Car Motor For Inboard

NO YOU CAN'T GO DIRECT DRIVE! Boston has given the basic reason; you might say that the prop needs a certain "swept area" (we call it propeller disc area) per kW, depending on the boat speed.

Without going into calculation details, you can check suitable propeller diameters on outboards of similar power as your engine. Now, first you say it is 75 kW, that is 102 hp. Then you say it will produce some 80 hp at 5000 rpm; there is 20 % difference! Anyway, you will find that outboards this size will use propeller diameters between 13.25" and 14". This is the statistical outcome from the most common applications of this engine size, giving reasonable performance and efficiency.

This means that for your engine, you have to use a step-down gear ratio of between 1.8:1 and 2.0:1 in order to match engine power to this propeller dia. A direct drive 5000 rpm would just stir a lot of water and produce no thrust; the maximum prop dia would be ~9". The only way to go direct drive would be through a waterjet, but even there, 5000 rpm is on the edge, depending on boat characteristics.

 

A 650 kg total weight, with 80 HP @ 4000 rpm, 1:1 gear. Propcalc shows a 10 1/2 x 11 P. resulting in 35+ mph @ 13% slip.
Shouldn't be a problem at all - Many years ago, I had similar situation, 100 + HP @ 4200 rpm, 850 kg, 11 x 13 prop, 46 mph.

PropCalc indicates 1.5 or 2.0 : 1 gearing is not recommended for this application [ 4000 - 5000 rpm engine speed]

 

D.A.R. [ DIA - AREA - RATIO ] Has little to do with power. Most 3 bladed props have a D.A.R. of 55% which means the total area of the blades is equal to 55% of the area of the circle created by the revolving prop. Outboard Props are larger in order to obtain the required blade area due to the Large Hub.

My current 2250# inboard runabout, with 450 HP uses a standard, no cup, 12 x 15 prop - 55% DAR, Turns up to 6800 rpm with LESS than 7% Slip, 87 mph gps. This "tiny" prop is able to instantly put the ENTIRE BOAT Airborne from idle speed. Hardly "just stirring a lot of water and producing no thrust"