Lets crank some numbers. These calculations are for a 6 hp (4.4 kW) engine peaked at 16 000 rpm.

What is the force on the strut in the traveling direction when going 55 mph?
P = engine Power [W], F = Force [N], v = velocity [m/s], n = prop efficiency. Power equals force times velocity. P * n = F * v, F = P * n / v. I am uncertain about prop efficiency but using 0.8 yields 140 N, which expressed in kilograms (not really a force unit) is about 14 kg,. Multiply by 2.2 to get 31 pounds.

What is the force on the strut perpendicular to the traveling direction? T = torque [Nm], w = angular velocity [radians/s], h = hull V-angle, more precisely this is a measure of how wet the prop is. Half prop in water gives h = 0, prop close to V hull gives h = 30 degrees. Power equals torque times angular velocity. P = T * w gives T = 26 kg*cm. Torque equals force times perpendicular distance, T = F * a. a is here the radial distance to the force center of the propeller. I do not know the exact force distribution on the blades but using F(r) = r^3 * (prop-radius - r) gives a = 2 / 3 times prop radius (70 / 2) gives a = 2.3 cm, F = T / a = 11.3 kg. But this is the force in the angular direction. Averaging in the left to right lateral direction involves integration but the ready to use formula becomes F = F * 2 * cos(h) / (pi + 2 * h) (h in radians) and gives for h = 30 degrees 11.3 * 0.41 so that the force finally becomes 4.7 kg, 10 pounds. For h = 0 degrees that would be the right number to use for quite a few boats the force is 11.3 * 0.64 = 7.2 kg, 16 pounds.

These are ballpark figures but worth knowing when building boats. The lateral force is partly countered by the hull, but fast boats have very little hull in the water and the rudder needs to be able to counter all the lateral force i.e. one needs to be able to hang several kilograms from the force center of the rudder (when boat lied on starboard side). The force center of the rudder is about halfway from lowest point to waterline when running, and somewhere between leading edge and middle line. And this is just for running straight forward, on top of this comes the forces needed to make turns.

Feel free to post remarks and questions ???

Interesting read! I will have to come back to it when my brain isn't so tired! I am out on business for a couple days and all these meetings are taking their toll! I like this though...will take another look at it when I am better rested.

Patrik,

Been following some of your posts, You are obviously quite a bright fellow :

Welcome to MGB,
Your input and posts have been quite informative and very deep in content. Keep it up ..... ;D

Marc and Scott, you are much to kind. It seems my posts tend to shoot beyond the scope of interest of most boaters. I am very glad that my input is of interest to some, and also to authorities in the field like yourselves no less.

With all this interest in the calculations I find it worthwhile to make them easier for everyone to use so I made ready to use formulas. Such formulas that I do not like to use without having seen the derivation, but the derivation you have already seen above. Below follows formulas to calculate the forces on the transom.

The forward force is easy

F = P * 165 * n / v (usually it is in the order of 5-15 kg)

Here the forward force F is in kilograms, multiply by 2.2 to get to pounds. P is engine power in hp. n is the propeller efficiency, I do not know this one but using 0.8 is close enough I guess. v is the boats velocity in mph. I know, strange mix of units and it gets worse. :-\

The lateral force is

F = 4320000 * P / ( rpm * propdia ) * H (usually it is in the order of 2-7 kg)

propdia is in the unit mm. H is the “wetness” factor of the propeller. It is calculated like this

H = cos(h) / (2 * h + 3.14)

Were h is how many radians ( = degrees * 0.0175 ) of the propeller above the horizontal plane through the prop center that is wet when running. I provide a chart of angles I guess are normal.

H = 0.32 for h = 0 degrees
H = 0.28 h = 10
H = 0.245 h = 20
H = 0.21 h = 30

Don't be a stranger

Holly cow Patrik bright is a understatement. That much info makes my head hurt trying to understand it! I guess thats why Im finishing props at 11:00 at night and not working on a calculator. Nice work!! Gregg

I guess i had to many beers to really understand the theory!The McEwans Scotch Ale is putting a hurt on my brain.