Displacement boat - speed

Does anybody have idea, suggestion about angel of the bottom plate of sponson in relation to water line? Also whats the value of lift force that I can except on speed of 8kn, and area of sponson flat bottom is 4m2?

 

Duco84,

By how much do you want to exceed 8 knots? You do not have a planing hull shape, so you are still restricted to displacement hull speed. The angle of attack for a symmetrical sponson/hydrofoil could start at 5%, or you could change the chord shape. Have you considered the increased drag?

http://en.wikipedia.org/wiki/Chord_(aeronautics)

It will have to be trial & error & if there is any increased speed at 2000 rpm, it's unlikely
to match your expectations after all the work & the fuel consumption will hit your wallet.

 

What is Your opinions regarding the Lift force calculation based on below mentioned formula, based on the fluid velocity, V , force
coefficients CL , the object’s dimensions or area A , and fluid density, ρ?

FLift = 1/2 * ρ * V^2 * A * CL

CL= 2 * pi * a (a- angel of attack)

F= abt 19000 N, for speed of 8kn, angel 5 degrees, and area 4m2

 

The only way this would be likely to gain any worthwhile extra speed is, if it is a relatively light displacement boat, and building stuff on to it will only increase the weight. Even then a couple of knots would be about the extent of it.

 

Those formulas are for an airfoil (or a flat plate at very low angles) with an infinite aspect ratio and flow on both sides. You have flow only on the lower side and aspect ratio is very low. Thust the force will be much lower, maybe 1/4.

You may want to look at planing boat formulas given by e.g. Savitsky. Those may give you the correct order of magnitude although the situation is quite different due to hull in front and below the plate.

 

NACA reports are an excellent source for experimental information. Here are a few:
http://naca.central.cranfield.ac.uk/reports/1958/naca-report-1355.pdf
http://naca.central.cranfield.ac.uk/reports/1958/naca-tn-4251.pdf
http://naca.central.cranfield.ac.uk/reports/1958/naca-tn-4294.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930088301.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930087531.pdf
http://digital.library.unt.edu/ark:/67531/metadc58286/m2/1/high_res_d/19930085949.pdf

 

Spoiler installed

 

And what difference in speed is there ?

 

I plan to increase pitch on same propeller, please advice propeller pitch. I have propeller D=25.6'', P=22.8 ", Z=4.

Max rpm of engine loaded and unloaded is same 2200 RPM. Gear ratio 1:2.9. Current speed max 8.5kn

 

Unfortunately there is no change of speed. Only there is no trim as before.

 

Were you expecting this to give you a speed increase?

 

If improvement trimming (he spoke about 5% trim) would be right to expect an improvement in speed. How much?, nobody knows.

 

I'd have thought so. He says no faster.

 

So perhaps the solution is to increase the trimming

 

So the governor is limiting the rpm and you are not getting the full power. What is the recommended rpm at WOT? Is the 2200 as a limit normal? Did you get the same 2200 rpm before you modified the boat?

With a very rough and fast calculation you should need about 70 hp at the propeller shaft to spin the propeller at those conditions. If that is correct, you can't increase the pitch very much. Maybe to 24-25", but that depends a lot on whether the speed increases or not, which is impossible to say at the moment.