Best approach for sheathing new design?

I realize that the world does not need another 20' launch/runabout design, but there is something appealing about developing an original vision. I have never designed or built a dedicated powerboat before, but have been doing my research, looking at what other people say and do on this forum, and think I am close to taking the risk in building a semi-planing "picnic boat" for day-tripping on freshwater lakes. There will be many questions along the way, but my first has to do with sheathing the hull. I will probably use 1/4" (6mm) plywood for the deck; 3/8" for the sides; and 1/2" for the bottom. The hull form is fully developable, and I want to use plywood for its strength and light weight (also dimensional stability for a trailered boat). I have built a 1/5 scale model using 0.1" thick plywood which conformed easily, but I doubt that 1/2" will conform so easily to the bottom of the full-size hull, particularly at the forefoot. So my plan would be to use two 1/4" layers. Getting the first layer on will be difficult enough: a 20' by 2 1/2' panel. Would it be easier to scarf it in place or handle a single panel scarfed beforehand? Then for the second layer; I think that I will have to use planks; just too much to handle, glue, clamp, and fasten in one operation. If planks are used, would I be better off to use solid lumber or scarfed plywood? Which is better for fastening the second layer, staples or (temporary) screws?

Another thing I am starting to think about is what size of O/B engine. The hull is approx. 5' by 18' on the water with a engine well cutout and splash pan integrated into the aft three frames. If I use a 15" transom height, I can probable incorporate a well cover blending with the deck contours. With a 20" transom, I'd probably leave the well uncovered. My goal is a speed of 12-18 mph. Having appropriate power/weight for hull shape will be important, and it is an area where I have little experience. The hull has some rocker but also a straight run to the keel. From what I have read on this forum concerning semi-planing hulls, there are a wide range of views.

I have already decided to move the forward cockpit slightly further forward, so the design is open to suggested changes.

Link to photos of model: http://picasaweb.google.com/Wayne.Grabow/DevelopableSurfaceBoatDesigns

 

Wayne, this is a very interesting wheel you have reinvented

Consider using 3/8" plywood and sheathing in a heavier fabric instead of relying solely on the plywood. Those bottom panels could be done in 6mm plywood, taped with biax tape, then sheathed inside and out with 12oz 45/45 biax for an overall thickness and strength comparable to your 1/2", with the added abrasion protection of glass.

Depending on your freeboard, the motor may or may not be able to be hidden below decks.

Please have a look at my blog (link at bottom), you might be surprised at the boat you find. The entire build process, start to finish is there, so get a cup of your favorite beverage, and settle in for a read. You will pick up a bunch of info on your design.

E

 

Just a one minute analysis. It should be a nice looking boat.

Looking at your model without hull lines, I see what appears to be closer to a planing hull than a semi displacement one. It might not like to run at the slower speed you give. The lines of a Hand based design like the Handy Billy might suit your speed goals better.

In any case, I agree that 3/8" is plenty for the hull bottom and 1/4" is good for the sides. I recently designed and built a full planing 13' runabout with similar lines to yours. It does not like speed under about 18mph to 20mph. I used 3/8" ply for the after bottom scarfed to 1/4" for the forward part with even more twist than yours. As was suggested, I laid 18oz biaxial glass on the inside of this forward part although the boat would probably be fine without it. On the other hand the deck will need some beams to support the 1/4" if it is expected to support a persons weight without deflecting a lot. My deck is crowned with 3/16" mahogany planks over 4mm ply with three transverse deck beams. It seems plenty strong.

http://www.bluejacketboats.com/scamp galleries.htm

Also it's difficult to find 15" outboards in the current crop of new ones, other than smaller hp units. A 15 should give you the speed you want with a true semi hull.

 

Wayne,

Sheath inside and out. Build the whole boat in BS 1088. Encapsulate totally in epoxy, paint or varnish to stop UV damage. Have fun.

http://www.duckworksmagazine.com/r/plansindex/fastpowerboats.cfm is why E mentioned re-inventing the wheel.

Tom,

Beautiful runabout. When did you build "Scamp"?

Pericles

 

Pericles,

I finished Scamp in the fall of 2005. Not a difficult build but probably not for a novice first timer either. I now power it with a 1981 Evinrude 25hp to keep that nice Merc Mark 20 out of salt water. The Evinrude is also much more user friendly with electric start and remote controls. Top speed is 31mph.

 

Sorry I didn't reply sooner, but with a three-week gap between posting and response, I had stopped monitoring the thread. Your replies were spot on, and I will be incorporating such help as I build this boat. If this posting registers (being such an old thread), I will follow up with some photos of the current state of the construction.

 

You can plank the boat in the bow with plywood in diagonal. That is, you put strips at 45 degrees or so pointing one way, then another layer at 45 degrees pointing the other way. Use screws to hold the layers together until the glue sets and then pull them out. It is pretty standard technique.

 

1/4" BS 1088, sheathed on the inside face with two layers of 8 to 10 ounce biax is just as stiff as 3/8", plus has slightly better penetration resistance. Calculate your loading and plank accordingly. Your deck at 1/4" will be the weak link in the "beam" consider using the same thickness as the topside planking, though your S/L is low enough to use 1/4". Diagonally planking the heavily twisted areas is a good solution as Gonzo mentioned.

 

I tried to post these photos earlier today, but apparently it didn't happen.

 

EStaggs: I think I read your entire blog sometime ago. My wife and I found the Spokane area attractive, with a lot more water than Colorado, so I also noted your experiences with the weather there. Trying to decide if we made the right choice. I like the idea of adding biaxial tape for more strength.

Tom: Yes, I've read your blog too. I liked your research efforts. I agree with your observation that the hull looks designed to plane. The aft 10' of keel is straight, but the chine has some curve to it, resulting in a continuously variable deadrise. This design is an experiment; I am curious as to just how it will perform. The deck has beams every 13.5" so it should be well supported. I am considering something like 3/16 planks to add thickness to the bottom.

Pericles: Will do. I ordered twelve sheets of BS 1088 today.

Gonzo: Actually, today when I was making paper patterns (preliminary, not final) of the sheathing, it appeared that the forward twist is fairly gentle and should not present too much of a challenge. I spent time getting the proper projection in the design stage so that I could get this result, but couldn't really tell if I'd achieved it until I could finally see it.

PAR: As stated, I ordered 12 sheets of 6 mm okoume today. I understand your beam concept. With 14 frames, continuous plywood sheathing, and glass inside and out, it should be pretty rigid. With rocky lake shores here, penetration resistance is a good idea.

Finished fairing the frame yesterday. Now I am working on the recessed transom. While waiting for the plywood to arrive, I intend to build a mantle for our home fireplace. It is great to finally have a decent shop.