"Overpowering?"

[RyanCT]

Curious. I can live with an 18 knot cruise but of course would love to cruise at 20+ on given days. So I am curious if a known hull cruises at say 18 knots with 370hp how does a higher hp engine run at that speed?

For example if you install a 480-500 hp engine in the same hull cruise speed will likely increase along with gph. But when the throttle is pulled back to 18 how do the 2 compare? I can answer 3 ways and justify each but dont know what is right?

1. The 370hp engine is more efficient because it is just not as thirsty as the bigger beast.
2. They are the same because it takes x gallons of fuel to make y amount of power regardless of the powerplant.
3. The larger engine is more efficient because of the lower load.

I guess I am asking if you install a higher hp engine do you pay for it everyday you use it? Or just on the upfront costs and maintenance. Appreciate any insight.

 

[GLA]

one issue I think of right away is that you loose your ability to go slow without trolling valves. yes the larger engine will work easier and last longer running at lower rpm. yet it has to be propped right to run full rpm at load , which will then affect your low end.

there may be a happy medium with an engine that develops most of its torque at lower rpm. say 1500 . then flat out at 2200, would make sense to run that at 1500 and use the torque available to you. then if you have a nice day run it at 2000 and move along.

 

[Brooksie]

No question the 370 is much more efficient at 18k. A diesel at near full power can be as much as 20% more efficient than one at 1/2 power. I can give exact #s if you wish.

Also and just as important the propeller on the 370 is able to use 370 the propeller on the 500 is designed to use 500 so at 370 the propeller hp curve is way away from the available hp curve.

Frictional and pumping losses in a 500 hp engine (and running gear) are there whether or not you are using 500 hp. It takes hp to turn a prop with no pitch or a shaft with no prop and the bigger the more hp

 

[captainlarry84]

This post is a little confusing. However IMHO power the boat for your needs. underwheeling and overpower does a diesel no good. You need to tach out a diesel boat once a day to full load to keep everything working correctly.

If you need a 370 Yanmar & put in a 440 Yanmar and under wheel it to use the same amount of HP. Your fuel burn will be the same, but your motors maintainces will not be. Things like slow turbos, carbon build up in the heads will occur. You need load for diesels to perform correctly.

 

[lobstercatcher]

Curious. I can live with an 18 knot cruise but of course would love to cruise at 20+ on given days. So I am curious if a known hull cruises at say 18 knots with 370hp how does a higher hp engine run at that speed?

For example if you install a 480-500 hp engine in the same hull cruise speed will likely increase along with gph. But when the throttle is pulled back to 18 how do the 2 compare? I can answer 3 ways and justify each but dont know what is right?

1. The 370hp engine is more efficient because it is just not as thirsty as the bigger beast.
2. They are the same because it takes x gallons of fuel to make y amount of power regardless of the powerplant.
3. The larger engine is more efficient because of the lower load.

I guess I am asking if you install a higher hp engine do you pay for it everyday you use it? Or just on the upfront costs and maintenance. Appreciate any insight.

The higher hp engines is supposed to be more efficient at the lower usage rpms. However, depending on the hull, it may have a hull speed. I remember a owner of 2 boats of the same builder. One had a 671 and the other had a 1271,, the 671 was the faster boat and used less fuel

 

[BillD]

Fuel burn and horsepower.

A "rule of thumb" or the "magic number" is 20 hp/gallon/hr.

A modern 4-stroke, direct injected, turbo charged/after cooled high speed diesel of modern design will consume 15 gallons per hr to produce 300 hp.

Doesn't matter if it's a 300 hp or a 600 hp motor. Mechanical or electronic diesel both will consume the same +- a % or two 15 gals/hr. to produce 300 hp.

Larry brings up a good point.

Put a 370 hp engine in a boat the requires 370 hp 90% of the time to accomplish what you want in the boat then plan on a new motor in a short time.

Bolt in a 500 hp engine to "get the 370" you need to go boating and you'll have a longer lasting engine.

Either way if you by any diesel engine regardless of hp the engine will burn 15 gals/hr to produce 300 hp.

A gasoline engine will consume more than 15 gals/hr to produce 300 hp.

Diesels are about 20% more efficient than gasoline engines.

That's a simple thermodynamic fact.

 

[F/V First Team]

If that 500 weighs considerably more than a 370 then you will need more power to pick that bow up to counteract this additional weight and the resulting wetted surface.

*shrug*

 

[Brooksie]

Fuel burn and horsepower.

A "rule of thumb" or the "magic number" is 20 hp/gallon/hr.

A modern 4-stroke, direct injected, turbo charged/after cooled high speed diesel of modern design will consume 15 gallons per hr to produce 300 hp.

Doesn't matter if it's a 300 hp or a 600 hp motor. Mechanical or electronic diesel both will consume the same +- a % or two 15 gals/hr. to produce 300 hp.

Larry brings up a good point.

Put a 370 hp engine in a boat the requires 370 hp 90% of the time to accomplish what you want in the boat then plan on a new motor in a short time.

Bolt in a 500 hp engine to "get the 370" you need to go boating and you'll have a longer lasting engine.

Either way if you by any diesel engine regardless of hp the engine will burn 15 gals/hr to produce 300 hp.

A gasoline engine will consume more than 15 gals/hr to produce 300 hp.

Diesels are about 20% more efficient than gasoline engines.

That's a simple thermodynamic fact.

You may be right about the longevity of a 500hp engine putting out 370 although diesels don't like to be grossly underloaded all the time.
But on the efficiency issue, and that was the question, you are incorrect. The 20 hp/gal/hr that you quote an ever so rough rule of thumb which may be close enough for rough sizing a fuel tank or something like that. It is a fact that a lightly loaded engine burns more fuel to make a given HP than a properly loaded engine. The additional frictional losses for the larger running gear are very real too.

 

[BillD]

You may be right about the longevity of a 500hp engine putting out 370 although diesels don't like to be grossly underloaded all the time.
But on the efficiency issue, and that was the question, you are incorrect. The 20 hp/gal/hr that you quote an ever so rough rule of thumb which may be close enough for rough sizing a fuel tank or something like that. It is a fact that a lightly loaded engine burns more fuel to make a given HP than a properly loaded engine. The additional frictional losses for the larger running gear are very real too.

Brooksie,

With all due respect,

BSFC (brake specific fuel consumption) is an accepted and universally used measurement for gauging power output in relation to fuel consumed. Typical units used by Cummins, Caterpillar, Yanmar, Isuzu, etc. would be lbs/hp/hr or grams/kW/hr. A standard wt for #2 diesel might be 7.001 lbs per gallon at 60 degrees F.

Pretty much all the diesels in marine service today and over the past 20 years fall in to a BSFC ranges of .450 to .325 (lbs/bhp/hr.)

With a little math, one can derive the "magic number" of 20 hp/gallon/hr.

How do you figure..........
"It is a fact that a lightly loaded engine burns more fuel to make a given HP than a properly loaded engine."

The fact is it takes X amount of fuel to produce X amount of hp.
Gears are load rotational load, pushing a hull is load, etc. etc.

 

[Brooksie]

Brooksie,

With all due respect,

BSFC (brake specific fuel consumption) is an accepted and universally used measurement for gauging power output in relation to fuel consumed. Typical units used by Cummins, Caterpillar, Yanmar, Isuzu, etc. would be lbs/hp/hr or grams/kW/hr. A standard wt for #2 diesel might be 7.001 lbs per gallon at 60 degrees F.

Pretty much all the diesels in marine service today and over the past 20 years fall in to a BSFC ranges of .450 to .325 (lbs/bhp/hr.)

With a little math, one can derive the "magic number" of 20 hp/gallon/hr.

How do you figure..........
"It is a fact that a lightly loaded engine burns more fuel to make a given HP than a properly loaded engine."

The fact is it takes X amount of fuel to produce X amount of hp.
Gears are load rotational load, pushing a hull is load, etc. etc.

Respect what you are saying but by your own numbers:
7/.325 = 21.5 hp/gal/hr
7/.450 = 15.5 hp/gal/hr
this is a 35% difference, that's a very rough rule of thumb and not what I am talking about.

The questioner was asking about the efficiency of running a 500hp engine @ 370hp vs: a 370hp engine @ 370hp. Here there are large SFC differences also; in favor of the properly loaded engine. Look up your favorite engine's propeller load specific fuel consumption at 50% load and 90% load.

As to the running gear load, think about it, how many hp does it take to turn a transmission, shaft, & propeller sized for 500hp vs: ones for 370hp. Answer: more and wasted if you are not going to use the hp.

 

[lobstercatcher]

If that 500 weighs considerably more than a 370 then you will need more power to pick that bow up to counteract this additional weight and the resulting wetted surface.

*shrug*

Specially in older boats and engines. Weights of engines had considerable differences for not much hp difference not long ago.

 

[BillD]

Respect what you are saying but by your own numbers:
7/.325 = 21.5 hp/gal/hr
7/.450 = 15.5 hp/gal/hr
this is a 35% difference, that's a very rough rule of thumb and not what I am talking about.

The questioner was asking about the efficiency of running a 500hp engine @ 370hp vs: a 370hp engine @ 370hp. Here there are large SFC differences also; in favor of the properly loaded engine. Look up your favorite engine's propeller load specific fuel consumption at 50% load and 90% load.

As to the running gear load, think about it, how many hp does it take to turn a transmission, shaft, & propeller sized for 500hp vs: ones for 370hp. Answer: more and wasted if you are not going to use the hp.

Yes I agree there is a "swing" in the numbers. The newer diesels are a bit more efficient than the old mechanical motors.
I suppose in the end you can "buy extra hp" and not run the engine on the edge all the time or buy a smaller engine and run it on the edge all the time.

Good discussion, good info

FWIW, Bill D

 

[lobstercatcher]

Yes I agree there is a "swing" in the numbers. The newer diesels are a bit more efficient than the old mechanical motors.
I suppose in the end you can "buy extra hp" and not run the engine on the edge all the time or buy a smaller engine and run it on the edge all the time.

Good discussion, good info

FWIW, Bill D

when I had the NB built. JH was reviewing the numbers,

Volvo 330vs 410 susi , different boat applications.

About the best I can do is 7mph @ 1gph
He claimed a nb38 could get up in the 7mph using .5gph with the 410.
Major difference in some applications on fuel consumption.

I guess I fall into the buy the extra HP . That is probably why I have no problem with the high rev Volvo that so many don't care for. I rarely use the Volvo inconsistent with way people use the slow turners. I have those rpms for emergency if needed.,not for normal commercial rated usage.

 

[F/V First Team]

We were talking to a couple who were considering building a 36' trawler style boat with twin 400 hp engines. Tried to explain why a 42' downeast was a better idea with a single 800 hp engine.

The husband was all heffered up because the trawler was supposed to cruise at 10 knots and have a wide open speed of 12 knots and we were telling him that he could get that at a high idle with the downeast.
"What the hell is all that other power for then?" he cried out.
"Emergency" was our truthful answer, since we were just keeping the original. horsepower requirements that he had laid out.
When his wife said "Oh that's a good idea" everything went south.

Apparently he didn't like being reminded that he had a health condition.

Some people man.

She was a really nice lady, showed her a picture of the boat and she said "Honey, this one is much better looking" I think I know who had the brains in that relationship.

 

[BillD]

Great discussion, this is what the forum is all about !!

 

[jerseysportfisher]

Great discussion, this is what the forum is all about !!

Bill D

I'm a little disappointed in you. This thread you could have had a perfect rebuttle to travis's (extra weight with more HP) argument by inserting the infamous V-drive clause

:-D

 

[Brooksie]

Not to beat a dead horse,

Found a few numbers on propeller load specific fuel consumption:
Cummins 6BT lbs/hp/hr .430 @ 2000 96hp, .408 @ 2200 127hp, .399 @ 2400 165hp, .397 @ 2600 210hp.

In this case, running at 1/2 the max hp uses 10% more gal/hp/hr with the same engine. Doubling the size of the engine then running at 1/2 load would be much worse, maybe 20% more fuel.

 

[plowin]

When I was considering an engine for my boat I was looking at an 6.7 cummins at 480hp or the 8.3 at 500hp. I did go with the 8.3 and the marginally higher horsepower but it was not for the "extra hp" reason. Most of the new diesels (under 8 litres) seem to be high rpm, 3000+ engines with a block that is not sleaved. Additionally the horsepower means very little, what is most important is torque and at what rpm it is delivered. In the case of 6.7 compared to the 8.3 the horsepower difference was only 20 but there is nearly 400 lbs of torque separating the two.
Just another spin, pun intended, on the thread.

 

[Brooksie]

When I was considering an engine for my boat I was looking at an 6.7 cummins at 480hp or the 8.3 at 500hp. I did go with the 8.3 and the marginally higher horsepower but it was not for the "extra hp" reason. Most of the new diesels (under 8 litres) seem to be high rpm, 3000+ engines with a block that is not sleaved. Additionally the horsepower means very little, what is most important is torque and at what rpm it is delivered. In the case of 6.7 compared to the 8.3 the horsepower difference was only 20 but there is nearly 400 lbs of torque separating the two.
Just another spin, pun intended, on the thread.

All good stuff. I'm with you on the lower RPM diesels. Seem to last longer and are much better for your mind during a long days steaming.
Torque and RPM are the factors that = horsepower. So looking at one w/o the other is meaningless. But if you are looking for power at a specific RPM then torque tells all.

 

[Powderpro]

Getting a true 18 knot cruise from a 370hp engine is going to require the boat be on the lighter side/smaller side. I imagine there aren't a lot of downeast boats used as cruisers or serious sportfishing boats that can do an honest cruise at 18 knots with only a 370hp engine, unless they are in 27'-31' range. From my experience, a 34'-38' downeast boat with only 370hp is not going to truly cruise at 18-20 knots. There are probably a few light weight, bare bones, not much fuel in them, not much amenities on them boats that size, that might get a cruise of 18 knots with 370hp, but those are very few and far between.

So you said you want/need an 18 knot cruise with the ability to bump it up a bit and cruise at 20 knots. In my opinion, "cruise" is where the engine is somewhere between 55-72% load. It's a load that you can run for multiple hours without putting undo strain or stress on the engine. What size of boat do you want, what kind of amenities/finish, what kind of gear, and how big of a crew will you have?

If you want a 30' Calvin Beal, built light, nice but basic finish, not a ton of gear, and say 4 adults, 175 gallons of fuel, and you want an 18 knot cruise, then I would say a 370hp engine would give you the cruise you want and a 500hp engine would be overkill.

If you want/need a 38' Flowers/Wesmac/NB/Lowell Brothers, finished comfortably, 4-6 adults, 250 gallons fuel, some nice amenities, then a 370hp engine will not give you a 18 knot cruise. In the case of the bigger boat, you would need the 500hp to give you the cruise you want.

Before deciding on HP, choose a hull you want/need, decide the level of finish, know what the approximate real world weight of the boat will be, and then you will have the information necessary to choose the correct engine.