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Old 05-06-2020, 05:34 AM   #1
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Prop size for 49-ft MT w/ 120FLs and RPM?

Looking for the factory or recommended prop size for my 49-ft MT. With a clean bottom, I am doing 8.5mph (not knots) at 1750 rpm. I thought this boat should be in the 10 knot range? At 2500, I still have a lot of throttle left. No, I do not know what my current props are. The PO put them on after having new ones (other than stock) reconditioned.



So the questions are:
What was the preferred prop from the factory for a boat like this, powered the same?
What is the typical RPM range you guys with FL120s are running? Most fuel efficient or is there a sweet spot?
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Old 05-06-2020, 06:56 AM   #2
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Quote:
Originally Posted by Redhook98 View Post
Looking for the factory or recommended prop size for my 49-ft MT. With a clean bottom, I am doing 8.5mph (not knots) at 1750 rpm. I thought this boat should be in the 10 knot range? At 2500, I still have a lot of throttle left. No, I do not know what my current props are. The PO put them on after having them reconditioned.



So the questions are:
What was the preferred prop from the factory for a boat like this, powered the same?
What is the typical RPM range you guys with FL120s are running? Most fuel efficient or is there a sweet spot?
Prop dimensions should be based on WOT, gearbox reduction and shaft size. A prop shop will be able to calculate best prop for you.
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Old 05-06-2020, 07:03 AM   #3
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Prop dimensions should be based on WOT, gearbox reduction and shaft size. A prop shop will be able to calculate best prop for you.

I am looking for a baseline that my boat started with.
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Old 05-06-2020, 09:08 AM   #4
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8.5 mph at 1750 for a 49’ boat sounds about right. I agree on using a prop calculator.
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Old 05-06-2020, 09:34 AM   #5
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If you get to 2500, and have more travel left on the levers, what does rpm do if you go full travel?
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Old 05-06-2020, 09:38 AM   #6
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If you get to 2500, and have more travel left on the levers, what does rpm do if you go full travel?

I do not want to take it over max RPM. Max RPM is 2500 on those.
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Old 05-06-2020, 09:52 AM   #7
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I do not want to take it over max RPM. Max RPM is 2500 on those.

2500 is rated RPM. Max RPM is 2650 from what I can find, and the governor on the injection pump will limit it to not exceed that. Ideally, if propped correctly, you'd turn somewhere around 2600 RPM at WOT under load, that way a little bit of bottom / prop fouling and some extra load on the boat doesn't prevent you from reaching rated RPM. Slight under-loading is better than overloading.
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Old 05-06-2020, 10:35 AM   #8
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You can't beat hull speed unless your hull can plane. Even then it takes more hp. If the hull can't plane it takes massive amounts of hp to get past hull speed. My boat cruises at 10 kts using about 300hp (2x150), but hull speed is 12. If you want to go faster in a displacement hull get a longer boat.
I served on WWII built destroyers. Hull speed was about 26 knots. It took half of 60,000 hp to run at hull speed. Using the other 30,000 hp, the ship only gained 7 more knots.
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Old 05-06-2020, 10:48 AM   #9
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Have you verified the accuracy of your boat tachometers?
If not then that should be step number 1
Otherwise you will have faulty data
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Old 05-06-2020, 11:29 AM   #10
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Have you verified the accuracy of your boat tachometers?
If not then that should be step number 1
Otherwise you will have faulty data

Yes, they are calibrated.
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Old 05-06-2020, 11:35 AM   #11
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Yes, they are calibrated.
Awesome.
I run anywhere from 1650 to 1800 with my single 120 Lehman. That seems to be the sweet spot for my boat. Speed would range from 6 to 7 knots.
At WOT I can just get to 2500 rpm with just a very light haze of smoke so I think I’m propped pretty well.
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Old 05-06-2020, 12:03 PM   #12
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Yep, 2500 is not max rpm. It is simply the rpm where engine makes its rated hp on a dyno. Nothing wrong with pushing it further. At some point the governor will start reducing fuel as rpms go over say 2600.

So finding out where it tops out is a big part of checking whether props are appropriate.

A diesel can have the throttle advanced fully in neutral, that is how we check governor function. It will just go up to governed rpm and sit there, no harm. I don't know what governed is on your engine, probably 2700-2800. Others may know the exact spec.
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Old 05-06-2020, 02:03 PM   #13
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Originally Posted by rslifkin View Post
2500 is rated RPM. Max RPM is 2650 from what I can find, and the governor on the injection pump will limit it to not exceed that. Ideally, if propped correctly, you'd turn somewhere around 2600 RPM at WOT under load, that way a little bit of bottom / prop fouling and some extra load on the boat doesn't prevent you from reaching rated RPM. Slight under-loading is better than overloading.
Been preach’in this for years.
Gald to see it coming from somebody else.
I just changed my prop yesterday. Haven’t done sea trials yet but I’m going forth w several variables. But I have lots of experience w both props.
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Old 05-06-2020, 02:07 PM   #14
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Quote:
Originally Posted by Redhook98 View Post
Looking for the factory or recommended prop size for my 49-ft MT. With a clean bottom, I am doing 8.5mph (not knots) at 1750 rpm. I thought this boat should be in the 10 knot range? At 2500, I still have a lot of throttle left. No, I do not know what my current props are. The PO put them on after having new ones (other than stock) reconditioned.



So the questions are:
What was the preferred prop from the factory for a boat like this, powered the same?
What is the typical RPM range you guys with FL120s are running? Most fuel efficient or is there a sweet spot?
Thinking about it I don’t see a 49’ trawler with 240 hp doing much over 10 knots at full throttle. I suspect that you are pretty well propped now. Go to boatdiesel.com and input your numbers and see what the prop calculator says.
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Old 05-06-2020, 02:15 PM   #15
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I did a little searching and found a mention of a 49 ft Marine Trader with twin FL120s having 26x19 props. If you've got somewhere around a 2.5:1 transmission ratio, that lines up nicely with the Michigan Wheel calculator for 120hp, 2600 RPM target and 10 kts boat speed. I found another ad mentioning an 11 kt top speed.

I'd suggest taking her out, running it up to WOT and see what you get for speed and RPM. If you're much over 2600 RPM with a normal complement of stuff on the boat, you're probably a little under-propped. Especially if you reach governed RPM before you hit full throttle travel and are seeing less than 10 - 11 kts at WOT.
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Old 05-06-2020, 02:55 PM   #16
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Originally Posted by Redhook98 View Post
I am looking for a baseline that my boat started with.
A lot of boats come from the manufacturer or dealer with the wrong props. You need to check out your top RPM with your normal load aboard and see what they are actually doing. Until you do that you are shooting in the dark.
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Old 05-06-2020, 03:45 PM   #17
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I always thought WOT on a 120 F.L. was 2,500. Why push them to 2,600, it won't add any speed. In fact I'm surprised you can get 10 Kn. much less 11 or 12.

Whats the hurry?

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Old 05-06-2020, 04:29 PM   #18
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I always thought WOT on a 120 F.L. was 2,500. Why push them to 2,600, it won't add any speed. In fact I'm surprised you can get 10 Kn. much less 11 or 12.

Whats the hurry?

pete
Per manual.

No load governed speed is 2650 RPM

Under load 2500 RPM
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Old 05-06-2020, 05:40 PM   #19
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FWIW My SWAG - Scientific Wild Assed Guess

Working backwards from 49 ft hull, assuming:
- waterline length of 44 ft:
- 16 hp per gallon per hour for a non-turbo old school diesel
- Props set to reach rated RPM at WOT
- typical semi-displacement trawler yacht hull.

SqRt 44= 6.63

6.63 x 1.34 = 8.8 kt/10.2 mph Theoretical hull speed

6.63 x 1.2 = 7.93 kt/9.2 mph Typical cruise around 1800 rpm, 2.0 gpm 32 hp each engine

6.63 x 1.1 = 7.29 kt/8.4 mph Economical cruise around 1700 rpm, 1.8 gpm 28 hp each engine
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Old 05-06-2020, 07:14 PM   #20
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Quote:
Originally Posted by Pete Meisinger View Post
I always thought WOT on a 120 F.L. was 2,500. Why push them to 2,600, it won't add any speed. In fact I'm surprised you can get 10 Kn. much less 11 or 12.

Whats the hurry?

pete
Pete 2600rpm WOT is ideal IMO.
With normal load inc. 3/4 fuel clean props and bottom.
Almost everyone will go south from there.
Max rpm at 2650 is comfortably under 2600 and you’ll only do 2600 on launch day. Two years down the Channel you’ll be at 2500 to 2525 both near perfect places to be. Then when you go 1500 to 2000 to cruise you’ll feel the engine/s working a little less.
And according to engineers that generate such information basically they all suggest to prop to rated rpm. But the idea is to be at rated rpm all the time. So if you start at 100rpm over your loading over time will be in the best zone.
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