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Old 30-12-2019, 10:43   #1
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Mileage Comparison

I am looking for way to compare the likely mileage of diesel powered models of trawlers / long range cruisers based on their hull shape and weight. I am seeing horsepower ranges of 4 to 40 horsepower per ton for boats listed as full displacement or semi displacement. This seems like a wide range.

There are some rules of thumb like 1 gallon to develop 20 horsepower for 1 hour. Given the wide range of engine horsepower ratings in what look like similar boats it seems likely at cruising speeds some engines are running at full power and others developing a small percentage of the rated horsepower. To estimate fuel consumption wouldn’t we need to know how many horsepower are actually being developed?

For comparison let’s assume all boats are at say 90% of rated weight and all operating in near still conditions. To predict mileage it seems you would need to know how many horsepower to move a particular boat thru the water at various likely speeds.

Any ideas where I can find this information or am I looking at this wrong?

Thanks
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Old 30-12-2019, 11:06   #2
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Re: Mileage Comparison

It is confusing for sure. One thing I discovered is that having too much HP, even if you don't need it, will increase fuel consumption vs less hp.
I had a little 30 ft trawler that had a big 80 hp diesel. It was very hard to get consumption below 1.25 gallons an hour. Often more like 1.5 gallons an hour.
I had a 44 foot sailboat with a 51 HP engine and it used .65 gallons an hour consistently.
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Old 30-12-2019, 11:23   #3
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Re: Mileage Comparison

Quote:
Originally Posted by AdkChris View Post
I am looking for way to compare the likely mileage of diesel powered models of trawlers / long range cruisers based on their hull shape and weight. I am seeing horsepower ranges of 4 to 40 horsepower per ton for boats listed as full displacement or semi displacement. This seems like a wide range.
You may want to ping TrawlerForum.com, sister-site to this CF site. You will get more ratios and general-rules that you can imagine, none of which will be particularly accurate or useful. Comparing older diesels such as my naturally aspirated Perkins 4.236 (80hp) with newer common-rail with turbo's and after coolers is akin to comparing an old Chevy 350 block that was lucky to get 12 mph with the newer hi-tech engines powering modern cars with close to 30 mpg.

I spent about 5-years delivering yachts full time on the Pacific Coast. Two rules all delivery captains cite: (i) owners are always wrong about their fuel consumption; and (ii) they are always optimistic.
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Old 30-12-2019, 11:42   #4
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Re: Mileage Comparison

Would be interesting to compare Brake-specific fuel consumption (BSFC) between generations of gas engines. I can find quite a few BSFC graphs for diesel engines but not much for stock car engines.

I wonder how much of the mpg improvement in cars was engine related and how much was due to improvements in transmissions, rear end gear ratios and other things. Just going from a 2 speed trans with a high stall torque converter to a 4 speed overdrive trans with a lockup torque converter had to make a big difference.

People can and do repower boats but it is hard to change the hull shape! How to rate the hull shapes and how much difference does it make?
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Old 30-12-2019, 11:53   #5
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Re: Mileage Comparison

Based on this:
https://en.wikipedia.org/wiki/Brake-..._shaft_engines

It seems that any engine we are likely to have in our boats is about 30% to 45% efficient.
There rest of the difference in fuel consumption would be related to boat drag, poor application and operation.
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Old 30-12-2019, 11:54   #6
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Re: Mileage Comparison

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Originally Posted by AdkChris View Post
People can and do repower boats but it is hard to change the hull shape! How to rate the hull shapes and how much difference does it make?
Technical term to differentiate hull forms is "Prismatic Coefficient." In practical terms, rare for designs to to include this factor in their specifications. It is the reason the many pundits on forums like this are dead wrong when they say there is no appreciable difference in consumption of twins vs single; and that moving a semi-displacement boat at less than 1.2 LWL would be the same as a displacement boat at same 1.2 LWL with similar HP.

Michael Kasten presents a decent discussion - see pages 3-4 on the attached.

http://www.kastenmarine.com/_pdf/coe..._equations.pdf
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Old 30-12-2019, 12:09   #7
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Re: Mileage Comparison

There are various formulas for figuring out/estimating/guessing boat HP per speed length ratio. Once you know HP then one can look at the engine specs and figure out the MPG.

When we started digging into buying a boat this was one of my first questions since the HP in displacement hulls are all of the place.

Semi displacement hulls are going to need more HP. There are equations for this but we am not interested in semi displacement hulls so I can't provide a pointer though there might be a formula in Skene.

Get a copy of Beebe's book, https://smile.amazon.com/Voyaging-Un...2&sr=8-1-fkmr0 and he has a way to calculate HP based on the boats characteristics.

Skene's book also has the HP calculations, https://smile.amazon.com/Skenes-Elem...audible&sr=8-1

Beebe and Skene's numbers are seldom/never the same but they are close to each other.

Later,
Dan
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Old 31-12-2019, 11:05   #8
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Re: Mileage Comparison

Thanks Dannc

Working on getting a copy of Beebe's book.
Looked at preview of Elements of Yacht Design and did not see much about motor boats which is my current interest. Did I miss something?

BTW good to talk about real information not opinion.
Statements like:
My boat is very cheap to run.
I really enjoy it so I don't care how much fuel I burn.
Are informative in a way but don't really help selecting between boats.

Even Bill Gates shows concern for the costs of things. Granted bigger things. Bill G talks about what he can do with a few hundred million. AdkChris talks about what he can do with a few thousand.
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Old 31-12-2019, 13:15   #9
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Re: Mileage Comparison

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Originally Posted by AdkChris View Post
Thanks Dannc

Working on getting a copy of Beebe's book.
Looked at preview of Elements of Yacht Design and did not see much about motor boats which is my current interest. Did I miss something?

...
In my copy of Skene, chapter 9(IX), "How to find shaft horsepower required for displacement-type boats", is on page 85. Key words are "displacement type" boats so the formula is the same for trawlers, as long as they are not semi displacement, and sail boats.

I have the 8th edition of Skene.

One of the variables in Skene's equation is propeller efficiency which Skene says to use 60-66% for displacement-type power boats. Then one has to increase the HP by some factor, Skene uses a third, for displacement-type boats, to get a final HP.

Then you have to figure out the engine fuel burn for a given HP at a given SLR(Speed Length Ratio) to figure out how fast the boat would go to finally figure out the MPG. In flat water and no wind.

Use a spreadsheet for all of this to make it more easy.

Skene's and Beebe's equation get into the ball park for required HP. I have compared their numbers with boats doing passages and the equations seem close enough. There are other rules of thumb equations but Skene and Beebe seem more accurate.

I happen to be looking at particular sail boat and I plugged it's numbers into the equations. Skene says the boat needs 80 HP for a hull speed of a SLR of 1.34. Beebe says it needs 75. The designer has specified a JD 4045 M3 engine that produces 135 HP.

For another sail boat, the designer says to use a 75 HP or 100 HP engine where as Skene says 66 HP and Beebe says 68 HP for hull speed.

I have a four equations from Gerr for computing HP equations. Looking at the last boat where the designer said to use 75/100 HP. Gerr's formulations say to use 83, 87, 91 and 100 HP for hull speed. So for this boat, all of the formula's and the engine spec are in the ball park.

For the first boat I mentioned, where the design has a 135 HP engine and Skene/Beebe said use 80/75 HP, Gerr's HP formulas where 95, 101, 106 and 127. For this boat, I wonder why 135 HP. Might be an owner's wish to increase HP a bit. In any case, the higher HP would not be an issue for me even though it is at the outer range of the HP calculations.

Later,
Dan
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Old 31-12-2019, 14:36   #10
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Re: Mileage Comparison

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Originally Posted by mvweebles View Post
Technical term to differentiate hull forms is "Prismatic Coefficient." In practical terms, rare for designs to to include this factor in their specifications. It is the reason the many pundits on forums like this are dead wrong when they say there is no appreciable difference in consumption of twins vs single; and that moving a semi-displacement boat at less than 1.2 LWL would be the same as a displacement boat at same 1.2 LWL with similar HP.

Michael Kasten presents a decent discussion - see pages 3-4 on the attached.

http://www.kastenmarine.com/_pdf/coe..._equations.pdf


And the qualitative Cliffs notes for this are as follows:

Very long, very narrow displacement boats are more efficient.

Looking at the total wetted surface area is also a great indicator of displacement hull efficiency.

Entry and exit are also important. The least turbulence possible while running makes a difference. Your displacement boat should leave no wake and hardly a ripple if it's efficient.

As you can imagine, my performance cat has all these in spades. Which is why I struggled with the interior design. Everything is a trade off.

But if you want efficient, OP, the factors above are it.

Note: my 50' boat goes 8-9 knots with a single 30hp outboard (it has 2 of them). And, it's underpropped! Runs out of RPMs before hitting max hull speed.

Think of those items above, which are results of a good prismatic coefficient.

Edit: I had a fuel supply issue during some testing on a windy day, working out kinks in the fuel system. Thought we were screwed as there were bridges everywhere. As it turned out, the boat also sailed well at 3-4 knots without any sails or a mast or any engines. Ha ha ha. We just "sailed" a few miles to the next anchorage a few bridges down the ICW.
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Old 31-12-2019, 14:54   #11
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Re: Mileage Comparison

Cliff Notes version of the Cliff Notes:

It doesn't matter.

For similar size and weight hulls, fuel burn at a given speed is going to be close enough that the difference shouldn't be a primary deciding factor.

There are so many other things to consider, and so many much higher costs. If you have to worry about incremental fuel costs, don't buy a boat.
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Old 31-12-2019, 15:00   #12
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Re: Mileage Comparison

And then, you need to factor in the other things you are using your engine to do, like charging the house batteries and hauling extra fuel around.

Calculate all you want. The important number is empirical. It is miles nautical, as you use the boat, per gallon consumed. Log the miles, log the hours, and figure out a way to actually measure how much fuel remains in the tank. That's what tells you whether you need to fill up before the next leg, or are carrying unnecessary weight.
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Old 31-12-2019, 15:39   #13
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Re: Mileage Comparison

There are 3 primary factors which determine duel efficiency of a displacement hull:
1. The drag of the hull going thru the water. Displacement of the boat is primary here. For a given displacement, as was mentioned above, long and narrow will have less drag than short and fat. Clean runs help. A semi-displacement hull, with its transom, will always have more drag than a full displacement hull so long as you are operating at displacement speeds.
2. Propeller efficiency - the larger, and thus slower turning a prop is the more efficient it will be. This is a bigger factor than is sometimes recognized.
3. The speed you are running at - this one is huge. Speaking about displacement hulls, increased speed greatly increases resistance and thus decreases mileage. Any speed above about 1.0 S/L ratio is going to be costly in mileage. There is a generalilty, and it is a generality but not a bad one, that every knot of speed increase doubles your fuel consumption.

Light, narrow, long, and slow with a big, slow prop will produce the highest mileage.
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Old 01-01-2020, 07:01   #14
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Re: Mileage Comparison

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Originally Posted by AdkChris View Post
I am looking for way to compare the likely mileage of diesel powered models of trawlers / long range cruisers based on their hull shape and weight. I am seeing horsepower ranges of 4 to 40 horsepower per ton for boats listed as full displacement or semi displacement. This seems like a wide range.

Any ideas where I can find this information or am I looking at this wrong?

Yes, typical discussion on trawlerforum.com.

But taking a sideways look at your questions...

1) If you're buying a used boat, your choice of engines will be determined more by the market than on fuel economy issues.

2) Fuel is often not the biggest on-going expense factor... unless you're always going to be anchored out, never ever in a marina, carry only liability insurance, etc.

3) One approach is to find boats with features you need/want -- and without features you abhor -- then think more about available engines in that boat model.

-Chris
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Old 01-01-2020, 08:45   #15
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Re: Mileage Comparison

A lot of practical boat buying thoughts on here, but some of us just love efficiency. I'm assuming the op may be enamoured with efficiency and possibly concerned about air pollution.
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