power calculations, I/O, IB, OB

Been running a Savitsky based excel program to determine power requirements of my 23 ft aluminum runabout. The program correctly projects my P effective to be 115hp to achieve a max speed of 30knts. I have a 115 OB and my top speed is 30 knts. My question is that my OB was rated with the old crankshaft horsepower, by modern rating my OB is only 90HP at the shaft.

The Savitsky models are based on Inboard power. Is there an effeciency loss with inboard power compared to OB & I/O power that is factored into the model or is the model off by 25 hp?

 

Hi Regal,your Stavisky program does it give you a result in shaft horsepower?Did you find 115 shaft horsepower? Do you know the weight of your boat (include you outboard engine),the load waterline line length,capacity of fuel tank.Doyou know the angle that makes the bottom of the hull with an horizontal line@transom and@boat amidships.Is it a boat you designed yourself? lucas

 

Well the spreadsheet gives P effective which is the power required to tow the boat.
Weight is about 2300lbs, waterline length is 20ft, deadrise 0 * transom and 7amidship.

 

Regal,well quite difficullt to to tell the right power to install.Indeed,I use a formula I think a bit different to Satvisky's formula but very close however and we need distance from Transom to Longiytudinal center of gravity(at least estimation)and we need the beam at chine(max).Idon't think that for an inboard 115 hp or an Outboard 115hp,there would be a difference concerning shaft horsepower.Some power is loosed thru racks of transmission and reduction and thru bearing struts too.And both will maybe give 99 shp.See the picture Ijoined and once again simplyask to a boat repairer cause there are use to do repowering.bye

 

outboard size

I do not know the formula but I have a 20ft aluminum bracket boat with cabin and 115 hp is very very light for power unless you have no load. Also my 4 stroke weighs more than a 2 cycle which adds to the load. I did run a smaller horespower motor but it had to be in the higher rpm's to ensure it stayed on step effeciently.

 

115 should be fine for a 2300# flat bottom unless you like to go real fast or tow slalom skiers. A 115hp 2 stroke seems stonger than a 115 4 stroke BTW IMHO. Bribe a boat owner for a ride with a setup like you are considering. Suitable bribes would include food, beverages or a tank of gas.

 

I would venture a guess and suggest that Savitsky is more likely to correlate to inboard powered boats.
There is a significant difference between the efficiency of an outboard (or sterndrive) and an inboard. Shaft angle and the appendage drag will almost always mean that you will require more power when running an inboard powered planing boat.

 

ACCORDING TO MY FORMULA THE SPEED FOR A BOAT WITH A DISPL. OF 1044 KG AND 115 BHP OUTBOARD POWER IS 31/32 KNOTS.
THE ADDITIONAL RESISTANCE OF THE OUTBOARD IS VERY SIMILAR TO THAT OF SHAFT + RUDDER + BRACKETS.

RANCHI Otto