Displacement hull power requirement

[Davy_S]

Does anyone know of a rough formula for the power needed to get maximum speed from a displacement hull?
I know how to work out the square root of the waterline and multiply by various factors, also that some hulls are slippery and some are not, also weight would have to be considered as well. When you think that smaller yachts can make 6 knots with a 10 hp diesel, there must be a simple answer.
I am mainly interested in a displacement cruiser (not yacht) say around 30 ft.
Anyone know the ballpark figure?

 

[Assassin]

As a simple rule, i would suggest seeing what the manufacturer fits as standard as thiswill give you most of the information without calculations. Most manufacturers have a power range, again this gives you the option to select a power unit at the upper end of this range.

 

[jimg]

Having a quick look at Dutch broker sites, 50hp seems to be fine.

 

[Tanqueray]

The range of 3 to 4hp per ton is a (very) rough starting point - if you have more details I'll run them through the calculator for you.

(need displacement, o/l, wll, hull shape, number of shafts etc.)

Steve

 

[Davy_S]

Thanks for the replies (and kind offer) I have no details yet, the reason I ask is because I missed out on a restoration project, it was a double ended fishing boat hull around 30 ft, it had no engine (single engine shaft drive)
I had the idea of converting it to a small economical cruiser/ dayboat for two people, I had thought around 50 hp would have done nicely.
It has simply got me thinking, I know i can buy a cheap 10 grandish yacht out here, but a restoration project has a certain appeal.

 

[Latestarter1]

The trick is getting an accurate displacement.

Boat speed is a function of power ( in pounds weight per Hp )

SL Ratio = 10.665 divided by the cubed root of weight in lbs, times SHP

Where

SL RATIO = Speed-length ratio and
SL RATIO = Knots divided by the square root of WL (waterline length )
SHP = Shaft horsepower @ THE PROPELLER
WL = Waterline length in feet

The speed predicted by this formula assumes that the propeller gives between 50 and 60% efficiency, with 55% being a good average.

PM me with GOOD data and I can run the #'s for you.

 

[PCUK]

50hp would be fine and not overstress the engine although 20hp would probably do it. Max speed about 7 knots?

 

[Davy_S]

50hp would be fine and not overstress the engine although 20hp would probably do it. Max speed about 7 knots?

Its sounding a bit better all the time! The reason I asked in the first place I was seriously tempted when I saw the double ended hull, The hull was sold but it has got me thinking, if I saw something similar i could be tempted, but the cost of fuel has to be a consideration. 50 hp or less would be ok on the pocket as it would use less fuel than my Honda outboard !



ps, thanks for the offer Latestarter.

 

[nedmin]

To give you an idea I used to have a 34ft Steel Pedro which weighed about 9tons.It had 2 62 hp diesel engines.Running on one engine, without stressing it, it would do 6 kts.

 

[Latestarter1]

Its sounding a bit better all the time! The reason I asked in the first place I was seriously tempted when I saw the double ended hull, The hull was sold but it has got me thinking, if I saw something similar i could be tempted, but the cost of fuel has to be a consideration. 50 hp or less would be ok on the pocket as it would use less fuel than my Honda outboard !



ps, thanks for the offer Latestarter.

Using the formula MINIUM power erquired to drive a 30ft hull 26 ft on W/L 6 tonnes displacement a maximum hull speed of 6.8 knots requires just 26.2 SHP.

Biggest problem is dealing with lies and damm lies on manufacturers spec sheets.

Whacko ISO standards 25 Degree C fuel instead of 40 Degree, metric Vs proper Hp, continious vs intermittent, gearbox losses. Remember engine spec sheets are written by marketing people not engineers, which is why so many vessels end up over propped if data sheet power is used without reading the small print.