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Old 01-12-2017, 15:49   #1
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Urgent question re prop size for 1973 morgan oi 41

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

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Old 01-12-2017, 16:55   #2
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Re: Urgent question re prop size for 1973 morgan oi 41

Our Morgan OI has a 18x11LH prop, but the answer depends on what engine you have, what transmission and reduction you have. For example, when we replaced our engine we went from a 18x15 to 18x11 because the new engine was higher revving and had less reduction so the prop ended up spinning faster than the previous setup
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Old 02-12-2017, 03:00   #3
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Re: Urgent question re prop size for 1973 morgan oi 41

We still have the original perkins 4108. So do think the 18 x 15 will work?

Thank you,

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Old 02-12-2017, 03:29   #4
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Re: Urgent question re prop size for 1973 morgan oi 41

I'm not too picky at this point on pitch etc. We just came out of a boat yard and sail/motored for 250 miles with no problem. I can't figure out how this prop even came off with 2 nuts cinched down and a cotter pin. Starting to wonder if I'm cursed.
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Old 02-12-2017, 06:33   #5
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Re: Urgent question re prop size for 1973 morgan oi 41

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I'm not too picky at this point on pitch etc. We just came out of a boat yard and sail/motored for 250 miles with no problem. I can't figure out how this prop even came off with 2 nuts cinched down and a cotter pin. Starting to wonder if I'm cursed.
You need to be picky. Wrong pitch and you will either allow the engine to rev to high or not get the rpms you need.

If you know some basics about your boat, the engine max rpm, and transmission ratio, you can use this calculator to find the right prop size/pitch:

https://www.vicprop.com/displacement_size.php
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Old 02-12-2017, 06:50   #6
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Re: Urgent question re prop size for 1973 morgan oi 41

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.
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Old 02-12-2017, 10:17   #7
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Re: Urgent question re prop size for 1973 morgan oi 41

Pitch perfect!

It’s important! Get the right prop, install and run it checking the max rpm. If too high or too low dive down, take it off and head back to the prop shop to get a few degrees added or removed then lather rinse repeat till it’s right.

I didn’t think this mattered much but on ole Scarlett the partner went to all this trouble and he was right. She motored great after that and we put a lot of happy miles on her.

Oh yea, have a spare 😉 Now you know why!
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Old 02-12-2017, 10:41   #8
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Re: Urgent question re prop size for 1973 morgan oi 41

Yes indeed you need to be picky! Someone gave you the prop calculator used by Victoria Propeller Ltd. Use it! Around here, it is what all mechanics use. As you see, it asks you for some basic parameters. In the binder of "Ship's Documents" you carry at your nav-station, you should already have these. If you don't, this is an excellent opportunity to start compiling them.

Here is a basic page of specifications you can simply print out and insert in the binder of documentation: MORGAN OUT ISLAND 41 sailboat specifications and details on sailboatdata.com
For BWL, which is not given in the overview, use 11 feet - that'll be justabout right.


Someone spoke of a left-handed wheel. Whether yours is right- or left-handed will depend on your particular engine installation, but you gotta know it! Put your tranny in forward gear, look at the prop shaft or the flange where it comes out of the tranny. Have someone crank the engine a turn or two without actually starting it. Note which way the shaft turns.

If the top-side of the turning shaft would move to your right if you were standing aft of the ship and looking forward, i.e. if so viewed the rotation is CLOCK-WISE, then you need a RIGHT-handed prop. If the topside of the shaft moves to the left, ie. the rotation is COUNTERCLOCK-WISE, you will need a LEFT-handed prop.

Once you determined the right specification for the prop you'll include THAT in your documentation, right :-)?

Note that at wide open throttle in forward gear you should be able to get 95% or so of RPM (max) specified for your engine. If you can get more, your prop is "too small" or to use the jargon, you are "underpropped". If can cannot get that much, you are "overpropped". Both conditions are deleterious to engine life and, less importantly, to boat performance under power.

As for losing your prop: Someone goofed! When you were in the yard, did you PERSONALLY inspect the prop installation and make sure that 1) the taper on the shaft was clean and bright, and the interior taper in the prop hub likewise? 2) That the key that positions the prop on the shaft was correctly inserted and extended the entire length of the keyway in the hub? 3) that the retainer nut was the "castellated" type, i.e. had slots cut in it through which you insert a "split pin", also often called a "cotter pin", that goes through a hole in the shaft so the nut can't "unscrew" itself?

My position in regard to such important, safety related tasks is that you should never leave them to "professionals" without monitoring and supervising them closely. :-)

All the best.

TP
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Old 02-12-2017, 10:50   #9
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Re: Urgent question re prop size for 1973 morgan oi 41

Call Michigan prop and they will tell u exactly what you need. Have all your drive train info available.
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Old 02-12-2017, 10:54   #10
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Re: Urgent question re prop size for 1973 morgan oi 41

I think a prop shop will have good specs for that particular boat and engine... there were so many made. You dont have to be spot on. Actually, keep in mind what rpm you like to run your engine. I ran my 4-108 in a 44 ft cutter (see avatar) at 2400-2600 rpm. and didnt prop the boat to make the high max rpm specified. No over heating, plenty of power and very economical and smooth. We get up to 8 knots current up here too.
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Old 02-12-2017, 11:04   #11
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Re: Urgent question re prop size for 1973 morgan oi 41

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I think a prop shop will have good specs for that particular boat and engine... there were so many made. You dont have to be spot on. Actually, keep in mind what rpm you like to run your engine. I ran my 4-108 in a 44 ft cutter (see avatar) at 2400-2600 rpm. and didnt prop the boat to make the high max rpm specified. No over heating, plenty of power and very economical and smooth. We get up to 8 knots current up here too.
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.
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Old 02-12-2017, 11:22   #12
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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.
.... it's all about how much HP you need. (..and by the way, I propped my two 3gm30 Yanmar's for their specified 3600+ rpm. Both engines failed in less than 2500 hours, but that's another story.)
Here's another example I've mentioned multiple times here:
I have specified diesels for commercial boats. If you go to the fisheries/commercial trade shows, you find the engine manufacturers actually specify a different rpm for the same engine depending on it's use. So for a engine for recreational use they may specify 2800 rpm. For a 24/7 use in a commercial boat they may specify 2300 rpm. SAME ENGINE.
What people dont realize is engines like the high RPM Yanmar's they advertise as "light weight!". ...are really just over rated. So they take an engine that produces 18 hp engine at reasonable rpm and call it a 26 hp engine by allowing you to run it at 3600+ rpm.

Of course there are a multitude of other things relevant too...
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Old 02-12-2017, 11:58   #13
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Re: Urgent question re prop size for 1973 morgan oi 41

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.... it's all about how much HP you need. (..and by the way, I propped my two 3gm30 Yanmar's for their specified 3600+ rpm. Both engines failed in less than 2500 hours, but that's another story.)
Here's another example I've mentioned multiple times here:
I have specified diesels for commercial boats. If you go to the fisheries/commercial trade shows, you find the engine manufacturers actually specify a different rpm for the same engine depending on it's use. So for a engine for recreational use they may specify 2800 rpm. For a 24/7 use in a commercial boat they may specify 2300 rpm. SAME ENGINE.
What people dont realize is engines like the high RPM Yanmar's they advertise as "light weight!". ...are really just over rated. So they take an engine that produces 18 hp engine at reasonable rpm and call it a 26 hp engine by allowing you to run it at 3600+ rpm.

Of course there are a multitude of other things relevant too...
Following your logic everyone should prop their Yanmar to reach hull speed at 500 rpms. Over propping a boat is the same as running your car up a steep hill in fifth gear all day long.
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Old 02-12-2017, 12:09   #14
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Re: Urgent question re prop size for 1973 morgan oi 41

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Following your logic everyone should prop their Yanmar to reach hull speed at 500 rpms. Over propping a boat is the same as running your car up a steep hill in fifth gear all day long.
Yeah, as I said, there are other things involved too. Going to extremes isnt a solution. But you have to realize that engine makers dont make engines in 1 hp increments. A boat may need 35 hp but the engine choices may be 51 hp or 29. Not my logic commercial industry logic.
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Old 02-12-2017, 12:18   #15
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Re: Urgent question re prop size for 1973 morgan oi 41

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Following your logic everyone should prop their Yanmar to reach hull speed at 500 rpms. Over propping a boat is the same as running your car up a steep hill in fifth gear all day long.
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...
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