speed.. and digging a hole

[rustybouy]

i do apologize for all these questions tonight but i am a bit miffed with working out how to re-power my boat, iv'e done the calculations and the max speed for my displacement hull would be 6.3 knots, but im not sure if i am a full displacement hull, i think iv'e read too much online about semi-displacement and i'm now doubting myself,i have an old 15hp lister which pushes me along at 5 knots maybe six with the tide up her ar$e, here's a few pics of the hull (or likenesses) does anyone know the score? it's driving me nuts , i would only like to fit a 20hp engine and maybe a new prop, but im not doing that just to bury the stern under water for the sake of 1.3 knots that i seem to lack. any suggestions would be helpful thanks

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Pete

 

[sarabande]

fine looking boat. It's definitely displacement.

A 20hp will make it more likely you will be able to achieve max hull speed, but the main thing is that you will have power in hand to maintain cruising speed against the wind or in lumpy conditions. Correct choice of prop pitch, size and number of blades will be very important. I remember being very impressed by a Cheverton workboat with a 4 blader.

 

[William_H]

As said it is a displacement hull. Therefor the rules about max hull speed will apply. Of course the concept of hull speed is that on the graph of power versus speed you get to a point (more a range) where a lot more power gives very little additional speed. But it will give some more speed. Hence your calculation can only be a guess. You might reach 6.3Kts with a lot of power or just a little. You might conceivably get another half knot going from 15 to 20 HP and certainly a better ability to maintain speed into a strong wind or waves. A contrary tide is something you can not overcome because we are talking water speed not SOG. A clean bottom will also make a lot of difference.
You might find it desirable to go to an engine with more cylinders ie 3 for a much smoother quieter engine than for actually more power. But no basically you won't get much more speed for a lot of cash. olewill

 

[rustybouy]

Thanks for replying, she is "a fine wee boatie" sarabande ,yes, thanks to both of you for clearing that up, i was dithering a bit, all taken in and noted, iv'e found some direction now

All the best

Pete