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Old 04-05-2012, 18:50   #1
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Join Date: Aug 2011
Posts: 4
Re-Power, Prop questions?

Hi all,

We have located a possible replacement engine for our Gulf Island 29. Originally there was Ferymann A-30 with 2.10:1 gear installed. The previous owner pulled it out on its last legs, claiming it was very underpowered.

The boat is fitted with a campbell 3 blade 13" - 12 prop.

The motor we have our eye on is a Yanmar 3HM35. I realize this is pushing the limits in terms of size for the boat ( it will fit). Do people think it is far too big? I have seen a few other gulf island 29's with 20-30 hp's. The gear is 2.14 :1.

Unfortunately, the 13" prop appears to be about the maximum diameter for the aperture.

What are peoples thoughts?
islandfellow is offline   Reply With Quote
Old 05-05-2012, 04:34   #2
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Join Date: Dec 2008
Location: Oriental, NC
Boat: Mainship Pilot 34
Posts: 1,461
Re: Re-Power, Prop questions?

I plugged some assumed and given numbers into boatdiesel's prop calculator- 8,000 lbs, 27' lwl, 2.1:1 ratio, 3,600 rpm max engine, 27 hp, 2.1:1 ratio. It calculated 16 hp required to hit hull speed of 7kts which will be 3,000 rpm. That's not so bad as it gives you another 11 hp for head seas. The 2 cyl Yanmar will be bit too small for that boat.

The 13" prop diameter works but the calculator says 11" pitch for a 2 blade. Check your shaft diameter and material, ie Aquamet 22, etc. It may be too small for the higher torque.

Post actual numbers and I will run again for you. Or join boatdiesel for $25 and run your own. Best $25 I ever spent. It is a general marien diesel forum.

djmarchand is offline   Reply With Quote
Old 05-05-2012, 05:35   #3
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Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 19,032
Re: Re-Power, Prop questions?

You can also download an xls to help you get to the right prop. Propking is what I played with.

We are often limited by the aperture size so then the only variable becomes the pitch. Some props will be better for going very fast, others for going very slow. There is no one 'best prop' for everything - unless you have a variable pitch prop. There are also two-pitch props.

I would start with the existing prop and if this one is no good, I would very carefully calculate the most adequate one for the job at hand.

If you buy one from e.g. Vetus or VP, they will do all the calculations for you.

barnakiel is offline   Reply With Quote

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