Flat bottom planning hull speed

[bradkitcher]

Hi
I need to make my boat go faster. All I need is another 5 to 10%.

The specifications are currently

15hp semi surfacing on jack plate.
Hull length 10.5 feet
Planing width 3.11 feet
Planing length 8.5 feet
The planing surface is perfectly flat 100% flat in all direction to plus or minus 2mm. There is no dead rise!!
Hull weight including engine, 5 gallons of fuel, two people 550 pounds.
Current hull speed 32 knots.
Things that are not an option, foils, multi hull etc.
Options steps single and double tunnel hull forced air or changes to dimensions.
*****
Attached is an upside down photo of the hull taken from the bow.
Please put on your thinking caps please.
brad@kitchergroup.com

[pistnbroke]

sorry mate the photo does nothing for me too indestinct ...change for a 25 hp ....like this...

please put your shirt on ..why the F is it on the hull .. and put the hull in the sun where we can see it ..also the topside

[bradkitcher]

http://www.youtube.com/watch?v=AkjjtrQqfeM

Actually know it is not a joke. Top boats are going 34-35 knots. I am maxing out at 32 nearly 33knots.

Please see you link above "avon descent 2009"

More horsepower is not an option.

Help

[lewisboats]

length of planning area and weight of boat are the factors that need to be addressed. Reduction of both is what is needed. For this...you need to increase the beam and reduce the length of the planning area as well as reduce the overall weight of the boat to decrease the loading of the available area. There is a point where your wishes will NOT overcome the physics involved and you will have to settle for what physics allows.

[bradkitcher]

The weight is at an absolute minimum.
What about steps. Would these work on a flat hull.

[tom28571]

As far as the hull is concerned, the speed limitation in this mode is water contact area and the friction it generates. Your original planing surface listed is obviously wrong and far greater than that shown in the video. The porpoising attitude of the boat in the video can't be optimum.

How do you know that your power/prop system is optimized as well as the others that are faster?

One possible mod would be an interceptor. Not too hard to fit but would call for air inlet through the bottom. There are some here who know a lot more than me about these. I think the interceptor would fit just aft of the water contact point.

[Ike]

Why two persons in the boat? Try it with one. You will probably pick up your 2 mph. If it must be two, have both as far aft as possible. You want the center of gravity as far aft asyou can get it.

[tom28571]

Originally Posted by Ike

Why two persons in the boat? Try it with one. You will probably pick up your 2 mph. If it must be two, have both as far aft as possible. You want the center of gravity as far aft asyou can get it.

Having the crew so far aft is why the boat in the video is porpoising so much.

[liki]

Have you measured the performance with the bottom in perfect shape? As smooth as possible, and ensure also that the transom edge is sharp.

The bottom seems to be a consumable item in your sports.

[gonzo]

Go on diet or find a lighter friend. Weight is the most crucial thing

[Yellowjacket]

You guys don't get it, the racing class obviously requires 2 aboard, and the power is limited also by the rules. An interceptor isn't going to help since that is only going to pitch the nose down and add drag and that isn't what they need either.

The performance of hulls like this has been optimized by the APBA guys racing runabouts for the last 40 years or so, and a flat planing surface is what you have and what you want.

If you are running slower than the competition with similar hulls you need to make sure that you have the right prop, and the engine is turning the right rpm's. That is the first thing to get competitive here. There are small cheap tach's that you can add and use for testing to check this out. Find out what rpm's you need to get max power out of your engine and then get a prop to do that. You didn't indicate if the props are governed by the rules, but that is the first place to look for speed. A fishing prop isn't going to do it, and if the rules tell you what prop to use, get one and then have a good prop guy work it over to get what you want.

After that look at the trim angle of the motor and experiment with that over a range and see how the boat responds to the trim angle changes, again use your tach to see if you are getting more rpm's with each change.

Your hulls are pretty thin and with two aboard and the speeds you are running, it isn't too likely that you will flip the boat if you get some air under it. The APBA doesn't allow rails that run into the water to help trap air under the hull, so you might look at putting a couple of rails at the outer chines that would extend a inch or so down into the water for the last two feet or so of the hull. Think of using something like aluminum angle channel 1 inch on a side, and bond it on with RTV to try it. If it doesn't work it is easy to take off.

A step might help since it could insure that the hull remains at the optimum trim angle, at all times. Again, the ABPA doesn't allow steps on the runabout classes for a reason, so that is something that you could consider, but you need to get some help here in designing a step system.

[bradkitcher]

Originally Posted by liki

Have you measured the performance with the bottom in perfect shape? As smooth as possible, and ensure also that the transom edge is sharp.

The bottom seems to be a consumable item in your sports.

Yes the transom angle is very sharp and perfect.

[bradkitcher]

Motor is at a max. The propeller are high performance custom made semi surfacing in bronze and stainless steel.
The boat seems to go faster when porposing. Short little porposing about 4 to 8 inches. It is as if the least amont of contact with water goes faster.
We sit back as far as possible.
What can we do to reduce wetted surface area. I need help and ideas guys.
Steps? Who is he flat bottom boat expert here?

[bradkitcher]

We use tiny tachs and gps?
Does anyone know racing fuel formula for two stroke??
Plug choice etc.

[bradkitcher]

Originally Posted by tom28571

As far as the hull is concerned, the speed limitation in this mode is water contact area and the friction it generates. Your original planing surface listed is obviously wrong and far greater than that shown in the video. The porpoising attitude of the boat in the video can't be optimum.

How do you know that your power/prop system is optimized as well as the others that are faster?

One possible mod would be an interceptor. Not too hard to fit but would call for air inlet through the bottom. There are some here who know a lot more than me about these. I think the interceptor would fit just aft of the water contact point.

Hi. Thanks for your input. The dimensions of the hull are definately correct.
Most hulls range from 950mm wide to 1000mm, planing length 8.5-9ft.