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Old 11-20-2020, 11:34 AM   #1
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prop size and clearances

I have been searching for a thread on props sizes and hull clearances .I have a single engine full keel trawler with a 24" prop and only 1.5" clearance. I know I have seen this in another post but can't find it .
also I took out my drive saver until I do a shaft alignment after the boat is splashed . debating on leaving it out. I don't know how moving it an 1.5" toward the keel and away from the rudder will effect the boat. any advice is welcome.
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Old 11-20-2020, 11:58 AM   #2
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I have been searching for a thread on props sizes and hull clearances .I have a single engine full keel trawler with a 24" prop and only 1.5" clearance. I know I have seen this in another post but can't find it .
also I took out my drive saver until I do a shaft alignment after the boat is splashed . debating on leaving it out. I don't know how moving it an 1.5" toward the keel and away from the rudder will effect the boat. any advice is welcome.
If I remember correctly you want a minimum of 10% and preferably 15% clearance.

We took our drivesavers out and replaced them with spacers from Spurs. They mad the spacers to fit and they fit perfectly. We didnít have room to slide the prop shaft forward. You should have more shaft length behind the strut in between the strut and prop than the thickness of the shaft. Out shafts are 2Ē so our prop should be within 2Ē of the strut.
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Old 11-20-2020, 12:11 PM   #3
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Yes, 10%D is the absolute minimum clearance above w/ 15% being better. 10% below to the skegbar is enough. If you stand on the hull above the prop at speed and feel strong vibration or hear sounds like someone playing a hose on the hull, you may be too close.
I lowered my skeg and shaft on my downeast lobsterboat to add 2" above and 1" below my 22"D prop. This resulted in almost 1k in crusing speed and 1k at WOT plus much less vibration and prop noise w/ no other chnges.
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Old 11-20-2020, 12:14 PM   #4
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ok I definitely don't have 10% of dimeter maybe of the radius . looks like I need to do some planning for the next haul out. out of money this year well that type of money.
I do have the over 2" to the bearing but the donut zinc is going to take up a lot of it.
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Old 11-20-2020, 02:29 PM   #5
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Looks to me like you’ve got too much prop.

How much power is in that boat?
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Old 11-20-2020, 02:36 PM   #6
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What is the distance from the back of the skeg to the front of the prop. It should not be more than 1 shaft thickness. Out shafts are 2Ē so I canít go more than 2Ē unsupported shaft from the last strut, in your case the skeg, to the front of the prop. Otherwise if it is too long you may break the shaft. Our props are very close to the last strut so I had to have spacers made when I removed the drivesavers. But I was able to align the engine properly and did loose some vibrations.
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Old 11-20-2020, 07:58 PM   #7
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Itís exactly the shaft diameter. I guess it never should have had the drive saver . That prop has probably been on the boat 20 years
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Old 11-20-2020, 08:01 PM   #8
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It’s a American diesel 136sn or something like that pretty much a 135 Lehman
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Old 11-20-2020, 08:08 PM   #9
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I have been following the prop pitch threads and economy ones I only get about 7kts at 1900 I thought a 38 should have a better hull speed than that . I figured I was under proped maybe my power is just beating into the hull and skeg
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Old 11-20-2020, 08:47 PM   #10
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Under propped will probably get you the most speed ... from the most power.

What’s your rated power rpm?
And what’s your WOT rpm?

I’m guessing now but suspect you need to reduce pitch.
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Old 11-20-2020, 08:57 PM   #11
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I think that you need to reduce the diameter so you get at least 10% clearance first. Then see what the RPMs are at full throttle. I have SP225s so I am not sure what your max RPMs should be but you want to hit the max. Boatdiesel.com has some calculators as to what size would be a good start. There is a yearly membership fee on boatdiesel so maybe you can find a free calculator somewhere else. Good luck and keep letting us know how it is going.
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Old 11-21-2020, 07:49 AM   #12
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I have been following the prop pitch threads and economy ones I only get about 7kts at 1900 I thought a 38 should have a better hull speed than that . I figured I was under proped maybe my power is just beating into the hull and skeg
Actually that rpm/speed sounds about right to me.
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Old 11-21-2020, 02:09 PM   #13
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On the survey it reached 2700 the ratesd rpm is 2600 . I have never pushed it past 2100I just saw the prop markings itís a 24 L20.
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Old 11-21-2020, 02:39 PM   #14
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My boat Possum had very little clearance between the skeg and the prop tip. The wash off the prop tip actually blasted a shallow dimple into the solid glass skeg.
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Old 11-21-2020, 04:26 PM   #15
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On the survey it reached 2700 the ratesd rpm is 2600 . I have never pushed it past 2100I just saw the prop markings itís a 24 L20.
Sounds like that is propped pretty good as long as the tachs are accurate.
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Old 11-21-2020, 04:58 PM   #16
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My boat Possum had very little clearance between the skeg and the prop tip. The wash off the prop tip actually blasted a shallow dimple into the solid glass skeg.
That is one of the reasons I think that he should go smaller on the diameter.
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Old 11-21-2020, 06:33 PM   #17
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Actually it may be best to go smaller on the diameter.
Cut it down. Just change the diameter.

It’s almost a square prop so no doubt suffers from friction water to blade.

If it’s a 2.57-1 gear going to the 2.93 (or whatever it is) and increase pitch as required would probably get-her into the good ratio zone.

timb wrote;
“On the survey it reached 2700 the ratesd rpm is 2600 . I have never pushed it past 2100I just saw the prop markings it’s a 24 L20.”

Well I’m say’in run her up there and verify. That’s perfect by the way ... 100rpm over rated.
Don’t change anything till you get the;
1. Rated max power rpm.
2. Check your tach and then go to WOT. Since it concerns you only stay at rated rpm for about a minute. As soon as it stabilizes observe the rpm. Make sure it isn’t still climbing. It won’t hurt your engine if there isn’t anything wrong with it. You can safely go 2 or 300rpm over rated rpm. A govoner in the engine’s injection system to prevent over reving. Not to worry.

Let us know or post before then if you like.
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Old 11-22-2020, 01:35 AM   #18
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Quote:
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I have been searching for a thread on props sizes and hull clearances .I have a single engine full keel trawler with a 24" prop and only 1.5" clearance. I know I have seen this in another post but can't find it .
also I took out my drive saver until I do a shaft alignment after the boat is splashed . debating on leaving it out. I don't know how moving it an 1.5" toward the keel and away from the rudder will effect the boat. any advice is welcome.
What was the reason the drive saver had to come out until you align, did that not bring the prop forward?
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Old 11-22-2020, 07:30 AM   #19
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I took the drive saver out to do prop shaft alignment and try to get measurements for shaft estimates. ( it needs replacing later)It seems who ever added the drive saver just stuck it in there and didn’t modify the shaft .
I will try to get the information on wide open after I check tacks and run wide open. Not really worried about the engine it seems fine just trying to be easy on it . I couldn’t afford a new one if I lost it.
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Old 11-22-2020, 07:44 AM   #20
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"Not really worried about the engine it seems fine just trying to be easy on it . I couldn’t afford a new one if I lost it."

Regardless of the RPM reached at WOT if you are down 10% in RPM ( I use 300RPM to be safer) you should l not be overloading the engine.
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