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Old 05-17-2013
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Prop size for 4-107

Anybody have any idea what prop size would be correct for a Perkins 4-107 in a 41' Gulfstar. From what I can tell the 4-107 should have an rpm of 3000, but I can only get 1800. I have a 16x16 prop and I am wondering if it is wrong. Been running this way for years and seems ok, I can only get about 5 knots out of it. I always thought that was all I would get. But, I started to wonder ifI the prop my be an issue and I wanted to see if anyone might know.
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Old 05-17-2013
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Re: Prop size for 4-107

18 by 12 for two blade
15 by 10 for three blade
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Old 05-18-2013
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Re: Prop size for 4-107

I have the 4-108 on a Tayana 37, which is smaller than your Gulfstar but probably a bigger displacement. It came with the 17x11 3 blade, however from day one I switched over to the 17 inch max prop. The max prop has flat blades so the efficiency is not as great as the curved blades of a fixed prop so the pitch will be somewhat different. I played around with the pitch and when I matched the pitch to the original prop I was not happy with the performance. I wanted more speed at a lower rpm. For the last twenty plus years I've have the prop set at around the 14 inch pitch mark. The rpm of the 4-108 is rated at 3600 continous and I can not achieve that with the 14 inch pitch. I'm happy to cruise at hull speed doing 2500 rpm which is 80% of what my max achieveable rpm under that 14 inch pitch. Most people believe that you should be able to achieve the max rpm rating of your engine....which I can not.
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Old 05-18-2013
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Re: Prop size for 4-107

When I try and push the engine above 1800, it starts to smoke a little and then the temperature starts to climb. Not necessarily to overheat, just higher. I usually will do that only if I need to get more speed for a short distance, like trying to make a drawbridge are something. But, for the most part the engine likes running at 1800. If I change the prop to get higher rpms will that effect the engine now that it has gotten use to operating at the speed it does?
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Old 05-18-2013
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Re: Prop size for 4-107

Smoking and overheating are signs of too much prop. You probably se some black oil in your exhaist also, right? Change the prop to an 18 by 12 and you should be fine. If you prefer the performance of a three blade 15 by 10 will work and give you more tip clearance for less vibration. You will be doing your engine a a favor by doing this.
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Old 05-18-2013
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Re: Prop size for 4-107

Check the tachometer as well. The tach on my boat is shot and reports 2000 rpm when the engine is actually turning 2800 rpm. I tested it using a digital tach measuring a point on the flywheel.

I've adapted, but I should also replace the tach someday.
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Old 05-18-2013
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Re: Prop size for 4-107

Quote:
Originally Posted by bobperry View Post
Smoking and overheating are signs of too much prop. You probably se some black oil in your exhaist also, right? Change the prop to an 18 by 12 and you should be fine. If you prefer the performance of a three blade 15 by 10 will work and give you more tip clearance for less vibration. You will be doing your engine a a favor by doing this.
Yes, the smoke is black.

My 16x16 is a three blade prop. I like it but I do have an issue with prop walk in reverse that really makes the boat uncontrollable. Wonder if a two blade prop would help. I'll check into the 15x10.
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Re: Prop size for 4-107

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Originally Posted by Alex W View Post
Check the tachometer as well. The tach on my boat is shot and reports 2000 rpm when the engine is actually turning 2800 rpm. I tested it using a digital tach measuring a point on the flywheel.

I've adapted, but I should also replace the tach someday.
I have checked the rpm's with a digital tach, it it appears fine. However, I do have a problem when my batteries are full, the alternator doesn't put out so my tack reading is off until I turn on a blower or something.

I guess a side question would be, anyone know how to get the tach reading from the engine and not the alternator?
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Old 05-18-2013
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Re: Prop size for 4-107

I don't know what gear redcution you have but a 16 by 16 three blade sounds like way too much prop. On the V-40's we found a 15 by 10 three blade (as I keep recommending) worked best. I think the reduct ws 2.57 to 1 as I recall. You can go with a two blade prop at the size I have already recommended but I don't think that will "fix" prop walk but it will give you the engine rpms you need and good boat speed. Your excessive prop walk may have to do with all that pitch you now have. What you have now is what we call a "square prop" i.e. diameter and pitch the same and they seldom work well.
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Last edited by bobperry; 05-18-2013 at 06:57 PM.
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Old 05-18-2013
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Re: Prop size for 4-107

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Originally Posted by bobperry View Post
I don't know what gear redcution you have but a 16 by 16 three blade sounds like way too much prop. On the V-40's we found a 15 by 10 three blade (as I keep recommending) worked best. I think the reduct ws 2.57 to 1 as I recall. You can go with a two blade prop at the size I have already recommended but I don't think that will "fix" anything but it will give you the engine rpms you need and good boat speed. Your excessive prop walk may have to do with all that pitch you now have. What you have now is what we call a "square prop" and they seldom work well.
Gotcha, thanks for the info I will see what I can do to find a prop matching what you say.

Also, the ratio is 2:51 to 1.
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