boat speed

Buttock

We see a buttock around to 8 deg.

 

Speed

Mean continuous Speed:

Light Ship (1/4): less than 11.5 knots.
3/4 Loading less than 10.5 knots.

Propeller's Reduction Ratio 1:3

 

That hull will be very hard pressed to over come it's own wave train, even with 650 HP. It has way too much by way of buttock angles, as Adler has noted and a huge amount of wetted surface, plus appendage and gear drag to even think about more then 1.5 S/L (less then 10 knots). Sorry, but she'll be a comfortable displacement cruiser, that will get best efficiency at about 8 knots. Pushing her to 10 knots will double her fuel usage, just for a 20% increase in speed.

 

PAR
Am I correct in thinking that HP absorbed at these speeds is way below capacity of even one of those two engines?

 

I read "Cummins engine" (not "engines") in the first post, and the power of 325 HP. But even if it was 650 HP, I remain of the opinion that she'll hardly be able to make 20 kts with 15 tonnes weight. She'd need (at least) to be stripped-down of few couples of tonnes first.

Besides that, it's round-bilge form makes it susceptible to dynamic instabilities at 20 kts speed, as the resulting Fn,L would be 0.9 .

The aft buttock is too high and appears to rise abruptly from the station at 40% of the LWL. The consequent kink will be a point of low pressure at high speeds, creating drag and bow-up trim. It might also become one of origins of the said dynamic instability.

Finally, we don't know where the CoG is, and that parameter will also play a role in the estimate of the maximum speed.

Cheers

 

Suggestion

Dear Sprayed,

Regarding to feel the way (in practice) of the speed effects at 20 knots you asked to reach and
Based to Daiquiri notes, you have to consider when you try to increase the power more than 430HP per engine,
that should be changed the parallel installation of the propeller shafts to an angular one,
where the shadowed elongation of the Propellers' Shaft Lines should be meet each other
the frontal point of attack at the WL level to even keel.

Add to that the vertical position of the rudders' axis should be inboard closer to CL at 1/4 Propeller's Radius.

A wedge-flap should be installed also. Please see the attached file.