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Old 05-07-2020, 07:08 AM   #21
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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
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).
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Old 05-07-2020, 08:06 AM   #22
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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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Old 05-07-2020, 09:10 AM   #23
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Originally Posted by Redhook98 View Post
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.
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.
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Old 05-08-2020, 07:08 AM   #24
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11 mph = 9.5 knots (rounded off)
10.5 mph = 9.1 knots
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Old 05-08-2020, 10:27 AM   #25
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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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Old 05-08-2020, 10:58 AM   #26
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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.
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Old 05-08-2020, 12:42 PM   #27
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if my engines both have a sweet spot of 1700-1750rpm, wouldnt that be where i want the most prop effiantcy?
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.
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Old 05-08-2020, 02:10 PM   #28
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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
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Old 05-11-2020, 02:37 PM   #29
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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.
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Old 05-11-2020, 04:30 PM   #30
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Yep I'd worked that out, it was successful on my boat too.
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Old 05-11-2020, 05:15 PM   #31
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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.
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Old 05-11-2020, 05:27 PM   #32
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Originally Posted by Nomad Willy View Post

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.
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!
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Old 05-11-2020, 06:59 PM   #33
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Originally Posted by SeaMaster101 View Post
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

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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Old 05-11-2020, 07:05 PM   #34
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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.
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?
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Old 05-11-2020, 08:09 PM   #35
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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.
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Old 05-11-2020, 08:31 PM   #36
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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.
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!
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Old 05-11-2020, 09:59 PM   #37
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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.
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Old 05-11-2020, 10:42 PM   #38
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Yes but the manufacturers curves are at WOT. Usless
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.
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Old 05-11-2020, 10:50 PM   #39
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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.
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...
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Old 05-12-2020, 12:40 AM   #40
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I see you’ve got less than 50 posts.
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)
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