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?

Redhook98 said:

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?

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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.

Nightsky said:

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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I am looking for a baseline that my boat started with.

8.5 mph at 1750 for a 49’ boat sounds about right. I agree on using a prop calculator.

If you get to 2500, and have more travel left on the levers, what does rpm do if you go full travel?

Ski in NC said:

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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I do not want to take it over max RPM. Max RPM is 2500 on those.

Redhook98 said:

I do not want to take it over max RPM. Max RPM is 2500 on those.

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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.

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.

Have you verified the accuracy of your boat tachometers?
If not then that should be step number 1
Otherwise you will have faulty data

jleonard said:

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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Yes, they are calibrated.

Redhook98 said:

Yes, they are calibrated.

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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.

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.

rslifkin said:

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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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.

Redhook98 said:

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?

Click to expand...

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.

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.

Redhook98 said:

I am looking for a baseline that my boat started with.

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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.

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 Meisinger said:

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

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Per manual.

No load governed speed is 2650 RPM

Under load 2500 RPM

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

Pete Meisinger said:

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

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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.

Pete Meisinger said:

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

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It's not to add speed. It's purpose is to add a "cushion" of sorts to prevent one from being over propped and thus adding strain to the engine, high egt, overheat, etc.
I agree to a point. In my case the 2500 rpm I can get is with the boat fully loaded. I also maintain the cooling system and haul out after a summer season so I don't get much growth on the bottom.

By the way the top speed I have seen is around 8.5 knots (without help from a current).

I was able to get to my props yesterday. They are 25x20, three blades. I may be in the ballpark based off all the info from you guys. I will do the WOT test when it splashes. When I backed off at 2500 RPM last time, I was at 10.5-11mph.

Redhook98 said:

I was able to get to my props yesterday. They are 25x20, three blades. I may be in the ballpark based off all the info from you guys. I will do the WOT test when it splashes. When I backed off at 2500 RPM last time, I was at 10.5-11mph.

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MPH and knots are different by about 15%. So your full throttle RPM was about 9 knots. Sounds like you're about right where you want to be.

11 mph = 9.5 knots (rounded off)
10.5 mph = 9.1 knots

if my engines both have a sweet spot of 1700-1750rpm, wouldnt that be where i want the most prop effiantcy?

Save yourself all the mental masturbation, gather all your details and then Google search Victoria Propellers, Vicprop and it will calculate everything correctly for you.

Rejuvination said:

if my engines both have a sweet spot of 1700-1750rpm, wouldnt that be where i want the most prop effiantcy?

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The expression “sweet spot” is used extensively here on TF. Many also refer to their engine being “happy”.

I don’t think it has anything to do w efficiency and all to do w vibration and perhaps to some degree noise.

But nobody refers to their “smooth spot”. But after thinking all were talking about vibration ... if it is something else I’d like to know what it is.

Your absolutely right Nomad Willy. If I'd designed my boat I would have fitted a Halyard Marine flexible coupling with super soft engine mounts to get that 'smooth spot' but as the boat was second hand I replaced all the engine mounts and wanted to buy a 5 blade prop to eliminate sympathetic resonance for 'smooth' running.
As it turned out the 5 blade prop was too expensive so I designed a 4 blader which got copied and is now called the Europoise. I was a bit concerned about sympathetic resonance with a 4 blade prop and 4 cylinder engine but in fact the gearbox reduction eliminated that aspect. As I had excess torque from the engine I over propped the boat to bring the engine noise/vibration down to its 'smooth spot'.
Apologies for rambling on but its been a great success.
Each boat and what we require of are different of, cruising ground/weight/economy/balance etc

Funny enough no one ever asked what is the goal here?
If the goal is maximum speed for WOT, then one should ail roughly to achieve 150-200 RPM less than what high idle of these engines is. So, if High Idle is 2650RPM, then approx 2500 will give you top speed and the engine will be correctly loaded.
Here is the catch: Correctly loaded at WOT! So what percentage of the time you will be driving the boat with WOT? If the answer is more than 5% of the time, then I would prop it as specified above (of course these are rules of thumb, and prop calculation SW will give you the exact results based on engine HP and RPM, transmission ratio, hull WLL, vessel weight, hull type etc.).
But if you don't ever intend to drive the vessel with WOT, this prop-ing will be a waste! It is the save bet, but inefficient setup...
What do I mean by that? The Diesel engines (every engine as am matter of fact) has a torque and HP curve that looks like a hump - see the attached graph. On the other hand, the propeller load curve looks like inverted hump. In the first case with prop calculated for WOT, the loaded RPM HP generation and the propeller load curve should mach at the loaded max RPM, therefore you will not be exceeding allowable load on the engine regardless where the throttle lever is. But if you look at the lower RPM, you will see that the engine is capable of producing tons more power that what the propeller loading curve is requiring at these lower RPM. So what does that mean? It means running your Diesel engine under-loaded for the majority of the time time you are cruising... Not so good! can lead to many issues, such as excessive carbon-ing, cylinders wall glazing, etc. Funny how everyone is more worried about overloading that under loading which takes place in 80-90% of the displacement boats use time.
I put about 12,000NM on my 47' trawler crossing the Pacific Ocean. My major concern when I was preparing for the trip was to make sure I get the most efficiency of my running gear.
So with my goal being efficiency, I decided to over-prop, knowing that I will never be running my engine for long on WOT. Just to be safe, I also installed a pyrometer and it was connected to my vessel monitoring system(6700 Vitals Vessel System Monitoring & Smart Alarm Device ), that gives you clear indication when your engine is being overloaded.
So my top speed dropped by 1knt after is re-proped with over-proped wheel, but I gained in efficiency. It allowed me to use more of my engine at lower RPM. Praveling with 7.5knts before I needed about 1350RPM. with new wheel configuration, I do 7.5Knts with about 1100RPM. with old prop I could rev to about 2000RPM at WOT, now I can reach only 1800RPM. Due to these changes I managed to go 2,700NM from Cabo San Lucas to the Marquises islands with 912US Gallons.

I do not advice the over-prop your boat, if you want to be safe, but if efficiency is the goal, propping the boat the "safe" way does not going to be optimal. Of course if you decide to over-prop, you or anyone driving the boat should be aware of the "NO WOT" for more than 5-10min rule and definitely have a pyrometer to monitor the EGT while pushing the motor hard at close to WOT.

This is my 25c and my vessel is in NZ now with 12000+ NM over-proper configuration, set for best long range efficiency.

Yep I'd worked that out, it was successful on my boat too.