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Old 02-12-2017, 11:56   #16
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Re: Urgent question re prop size for 1973 morgan oi 41

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Originally Posted by Empty Pockets View Post
No, it's not. It's like shifting into fifth gear early on flat ground to improve mileage. No damage. Your example would be correct if the engine were over propped and the operator loaded it up till it lugged. As mentioned before, you're really just limiting the horsepower available for use. My engine is rated at 55 hp at 3600 rpm. I'm over propped in that I can only get 32-3300 wide open. So my boat operates as if it's got maybe a 40-45 hp engine. I run my boat at 22-2300 at about 6.5 knots. Nice and quiet and economical...
Same here logic. Blue has 46 hp @ 3600. I run at about your RPM, needing 32 hp on the power band. Quiet, smooth, economical turning a 17 x 13 3 blade with 1.88:1. Mitsubishi block is used in Case tractors and skid loaders, which work normally at low RPM.
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Old 02-12-2017, 13:22   #17
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Re: Urgent question re prop size for 1973 morgan oi 41

I have a Perkins 4-108 and for the past 32 years I've been over propped. The engine is still running well after all those years. Thanks to Blue Stocking and Empty Pockets for an explanation that is easy to understand.
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Old 02-12-2017, 14:16   #18
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Re: Urgent question re prop size for 1973 morgan oi 41

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Originally Posted by pcmm View Post
Pitch and diameter are relatively important. not enough pitch and you won't get to the speed you want. too much and the engine will lung, smoke and eventually do damage to it! what gear reduction is your transmission?

The only way your prop could come off as I see it, is if it was improperly secured.
Could be electrolysis ?
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Old 02-12-2017, 22:39   #19
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Re: Urgent question re prop size for 1973 morgan oi 41

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On a sail from marthon to stuart, we lost our prop. Gonna set sail back to boot key and dinghy in to find one. Does anyone know the prop size for this boat?

Thank you

Paul
If the prop parted company while motoring in forward it took the tail of the prop shaft with it.

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Old 02-12-2017, 23:44   #20
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Re: Urgent question re prop size for 1973 morgan oi 41

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Same here logic. Blue has 46 hp @ 3600. I run at about your RPM, needing 32 hp on the power band. Quiet, smooth, economical turning a 17 x 13 3 blade with 1.88:1. Mitsubishi block is used in Case tractors and skid loaders, which work normally at low RPM.
Your boat must have a tremendous amount of drag. My ex.timber 43'LOA, 33' WL, 13'3" beam, 7'1"Draft, SS Design with a 4 108 Perkins 1900 rpm which equals approx. 16-17hp which is also at max torque and makes 6.3 knots. The boat I have now 42'LOA 12'6" Beam 4' Draft and steel with a Yanmar 3JH4e is under pitched 6 knots at 2150 RPM she only needs 12 hp to make six knots. So needs to be pitched to make hull speed at max torque 1850 RPM which is to much HP wasting fuel but will not glaze up or be overloaded.

"Cheechako, yes I agree" Perkins 4 108's start at 30 HP and go up to 50HP. The timber boat mentioned above, when I first got it, had to run at 2800 RPM to achieve 6 knots, she had glazed bores and ate fuel 6.5 litres/hour rebuilt motor, re-pitched prop (twice) and under 4 litres /hour for 6 knots.
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Old 03-12-2017, 02:00   #21
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Re: Urgent question re prop size for 1973 morgan oi 41

Thanks you guys very much. When I said I'm not too picky, I meant that I'm stranded in the middle keys with no prop and have to make Stuart FL in 4 days. Not many prop shops open here since Irma. If I can pick up an $100 prop that will assist me in getting to my next port, I'll gladly replace it with the ideal prop. For now, I need the ability to motor till I get somewhere with more options.

My perkins is 37hp but can't be run higher than 2000rpm due to temp, and I get 6.5 to 7 kts with approx 32 - 34 hp at that rpm.

My boat was in a boat yard for 6 weeks where I did a bottom/deck/topside job, replaced spreaders, added lazy jacks, replaced cutless and midshaft bearing. If there were user error on the prop it was mine. I cleaned the shaft, winched down both nuts, and inserted a cotter pin. Made it 300 miles before it came off. Not sure why or how.

As always, your input is invaluable. My boat probably wouldn't be floating without help from you guys. I really appreciate it!

Cheers,

Paul
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Old 05-12-2017, 05:13   #22
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Re: Urgent question re prop size for 1973 morgan oi 41

So I found an 18 x 14 prop in key largo. Problem is that it's a 1 3/8 shaft size. The prop shop guy said putting a bushing (which he provided) would work fine to correct the difference. Does anyone have experience with these bushings?

Thanks,

Paul
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Old 05-12-2017, 08:02   #23
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Re: Urgent question re prop size for 1973 morgan oi 41

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So I found an 18 x 14 prop in key largo. Problem is that it's a 1 3/8 shaft size. The prop shop guy said putting a bushing (which he provided) would work fine to correct the difference. Does anyone have experience with these bushings?

Thanks,

Paul
A bushing of the proper size will work fine.
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Old 05-12-2017, 08:51   #24
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Re: Urgent question re prop size for 1973 morgan oi 41

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So I found an 18 x 14 prop in key largo. Problem is that it's a 1 3/8 shaft size. The prop shop guy said putting a bushing (which he provided) would work fine to correct the difference. Does anyone have experience with these bushings?

Thanks,

Paul
I had one with a bushing. The bushing was plastic. I wasn't that fond of that solution but it did work. Remember also, you can a lower diameter and higher pitch for pretty much the same result. Maybe 17 x 15 etc.
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Old 05-12-2017, 09:30   #25
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Re: Urgent question re prop size for 1973 morgan oi 41

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Bad advice. Over propping your boat because you like to run at lower rpms is a common mistake. Don't do it. It's hard on your motor.
Bad advice for a 4108. Running at higher revs means it will leak more oil.
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Old 05-12-2017, 23:37   #26
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Re: Urgent question re prop size for 1973 morgan oi 41

I'm not that fond of a plastic solution either but is on and seems to be tight. I'll probably replace it with a 1.25 inch at my final port of call. Hopefully it gets me there if I don't use reverse.
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