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Old 17-07-2024, 10:34   #16
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Re: Prop sizing yanmar 3gm30f

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Originally Posted by Mg451 View Post
Would pitch or diameter be more desirable? Wonder if momentum or torque would serve me better if I were being stopped by headseas or hobby horsing. I mean if I were going faster I'd plow though it more or would I need the torque to reaccelate...God I hate overthinking things this is what information in the hands of the ill informed does��
Do the testing first and see what the speed and RPM numbers look like at WOT. That'll better inform where you stand now and guide what might be worth changing.
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Old 17-07-2024, 13:23   #17
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Re: Prop sizing yanmar 3gm30f

Hint: Less diameter and more itch will be your friend. This will also reduce prop drag when sailing.
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Old 17-07-2024, 22:57   #18
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Re: Prop sizing yanmar 3gm30f

430x270, 3 blade is what Yanmar thinks you should have for 7 knots. From the 3GM30 specs in the installation manual
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Old 18-07-2024, 07:15   #19
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Re: Prop sizing yanmar 3gm30f

So in inches 17 by 10.6 I have a 16 x 11 now I wonder how much of a difference 1 inch bigger diameter would actually make?
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Old 18-07-2024, 07:29   #20
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Re: Prop sizing yanmar 3gm30f

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So in inches 17 by 10.6 I have a 16 x 11 now I wonder how much of a difference 1 inch bigger diameter would actually make?
It would give a bit more blade area, but I wouldn't expect that difference to matter much for top speed. It might have slightly better thrust at low speeds, assuming both props are of the same design and just a different size.

Regardless, it makes sense to do some testing with what you have now before thinking about changing anything.
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Old 18-07-2024, 14:24   #21
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Re: Prop sizing yanmar 3gm30f

I run a Yanmar 3gm30f in my Pearson 30. It is 9000 pounds with a 2.61:1 gearing and a 14x10 3 blade prop. My prop was custom made for my boat and driveline by a prop shop. We run 2000 RPM cruising and run WOT 3600 RPM when bucking a current. Yes it's noisy, but it's happy.

I had a Downeaster 38, It needed a lot of power to move.
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Old 18-07-2024, 15:04   #22
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Re: Prop sizing yanmar 3gm30f

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So in inches 17 by 10.6 I have a 16 x 11 now I wonder how much of a difference 1 inch bigger diameter would actually make?
Here’s the pages from the installation manual regarding propeller selection.
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Old 22-07-2024, 08:29   #23
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Re: Prop sizing yanmar 3gm30f

I would also look at the engine.
Maybe it is not capable of producing the necessary power under load...

Some areas:-
-Valve clearance adjustment
-Fuel injector spray pattern & firing pressure
-Mixing elbow blockage
-Low compression

The first two are considered maintenance items and are often overlooked by previous owners.

On my boat (Universal diesel), the valve clearances were WAY off.
I took the injectors to a shop to be tested and all three injectors had issues with either a bad spray pattern (squirt vs spray), not sealing completely after firing and/or firing at the wrong pressure.

After I installed factory-new injectors and adusted the valves it was a completely different engine. More power, less vibration.

My two cents

cheers
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Old 22-07-2024, 09:47   #24
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Re: Prop sizing yanmar 3gm30f

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Do the testing first and see what the speed and RPM numbers look like at WOT. That'll better inform where you stand now and guide what might be worth changing.
I agree, do the wide open throttle test to see if the engine can just reach its rated speed. If so, then you will be able to use the full power of the engine, maybe to get you out of trouble, beating into a head wind and sea and so on.

Propeller A is over-propped, too big, in that it won't ever let you use more than a proportion of the engine power. You will not go as fast.

Propeller C is under propped, too small, in that it will reach the rated speed of the engine before wide open throttle. Again you will only ever have a proportion of the engine power and will not go as fast.

Good luck


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Old 22-07-2024, 12:55   #25
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Re: Prop sizing yanmar 3gm30f

Wrong prop
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Old 22-07-2024, 13:04   #26
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Re: Prop sizing yanmar 3gm30f

To be sure - propeller fouling is ANY growth on prop. Hull fouling is anything more than a very fine scum on the underside of the hull. Makes a substantial difference in hull speed, especially if you’re bucking waves.

So I’d 1)use optical hand tack to verify tach accuracy 2)clean hull & prop 3) do WOT run.

I never run WOT except in exceptional need……high rpm simply reduces engine life. Longest lasting marine diesels are low rpm designs.
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Old 22-07-2024, 14:54   #27
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Re: Prop sizing yanmar 3gm30f

It would seem to me that 2.6: 1 reduction would leave your shaft turning correspondingly slower at a given RPM than a transmission having a lower reduction ratio, for example 2.1:1. Check that your tachometer is accurate, if so, the move normally you would add probably 2 inches of pith based on your mis propped description. Face area being less likely the problem but in the extreme maybe both. If pitch is already too high or diameter is too low (the latter not being as likely) your prop is likely already cavitating due to excessive pitch and you wouldn't achieve speed. Thus, For many hull shapes the proper three blade prop will be a smaller diameter than a two blader would. Your problem is not due to drag. As for me I have 19 x 12 LH two blade prop on a boat is displacing about 18,000 lb, it reaches hull speed at about 2300 RPM in flat water. A smaller and lighter boat such as yours might do best with something like15 x 15 if it is a three blade prop or perhaps a 15 x 14. Do not exceed tip-hull clearance guidance as being somewhere between one fifth and one sixth of the prop diameter. There is an answer for why your boat isn't reaching hull speed, I don't claim to be a prop expert, just a boater that happens to be an engineer with a lot of experience boating. It is my opinion that if propped correctly, a displacement hull should reach its hull speed at roughly 80-85% of the engines rated max RPM.
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