# Confused with hull design and engine HP..

Discussion in 'Powerboats' started by SoNew, Nov 18, 2012.

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Just read an article on Honda's new 1.7 l diesel. A little over 100hp. Any thoughts on using that in a boat?

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### Mark MainNew Member

I used Dave Gerr's formula and calculated for SHP (shaft horsepower) and increase the 5% for the BHP (brake horsepower) per the description. 110% to 120% could also be used as a rule of thumb increase.

D = Displacement Lbs
SL = Speed Length Ratio, 1.34 can be used or calculate using: speed knots / Sqrt(LWL feet)

SHP = ((D × SL^3)÷10.665^3)
BHP = SHP x 1.05

The calculation assumed 0.333 = 1/3, which later became cubed when solving for SHP; this alters the result slightly, if you want to use 0.333 instead then replace the ^3 with ^(1/0.333)... SHP = ((D × SL^(1/0.333))÷10.665^(1/0.333))

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### Easy RiderSenior Member

As an example of small boat power my Willard 30' is 8 tons and has 40hp (5hp per ton)

She's overpowered but not by much. Most of the Willard 30' boats were sold w a 36hp engine but were over propped so really only had 33hp. Never heard anyone complain about too little power. And the boat would do fine (even in heavy weather) w 30hp.

But this is a full displacement hull with no semi-displacement ambitions. So for a real world number to shoot for IMO 4hp per ton seems perfect.

Can't see the hull well in the pics but it looks flat w a little rocker. If the rocker is minimal I'd see it as a SD hull w a little angle of attack. Or a dirty FD hull at 4 knots. So I'm thinking 6 to 12hp should be fine. Just my opinion. Can't really go wrong unless she gets overpowered and overdriven.

Speaking of opinion if this little boat were mine I'd put a 6 to 10hp outboard on her. Lots more space in the boat and LOTS less vibration. And if I was building her I'd take the previous suggestion and stretch two feet ...... or 3'.

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