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Old 30-03-2018, 07:04   #1
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Correct prop size

Can anyone please verify that a boat has the correct prop size if the engine (s) can achieve 90% of rated rpm? I have been told this in the past, and if true, I am underpropped (I can achieve almost 3300 for a 3200-rated engine). I appreciate any technical insights.
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Old 30-03-2018, 07:15   #2
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Re: Correct prop size

Are you using the tach to measure engine speed?

If so, verify with a photo tach.

What is your lwl and what speed to you achieve under power?
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Old 30-03-2018, 08:15   #3
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Re: Correct prop size

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Originally Posted by Sailmonkey View Post
Are you using the tach to measure engine speed?

If so, verify with a photo tach.

What is your lwl and what speed to you achieve under power?
yes, I'm using the installed tachs. it is a Leopard 40 (LWL 37.5). approx 5.5 kts at 2800 rpm
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Old 09-04-2018, 12:49   #4
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Re: Correct prop size

I have asked local prop shops about proper sizing (and the 90% rule). The most sound direction I have been provided is from an analysis done by Boat US, which reads in part, "The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum r.p.m. where the maximum horsepower is produced, with ideal engine loading. Using your existing propeller, determine your maximum obtainable r.p.m.. If during this test, you begin to exceed the maximum rated r.p.m. of the engine, reduce the throttle setting. If the engine over-revs beyond the maximum recommended r.p.m., you may need to increase the pitch of the propeller. Increasing the pitch increment by 2" will result in approximately a 200-400 r.p.m. drop."


Obviously, I am over-revving by approx. 600 rpm with the current 16X11X3s. What I am unsure of from this Boat US data is whether a 3 bladed prop analysis is different. If a 3 bladed prop is applied to the analysis, I wonder what the increased pitch requirements are to reduce over-rev by 600 rpm. Would a 16X12X3 be more appropriate than a 16X13X3? I know Flexofold suggests a 16X13X3 for the 40 hp version of my boat (mine has the Volvo D1-30 [29hp] version engines).

Anyone have any thoughts who may have dealt with similar issue??
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Old 09-04-2018, 12:51   #5
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Re: Correct prop size

Data point correction to OP...my engines (Volvo D1-30) are rated at 3200, but I am seeing about 3800-3850 at max power with the 16X11X3 props.
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Old 09-04-2018, 13:25   #6
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Re: Correct prop size

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Originally Posted by gbillman View Post
Data point correction to OP...my engines (Volvo D1-30) are rated at 3200, but I am seeing about 3800-3850 at max power with the 16X11X3 props.


You really need to check actual engine speed with a photo tach to the crank pulley before you assume you’re over revving by that much. The governor should hold the engine speed down more than that.
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Old 05-06-2018, 03:10   #7
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Re: Correct prop size

There are people calculators online
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Old 05-06-2018, 06:42   #8
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Re: Correct prop size

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Originally Posted by sonofasonofasailer456 View Post
There are people calculators online


AutoCorrect - My worst enema.
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Old 05-06-2018, 08:23   #9
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Re: Correct prop size

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Originally Posted by Sailmonkey View Post
You really need to check actual engine speed with a photo tach to the crank pulley before you assume you’re over revving by that much. The governor should hold the engine speed down more than that.
Like he^^^^ said.

It is unlikely that your engine is reving that high. You have to check it with a real trustworthy tachometer. The instrument panel gauge is NOT to be trusted.

If you ARE running that fast, you are way out on the prop size.
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Old 05-06-2018, 12:49   #10
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Re: Correct prop size

AHHHHH BLASTED AUTOCORRECT¡!!!$#@&_+()/
there are prop calculators online, punch in your information they will tell you what you need. Have you checked the tacks accuracy? Not all are created equal, I know factor tacks need to be corrected.
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