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Old 01-01-2010, 05:35 PM   #1
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HP vs. RPM vs. Economy

Hi Guys,


Still looking for that illusive boat. So far Ive only looked at 3, so Ive got a ways to go, but Im getting a little more knowledgeable with each viewing.


So far I havent considered boats with bigger engines, like the Sea-Rays and Vikings because of fuel economy. But this has gotten me to thinking; does a boat with big engines operating way below its max operating speed really use more fuel than a similar displaced boat operating at the same speed, but with smaller engines operating at its cruising speed. For example: Consider a 36 Island Gypsy, 22,000 lbs with *twin 135 HP diesels running at 8 kts, probably at about 2000 rpm. Compare that with a 40 Sea Ray, same weight with twin 450 hp diesels running at 8 kts, at probably around 1100 rpm.

My guess is the Sea Ray will use more fuel because those massive engines are slinging a lot more internal iron, but how much more? I know an exact answer is not possible because there are many variables, especially the design of the hull, but can anybody give a general rule of thumb on this?


Heres my guess, but with no data or experience to back it up; Big diesel engines (450 hp) operating at just above idle to 1100 rpm are pretty economical, perhaps very close in fuel flow to a smaller engine operating at cruising speed. Above this speed though the fuel use on the big engine goes up astronomically along with just a modest increase in boat speed. *The smaller diesels are pretty economical all the way up to around 80% WOT which should result in a boat speed of around 8-10 kts in my above examples.
Anyone care to chime in on this.


Part 2:
Does continuously operating a diesel engine well below its normal operating rpm do permanent damage?
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Old 01-01-2010, 07:06 PM   #2
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RE: HP vs. RPM vs. Economy

Hey TJ,
Your part 2. Wev'e beat that one into the ground over time. See the old posts. I think so but I'm not totally convinced and others don't think so and I'm sure they aren't totally convinced. Two things will be well worth considering. Very few of us have known of engines that have died from underloading (I have) but on the other hand all modern and most all older diesel engines can operate happily, dependably and economicaly at very high levels of loading (in the range of 80-90%). I think a diesel engine should be run around 70% load.
Heavily loaded gasoline and diesel engines are about the same in efficiency but lightly loaded diesels are much more efficient than lightly loaded gas. The difference is highly related to fuel mixture over a range of load.
Big engines loafing are quite pleasant but less efficient for two main reasons. There is much more friction in the large engine (even turning higher rpm) but most importantly there is much or much much more surface area to disipate* heat. And since wer'e talkin about heat it should be remembered that heat loss is directly related to efficiency. Tough questions but there are many known facts. Good luck with it.
Eric Henning
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Old 01-02-2010, 05:09 AM   #3
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RE: HP vs. RPM vs. Economy

If you wish to have a hugely powerful engine and maintain efficiency at high idle GASOLINE is a better choice than any diesel..

The problem is not only the inefficiency of a huge engine doing nothing it is also a hull design that is not efficient at crawl speeds .

Poor ride, as well as poor economy.

Decide what TYPE boat you want and go for it.

Hammers make very poor screwdrivers.

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Old 01-02-2010, 12:45 PM   #4
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RE: HP vs. RPM vs. Economy

I agree with FF, the fuel efficiency is dependant on the hull type, full, semi or planing.* Then the engine/engines should be correctly sized to the type of hull.* *A single is usually more efficient than twins even if the total HP is the same.* So decide on the lay out/design you like, pilot house, tri cabin, sun deck, then hull type/brand name*and that will decide the engine HP, RPM and efficiency.***

Your sail boat hull and engine is probable close that meets those requirements.* The fast you want to go over hull speed the more the house power, faster the*rpm*and high fuel consumption.* Most trawler are semi displacement weather single or twin engine.

Our trawler is 58 ft, 86,000 lbs, power by a single 671 DD 165 hp, 8 to 10 knots at 1500 rpm*so it does not take much power to push a boat throught the water at hull speed.
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Old 01-02-2010, 01:44 PM   #5
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RE: HP vs. RPM vs. Economy

Just an anecdote, so draw only the conclusions you want from it:

145 hp twins designed in the 70s, used to give me 5 gph at 7.5 knots, 2750 rpm: 1.5 mpg.
In the same boat, I switched up to 200 hp twins, designed in the 80s, and increased my speed to 8.2 knots, repitched the props to get my rpm down to 2150, and I now get under 4 gph, ie better than 2 mpg.

From that you might conclude that more hp, loafing, is more economical. Or not, as other design advances incorporated in the newer engines have made a significant difference in overall performance. If I am now suffering inefficiencies from under performing at low speed, those losses are hidden in the bigger improvements in performance from the newer design.

So check on the design of the engines. If older design, they will typically have much lower injection pressure, which by itself isn't a good indicator of economy, but goes with other advances that do give good economy, and less smoke.

You may find boats with great engine choices and lousy hull shape that give the same economy as ones with great hull shape driven by the wrong engine(s).
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Old 01-02-2010, 04:42 PM   #6
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RE: HP vs. RPM vs. Economy

Koliver,
Two 145hp engines at WOT (and at 2750rpm you must be at WOT) should burn about 13gph. Why did yours burn only 5?

Phil fill
Yes. FF is right. Most of the efficiency of most boats like ours is mostly dependent on hull shape. If you had a GB the same displacement as yours your 165hp DD wouldn't come close to the performance you enjoy. On the flip side if you had a modern Deer in your boat you'd enjoy lower fuel burn.

Eric Henning
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Old 01-02-2010, 07:13 PM   #7
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RE: HP vs. RPM vs. Economy

Eric:
WOT was something like 3600. And no I never did WOT except to test for black smoke. Why would anyone want to burn 13 gph when you could get where you are going comfortably at much less? Or to put it another way, if I wanted to burn 13 gph I wouldn't have a trawler, I would have a planing boat.
WOT only got me 10 knots. With 200 hp each, WOT (still 3600) now only gets me 10.5 knots. I can do 8.2 for less than 1/3 of the cost of 10, so that is what I do. I occasionally push the throttles ahead and check the smoke, but after 15 years of going slow, problem free, and watching my friends with their faster boats with costly engine repairs, I will remain of the view that excess hp to my needs is a good thing.
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Old 01-02-2010, 08:56 PM   #8
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RE: HP vs. RPM vs. Economy

Koliver,
Ye gods Koli what engines do you have that turn 3600? Unlike most on this forum I have no problem w high reving engines but I assumed you had Lehmans or equivalent.* Sorry. Without the info one tends to assume. Not good.

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Old 01-02-2010, 10:01 PM   #9
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RE: HP vs. RPM vs. Economy

Tamd41 Volvos
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Old 01-03-2010, 05:00 AM   #10
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RE: HP vs. RPM vs. Economy

Thanks Guys for your comments. I've gotten really good comments on this forum and it has been my primary source of info.
From what I've read so far, it seems that buying a boat correctly powered to cruise at trawler speeds is more fuel efficient than buying a boat thats designed to plane and operating it at trawler speeds. That answers my question.

FF sums it up nicely: hammers make poor screwdrivers.
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Old 01-05-2010, 03:02 PM   #11
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RE: HP vs. RPM vs. Economy

Charles in open water what do you run for RPM,s and what do you make for speed?
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Old 01-06-2010, 01:02 PM   #12
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HP vs. RPM vs. Economy

I have a full displacement hull. When I kick up the RPM's my bow rail goes down. *i.e. I am making a bigger hole in the water.*
****
It is like the Bernoulli's effect As demonstrated with a spoon and a stream of water.* Say from a faucet. If you hold the back of the spoon under the stream of water. *Rather than knocking the spoon away it sucks it in as the pressure of the water flows around the camber of the back of the spoon. creating lift. The same way a wing generates lift* So in effect this sort of helps pull the hull thru the water.
******
*As you reach that sweet spot where your hull speed matches this effect . You have optimal performance reguardless of the RPM.* If you are pushing more than hull speed you are just burning fuel and fighting the physics of fluid motion.

Hey streighten me out here if I am wrong. I have been wrong before.* once

SD*

-- Edited by skipperdude on Wednesday 6th of January 2010 02:13:11 PM

-- Edited by skipperdude on Wednesday 6th of January 2010 02:19:59 PM
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Old 01-06-2010, 03:06 PM   #13
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RE: HP vs. RPM vs. Economy

Aaaaaaaaaaaaahhhhhhhhhhhhhhhhrrrrrrrrrrrgggggggggg hhhhhhhhhhhh!!!!!!!!



3..........2..........1.........
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Old 01-06-2010, 03:27 PM   #14
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RE: HP vs. RPM vs. Economy

Quote:
baker wrote:

aaaaaaaaaaaaahhhhhhhhhhhhhhhhrrrrrrrrrrrgggggggggg hhhhhhhhhhhh!!!!!!!!



3..........2..........1.........
x2

*
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Old 01-06-2010, 03:30 PM   #15
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RE: HP vs. RPM vs. Economy

What?
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Old 01-06-2010, 04:44 PM   #16
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RE: HP vs. RPM vs. Economy

Quote:
skipperdude wrote:

The same way a wing generates lift*
Not going to get into this discussion here, but just to correct you, a wing does not generate lift because of the Bernouli effect.* It generates lift because of Newton's law of action and reaction.* You can PM me or OTDE if you want to discuss this farther.

*
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Old 01-06-2010, 07:27 PM   #17
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RE: HP vs. RPM vs. Economy

Ok. So I was really wrong on this one. it is not lift that is generated. the annalogy of a wing was way off. The Bernoulli effect deals with fluid not air.* Forces are still at play. The curvature of the hull has it's own effect on the way it moves thru the water.*

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Old 01-06-2010, 08:46 PM   #18
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HP vs. RPM vs. Economy

Air is a fluid.


"A fluid is a substance that continually deforms (flows) under an applied*shear stress. All gases are fluids, but not all liquids are fluids. Fluids are a subset of the phases of matter and include liquids, gases,*plasmas, and, to some extent, plastic solids."


-- Edited by Marin on Wednesday 6th of January 2010 09:51:51 PM
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Old 01-06-2010, 09:24 PM   #19
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RE: HP vs. RPM vs. Economy

Quote:
dwhatty wrote:Baker wrote:AAAAAAAAAAAAAHHHHHHHHHHHHHHHHRRRRRRRRRRRGGGGGGGGGG HHHHHHHHHHHH!!!!!!!!3..........2..........1....... ..
X2
DITTO>>>>>>>>>>>>>

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Old 01-06-2010, 09:24 PM   #20
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HP vs. RPM vs. Economy

Marin:

There are 3 phases to consider, gaseous, liquid and solid. Is there such a thing*as a fluid phase? *And Marin, do*you mean Newton's second law where f=ma, or another Newton law? Does not an airplane require all 3 Newton laws to successfully take off, fly over the ocean and land? And how would you describe, using Newton's laws, *the lift a hydrofoil develops? So back to the teaspoon effect alluded to by our friend from the frozen North, don't dismiss it too quickly as it pertains to air and hydro foils. And to add confusion, Bernoulli is alive and well regarding wings.**The Bernoulli Principle can indeed be used to calculate the aiflow over a foil. Please forgive me for my ignorance, I did not go to forestry school in CO.

-- Edited by sunchaser on Wednesday 6th of January 2010 10:34:13 PM
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