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-   -   Working out which prop! (https://www.cruisersforum.com/forums/f114/working-out-which-prop-24827.html)

ciclon1942 26-03-2009 00:01

Working out which prop!
 
Hi,

I have a westerbeke 4-107 which is governed to 1850 RPM, we are using a Hurth HBW10 to transfer the power but have two problems, firstly the gearbox is leaking oil from the back seal and secondly we are not sure if the prop which is on is right as our speed drops from 5.5 knots to about 2.5 knots when we go to windward.

I have been offered a ZF12M gearbox and have been told that it will slot straight in and that the ratio's are, 2.14:1 - 1.95:1 but I am unsure of the correct calculations required to give the correct prop size, if there is anyone who can help it would be much appreciated.

over40pirate 31-03-2009 20:48

Call a good prop shop and give them the specs and they will tell you the prop you need. Or should need!
Call a few shops and I bet you get the same answer.
If they sell you a prop, they usually will repitch it for free if needed.

SkiprJohn 02-04-2009 00:34

Do you know the size of your current propeller? What is the gear ratio of the transmission you have now?
I've had your engine in a Mariner 35 ketch but I had a Borg Warner transmission. The prop was a 15" but I can't recall the pitch. Of course, turning to windward with a good sea will slow you a bit but should be more than 2.5k.
Kind regards,
JohnL

Cowboy Sailer 02-04-2009 01:13

Ciclon1942,
Why are you governed to 1850 rpm? Is not the engine rated at 40 hp at 3000 rpm?

SkiprJohn 02-04-2009 11:31

Cowboy makes a good point. You are essentially using only 2/3 power so, of course, you'll drop speed going to windward. My engine was a Perkins and more or less the same. There is another thread here on the forum which will help you choose a better prop size if you need to but being able to at least get your engine speed up to 2500 if needed will help a great deal.
Kind regards,
JohnL

Badsanta 04-04-2009 21:05

There is a prop calculator on boat diesel.com you have to join and you did not give enough info for me to do it. There may be other prop calculators on line. Its a cheep place to start. this is some info you need. A program to determine the correct diameter and pitch of a propeller for any given vessel and engine.

Define your:
Vessel - displacement, semi-displacement or planing. Water line length in feet or metres and displacement in lbs, kgs, tons, tonnes or long tons
Engine - output in SAE hp, kW or metric hp, maximum rpm, manufacturers rating of either pleasure, light, medium or heavy duty.
Transmission - type and ratio.
Propeller - number of blades and if need be define a fixed diameter.

The program will then calculate: Maximum achievable speed, estimated fuel consumption, recommended propeller diameter and pitch, a propeller pitch based on a required fixed diameter, plus it will suggest alternative propeller sizes using different gear ratios to provide better propeller effeciency.
If you define a cruising rpm, the program will calculate the power produced, estimated fuel consumption and cruising speed you can expect with this same propeller.
Good luck

ciclon1942 05-04-2009 14:00

Thanks for your help, the reason why I say 1850 RPM is that the plate which is on the Lucas/CAV pump states that it is 1850, I assumed that this was the maximum the pump would allow the engine to rev to?

From memory I think that the prop is 16" but can not tell you the pitch as she is in Turkey and I am in the UK until mid May. Any suggestions regarding the prop size would be much appreciated, I would also like to know if the ZF 12M will fit where the old HBW10 once fitted. The ratio's for the HBW10 are position A 2.045:1 position B 1.864:1 and the ratio for theZF12M are 2.136:1 and position B 1.955:1

r.furborough 06-04-2009 03:12

The injector pump runs at half engine speed and is therefore safe to operate at up to 3,700 engine RPM's


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