Mileage vs Consumption

About the most interesting thing is that a 40 ft lwl boat weighing about 50,000 lbs going 7 knots will get about 2.5 to 3 mpg. Hull type, number and size of engines or color of the bootstripe are insignificant. So, how do I now go about touting my trawlers assumed superior efficiency?

kulas44 said:

About the most interesting thing is that a 40 ft lwl boat weighing about 50,000 lbs going 7 knots will get about 2.5 to 3 mpg. Hull type, number and size of engines or color of the bootstripe are insignificant. So, how do I now go about touting my trawlers assumed superior efficiency?

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Call it all a Wallow of Fruit Salad?

I like fruit salad. I cant tell you how many of those little cans of DelMonte fruit salad I can eat per mile, whether statute or knautical, but probably 2.5 to 3. Merry Christmas all, and to all a good night.

kulas44 said:

So, how do I now go about touting my trawlers assumed superior efficiency?

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My stupid statements for Christmas on this subject are:

• I really don't care about "efficiency" as I'm confident Art DeFever did the hull design and engine/prop selection correctly.
• My fuel bill is normally less than 15% of my yearly boating costs before depreciation so why worry.

I think the boat diesel calculator is reasonably good. You do need to select the right hull type, and it might not be what you would think.

Some actual numbers below. I'm a little longer LWL ~46', a little heavier D 60,000# half load, semi-displacement hull.

For me, 1.1 is a bit under 7.5 kn. At that speed I am using about 60 HP and burning 3.3 gph. So NM/G is around 2.3.

As best as I can scale off my graphs from sea trials, at 1.0 I would be at 6.8 kn, using 44 HP burning 2.3 gph and have NM/G of a touch under 3.

Yes, speed makes a big difference as pointed out in earlier posts. Portuguese, to me you calcs look in the ballpark, although maybe a touch optimistic. I think that if you want, say, 2.5 NM/G then you will need to be a little below 7 kn.

Without spending mega bucks , about the only control over the fuel bill per trip is the Throttle.

Clean bottom ,clean prop , GPS flowscan, and cruising prop can all help, but the throttle position is easiest to change.

Thank you all for the opinions and wise words put in this thread.

Merry Christmas to all

What's the origin of the figure of 1.1 x hull speed that's been mentioned here?

I'm getting 8.47 knots as hull speed for a 40' LWL boat, using the only formula I've ever known: 1.34 x sqr(40)). I'm assuming a "typical" monohull, and putting aside the debate about what that means.

1.1 times that would be over 9 knots, and most certainly wouldn't be as efficient.

Sorry if this is a dumb question, just that I haven't run into that 1.1 value before.

As for the other debates, I find myself agreeing with Twisted. The goal is to figure out how far you can go on a given amount of fuel. NMPG at a given speed is the ONLY important number when you're trying to plan your route, and your fuel stops.

You can factor in the effects of wind, waves and current, or even an inexperienced helmsman, but you need a starting point. That's NMPG.

Technically, a better number would be gallons per 100 NM. It's sort of the same thing, but it's a more useful way to think of it. A slight change in NMPG might sound significant, while the same figure expressed as a change in Gallons per 100 NM might not. I can't explain it any better than that, but try it and see if it works better for you.

One more thing, back to the hull speed calculation. Someone above was suggesting that hull configuration was irrelevant. While minor hull shape changes do have only minor impacts on efficiency, rest assured that a multi-hull will throw all these calculations out the window. Hull shape does matter.

1.35 is as arbitrary a number as 1.1....I believe THAT number comes from the prismatic coefficient and well as a few other "design features" of your boat.

IT IS NOT A CONSTANT FOR ALL BOATS!!!!!!! Only people that have NOT really studied NA very much keep spouting off that 1.34 is a real number to use (usually (really have one person in mind) or that there is such a thing as hull speed that can be ACCURATELY CALCULATED for any given boat. I'm pretty sure it can be approximated and has to be tested with tank models then ACTUALLY run on your finished outfitted boat....which is why Rick keeps pointing out that most of all this is just mental you know what for a lot of different reasons!!!!

from.... Professor Robert B. Laughlin, Department of Physics, Stanford University

"Obviously this analysis is oversimplified. Naval architects and professional shipwrights perform much more sophisticated analyses of their craft to account for interaction with ocean waves and wind, the precise shape of the hull, the modeled shape of the wake, and other factors. However the concept of a speed to length ratio is so useful and fast that many professionals work out a value for a specific hull shape and then refer to it as they scale the model up or down in size. Values range from 1.18 (in nautical units) for barges to 1.42 for very long, sleek vessels. Most amateurs use 1.34 as a good approximation for most common hull shapes"

CaptTom said:

What's the origin of the figure of 1.1 x hull speed that's been mentioned here?

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I think it's just the max speed that a lot people target for a boat. You might use it when selecting the max power for the engine.

You can factor in the effects of wind, waves and current, or even an inexperienced helmsman, but you need a starting point. That's NMPG.

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I think NMPG is an interesting way (better way, in my opinion) to operate a boat for a passage. You know the distance, assuming you don't reroute for weather etc. And you know how much fuel you have. The big variables/unknows are wind, current, and waves, and the extent to which they will cause you to burn more or less fuel than planned.

But knowing the distance and available fuel, you can easily calculate the max allowable NMPG to reach your destination with an acceptable reserve. If you operate at or above that NMPG rate, you will get there on the available fuel. Maintaining the desired NMPG rate will mean adjusting speed to compensate for all the above variables, but I think it makes it very easy giving just one number to watch. Of course this depends on having accurate instrumentation for fuel burn and course made good, and preferable something to do the math for you, but newer chart plotter can do it, and new engines report their fuel burn very accurately without floscans and other such stuff.

Food for though.

Technically, a better number would be gallons per 100 NM. It's sort of the same thing, but it's a more useful way to think of it. A slight change in NMPG might sound significant, while the same figure expressed as a change in Gallons per 100 NM might not. I can't explain it any better than that, but try it and see if it works better for you.

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Sure, use whatever units works for you. I think the key it to have some form of damping for the numbers so you don't have to do it all in your head. But we are all probably pretty used to averaging out speed in our heads from a bouncing GPS reading..

One more thing, back to the hull speed calculation. Someone above was suggesting that hull configuration was irrelevant. While minor hull shape changes do have only minor impacts on efficiency, rest assured that a multi-hull will throw all these calculations out the window. Hull shape does matter.

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Yes, I said that. It was meant in the context of displacement trawlers.

....Sooooo, instead of 1.34 x Square root of LWL = full displacement Hull Speed.

I have chosen to cruise at 1.1 x Square Root of 40.2 which is my LWL (6.97 almost 7 knots correct?)

Portuguese,
A big yes in my opinion. Or as so many are saying these days "perfect".

Portuguese said:

....Sooooo, instead of 1.34 x Square root of LWL = full displacement Hull Speed.

I have chosen to cruise at 1.1 x Square Root of 40.2 which is my LWL (6.97 almost 7 knots correct?)

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I'd have to go back and look at Voyaging Under Power to confirm, but I think it's a little different than what you said.

I've always understood it to mean 1.1x the "Displacement speed", where the displacement speed is 1.34 x sqrt of LWL. I'm not saying it's the right or wrong way to look at things, but they calculate the Displacement speed, then look at going 0.8 x displacement speed, 0.9x 1.1x, etc.

I've always applied the "not so constant" right in the formula...1.1 times the square root of the waterline length.

Thank you Fernando, you clarify a question

Well.......Ebbtide is happy at 1650 rpm and varies between 7 and 8 knots depending on the conditions. We average 7.5 kts most of the time and burn an average of 1.75 gallons per hour. 120 Lehman, 4 blade prop 24x16 LH, full fuel and water tanks.

A DD 8-71 will burn significantly more fuel to generate 70 HP as compared to a John Deere 4045T (4 cyl).

The use of a DD 3-71 would have the fuel burn near identical. DD requires operation over 60% of rated power for efficiency.

The SQ rt of the moving LWL is known as SL.

SL times 1.34 or so is where only sailboats go with displacement boats , as a big down wind is free.

Most marine motorists will refuse to pay for the fuel needed to dig a big hole in the water and attempt to climb over the bow wave.

Depending on how FAT and HEAVY the boat is the rule of thumb for Long Range Cruise is SL x .9 to SL x 1.15.

Somewhere with a normal boat the best compromise of speed and range will be found there .

Skinny very light boats with a Length to beam ratio of at least 6-1 can travel a bit faster with the same fuel burn .

8-1 seems the limit unless high expensive speeds are going to be the normal operation , like a ferry.

FF said:

A DD 8-71 will burn significantly more fuel to generate 70 HP as compared to a John Deere 4045T (4 cyl).

The use of a DD 3-71 would have the fuel burn near identical. DD requires operation over 60% of rated power for efficiency.

The SQ rt of the moving LWL is known as SL.

SL times 1.34 or so is where only sailboats go with displacement boats , as a big down wind is free.

Most marine motorists will refuse to pay for the fuel needed to dig a big hole in the water and attempt to climb over the bow wave.

Depending on how FAT and HEAVY the boat is the rule of thumb for Long Range Cruise is SL x .9 to SL x 1.15.

Somewhere with a normal boat the best compromise of speed and range will be found there .

Skinny very light boats with a Length to beam ratio of at least 6-1 can travel a bit faster with the same fuel burn .

8-1 seems the limit unless high expensive speeds are going to be the normal operation , like a ferry.

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Fred - Happy Holidays!

With your in-depth calculation methods how would you recommend to calc our Tollycraft's "hull speed". Also, what do you feel our boat's hull speed actually is (before it begins pushing water to climb up onto plane)?

Boat Stats: Loaded weight 21K lbs +/-, OAL 34', moving LWL 32', OAH from WL 12' 6", Deck Beam 12' 6", Bottom beam 10' 6", 2' 9" draft, dead rise 11 degrees, short keel with 9" max depth 5' from stern, full length hard chine...

With our Tolly's (off plane) moving LWL 32' I mathematically calc "hull speed" at 7.58 knots. Having run different speed-to-fuel use tests I've found that while using twin gassres both running an economical fuel-use speed is 6.5 to 7 knots... 2 +/- nmpg With only one screw running most economical speed is 5 to 5.5 knots... 2.75 +/- nmpg. On full plane at 16 to 17 knots 1 +/- nmpg.

Your marine knowledge always interests me. I look forward to learn what you feel our Tolly's hull speed is, and how you calc it.

Art said:

Fred - Happy Holidays!

With your in-depth calculation methods how would you recommend to calc our Tollycraft's "hull speed". Also, what do you feel our boat's hull speed actually is (before it begins pushing water to climb up onto plane)?

Boat Stats: Loaded weight 21K lbs +/-, OAL 34', moving LWL 32', OAH from WL 12' 6", Deck Beam 12' 6", Bottom beam 10' 6", 2' 9" draft, dead rise 11 degrees, short keel with 9" max depth 5' from stern, full length hard chine...

With our Tolly's (off plane) moving LWL 32' I mathematically calc "hull speed" at 7.58 knots. Having run different speed-to-fuel use tests I've found that while using twin gassres both running an economical fuel-use speed is 6.5 to 7 knots... 2 +/- nmpg With only one screw running most economical speed is 5 to 5.5 knots... 2.75 +/- nmpg. On full plane at 16 to 17 knots 1 +/- nmpg.

Your marine knowledge always interests me. I look forward to learn what you feel our Tolly's hull speed is, and how you calc it.

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Unless you know your hull's constant at something between 0.5 and 2.0 at those lower speeds...using 1.34 is as good a guess as anyone can make (per the one of thousands of supporting articles like the one I linked) about your vessel.

What I think Fred, myself and a bunch of others are always referring to the 1.0 (plus/minus) number...is just running well enough below your hull speed for economy reasons....but knowing exactly where that number is for your boat would take many test runs and a careful plotting of fuel consumption versus speed to see if there even is and what that magic number is. It's the point where that professor in my linked article says

" Typically the energy required to speed up a displacement hull then becomes exponential in speed rather than quadratic"

Art,
I think HS has little to do w boat performance on a hull that is not FD. Faster burns more fuel ... slower burns less.

With a SD hull at hull speed the stern will be right on top of it's following wave and the transom is deep in the water creating so much drag that the usual thought of benefits of HS aren't realized. It's just another speed w no special benefits. A FD hull has a stern shape that fits the face of the following wave and it surfs a bit and the following wave (to some degree) pushes the boat fwd. And the transom is high (or even out of the water) creating little drag. It's a bit like the Bart using it's electric motors as generators and brakes to recap some of it's energy spent. The FD hull reclaims some of it's energy lost making the bow wave.

The only reason I track fuel use is for planning fuel stops while traveling. When I boated on a inland lake I never worried about it. I can guess within 5 gallons when I go to the fuel dock how much it'll take to top off the tanks. I'am not going to stop boating because of the cost of fuel, as a matter of fact I wish I was able to be out burning some right now. So the way I figure "It is what it is" & I'll live with it.

River Cruiser said:

The only reason I track fuel use is for planning fuel stops while traveling. When I boated on a inland lake I never worried about it. I can guess within 5 gallons when I go to the fuel dock how much it'll take to top off the tanks. I'am not going to stop boating because of the cost of fuel, as a matter of fact I wish I was able to be out burning some right now. So the way I figure "It is what it is" & I'll live with it.

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It's not always about money...sometimes it's just plain old what do I have to do to stretch it to the next fuel stop...or make it some place safe on just a few gallons because you got sold bad fuel and can't use most of it...I guess I could keep going...but......it's just nice to know your options....

If money was no object, I'd travel by Learjet.....and boat in several boats strategically placed all over the world, of all different kinds for all different kinds of boating....

so just back a hair off your bow wave a bit, shut down your engine and surf till your next stop.....

these discussions get better and better....

psneeld said:

so just back a hair off your bow wave a bit, shut down your engine and surf till your next stop..... these discussions get better and better....

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"Blow-boaters stuck in a trawlersman shell"

Ron I agree and I'd like to be burning too. But mine's on the hard.

However as we change boats or otherwise buy boats we have a huge say in how heavy and hull efficient our next boat is going to be.

But the value here on TF is how good the conversation gets and how much we can learn.

We're copacetic running at 1.1 of the square root of our waterline, meaning roughly 6.3 knots with almost 32-foot waterline using 1800 RPM of the John Deere 4045's maximum of 2400.

manyboats said:

But the value here on TF is how good the conversation gets and how much we can learn.

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The conversation is a great way to spend these winter months when some of us can't be on our boats & on a cruise somewhere. The amount of daylight is increasing each day now as is my attitude.

Art said:

Fred - Happy Holidays!

With your in-depth calculation methods how would you recommend to calc our Tollycraft's "hull speed". Also, what do you feel our boat's hull speed actually is (before it begins pushing water to climb up onto plane)?

Boat Stats: Loaded weight 21K lbs +/-, OAL 34', moving LWL 32', OAH from WL 12' 6", Deck Beam 12' 6", Bottom beam 10' 6", 2' 9" draft, dead rise 11 degrees, short keel with 9" max depth 5' from stern, full length hard chine...

With our Tolly's (off plane) moving LWL 32' I mathematically calc "hull speed" at 7.58 knots. Having run different speed-to-fuel use tests I've found that while using twin gassres both running an economical fuel-use speed is 6.5 to 7 knots... 2 +/- nmpg With only one screw running most economical speed is 5 to 5.5 knots... 2.75 +/- nmpg. On full plane at 16 to 17 knots 1 +/- nmpg.

Your marine knowledge always interests me. I look forward to learn what you feel our Tolly's hull speed is, and how you calc it.

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I'll be interested in Fred's views as well. His rules of thumb are pretty good.

For what its worth, I have found the Hawaii Marine Company spreadsheet for Displacement and SemiDisplacement hulls pretty good also.

Displacement and Semi-Displacement (or Semi-Planing) Hull Powering Calculations, Description

It uses material from Gerr's Propeller Handbook amongst other sources. It has a small fee and user licence so I cant post the xls file I played around with using some of Art's info. I had to guess a few things, but for Art's Tolly I got a figure for SLR of 1.37 from it, giving 'hull speed' of 7.7 kn.

The xls file seems to manage the FD versus SD hull type issue reasonably well. For SD hulls, like my own, I find that SLR of 1.2 is still pretty economical. For Art, 1.2 is a speed of 6.8 kn so it seems to fit for his Tolly as well.

Insequent said:

I'll be interested in Fred's views as well. His rules of thumb are pretty good.

For what its worth, I have found the Hawaii Marine Company spreadsheet for Displacement and SemiDisplacement hulls pretty good also.

Displacement and Semi-Displacement (or Semi-Planing) Hull Powering Calculations, Description

It uses material from Gerr's Propeller Handbook amongst other sources. It has a small fee and user licence so I cant post the xls file I played around with using some of Art's info. I had to guess a few things, but for Art's Tolly I got a figure for SLR of 1.37 from it, giving 'hull speed' of 7.7 kn.

The xls file seems to manage the FD versus SD hull type issue reasonably well. For SD hulls, like my own, I find that SLR of 1.2 is still pretty economical. For Art, 1.2 is a speed of 6.8 kn so it seems to fit for his Tolly as well.

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I highly doubt that a planning or semi-displacement hull has a SLR of 1.37 as that is above the average that is usually reserved for full displacement hulls with a slippery prismatic coefficient...

While I'm no NA or even self proclaimed expert.....just sounds high for a boat that can plane.

1.2 sounds a lot better.

I would like to see some real world figures. How about a 40 foot lwl boat weighing 50,000 lbs with a 14 foot beam doing 7 knots. No b.s. real figures. You guys with Krogen 42s will know this best. My opinion is that ANY boat that fits this criteria will get about 2 to 3 mpg. regardless of hull configuration, diesel that is, subtract 1/3 for gas. But gas is cheaper, so we could do cost per mile. New wrinkle.