Displacement boat - speed

Your boat looks more like 3,35 as I read from post #10 and as it looks, then C120 has 6 more square meters of lifting (0,55x11) and probably a more full shape of the hull, just for fun you could mount two outboards on the sponsoons but i don't think it would be approved from the register

 

TANSL
here, in the post n° 28, 16 and 35 you can see the answer that you asked about my design in the other thread in the same question

 

My mistake. On my boat B-4m, Bwl-3,5m

 

If it is a heavy boat, fiddling around with tabs or shaped additions to the stern is going to produce only marginal speed improvements at best.

 

With the modifications essentially turning the craft into a semidisplacement(if there is such a thing) you now need to add more thrust .I can see you are limited by what sort of propeller can be fitted .Normally a 100hp on a 3:1 reduction would swing a 900mm prop. So the existing set up was normal for a displacement craft EG.a 100hp motor loaded to 40hp Which could be tested with a flow meter Eg.250gm/hour per KW so 30kw should burn 7.5 L/H .That same motor should have no problem running at 70%.So A 2.5;1 box would be ideal for your present prop .But what has been the norm is a 2:1 box with 650mm 3 blade .I guess you could try a 2:1 box with present prop because there will be some cavitation trying to push your hull along so your motor might not be overloaded Eg not reaching 2000rpm or governers pumping fuel resulting in black smoke are signs motor is overloaded .Changing the pitch will I dont think give you much more thrust