Prop size for 49-ft MT w/ 120FLs and RPM?

Seamaster101 wrote;
“ 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.”


The goal is to load the engine correctly to a given prop.
High idle can be adjusted by a mechanic. It has nothing to do w loading or power.

The manufacturer is where you go to learn what the rated power is and at what rpm it is achieved. And the rated rpm is where the engine develops maximum power.

And maximum speed will be achieved w the ideal propeller running at rated rpm where max power is available. And if you have the ideal propeller maximum speed will be achieved if loaded to the rated engine speed where maximum power is achieved.

Nomad Willy said:

The goal is to load the engine correctly to a given prop.
High idle can be adjusted by a mechanic. It has nothing to do w loading or power.

The manufacturer is where you go to learn what the rated power is and at what rpm it is achieved. And the rated rpm is where the engine develops maximum power.

And maximum speed will be achieved w the ideal propeller running at rated rpm where max power is available. And if you have the ideal propeller maximum speed will be achieved if loaded to the rated engine speed where maximum power is achieved.

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This is exactly what most of people don't really get. Loading the engine to a prop is misnomer. If you read my post carefully and look at the graph I provided, you can see there is no easy way to match the prop load curve to the engine load curve. Matching it to the top power of the engine will be one way of dong it, but as I explained, pointless if you are never going to drive the boat at WOT.
People do it because it is "idiot proof". I'm not an idiot, so I know physics and I can think, therefore I could do adjustments to fit my prop setup to my particular use of the boat!
Cheers

SeaMaster101 said:

This is exactly what most of people don't really get. Loading the engine to a prop is misnomer. If you read my post carefully and look at the graph I provided, you can see there is no easy way to match the prop load curve to the engine load curve. Matching it to the top power of the engine will be one way of dong it, but as I explained, pointless if you are never going to drive the boat at WOT.
People do it because it is "idiot proof". I'm not an idiot, so I know physics and I can think, therefore I could do adjustments to fit my prop setup to my particular use of the boat!
Cheers

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The ideal solution is a controllable pitch prop. But with a fixed pitch, propping for correct RPM / loading at WOT is the only way you can use the full power output of the engine if needed. Yes, it may not be the most efficient elsewhere in the range, but unless the engines are greatly over-sized, it's likely not too terrible.

rslifkin said:

The ideal solution is a controllable pitch prop. But with a fixed pitch, propping for correct RPM / loading at WOT is the only way you can use the full power output of the engine if needed. Yes, it may not be the most efficient elsewhere in the range, but unless the engines are greatly over-sized, it's likely not too terrible.

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Fully agreed, but controllable pitch prop is way outside of my price range...
and yes you are correct that if you want max speed, you should prop in such way to be able to achieve full RPM/HP from the engine; if you want efficiency at cruising speed, then little over-prop will help, thus the statement in my earlier post: what is the goal here? top speed or efficiency at cruising speed?

Neither one sir,
It’s a matter or correctness. And there’s only one way according to manufacturers. Load the prop to rated rpm.
Sure you can overprop and save a tiny bit on fuel and you won’t harm your engine unless you ran around at half throttle. You can’t run your engine 80%, 75% or at what rpm your system becomes overloaded. But nobody knows exactly what rpm that is.

Why shouldn’t one adhere to manufactures Recommendations? They have every reason to desire everyone running their product gets the best service possible.

And underloading is usually done here using the throttle .. not advancing it far enough. Or skippers running at 45% load or less. At one point (about 2010) most here were running underloaded and overpropped. Now I’m guessing there’s still 25% in that category.


And one should run their boat at WOT for about one minute (better for 4-5) to make reasonably sure your engine is running properly. Yes one should run at full bore briefly, at times as a test. Otherwise you can run at the rpm that your engine recommends for maximum continuous power.

And re efficiency most people here can afford to burn 2-3% more fuel. Looks to me like you have a boat worth several hundred thousands of dollars. Do you overprop to save fuel? That MAY be justified if you are a Passagemaker type of boater. Only a few here cross oceans. This is Trawler Forum.

Nomad Willy said:

Neither one sir,
It’s a matter or correctness. And there’s only one way according to manufacturers. Load the prop to rated rpm.
Sure you can overprop and save a tiny bit on fuel and you won’t harm your engine unless you ran around at half throttle. You can’t run your engine 80%, 75% or at what rpm your system becomes overloaded. But nobody knows exactly what rpm that is.

Why shouldn’t adhere to manufactures Recommendations? They have every reason to desire everyone running their product gets the best service possible.

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Why would you say "nobody knows". it is not a secret. Rather it is a scientific fact. Every engine has a power ant torque curve available from the manufacturer. One can clearly know where the particular engine of interest power curve intersects the power demand curve on the prop provided by propeller calculation SW. If you do not exceed the RPMs where the power demand of the prop exceeds the possible power generated by the engine, there is never going to be and issue. And again, the best indication of how much diesel engine is loaded is the EGT. Acceptable range of EGT is also available from the engine manufacturer. It raises and falls fast based on the engine load and it is proportional to the amount of fuel that the governor trows based on the power demand of the propeller (in layman's terms).
If I were an engine manufacturer I would recommend the safe thing - match your engine power at the max RPM to the propeller demand. They (the engine manufacturers) will be crazy to recommend otherwise... they have to make sure that if one does not understand power curves, and runs the boat at full open throttle, their engine is not going to overheat the valves and blow up.
The ones that "don't know" do what the engine manufacturers say. If you actually speak with knowledgeable representative, or knowledgeable prop shop, they will confirm that "over-prop" does not automatically mean "overload"
I did say multiple times: if you want top speed, do match to the max engine operating RPM - this is where the engine develops the most horse power.
if you want max efficiency - match to engine torque (max torque is usually at way lower RPM. For some this may be black magic, but for many of us out there that have good understanding of how engine actually works, this is not a surprise or "tabu"
I would admit, if I had sports fisher, I would do exactly what engine manufacturers recommend, but I have a full displacement trawler that never will be ran at WOT, thus, I have options!
I think we have beaten this one to dead!
Cheers!

Yes but the manufacturers curves are at WOT. Usless
I don’t want max anything. I want my engine to sing along not feeling, sounding or being overloaded.
I think those that overprop think they are cheating and getting away w it. Oh happy day oh happy days. Do whatever you want.
I just took an inch of pitch off my prop and I know from experience I’m going to like it. I’m hoping I’ll get 3100 on my 3000rpm rated engine. And no I won’t be cruising at 3100.

I see you’ve got less than 50 posts. The archives are overflowing w posts down through the years. We started in 07 but don’t know how far back one can search now.

Oh,
I do agree w you re EGT. There are benefits from EGT meters whether or not you overprop.

Nomad Willy said:

Yes but the manufacturers curves are at WOT. Usless

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Sorry, I mean no disrespect, but I had to laugh at this one...
the power curve represents the engine running at different throttle levels on dynamometer. They take a data point (reading) at every 100 or often less RPM and represent the HP the engine can safely generate at these RPM. they do that starting from IDLE and going slowly to WOT. SO you are wrong there... and your statements are mostly on what you think feels good, not what the reality and the engineering calculations show.
Cheers.

Nomad Willy said:

I see you’ve got less than 50 posts. The archives are overflowing w posts down through the years. We started in 07 but don’t know how far back one can search now.

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Wow, I did not know we judge experience based on number of posts... I have more open ocean miles that number of posts you have. I guess to some, that counts for something...

Nomad Willy said:

I see you’ve got less than 50 posts.

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Seamaster, you need to spend less time on the water and more time on the laptop. If you randomly post to old messages for 4 or 5 hours a night, you should hit 20,000 in no time. (Actual knowledge is optional and the manufacturer's engineers know nothing about their products so don't hesitate to weigh in)

SoWhat said:

Seamaster, you need to spend less time on the water and more time on the laptop. If you randomly post to old messages for 4 or 5 hours a night, you should hit 20,000 in no time.

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I have the feeling that this is exactly what I want to do with my time...

SoWhat said:

(Actual knowledge is optional and the manufacturer's engineers know nothing about their products so don't hesitate to weigh in)

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Agreed, and how would I know that? Wild guess - I did speak with few of them...

You guys are really getting into the weeds here. I only wanted to know what the factory spec'd props were that would have come with my MT. Being 40 years old now, and as it was neglected most of its life, I wanted a good base-line to start from to ensure I had the correct props installed, or a base to start from. As for the sweet spot comment, we all know there is a good RPM vs fuel consumption spot for our boats. I was merely wanting some input on others with similar setups.



Not sure what this thread has become. Yikes! Bet you guys are fun at Yacht Club meetings and parties.

Just ran across this thread...may not help but I have a 1979 44 MT DC. Original owner kept great records. This boat had 24x17's 3 blade wheels on it when new and there was a chart of fuel economy at 100 rpm increments. The last owner put 24x19's and it was grossly over wheeled. I ended up with 24x18's on it and it gets better economy than original and it reaches full rpm's(which is important).

Your boat is bigger and heavier so I'm guessing less pitch than mine. I run between 1500-1700 rpm depending on how much range I need. Very economical in that range. Avg 9mph at 1700

thank you Always Late, the info was what i hoped for.