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Old 01-23-2016, 10:31 AM   #1
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Those with an FL120

I need to replace our prop and had a question. What size do others with FL120s run. We have a 3 blade LH 23" 20" pitch. It seems to be the correct size as WOT is 2480 rpms.

We have a spare prop that came with the boat that is a 3 blade 23" x 25". I know this is too large but was wondering if it was resized to a 22" x 23" if it would be close to the right size.

If you have an FL 120 what is your prop size and your WOT rpm? Also boat size, which brings up another question I have. Does the hull shape/size/weight change the prop requirements or do all FL 120s require the same amount of prop so to speak? Whether it is a 23x20 or a 22x23 or whatever the equivalent would be?

Gear ratio is 2.57 to 1

I have searched the web for a conversion table but so far have not been able to find one. If you know where one is please post.
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Old 01-23-2016, 11:24 AM   #2
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We own a Defever 44, 44,000 lbs. Boat has 25 x 20 4-blade props and achieved 2,450 RPM last Spring when commissioned and tested.
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Old 01-23-2016, 11:43 AM   #3
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You can do a straight line proportion of the two pitch numbers to estimate new max rpm: 20/25x2450= 1960, which is overloaded pretty badly. This is an approximation as there are other factors in play. But usually pretty close. If you only run hull speed at 2gph, it won't hurt anything. Try to cheat hull speed and engine will not be happy.

I would not want to repitch a 25" to a 20. Prop guys usually say 2" pitch change is far as they like to go.

Also, when discussing props and rpms, always give gear ratio too.
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Old 01-23-2016, 11:49 AM   #4
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Quote:
Originally Posted by Ski in NC View Post
You can do a straight line proportion of the two pitch numbers to estimate new max rpm: 20/25= 1960, which is overloaded pretty badly. This is an approximation as there are other factors in play. But usually pretty close. If you only run hull speed at 2gph, it won't hurt anything. Try to cheat hull speed and engine will not be happy.

I would not want to repitch a 25" to a 20. Prop guys usually say 2" pitch change is far as they like to go.

Also, when discussing props and rpms, always give gear ratio too.
Good point Ski, I added our gear ratio.
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Old 01-23-2016, 11:57 AM   #5
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A re-pitch is really only the outer 2/3 or so of the blade as the blade is not re-pitched at the hub for obvious reasons, it's part of the black art of props.

A re-pitched prop is not the same pitch as an factory prop of the same pitch, it is something less (or more) then stock.

It all depends on how the twist is done and over what percent of the blade is what pitch.
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Old 01-23-2016, 12:05 PM   #6
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An engineer or a prop man w lots of experience will have useable input.

Most typical trawlers will be good enough for comparison but hull differences will have some effect. And 6 cyl Perkins engines should be about the same too. Non-turbo to non-turbo.

Many builders overpropped in the day so your boat may never have been propped to rated rpm. Like my Willard ... it was 200rpm (about) overpropped and I discovered that by reading on the Willard Boat Owners yahoo site. Took out 1" of pitch and all was well at 3000rpm. But w a bigger prop and engine a 1" pitch change may not change max rpm 200. The prop guy knows how much though.
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Old 01-23-2016, 12:49 PM   #7
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Twin engine boat, four bladed props, 23" diameter, 16" pitch.
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Old 01-23-2016, 02:34 PM   #8
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I know I've only got one prop of 23" diameter and 16" pitch.

Ready2Go must have one of those "high speed" Marine Traders.
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Old 01-23-2016, 02:44 PM   #9
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Marin,
Your pitch/dia ratio isn't bad but I thought you switched to 3 blade props.
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Old 01-23-2016, 03:39 PM   #10
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Marin,
Your pitch/dia ratio isn't bad but I thought you switched to 3 blade props.
We were going to when we thought we needed new props. Fortunately we asked around and got the name of a far superior prop shop. Took our props to them and they said they were physically fine but had been very poorly set up (prior to us buying the boat). So we had them completely overhaul, adjust and balance the four bladed props.

We've never run our boat with three bladed props (which is how the boat was built) so we can't compare, but with a multi-engine boat we want the maximum possible prop walk. Four blades theoretically delivers more of that than three blades.
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Old 01-23-2016, 04:29 PM   #11
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Another data point:
We have a GB 32 w/ single FL120. 17,000lb displacement. Gear ratio 2.1:1.

A PO replaced the original 3-blade, 24" dia., 18" pitch prop with a 4-blade 22" x 16". Don't know why they made the change, but the newer rig seems to work fine. WOT is 2300rpm.
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Old 01-23-2016, 04:36 PM   #12
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Quote:
Originally Posted by Marin View Post
We were going to when we thought we needed new props. Fortunately we asked around and got the name of a far superior prop shop. Took our props to them and they said they were physically fine but had been very poorly set up (prior to us buying the boat). So we had them completely overhaul, adjust and balance the four bladed props.

We've never run our boat with three bladed props (which is how the boat was built) so we can't compare, but with a multi-engine boat we want the maximum possible prop walk. Four blades theoretically delivers more of that than three blades.

Several years ago I ran a large heavy dive boat with large 4 blade screws, I completely understand what you mean.

Was able to adjust the RPM and split the transmissions and make the boat move sideways to port or starboard that way. Just turned the rudders away from the dock, dockside in reverse and a bit more RPM, away side in and out of forward and sideways we would go. Slick as eggs thru a hen.
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Old 01-23-2016, 04:39 PM   #13
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2.1:1 gear and a 22x17 4 blade.


After some prelim homework....best to ask someone who will do the work on the prop....to get where you need to be may not be possible/economical with what you are starting with.
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Old 01-23-2016, 04:50 PM   #14
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Interesting about the prop walk Marin.

More pitch should increase propwalk and going from 4 to 3 blades would do just that. But your pitch/dia ratio is good. If you had 24X12" then I'd be saying a 3 blade would be better but your prop shop probably would have told you.

Interesting that it came w 3 blade props. Just a thought but the builder could have prefered the cheaper 3 blade.
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Old 01-23-2016, 06:43 PM   #15
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Interesting that it came w 3 blade props. Just a thought but the builder could have prefered the cheaper 3 blade.
Maybe. Three blades seem pretty typical for that early 70s era for this type of boat. The shop pitched the blades down by one and two inches respectively when the reworked the props. They had been 17 and 18 inches before, which were the same pitches as the stock three-bladed props.
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Old 01-23-2016, 07:01 PM   #16
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Twins, Michigan Federal, 3 blade, 24" by 17", OEM.
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Old 01-24-2016, 09:25 AM   #17
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Quote:
Originally Posted by Ski in NC View Post
You can do a straight line proportion of the two pitch numbers to estimate new max rpm: 20/25x2450= 1960, which is overloaded pretty badly. This is an approximation as there are other factors in play. But usually pretty close. If you only run hull speed at 2gph, it won't hurt anything. Try to cheat hull speed and engine will not be happy.

I would not want to repitch a 25" to a 20. Prop guys usually say 2" pitch change is far as they like to go.

Also, when discussing props and rpms, always give gear ratio too.

Ski, I am not understanding your math. Would you be kind enough to explain?
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Old 01-24-2016, 12:53 PM   #18
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40 Albin single engine. 1.9:1 gear, 20 x 16 4 blade. WOT rpm 2500
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Old 01-24-2016, 07:02 PM   #19
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Quote:
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Ski, I am not understanding your math. Would you be kind enough to explain?
Prop pitch is like marine gear ratios, or like gear ratios in car. 20 inch pitch means the boat will advance 20" with one prop revolution. 25" pitch means it will advance 25" per rev. Theoretical as water does not give "traction" like a tire, there is some to a lot of slip involved.

So the math basically calcs rpm like shifting a car from third to fourth gear, the rpm for the same speed will be proportionally lower.

So basically I took the ratio of the pitches of the two props and multiplied your existing full load rpm. 20/25= 0.8. 0.8x2480= 1980.
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Old 01-24-2016, 08:14 PM   #20
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Ski, got it. Thanks
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