HP vs. RPM vs. Economy

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Apr 9, 2009
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?
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
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.

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.
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).
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
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.
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.

Eric Henning
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.
Charles in open water what do you run for RPM,s and what do you make for speed?
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


-- 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

baker wrote:



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.

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.*

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
dwhatty wrote:Baker wrote:AAAAAAAAAAAAAHHHHHHHHHHHHHHHHRRRRRRRRRRRGGGGGGGGGGHHHHHHHHHHHH!!!!!!!!3..........2..........1.........


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
Bernoulli is a factor in the generation of lift. It is not THE generator of lift. You can have lift without the Bernoulli effect. But I'm no expert--- I'm simpy quoting what NASA and our aerodynamicists say.

As to the Bernoulli effect with regards to hulls, logic would suggest that since a boat hull is normally curved identically on both sides the Bernoulli effect would not come into play here since the pressure on both sides of the hull would be equal, regardless of how it was reduced by the Bernoulli effect of the water flowing over the curved surfaces. If there is no pressure differential, there wil be no "force" acting on the hull. Whether the Bernoulli effect on each side of the hull reduces the resistance of the hull to the water I have no idea. You'd need to find a hydrodynamicist for that.

But whether it does or doesn't, at 8 knots and 27,000 pounds it's not going to make a g*ddamned bit of difference to my boat or my fuel bill.
Marin wrote:Whether the Bernoulli effect on each side of the hull reduces the resistance of the hull to the water I have no idea.
In the pressure domain surrounding a hull there are high pressure areas forward and aft and a large low pressure area midships. This is what causes "squat" and "bank effect."*

Squat is the phenomenon of the draft increasing with speed in shallow water. Bank effect is the tendency of the ship to yaw rapidly when passing close to another ship or the bank of a river or canal. It is what really screws people up when trying to rapidly motor out of the Ballard Locks and they appear to be stuck to the sidewall.

The effect doesn't suck the boat forward and reduce the power requirement, just the opposite, it takes more power since the draft increases. In a large ship, you can tell from the depths of the engine room when the ship has entered shallow water as the vibration increases a great deal and the power output increases to maintain the same shaft revolutions.

"But whether it does or doesn't, at 8 knots and 27,000 pounds it's not going to make a g*ddamned bit of difference to my boat or my fuel bill."

It would if you ran around in shallow water all the time. You might not care and it might not show up as a big increase in fuel consumption but it would make a difference.


-- Edited by RickB on Thursday 7th of January 2010 06:44:21 AM
From operating narrowboats in the UK for many years I'm well acquainted with what a hull does in very shallow water since the clearance between the bottom of the 60', 20-ton boat we use and the canal bed is a couple of feet at best and often less. In fact the technique for not running aground when forced out of the middle of the dish-shaped channel is to slow down to reduce the "squat." However I was told-- perhaps erroneously-- that the stern squat of these boats when more power is applied is due to the propeller pulling water out from under the hull. Reducing the power reduces the amount of water being moved back by the prop and the stern will come up a few inches. In fact when powering the boat around in a tight turn with almost no forward speed, adding a burst of power to put a lot of thrust against the rudder also pulls the stern down enough to see and feel.

When these boats meet and pass, the width and contour of the canal usually requires them to pass very close--- perhaps about two or three feet apart. I have never noticed a tendency for the boats to be pulled together until they get about halfway past each other when the bow of each boat is pulled in by whatever the force is just beyond the stern of the other boat. You have to anticipate this and be ready to counter this inward yaw with the tiller.

Anyway, interesting forces at play here--- I know how to deal with them on a narrowboat but I don't really understand the dynamics that create them.

-- Edited by Marin on Thursday 7th of January 2010 11:45:50 AM


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