improve boat speed

[tatu_malta]

Morning all

Tatu is a Lohi 34 ( LWL 28.6 ) Centre cockpit ( launched in 76 ) with a yanmar 29 and a fixed 3 bladed prop. Am happy in all aspects other than speed under motor. At 2,300 revs she cruises at 5 knots or thereabout. would be happy to replace propeller if this woud improve perfomance but dont want to go to all the hassle if not. Any advice or similiar experiences ??

 

[tatu_malta]

Any ideas ?

 

[Twister_Ken]

Sounds slow to me, but not my area of expertise. I'd suggest consulting a prop specialist who'd be able to advise on pitch and dia.

 

[tatu_malta]

thanks. Can you recommend any pop specialist ?

 

[Tranona]

Why are you only using 2300? The maximum revs are 3600. At 2300 you are only producing around 18hp out of potential 29. Increase your revs to 3500 and you should get about 6.5 knots and cruise at 75% (Yanmar recommendation) you should get 5.6 at 2600 rpm. If you cannot get 3500 in flat water then your prop is too big (diameter, pitch or both).

You can get a good estimate of ideal prop size by inputting your boat's data into Propcalc on www.castlemarine.co.uk If you do decide to change, get your supplier to recommend the correct size.

 

[tatu_malta]

so what you're saying is that the recommended cruising speed is 2600/2700

 

[Tranona]

Yes. The Yanmar recommendation for its engines is 75% which is 2700. in practice your actual revs may be slightly different as installations can vary giving variations in smoothness. On my similar size Volvo 2030 I find 2600 is best. Small diesels like to be worked hard - low revs is not good for them.

The key when choosing a prop is be able to achieve max hull speed (in your case around 7.3) in flat water at close to maximum revs. You will then find that you get a good cruising speed at 75%. The 5 you are getting at 2300 is about right provided you can get max revs.