prop calculation

[Daddyo]

Does anyone have access to a prop calculator that can figure out the following prop pitching? My engine will make 2200 of 2500rpms with the following:
23x21 4 blade prop. 120hp 1.5" shaft. 2.5 reduction. 24,000lbs semi displacement. 33' waterline.
I'm thinking about reducing the pitch to get a flatter wheel and the ability to make some more rpms at the top end. The boat is for sale and surveys will require the higher rpms.

 

[Codger2]

A subscription to BoatDiesel.com will result in a one year use of their prop calculator. $25 well spent.

 

[koliver]

boatdiesel.com
I used their prop calculator to verify my guesses, they agreed, I did the change and it worked exactly as predicted.
May cost you the $25.00US membership fee, but worth it to get all the wisdom about engines and systems on that site for a year.

 

[RickB]

Daddyo wrote:

The boat is for sale and surveys will require the higher rpms.

I would suggest talking to a good prop shop.

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As far as the survey goes, a legitimate surveyor will simply note that the boat goes X speed at 2200 rpms.

Unless you are classed by Lloyds or ABS and the class rules require a certain engine performance spec be maintained, tell the little hammer wielder to go pound sand if he tells you that you have to meet some spec he has decided to apply.

If the surveyor is known for applying bizarre "requirements" to the boats he attends, don't let him onboard. If you do, make him document his findings and reference the "rule" that applies.

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[koliver]

DO
you could try dumping 1/2 of your load of water and using 1/2 of your supply of fuel. or dump all the water. A lighter boat may get you those extra rpm.

 

[FF]

First check is that the engine will reach WOT of 2500 or whatever the gov speed is supposed to be.

Would be a shame to haul and do a prop change , because a cable is not properly set.

30 seconds at WOT for the test will do no harm , as long as 10% over the required gov speed is not exceeded.

 

[Daddyo]

Engine made 2500rpm* @WOT no problem with prior undersized prop.

 

[FF]

Engine made 2500rpm @WOT no problem with prior undersized prop.


And now in the slip?


Have you borrowed a portable tach to check yours is accurate?

 

[troy994719]

Daddyo wrote:

Does anyone have access to a prop calculator that can figure out the following prop pitching? My engine will make 2200 of 2500rpms with the following:
23x21 4 blade prop. 120hp 1.5" shaft. 2.5 reduction. 24,000lbs semi displacement. 33' waterline.
I'm thinking about reducing the pitch to get a flatter wheel and the ability to make some more rpms at the top end. The boat is for sale and surveys will require the higher rpms.

Reducing your pitch by one inch will raise your RPM's about 200, so taking two inches out would be a little more than you need but it will get you to 2500. Is the Lehman 120 max RPM 2500 or 2550? Seems like the manual says 2600 RPM unloaded and 2550 RPM loaded for the max RPM???

I had a spreadsheet that I found online called propcalc.xls, you might try searching on Google for it. It worked pretty well, if i can find a link again I will post it.

 

[Marin]

The manual for our boat (1973) calls out the horsepower rating of the FL120 as 120 hp @ 2,500 rpm.

 

[AllanY]

Try this one
www.alberg30.org/.../MechanicalPropulsion/Propeller/propcalc.xls

Allan

 

[AllanY]

This might help

http://www.google.com/custom?q=prop...&domains=alberg30.org&sitesearch=alberg30.org

Allan

 

[Daddyo]

FF wrote:

Engine made 2500rpm @WOT no problem with prior undersized prop.


And now in the slip?


Have you borrowed a portable tach to check yours is accurate?

Don't know what you mean by now in the slip? I have an infrared tach. I was thinking the one to two degrees as the way to go.

*

 

[troy994719]

Are you getting black smoke in the exhaust at WOT now? If you only go one degree then it will probably still have smoke at WOT. I would go for two inches if youre looking to make sure the boat turns the full RPMs and doesnt smoke. *

 

[FF]

By in the slip I mean in neutral not underway ,


just advancing the throttle OUT of gear ,to find out where the top of the gov is set