New engine, new prop? HELP

Hello I just upgraded my diesel motor from single cylinger 9.9hp to 27hp diesel Ruggerini, I have a 38.5 cm brass propeller on my 6.50 wooden boat, do I need to change my propeller? even thought i dont have any space for a bigger one..

 

I expect that If you want to burn lots more fuel and make bigger waves then you should change the prop.

The required information to make a reasonable calculation is:
1. The displacement with the old motor and type of hull. Good to know the WL beam and the shape of the run aft - is there some chance it might have lift or will it squat.
2. The pitch of the existing prop.
3. The rpm for maximum power on the 9.9 and if you could reach or exceed this with the old prop.
4. The boat speed at full throttle with the 9.9.
5. The rpm at maximum power for the new motor.
6. Any increase or decrease in displacement with the refit.
7. Forward gear ratios for both motors.

Rick W

 

Thank you Rick. I don't know the full specs on everything. I guess i will see what happens. Thank you for the help.

 

My cryptic comment in the first sentence was to suggest that you should not expect too much from the bigger engine in the way of extra speed.

If the hull has some potential to plane then it might respond well to the extra power but odds are that 9.9HP was a good match for a 6.5m displacement hull. If the 27HP can be applied with the existing prop it may not give much more speed but you could make much bigger waves.

Am looking forward to what happens. If want to reprop after the test then the information I listed is what you will need.

Rick W

 

Thanks

 

Ok the new motor is in and seems to be working good after changing the fuel pump, one of the head gaskets and crank seal. Idles good and very satisfied with the power, prop is good for now but i can't test the full speed rpm before break in time. I dont think prop needs to be changed. I thought it was going to be quieter than my old motor but no surprise since it's air cooled.

 

I will be interested to see what speed you can get from the more powerful motor.

Rick W