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Old 10-11-2015, 10:45 AM   #21
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Have a read through W54 Project. There is a lot of good information on the site. I think they basically determine what power is required for cruise speed and select an engine so it 80% loaded at cruise speed.
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Old 10-11-2015, 10:57 AM   #22
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Read what Ski said again. Boats do not operate at the max torque curve they operate at the prop demand curve which is well below the max torque curve except at WOT or with a variable prop.


We cant validate Skis assertion that best efficiency, in a boat with a prop, will be at max torque rpm because the makers don't provide any efficiency data at power levels less than maximum . They do provide a prop demand fuel
curve but I haven't figured how to convert that to an efficiency curve.


We also must remember that even though makers measure fuel use on a dynamometer the prop curve they measure against is an approximation. So much so that various makers use exponents form 2 to 3 to calculate it. 2.7 seems common.
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Old 10-11-2015, 11:05 AM   #23
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Bigger prop is dock talk.

Most responsible people use the correct size prop. Determined by engineers through engine manufacturers and other sources of engineering. We've been through this 110 times FF and I haven't got the time or energy to go it again.

Bigger props may have more thrust at WOT but we almost never run at WOT.
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Old 10-11-2015, 05:02 PM   #24
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Quote:
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Bigger prop is dock talk.

Most responsible people use the correct size prop. Determined by engineers through engine manufacturers and other sources of engineering. We've been through this 110 times FF and I haven't got the time or energy to go it again.

Bigger props may have more thrust at WOT but we almost never run at WOT.
You may be missing his point. Take what ski said and what FF said. The max torque is usually significantly lower th an where we operate our engines. The only way you are going to extract max efficiency at a suitable cruising speed is to either over prop your boat or have a variable pitch propeller. There are safe scientific ways to over prop a boat that fits this regime. Obviously a boat like mine does not.
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Old 10-11-2015, 05:31 PM   #25
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JD: I don't know about that. Doesn't go into that much detail on the website. I've bounced around the site quite a bit and it just says the standard Beta Marine 85 HP 2,800 rpm or the optional Yanmar 110hp 3,200 rpm. Standard Beta top speed is 10 k and optional Yanmar tops out at 11k. I suppose if I looked into the Yanmar and Beta Marine websites it might tell me more. I'm not engineer enough to figure torque, shaft, prop size and angle etc. I know it makes a difference but I'm smart enough to know that I don't know why or how.
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Old 10-11-2015, 07:21 PM   #26
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Baker,
I see the point. I've seen it for years. Cars get better millage in OD. No doubt about it. I just disagree about doing it. I think all boats should be propped to rated power rpm. And manufacturers agree.

On the other hand boats can probably be safely overpropped but nobody really knows how far the throttle can be advanced on an overpropped boat. Somewhere on the rpm scale is a nearly perfect load and rpm for the underpropped boat but it's a guess what rpm that is. So a huge chunk of operating range can't be used. And somewhere on the rpm scale the engine starts to become overloaded and again that point is not really known. EGT monitoring gets you close but installing the sensor in exactly the right place is problematic too.

There's a lot more to it than this post too. I think propping correctly avoids all these problems and unknowns.
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Old 10-11-2015, 10:43 PM   #27
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[QUOTE=manyboats;378133... It would be interesting to see all our boats cruise rpm as a percentage of max power rpm. It would not however indicate a percentage of engine load. I often have viewed it as X # of rpm down from max rated. But that's not apples and apples unless all engines considered have the same max power rpm.[/QUOTE]

Best to use percentage of maximum fuel consumption to measure load. My normal cruise (1800 out of 2400 RPM) is about 40 percent.
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Old 10-11-2015, 10:46 PM   #28
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Higher HP Diesel

Manyboats: Would you mind posting a link to one of the 110 times you've been through it so this latecomer might try to catch up? Before I spend money on more engine than I need or want or will ever be able to use? Sorry I missed it the other 110 times.
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Old 10-11-2015, 11:11 PM   #29
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Wanderin Star,
All 110 times are in the archives.
PM me w questions after you get into it up to your armpits.
How much power a FD boat needs is easy. But you'll probably be looking at SD boats. Pictures and e-mail may be required for that. Whenever you look at a boat try to discover how much power, size and perhaps displacement.

Then when you see that lots of trawlers w two engines were offered w one it gets confusing. You can bet that most trawlers w twins are overpowered and most other trawlers are overpowered as well. If you talk to most anyone on this forum they will think I think a rowboat is overpowered. But I don't think there's anybody on this forum that says they just don't have enough power. That leaves me thinking there must be many or most that have too much power. There are almost none that are underpowered. And my 40hp 8 ton boat is not one of them.


Mark,
My Mitsu develops it's power at 3000rpm. I cruise at 2300 .. 700rpm down. I cruise then at 76% of rated rpm. Note though that it's about exactly 50% engine load.
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Old 10-11-2015, 11:18 PM   #30
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definitely not overpowered:

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Old 10-11-2015, 11:21 PM   #31
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Mark is there a 50hp engine in that box astern?
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Old 10-11-2015, 11:32 PM   #32
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...Then when you see that lots of trawlers w two engines were offered w one it gets confusing. You can bet that most trawlers w twins are overpowered and most other trawlers are overpowered as well.
Those aren't "trawlers" (capable of exceeding hull speed) in the Chapman sense. They're fast cruisers.

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Old 10-11-2015, 11:42 PM   #33
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Mark is there a 50hp engine in that box astern?
I'm clueless. How does the typical Turkish fisherman equip his boat? And I'm not certain what his secondary engine is: the oars or a possible engine.
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Old 10-12-2015, 05:38 AM   #34
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"They do provide a prop demand fuel
curve but I haven't figured how to convert that to an efficiency curve."

That is because you cant.

ONLY a BMEP or fuel map will allow one to chose and match efficient engine speeds with desired rational cruise speeds.
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Old 10-12-2015, 07:10 AM   #35
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"They do provide a prop demand fuel
curve but I haven't figured how to convert that to an efficiency curve."

That is because you cant.

ONLY a BMEP or fuel map will allow one to chose and match efficient engine speeds with desired rational cruise speeds.
The prop charts for our engines show prop demand fuel burn and power required at each rpm. With a hull speed versus rpm chart (owner generated) one can then get a pretty good idea of what's going on in terms of efficiency for that hull/engine combination. Guess you could substitute prop chart data for other engines to get a ball park comparison.
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Old 10-12-2015, 08:54 AM   #36
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"With a hull speed versus rpm chart (owner generated) one can then get a pretty good idea of what's going on in terms of efficiency for that hull/engine combination.

Not really , it is fine (if its accurate ) to know what the burn might be at some chosen loading, but weather the the egine is at high or low efficiency (fewest GPH/HP is unknowable. ON PURPOSE.
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Old 10-12-2015, 10:47 AM   #37
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"With a hull speed versus rpm chart (owner generated) one can then get a pretty good idea of what's going on in terms of efficiency for that hull/engine combination.

Not really , it is fine (if its accurate ) to know what the burn might be at some chosen loading, but weather the the egine is at high or low efficiency (fewest GPH/HP is unknowable. ON PURPOSE.
Depends what you're trying to assess. It is what it is for a given engine in a given boat, and the only way to move the engine to another place is with a different prop or variable pitch prop. Prop performance data is not readily available either...I've looked. While not a design tool for a "paper" boat, what I described is a method to compare one engine to another in an existing hull. Accuracy of the prop chart for the installed engine can be judged by comparing actual fuel burn to the prop chart at a few key points. The chart for our boat is surprisingly accurate.
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Old 10-12-2015, 10:59 AM   #38
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[QUOTE=markpierce;378410]Those aren't "trawlers" (capable of exceeding hull speed) in the Chapman sense. They're fast cruisers.


Mark,
What does Chapman say about trawlers and cruisers?
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Old 10-12-2015, 11:15 AM   #39
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Wandering Star,
A good place to get an idea what existing boats use for power look at Yacht World. The information is however frequently supplied by the owners. You will see fuel burn quotes that are way too optimistic (low) and speeds that reflect the owners style rather than the boats capabilities.

Searching may work better using the expression "hp per ton" as I use that expression quite a bit. Obviously you can read more of what other people say about power needs than me. Sorting out all the opinions is typical of any forum.
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Old 10-12-2015, 11:17 AM   #40
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Actually it is not at all difficult to determine when a particular prop on a boat will overload the engine. The calculations are simple if a bit tedious. Here are the results for my boat, engine and prop.



The black line is the maximum power output for the engine as a function of engine rpms. The red lines (solid and dashed) are the propeller power curves for the boat for calm and rough water with my 20" diameter by 13" pitch three blade prop. The kink in the black line at about 1850 rpms is the point where the engine puts out its maximum torque (about 72 ft lbs). At 1850 the propeller curve is at about 6.2 hp and 16.5 ft-lbs of torque. The engine power curve (black line) shows that the engine can put out about 25 hp at 1850 rpms. So the boat requires a lot less than the power available at 1850 rpms.

If you increase the pitch of the prop (i.e., over prop the boat), the propeller curves (red lines) will move up and intersect the engine power curve at less than the engine's rated maximum rpms. In that case increasing the engine rpms above the intersection point of the engine and propeller curves won't generate any more speed and will make a lot of black smoke (for a diesel).

Basically, I don't see the argument for running at the rpms that correspond to the engines maximum torque output since over most of the engine's rpm range the horsepower and torque required by the prop will be considerably less than that which can be produced by the engine at full load.

Incidentally, the relationship between horsepower and torque (in ft-lbs) is:

HP = Torque x RPM 5252

Finally, it is trivial to calculate the fuel burn from the propeller curve. For my engine, fuel consumption is about 1 gallon per hour for every 16.8 hp required.For my boat the specific relationship is:

Gallons per hour = 0.05962*hp+0.054814

where hp is horsepower determined along the propeller curve.
That is determined from the engines fuel consumption curve, which is published for the engine. Even if you don't have the engines fuel consumption data, using 17-18 hp per gallon per hour of fuel consumption will get you withing 5% or so.
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