Df20a propeller

[Idrabmol]

Hi, I have lately bought a Suzuki df20a, this is to be used as an auxiliary engine mainly for trolling. I was hoping i would be able to troll with around 6.3 knots but engine does not reach that speed 😒. It is installed on a 24 foot boat. At the moment I'm using the standard prop 9 1/4 10 pitch. With maximum speed of 6 knots at wot. Do you think I would be able to improve a bit the performance with a different propeller? I have hooked up a techometer and with my current prop engine revs 4300 maximum. Have Also tried a 9 pitch prop but top speed remained the same even though engine increasd it's rpm to 4900. Any suggestions?
Thanks in advance

[Bclaude]

You are on the right track. Your propeller must be pitched such that the engine can reach the RPM required to develop maximum power. The DF20 engine operating range is 5,300 to 6,300RPM. It is only in that RPM band that max power can be reached and speed maximized. What you should to is test your engine with various props until you can reach a max of 6,300 rpm with only you on board and a totally un laden boat. This will ensure that your engine can stretch it's legs enough to develop 20hp but is not allowed to over rev. With "normal" load on board and at WOT your engine should run somewhere between 5,300 and 6,300 rpm with the right prop. It is only then that you will be able to maximize speed.

[Stijn_T]

Agree with Bclaude, you should try to reach about 6,000 RPM with your engine. If you are at 4,900 RPM with the pitch 9, i would try even one or two steps smaller (pitch 7 or 8). There is a good explanation here on choosing your pitch related to WOT RPM. For lower hp engines its about 500RPM extra for every pitch smaller.