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 Boat Design Forums Confused with hull design and engine HP..

#16
11-21-2012, 02:03 PM
 seadreamer6 Junior Member Join Date: Oct 2012 Rep: 10 Posts: 29 Location: helena,mt
Just read an article on Honda's new 1.7 l diesel. A little over 100hp. Any thoughts on using that in a boat?
#17
06-23-2016, 09:08 PM
 Mark Main Join Date: Jun 2016 Rep: 10 Posts: 1 Location: Fort Lauderdale, Florida
Quote:
 Originally Posted by Eric Sponberg SoNew, A good reference book is Dave Gerr's "Propeller Handbook" that will more fully explain the speed-power relationship. In there he describes his "displacement speed formula" which is: Speed/Length Ratio = V/Lwl^0.5 = 10.665/(Displ/SHP at prop)^0.333
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))
#18
08-14-2016, 12:30 AM
 Easy Rider Senior Member Join Date: Oct 2009 Rep: 732 Posts: 912 Location: NW Washington State USA
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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