New Prop and engine speed

[DavidBolger]

Hi,

I have just replaced my prop on my Sadler 34. I have a Volvo 2003 28hp engine which has a max rev of 3,200. On my old 2 blade fixed prop, I was getting to about 2,800 revs under load and about 3,300 with no load. I bought a new 3 blade folding Gori and spoke at length with them to get the correct size prop. They recommended a 13.5in diameter with 16in pitch. I have the new prop on and I notice that under load the max revs are 2,300 to 2,400. I have not checked the max revs without load but assume it will be the same.

What do I do?
In a flat sea I am getting great power and am reaching cruising speeds in excess of 6kts where beforehand I rarely got above 6kts. How will the lower max revs handle in a lumpy sea? Will it be as effective?

Many thanks to the forum for all their advice to date on this issue. It is with this advice that I am able to appreciate the issues involved.

regards

David

 

[Robin]

David

If you are unable to reach the designed engine revs under load in gear then your engine is not going to be able to output it's rated horsepower which is achieved at 3,200 rpm. You are getting a higher speed at low revs simply because in effect you are in overdrive, OK in flat sea/no wind but not so good if you need the full engine output into a head sea or headwind. Getting prop sizes right is difficult but IMO essential, it is abour 80% science and 20% trial and error it seems. Does the Gori prop have an adustable pitch? I believe some do like the Maxprop but I don't know the Gori specs. I believe that approx 200rpm equates to 1" of pitch, if so it would seem that your Gori is quite a way out in the pitch dimension, ie 5" too much. I would suggest you approach the dealer and ask how to proceed.

Best of luck

Robin

 

[Das_Boot]

If the new prop has more bite it will be more difficult for your engine to turn it. At some point the engine wont have the power to spin the prop any faster. Have a look at the torque band on your engine for the max torque vs revs I think it would probably be arround where you are now arround 2300 revs. The more torque the more your boat will push through a lumpy sea and the more revs the faster it will go on a flat sea. The compromise is between these and fuel comsumption and engine life. Some people say you should run your engine at 80% some at 50% with only occasional short full power runs. The implication of this is that if you want max torque you will need to run at full power however if you look at most engine graphs the torque band reaches max at between 50 and 75% of max revs you will therefore only reach max torque at max revs. In my opinion fuel consumtion will be higher and engine life shorter.

All of the above assumes of course that your engine is ok.

I am in the process of puting in a new engine and have not bought a new prop because I want to solve this dillema.

 

[LeonF]

David look at my post of a couple of days ago re VOLVO REVS PLEASE....some of the replies may be relevant to you too...

 

[JackFrobisher]

You're getting a reasonable cruising speed because the engine is oversized. My Storm Cruiser has a 28HP volvo (also oversized) to allow for maintaining speed in adverse conditions. If you look in the VOLVO manual it will tell you approximately how much power the engine puts out at the RPM that you are achieving. Get them to size the prop again - a pain but it is necessary to reduce engine glazing and wasted fuel.