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Old 02-25-2020, 03:09 PM   #1
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Prop question

Hi All, I just purchased a 30 ft. Island Gypsy , and have a prop question. It's powered by a standard Ford Lehman 120. Has anyone figured out the optimum prop size and pitch. Former owner ran at about 1650 rpms. I'm not concerned about fuel use, but more about cruising speed. Thanks for any suggestions !!
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Old 02-25-2020, 03:13 PM   #2
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Welcome aboard. Does your current prop allow the engine to reach its rated max RPMs? That is the first place to start.
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Old 02-25-2020, 03:20 PM   #3
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Great question. WOT at dock was about 2400-2600 without load. Thanks
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Old 02-25-2020, 03:40 PM   #4
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Underway, what is the max RPM that you get? You need to obtain the max RPM underway otherwise the prop is either too big or too small. In neutral doesn’t tell you anything about the prop size.
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Old 02-25-2020, 04:09 PM   #5
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Quote:
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Hi All, I just purchased a 30 ft. Island Gypsy , and have a prop question. It's powered by a standard Ford Lehman 120. Has anyone figured out the optimum prop size and pitch. Former owner ran at about 1650 rpms. I'm not concerned about fuel use, but more about cruising speed. Thanks for any suggestions !!
You will need the transmission reduction ratio from the nameplate and weight of the boat as well.
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Old 02-25-2020, 04:24 PM   #6
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I have a 30 IG. Factory installed twin P90's with factory installed 21X16 props. Grossly over propped.

Vicprops has an online calculator that should put you in the ballpark.
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Old 02-25-2020, 05:57 PM   #7
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Great question. WOT at dock was about 2400-2600 without load. Thanks
2600 at the dock without load is fine, but if you're asking about propping, only WOT in gear away from the dock will tell you if you have the right prop or not.

That said, 16-1800 rpms seems to be a good range for the FL.

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Old 02-25-2020, 05:57 PM   #8
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All he has to do is go for a spin in this boat and observe rpm at WOT. It should be 2550 to 2600rpm. But 2500 will be acceptable. Anything under 2500 should be re-pitched.

But there’s a potential problem. One can only adjust a prop in one inch increments. On my boat w an 18” prop one inch pitch change will change the rpm 200. So if I’m at 2450 and want more rpm lowering the pitch 1” will probably get me somewhere in the neighborhood of 2650rpm. Too high. On a bigger boat w a 26” prop one inch pitch change will be much more likely to be the ideal prop. Not so much rpm change w an inch of pitch change.

But if this boat was mine I’d shoot for 2550rpm.

But to shoot for a cruise rpm is wrong. 1600 - 1800 may be on your want list but it’s wrong to adjust your prop to attain cruise rpm. Use the throttle for that. AFTER one adjusts the WOT rpm then you go whatever speed you want (engine or boat) as long as you run hard enough to keep the lube oil temp high enough and not so slow you cause underloading problems.

A way to adjust rpm down is to cup the trailing edge of the prop blades. Or to go up (rpm) grind off 1/8ths” (about) of the the prop. Just make it smaller. I’d probably grind off the leading edge. See your prop man. I don’t think cupping props is a good practice on a trawler.
If your prop has an abundance of pitch just re-pitch. But if it has too much blade area and too little pitch (common on trawlers) just adjust according to your prop man. Or switching to a prop w fewer blades will probably be the route to go. These are ways to fine tune to get just the right rpm.
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Old 02-25-2020, 06:04 PM   #9
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Why only 1” increments? I have seen props in 1/2” increments, but maybe that was from the manufacturer?
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Old 02-25-2020, 06:07 PM   #10
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Underway, what is the max RPM that you get? You need to obtain the max RPM underway otherwise the prop is either too big or too small. In neutral doesn’t tell you anything about the prop size.
Dave, To clarify, if a prop is too small it is likely to rev at the same as in neutral. From what I understand under load the max rpm should drop about a 100 rpm approx below neutral rpm max, give or take to show it is under load.

This prop thing is confusing for me as the vic-prop calculator cannot tell me the props I have are the ones I should have yet they work well and follow the above statement.
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Old 02-25-2020, 06:30 PM   #11
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First he needs to get a base line so he can tell where he stands. Take the boat out and go WOT and check the RPMs, and not just with the dashboard tach as they are often wrong. Need to get a tach that can read off the crank. Once you find out what RPMs you have at WOT then you can decide if the prop is correct or not. It may well be fine or maybe not, but until you try a WOT run, it is all guess work.

I had 1/2” pitch taken off my props when I had them rebuilt so 1/2” changes can be done.
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Old 02-25-2020, 07:18 PM   #12
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I have had 20" pitch props reduced to 18.5" by a Prop Scan company. That was 20 years ago.
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Old 02-25-2020, 07:52 PM   #13
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So Archie what was the results and how did you like it?
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Old 02-25-2020, 09:18 PM   #14
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So Archie what was the results and how did you like it?

WOT RPMs came up to rated (gas engines)
WOT speed came up 2 knots.
Cruising speed came up 1 knot for the same GPH (Floscan).


All in all beat my expectations.
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Old 02-25-2020, 09:48 PM   #15
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Thanks Archie,
Seems like a winn winn.
But do you think you’re surrounded by less noise, the same noise or more noise?
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Old 02-25-2020, 09:51 PM   #16
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Thanks Archie,
Seems like a winn winn.
But do you think you’re surrounded by less noise, the same noise or more noise?

No noticeable change in noise level. This boat was a Phoenix 29 SF.
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Old 02-25-2020, 11:09 PM   #17
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When I had my props scanned and tuned up I did notice a definite drop in vibration. I had not done real specific speed vs RPM tables so I can’t speak to that but vibration has definitely dropped. I did post the before and after measurements of the 2 props. I can post them again if there is interest.
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Old 02-25-2020, 11:16 PM   #18
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Archie wrote;
“No noticeable change in noise level.” Post 16

That’s what I’ve always thought.
I think noise from a diesel engine comes from load.
I worked in a powerhouse in 1960 w a 1400kw alternator driven by a 1400hp Enterprise diesel. It was noisy. And I noticed it was working at about 1/2 load. The straight 8 engine turned at 327rpm + or - 10. The governor was very very effective. It only lost 8-9rpm when it instantly went from 50% load to 90% load. It went instantly as it was the result of someone on the dredge flipping a switch turning on electric motors. And the noise seemed to double when they switched to the high pressure pumps down on the dredge. It sounded like you were near a diesel engine and instantly you were in a diesel engine. And almost NO change in rpm.
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Old 02-26-2020, 06:52 AM   #19
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On a 30 ft boat the LWL is probably 25 -28 ft.


Your cruise speed will be about 6K regardless of the prop installed.
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Old 02-26-2020, 07:19 AM   #20
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Archie wrote;
“No noticeable change in noise level.” Post 16

That’s what I’ve always thought.
I think noise from a diesel engine comes from load.
I worked in a powerhouse in 1960 w a 1400kw alternator driven by a 1400hp Enterprise diesel. It was noisy. And I noticed it was working at about 1/2 load. The straight 8 engine turned at 327rpm + or - 10. The governor was very very effective. It only lost 8-9rpm when it instantly went from 50% load to 90% load. It went instantly as it was the result of someone on the dredge flipping a switch turning on electric motors. And the noise seemed to double when they switched to the high pressure pumps down on the dredge. It sounded like you were near a diesel engine and instantly you were in a diesel engine. And almost NO change in rpm.
Agreed. More RPM increases noise some even with no load, but increased load has a much bigger difference with noise. This applies to gas or diesel for the most part.
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