Engine not reaching full revs by far

[rajjes]

Hi,

My engine reaches 3000 rpm on full throttle on neutral. On engaging gear the revs climb from 900 to 1600 rpm at ½ throttle, but do not go beyond that with additional throttle!

The specs are as follows:
2 cyl Kubota – 10Hp
Max revs – 3000 rpm
Gearbox ratio – 2.2
Waterline – 21 ft
Displacement – 6000 lbs
Propellor – 13 X 12

Using the available programs, such as the castle marine, the prop pitch seems about right, and there is no noticeable fouling on the prop, hull.

Is there anything I can check before trying to lower the prop pitch / diameter

 

[chwarae]

I replaced a 6hp yanmar with a 16hp beta (kubota) and the spec. was about the same as yours. LWL 20ft 4ins,gearbox 2.1, Displacement 5066lbs.
Original prop was 14 x13 but new engine would only reach 2000rpm in gear and 3600rpm out of gear. Changed prop. to 12 x 9 as advised by Beta and now ok reaching 3600rpm although generally runs about 2000/2200rpm.
Hope this helps.
Best Wishes,
Harry.

 

[rajjes]

Thanks, it does help!

I guess I will have to go for a lower than that with 10HP?

 

[Guest]

Any black smoke when tied to the dock at full power? For example if I overload the generator it will slow and produce black smoke. Same with the engine when tied to a dock and pushed past 2000rpm

Before going down the route of new props etc. make sure it is not limited by fuel supply. No constrictions?

 

[tonykerry]

This may be a silly question but have you cleaned the prop recently? If your prop is fouled, especially with barnacles, you will get the effect of limiting your revs, and cleaning and polishing the prop will restore performance to normal.
Regards, Tony Kerry

 

[Alex_Blackwood]

We had a similar problem with the same engine (Beta 10) a couple of years ago. Cause was dirty prop. You don't say how long you have had the engine/boat or if things have been OK previously!
We also had a recent problem with the engine, run OK off load but died on load. We had to replace broken springs in both the injector fuel pumps. This engine is prone to this fault.
You don't say where you are based but you could talk to Beta. betamarine.co.uk
or try the yellow pages for Kubota agents. The Portsmouth one is Woodleigh Power in Horndean.

 

[bendyone]

I would say you are overpitched.

mine is a 14Hp everything else about the same and have a 13x9 prop which on performance is slightly over pitched, I will be reducing to 12 inch dia over the winter

 

[johnalison]

I bow to the others regarding prop pitch etc, but I heard of a Volvo owner with a similar-sounding problem which turned out to be coking-up of the exhaust elbow. This had defeated several engineers before being discovered.

 

[hollandd]

i run a 11by10 prop on my snapdragon 747 fitted 2 years ago with new engine and no problems

 

[oilyrag]

You're way over-propped - you should be running nearer 13 x 8. To drive a 13 x 12 you need nearly 20HP!

 

[oldharry]

I agree - 13x12 is far too big for 10hp at 1300+ rpm shaft speed - 13x8 or 13x9 is the largest it can take.

My Bukh 20 would only just take a 14x12