pontoon that masquerades as a powercat -overview

Manfred's responses on the 6m low power thread about kohler were the ones that got me thinking about it. I used to build and fly stick built rc airplanes so I followed the diagrams referencing airfoil shapes much better than some of the stuff Ive read that was hull specific. So adding lift sounds nice but it would then drive the bow down, no? and it would only have the effect while in motion. I would guess then, that at rest, more rocker would be less displacement and the stern would sit lower than if it was flat bottomed?

 

I think on Kohler's Dove Tail bottom is where the math to know how much lift it creates enters in. I don't know the math so, I'm going to stay away from it. I have about reached the point where I am saying, "This will work, so I'm staying with it" which translates to long square boxes with sharp points. If it don't leak, it will float. Most of my "I wonder if" is from the hulls up.

 

i was mainly just thinking the shallow v at the bow area might push the water like a plow rather than cutting through. So if i steepened the angle at the bow it would be better. I just don't know for sure if I am interpreting & applying everything I am reading properly. After the comment about starting over I have been re-reading the basic stuff on hull shape and am getting kinda confused. It doesn't seem that the shape i laid out is much different to Groper's. So is the issue the weight of my build method?

 

Pirate, I know this will make a few of the people on here mad but, they think if you don't have a degree or buy a plan from someone who does, that it will not float. You don't have to have a degree to copy what hundreds of others have done closely enough that it will work. It may not be as fast as theirs but that is not the point in my mind. They seem to look at things in a what can I find wrong, instead of what can I find right. I trust people like Manfred, he takes the time to explain it and gives you a picture. Sometimes I just have to keep reading it until I get it.

 

Ok, Ive been a bit paranoid that I had made some grave newb hull shape mistake. So assuming this shape isn't going to kill anyone, would this thing be better off with the v angle increasing the closer it gets to the bow, with the bottom of the bow being 13 " above the bottom of the hull over a 5 ft span? The angle of the taper isn't incredibly steep so I am unsure if there would actually be much difference.

(I have been playing with freeship but was having difficulty, delftship looks muuuuch nicer)

 

I like the looks of the Woods Skoota 28 for the bow and the aft end will be the box or rectangle. 18" wide and 24" tall. Keep in mind that my length will only be 16' the beam will be 8-1/2' I'll try to make it look wider so the Highway Patrol guys can earn their money. The beams will be hollow plywood. I have been playing with 1/8" plywood and blue Styrofoam sheets with 1 x 2's on edge for the deck and will use three 4 x 8 sheets for a deck. The open span will only be 5 feet and there will be four beams under them. Once I get to that point maybe I will make up my mind for the rest of it.

 

If I had the cash and wanted to get an existing design, the Skootas would be in the top 3 easy. I am mainly wanting to build something I haven't before that I can use. (I have boxes of stuff I've turned on the lathe that we ran out of uses for.) Plus I can see if this is what I really want or if i only think it is. Then I can get a real designer to do a proper hull setup and build plan and make everything look nice. Its a really rough concept model/test bed. a pontoon boat made to look like something other than a front porch.

 

If I bought even a small new catamaran I'm sure my two kids would try to have me committed to a mental institute. I have somewhere between 40 and 60 guitars, two Jeeps that are now sitting on a friends car lot for sale. A 1800cc Honda Goldwing, a Harley Sportster. a Mini Mate camper to pull with my Goldwing. Two other motorcycle luggage trailers, a mini Chopper motorcycle, a car, a van and at least 4 rollaways full of tools. 20 or so pistols and rifles, enough ammo to win a war with. I don't think they like me spending their inheritance. I can get away with buying the material and building something because they are on my case to exercise. Of course if I spent the money on something they wanted it would be fine.

 

So i found " Centerline length times transom width times 2 minus 90. thats' in feet (such as 14.2 feet) transom width is the maximum width including rub rails." on another thread. http://www.boatdesign.net/forums/outboards/bigger-motor-56541.html If doing transom widthx2 for mulithull works, then max hp for this would be 70. Having trouble finding the constant so I can do Crouch's formulae to see if this is capable of planing.

 

Will this work http://www.go-fast.com/boat_speed_predictions.htm

 

it helps, tyvm. So with the max hp at 70 it wasn't looking good. Change the back end design a little puts me at 110 max hp....

 

Nope, that formulae is only for monhull. Still looking for proper formulae for multi so I can redo the calculations. Hull speed for the whole wavelength deal works out to be about 6 knots. So if only running on displacement that's about it and probably wont even need a 15 im thinking. But, I did find the Bolger sneakeasy, which, with the exception of my shallow v/pad setup and my taper up at the nose, is just about dead on to what I was working towards. Says it was designed for an 8-20 but people seem to do really well with a 25 hp getting a little over 20 mph. Thats not a bad point a to point b speed. But, that boat is lighter with less wet surface area so, my turd probably can plane but still unsure of the hp needed and therefore if the gas is worth the plane.

 

Kohler's Eco 75 is very similar as far as bottom hull shape goes. http://duckworksbbs.com/plans/kohler/eco75p/Sampledrawing.PDF His deadrise angle appears to be greater overall and gets much steeper at the bow. I laid my deadrise out using pitch ratios like I would for a house roof at 1" rise to 3" run. I think that figures to 18.4 degrees. My understanding is the greater the deadrise the less slap. I have room to increase the degree some overall and I can still increase the angle quite a bit at the front 5 ft. Will likely increase LOA overall as well to somewhere around 24-25 ft. With current calculations that will give me another 450 pounds or so buoyancy per hull.SO..... any thoughts or suggestions?

 

I don't know about the deadrise and less slap, but I have thought that the reason for a "V" bottom was to decrease the slap. The aft end is not going to be coming out of the water to slap, so you can make it flat. So you make the front pointed to cut the water, "V" the bottom to lessen the slap and give and take for a lot of hours. Then at some point you say this is it.
That's the total of my knowledge and it's probably wrong, but it will float and move.

 

http://www.boatingmag.com/all-aboard-boats-whats-deal-deadrise That about sums up everything I've read about the deadrise angle. Just not sure in a general way how bad the shallow I have would be.