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Old 11-16-2019, 04:15 PM   #1
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Optimal Engine(s)

Trying to make sense of the wide range of engines I am seeing in trawlers of what looks like similar size and weight.

Is there any way to estimate how much power it will take to move a boat thru the water at 70 or 100 percent of hull speed?

Is there a listing of the BSFC, Brake Specific Fuel Consumption for likely trawler engines?

Seems the optimal engine for a trawler would be developing maybe 50 to 75 percent of rated horsepower at cruise speed. This would provide extra power if needed and not work the engine hard enough to reduce life expectancy. I would think oversize motors just add weight, take up room and never get into their optimal operating range.

This is for doing the Great Loop so I donít expect huge storms or tidal surges. I just want to go far and have money left for other things.

Thanks
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Old 11-16-2019, 04:40 PM   #2
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Originally Posted by AdkChris;821155.
Is there a listing of the BSFC, Brake Specific Fuel Consumption for likely trawler engines. Thanks
Try boatdiesel.com in the engines section. Best source of data for a wide variety of marine engines I've found.

My view is that one's ideal engine preference gets tempered real quick by what is commonly installed by the builder and available in the market place you're shopping.
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Old 11-16-2019, 04:46 PM   #3
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Boat diesel also had a calculator that will tell you how much HP is required to push a boat any given speed. You enter hull type, LWL, displacement,and push the button.

For BSFC, a reference point is that my Scania runs between 190-200 grams per KWh. The consumption is remarkably flat across the power range, making optimizing the operating point for the sake of fuel consumption a foolís errand.
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Old 11-16-2019, 04:52 PM   #4
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Turbodiesels tend to have a pretty flat efficiency curve except at very, very light loads. N/A diesels can come close, but tend to see the light load efficiency drop off a little more. Gas engines, on the other hand, have pretty terrible light load efficiency relative to what they do at higher loads.
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Old 11-16-2019, 05:45 PM   #5
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About 20 hp hours per gallon for modern 4 strokes. Brand doesn’t matter
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Old 11-16-2019, 09:23 PM   #6
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Joined boat diesel, as a guest for now.
Looks very interesting so far.
Not sure yet what level I have to commit to to get the info I am looking for.
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Old 11-16-2019, 09:29 PM   #7
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Joined boat diesel, as a guest for now.
Looks very interesting so far.
Not sure yet what level I have to commit to to get the info I am looking for.
$25 gets you to level 1 and the engine data.
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Old 11-16-2019, 09:36 PM   #8
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George Buehler had some calculations in his book. The numbers were much lower than you’d think. 30-50 hp for a 40 footer. I believe reserves are necessary to go into sea/weather.

As an example, my 54 footer has 143 hp with a displacement hull and it’s almost overkill.
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Old 11-16-2019, 10:13 PM   #9
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We have 54hp x 2 (NA) for a 50,000lb full displacement boat. Perfect for our needs.
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Old 11-16-2019, 11:37 PM   #10
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My <50,000 pound semi displacement boat, 45' water line length (50' LOA) goes 7 knots at 2 GPH (40 HP), 8 Knots at 3.5 GPH (75 HP). The 135 HP 4 cylinder John Deere does a good job.

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Old 11-17-2019, 07:46 AM   #11
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"Is there any way to estimate how much power it will take to move a boat thru the water at 70 or 100 percent of hull speed?"

The hard part is finding the total weight of the vessel.Then its 2 to 3 HP per ton( 2240 lbs) for normal cruise , 50%to 100% more for "hull speed".

"Is there a listing of the BSFC, Brake Specific Fuel Consumption for likely trawler engines?"
Yes and no. For larger engines used in big gen sets, pumps and as propulsion engines , no problem.
Power output levels per hour are also available.

At the power level required for displacement boats , probably NO.

Purchasing a custom new build you have engine choices , for a used boat engine repair costs would be the concern.

The small engines are sold based on wholesale cost to the boat assembler.

A 20% difference in fuel burn does not come up to much in a 40HP engine used 200hours a year.
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Old 11-17-2019, 11:25 AM   #12
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I’ve always used hp per ton of displacement.

Many of the numbers above in this thread are for FD boats. For SD trawlers you’ll need to come close to doubling the standardized 3hp per ton power to about 5.5hp per ton of disp. But if you look at sailboats it’s clear that 3hp per ton is in excess. Most sailboats are powered by less. That’s because there are many different variations of FD hullforms. My Willard as a rec-trawler fits nicely into the 3hp per ton category however she’s a bit overpowered w about 40hp (5hp per ton). But she’s only overpowered re the not very well specified general hull type called full displacement. I’ve seen some larger vessels w 2hp per ton. One recently w 1hp per ton and a Great Lake ore boat may be propelled by as little as 1/2hp per ton of disp.

Typically a SD rectrawler will have 7hp per ton ... and up. Some SD hulls are very close to FD and some SD hulls are close to planing hulls. A lot of quite different hulls all in one category. There’s very little specifics and almost all grey zones of hull/speed definitions. The way I have evolved into a useable identification protocol is to make hundreds and more observations of hull variations and the amount of power installed in the vessels to expand my concept of power required for a given hull shape and displacement. Many many known examples over time results in the ability to be able to predict power requirements.

But again the whole range of power requirements is clouded w “mostly grey zone”. One can’t be fully specific about subtle differences in hull shape. But one can get close enough to be good enough for most practical purposes. Perhaps a sculpturist like my mother would have the best eye for identifying variations in forum. But many numbers like hull speed and prismatic coefficient and all the comparison ratios combined get us to a place where the grey zones are minimized.
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Old 11-17-2019, 11:47 AM   #13
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AdkChris wrote;
“ Is there any way to estimate how much power it will take to move a boat thru the water at 70 or 100 percent of hull speed? “

Be informed that most all FD boats cruise at a knot below hull speed. And most that are deep into the FD category are incapable of reaching hull speed unless overpowered. Only FD boats that are very significantly overpowered can get up to hull speed. I don’t know if my 30 foot Willard is capable of her hull speed of 7 knots. I know I can go well over 6.5 knots continuously but to know exactly what my top speed is has almost no value. Knowing what engine speed is tops at WOT is very important to me and I always know that rpm. But seawater is always subject to tidal currents and presents a challenge to speed estimates. I’ve done the two way estimate many times but it’s not very accurate. There use to be a knotical Mile marked off on the Lake Washington bridge in Seattle but I’ve not managed to get there and the marks are not there anymore.
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Old 11-17-2019, 11:59 AM   #14
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For a boat to move through still water and air, hp needed is quite low. Once you throw in sea state, windage, driveline losses, hang belt driven accessories on the engine, power requirements go up. The 45' boat I'm building only needs 80hp to move around. I'm adding hydraulics, air compressor, at least one big alternator and possibly two. I've nearly doubled the hp needed to run my boat and drive it's accessories. So there's really no one right answer unless you work out the math for each specific boat and its set up. Most people, including manufactures, put in more power than need just in case the owners add on power consuming accessories.


Also, make sure you're comparing similar hulls. A displacement hull doesn't need much power at all. A planing hull needs crazy amounts of power to get the boat up and out of the water to plane effectively. A semi displacement hull can go either way. They need tons of power to kind of semi plane.
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Old 11-17-2019, 12:22 PM   #15
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I know I can go well over 6.5 knots continuously but to know exactly what my top speed is has almost no value.
I'd say it's somewhat useful to know, as knowing you can push for 7 kts in a pinch instead of 6 is a significant difference if you want to go somewhere with a 4kt opposing current to get in or out, for example. And knowing how much extra fuel it takes to push for that last knot also determines if it's worth it. In the 4kt current example, if you burn less than 50% more fuel to go 7kts instead of 6, you'd be better off going for 7. If it's more than 50% more, 6kts would be better (as you're making 50% better speed over ground, 3 kts vs 2).

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Also, make sure you're comparing similar hulls. A displacement hull doesn't need much power at all. A planing hull needs crazy amounts of power to get the boat up and out of the water to plane effectively. A semi displacement hull can go either way. They need tons of power to kind of semi plane.
And even within the realm of planing hulls, amount of power needed can vary widely depending on the hull and weight.

At one end of the spectrum, we have my boat, which even at a little over 25k lbs (fairly heavy for a planing boat of its size) will make 27 - 28 kts flat out with 680 total hp. But it's a fairly low deadrise hull that makes a good bit of lift, rather than digging a big hole in the water in exchange for better wave smashing performance.

And then at the other end, we have some of the sport fishes with draggy deep V hulls for pounding through rough water. A similar vintage 38 foot Hatteras sport fish, for example, at 32 - 33k lbs (so very heavy) only does 25 kts with 900 total hp.

If I estimate the hull constant of my hull at 175 based on its known performance and then turn up the weight to match the Hatteras, estimated top speed drops into the 24.5 kt range. But that's with 220 less hp than the Hatteras needs for about the same speed at the same weight. Not a small difference by any means (about 32% more power to move the same weight at the same speed).

I intentionally ignored express cruisers in that comparison, as by their nature, they tend to be much lighter weight and often narrower. My dock neighbor has a 40ft Formula express cruiser, so similar in overall size to mine other than his beam being 3 feet narrower. His boat is 6100 lbs lighter in dry weight with a bigger difference when loaded (I carry an extra 170 gallons of fuel, a bit more water, etc.). So figure loaded, the weight difference is probably more like 7500 lbs.
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