Designed and built a 3 pointer - It won't plane!

Hi!

Been into boats for a long time and for years I have thought about building a Minimax or Minimost. Well, finally decided that now is the time to do it but figured I'd design a simple to build, stable three pointer and if it was any good to make the plans available for free on the net so others could build one too.
Having designed and built a good few RC 3 pointers I used a similar design but scaled up to 6' wide and 8' long ( see here for the design https://www.tinkercad.com/things/hnZmNdvb1TJ-three-pointer ). The only difference between the 3D drawing and the build is it's got curves on the real thing - CAD isn't my strong point!
Anyhow, We've (my long suffering brother in law and I) built it, it comes in at 72lbs and is constructed from 1" square frame, 1/2" ply bulkheads and 1/8" ply skin. The internal corners are glass mat reinforced and the whole thing is painted in 2 pack.
We launched it with a 4.5HP two stroke outboard and it floats in next to no water even with me onboard and it's amazingly stable too. With the engine running at relatively low revs the front out riggers are planning and as the revs increase I expected the rear to plane as well - but it doesn't! I tried shifting my weight as far forward as possible but the back end just won't get up on the plane.

So, I'm thinking of a couple of possible ways forward (as it's no good like it is, my three kids were quicker than me in a Zodiac with a Suzuki 4HP on it!)

1) I'm going to put large trim tabs on back to give it a larger flatter area to lift the back edge.
2) Rake the transom back at top so the motor is generating lift
3) Larger outboard - Ain't no replacement for displacement!

Feel free to chip in with any ideas or advice

 

This is what it looks like!

 

You got
- hook in the back
- rocker in the front
- no running surface on either
- my bet is it would never plane properly, regardless of power

 

I seriously doubt that 4.5 hp would plane much anything. The key to this problem could be the weight of the driver !

 

10 hp was the minimum on small class hydros!

 

Hi! Thanks for the replies.

Looking on youtube and from my own experience I can confirm that 4 or 4.5 HP will get boats planning, my Zodiac with it's blow up keel does quiet happily with me and one of my boys on board with my Suzuki 4hp 2 stroke.
I have used this design ( all be it much smaller ) in an RC boat and it belts along, although the power to weight ratio is obviously massively different with a .46CI two stroke motor on a tuned pipe at 20000rpm in a model that weighs 2lb ( And Yes, the comment about driver weight being a big factor ).
When we tried it the two front out riggers are definitely planning, leaving lovely smooth water behind them but the rear just digs in.
I'm thinking that the comment about having no running surface at the back is the problem (Well, that and 10hp would be better!).
So, my plan is either to build two flat bottomed float boxes and fit them to the transom either side of the motor that would give a flat running area at the back and more buoyancy at low speed or a pair of 12" square trim tabs to give a flat running surface, I thought about making them from 1/2" ply then giving them a layer of 1" thick polystyrene on top for some added buoyancy at low speed.

 

The bonus of trim tabs being I could hinge them to give some adjustment and I could adjust the height slightly to give a step before them to run on bubbles?

 

It's a poor design. That rolling wave bottom is your problem. How did you ever come up with that idea? Try flattening it out like this and put a 12 to 14 degree angle on the transom.

The transition from the bottom to the transom isn't shown so no comment on that.

And strap down the gas tank.

 

Thanks SamSam - explains it best
You got
- no hook in the back
- no rocker in the front
- good running surface on both
- my bet is it will plane properly and go real good.

 

Thanks for the positive comments JSL, you're a star.......

 

Some forward rocker is acceptable and desirable, as it helps get the bow clear off the line and in turns. The hook and trim tabs are just adding drag (lots of it). It would be wise to study full size examples of these, because models need to use cute little tricks to make them work in a small scale.