engine question

[FAST FRED]

If we base the culculation on Max RPM what will happen to the efficiency of the propeller when we operate at 85 % Maximum Rating. ( I have been thiking about it but I am not sure what will happen )

The load reduction of 10 or 15% is to keep the engine from working at max power output, for a far longer life.

The big engine folks I talk to prefer 80% engine power at 90% of rated rpm, for longest service.

Run the prop numbers at these power settings , they will be pretty close to what you already have.

No matter how well you do the math , it usually takes 3 prop changes to get it just "right".
Boats frequently dont perform as well as the math sez, so changing the props is std.

FF

[RANCHI OTTO]

3 changes to get right....is a little bit pessimistic.

For 30m yacht model tank tests are foreseen because the influence of a wrong resitance estimation is the most important problem in order to have the right propeller.

If the recorded rpm during sea trials and the gas temperatures are not far from the estimated, only a pitch correction has to be done on the propeller.

[nautical]

Fast Fred says to run the numbers on 80 percent ratings.

Gerr says to run the numbers on Maximum ratings.

????

Ranchi thanks for ur help Now I am going to work on a Rudder

[RANCHI OTTO]

Have a good rudderday & night

[kjell]

Originally Posted by FAST FRED

No matter how well you do the math , it usually takes 3 prop changes to get it just "right".
Boats frequently dont perform as well as the math sez, so changing the props is std.
FF

You are right 90% of new boats have the propeller wrong calculated. But with my program PROPCALC it is easy to calculate the right propeller if you don’t try to get blood out of the stone.
Kjell

[nautical]

Kjell thanks very much for the program again I tried to varify my values with r program. can you check it with ur program .. I have posted all the details in this thread

[kjell]

This is my calculation for one Volvo Penta engine D 12-675 with Gearbox ZF 325A –EB Ratio 1:1.73.
Kjell

[nautical]

where are the values .

its 2 volvo penta D12-615 ZF 350 1 IV gearbox

[kjell]

Can you give me the gear ratio of the ZF 350 1 IV and why 2x 615 engines?

[nautical]

because total power is 900 HP extra a little of extra power to operate at low RPM for fuel consumption (Ranchi) gear ratio 3. Maximum Engine RPM 2100 rev/min

[kjell]

If you are looking for best fuel consumption You have to run the boat no more than 13 knots and you only needs 500 SHP