Wondering about some improvements

I recently acquired a fully reconditioned 1992 Grumman Fun Ship 18' pontoon with a 1996 2 smoker Johnson 115 SPL. It's mission is hunting/fishing and does a pretty good job. It does sit low in the rear as all of the old 2 log pontoons with a big(ish) engine do.

Top speed is 30 and I would like to do a couple of things with some sealed sheathed ply pontoons. Increase the flotation to sit higher in the rear, increase efficiency/top speed/cruise speed with flat bottoms that are wider than the round logs (currently 25" round). It's all bolted together so it would just be a matter of unbolting the old ones and bolting new ones on once they're constructed.

I'm thinking 1/4" exterior ply for bottoms and sides. 3/8" for flat tops that will be 6" wider than the sides allowing for 3/8" stainless steel through bolts on each side at the cross members which are at 16" spacing. 6" glass tabs, 12 oz biax sheathing with epoxy. 3/8" frames every 36" or so.

Thoughts on shape/size to get the most speed out of it? I'm looking for light weight of course and additional buoyancy.

 

Are you laminating with fiberglass? Otherwise 1/4" is very thin. Seems like a lot of effort to save a deck. You could try lifting strakes.

 

Yes. 6" roll at seams and 12 oz biax sheathing.

 

I think you can make improvements without replacing the hulls.
Move the fuel and batteries forward first.
Then maybe try large lifting strakes.

 

That's the first thing I'd do.

Note that to lift onto plane most vessels (and all displacement hulls like these) need to have some bow up angle to them. 18' and 30 knts is actually doing pretty good; you would really, really need a good design to push much beyond that.

 

Agreed, don't fix what ain't broke!

However, if after all that you're still not convinced or require more buoyancy and/or speed.
You could try the following:



Extend the aft section, for additional volume, by the amount of buoyancy you calculate that you need.
Then shape the lower part to be a shallow Vee....with the Vee slowly changing shape as it goes fwd to butt and blend into the circular shape.
The "chine" part of the Vee, could then butt into a spray rail/chine that is added fwd of the extension by around 1/3 rd length of the hull that extends fwd to help facilitate better flow at higher post hump speeds.

 

That's a lot of money, effort and time for limited improvement.

Buy another full or partial length pontoon and bolt it in the center, they are readily available. Or buy a different boat altogether.

We extensively modified a pontoon boat for guiding purposes, they are cheap and easy to find. We didn't go with an 18' because they suffer from that exact problem, and there's no great fix for it. The next size up works far better.

 

Or unbolt the pontoons and rebolt them further back.

 

Found similar with a different style of power cat, the 18' was hard to like and the cut and stretched 20' did the job very well.

 

Thank you gentlemen for the information. She runs 30mph - about 26 knots which is quite good. She's been lightened about as much as can easily be done. All furniture removed, 1/2" exterior decking instead of 3/4".

The engine is a bit low - it throws water on the deck above 20 mph so the engine height needs adjusting but I wanted to get any changes that would affect how she level sits done before moving the engine. Once that's done, I'll optimize the prop. I think strakes will be in her future after that.

I'm thinking I'll put a partial third pontoon under her at the rear with an engine mount.

 

Those hydrofoil plate devices that attach to the outboard might help.
They are not the cure all that they are touted to be, but in some instances they can improve performance.
It would be a low dollar gamble to try?