If it's similar to the airboat in the youtube clip, then you can use disposable runners (HDPE or hardwood), which should be closely spaced and again supported with reinforcement on the inside of the boat. These can be replaced as needed and if like the airboats we use down here, twice a season get renewed.

[COLD-EH']

Yeah it is the airboat in the youtube clip. My concern with hdpe is tearing from the jagged rocks. My plan was new hull built out of plywood, 3 layers at least for the bow but probably for the whole thing. 12 gauge steel is 4.4 lbs/ sq ft and plywood would be just over 3. I have 7 stringers on a 92" bottom on this hull and I would like it to be lighter as well.

[PAR]

More stringers and more frames in the forward sections, deeper rub strips (standing farther off the hull) and expect to tear up the boat fairly quickly with this type of service. Using the hull's bottom as a road grading blade isn't the best thing for it.

A HDPE skid plate on the bow and part of the forward sections isn't a bad idea, but it too will get torn up. You decide which you prefer the hull or the HDPE getting a beating.

You can forget about "I would like it to be lighter as well.", because adding structure will not accomplish this.

The guys I know that run them down here call them "disposable", probably for the same reasons you do.

[COLD-EH']

In total agreement PAR! that's why I want to build a new hull! I am of the opinion that I need density like a thick ply construction to deal with the point loads of a boulder. I know I have to relax a little in regards to the speed I nail some of these rocks. My future piloting plan is to stay on the water unless I need to deviate from it and be less of a show off. Southern guys I have chatted with will generally run 3/16" with HDPE over rock, Texas guys, and they say part of maintence is beating the hull straight again.

I need lighter, tougher, easy to build, inexpensive. 1/4" aluminum is about the same weight as 12 gauge steel but needs HDPE or the guys up here also sheet it with steel to slide on rocks, So too heavy as I imagine I could tear 1/4"-3/8" HDPE open with one bad rock. Figure 1/2" would be the minimum and that's 3 lbs/sq'. too heavy at 7 lbs/sq' just for bottom.

3/8" exterior fir is about $14.00/sheet here. I figure 3 layers laminate bottom because of easy bending, alternating sheet joints to reduce/eliminate scarf joints, inexpensive glues for majority of joining work, maby even vacuum bag for ease of laminating, elimination of mechanical fasteners. Same deckover kind of boat with plywood/built-up truss similar to steel boat. Layer or 2 of glass for abraision resistance. Fix divots with kitty hair and glass? 9/8" plywood works out to 3.5 lbs/sq' plus a layer or 2 of cloth. I imagine with 9/8" thick hull material number of stringers would be less of an issue?

No rub strips on these baby's! or at least that isn't the trend. Southern guys pretty much cover complete bottom with HDPE if they are doing rough stuFF.

[ancient kayaker]

For the kind of application you are showing whatever you put on the bottom is going to be a consumable, like the skirt of a hovercraft.

[Easy Rider]

Just an opnion .. You need stiffness and lightness .. plywood. You need abraision resistance .. Dynell. I covered a wood propeller w Dynell cloth and epoxie and I had to throw it away .. couldn't even sand it!

[GTO]

Following A.K.s theme, why not just build a hovercraft, if you are going to all the trouble of building a new hull?

Even better, a ground effect craft. You won't even hit the rocks.

http://www.hovercraft.com/content/in...index&cPath=53

[Submarine Tom]

COLD-EH,

You're in a tuff spot there. More resilience without adding weight.

How about a spring mattress approach?

Or, like you said earlier, pounding out the dents is just part of the game.

Well engineered carbon fibre might work too.

What about loosing the void space up front in the impact zone and puting all

your weight into one, thick impact plate...?

I really like the ground effects idea. I can just see the stubby little wings

out the side complete with computerized ailerons to keep you level.

-Tom

[Easy Rider]

My father built a 50' Jet sled w Kevlar. Must work well as I heard about the boat powering over log jams. Maybe this is a throw money at it situation .. meaning Kevlar. My Kevlar kayak and canoe both have a leathery quality that is more important to me than the lightness. Fiberglass is extremely brittle by comparison.

Easy Rider

[COLD-EH']

I actually like the ground effect idea and always thought that would be something to experiment with. Fold out wings rear, fair bit of diheadral, canard up front. Try to design it so it's self limiting in some respect? actually if I don't see rocks sticking out, I don't hit rocks,It doesnt matter how shallow the water is. . It creates a bow wave and it's just like a beach skimmer. Sometimes I just get silly, how many of you would decide to go across a field of 1' to 2' boulders? Dynel looks like it's in my price range!
Thanks for the ideas!