Comments about Fuel usage

Pseudomind

Pseudomind

Active Member
Jul 1, 2008
2,122
Jacksonville, FL
Boat Info
2011 Hurricane with Magic Tilt Trailer
Engines
115 HP Yamaha Four Stroke
I read this article and also another article which I cannot find, but this is interesting, but as always, what are your comments about this article (It's short)

Comments about fuel usage article

What do your calculations show? :huh:

:thumbsup:
 
We used the "slow down" rec., this year we had more "hours" than previous years, more "away from the dock time"... and rarley went on plane, and used the least fuel than the previous 4 years... Hoever, we also didn't go to any "far" destinations for lunch, dinner etc... reducing the # of "just go for a ride" trips... saved mucho $$$$$ :smt038
 
If i calculated it id probably freak out. its best to just let it be! lol
 
I usually like to figure it out in terms of GPH. Take the 40 hours we're on the boat over a typical weekend from Friday night until around 3:00 on Sunday and divide by the number of gallons used. So 5 gallons divided by 40 hours or .125 per hour makes it looks pretty good! Even 10 gallons over 40 hours is still only .25 per hour. When we were in the San Juans, it wasn't nearly as good, but still made it look better than actual running time! :grin:

Of course, I don't include the towing mpg in that figure. That would probably push that up and I don't want to ruin my figures! I figure towing costs are roughly equal mooring costs. But if I don't use the boat, I'm not paying anything for moorage!
 
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Good article. Mostly correct, but the first speed versus fuel comparisons don't take into account that your boat may not be able to sustain a plane at those speeds.
 
That article has some correct points but the drag calculations and fuel savings are total BS. For a planing hull, the TOTAL coefficient of drag (CD) increases as the boat slows down and sinks in the water more. His assumption is that skin friction drag and wave drag (something for displacement hulls) is the same for planing hulls regardless of speed and it is not. The pushing of water dominates the drag calculations. As an example, my boat get the same "MPG" if I go 23 knots or 25 knots or 21 knots... it's because the faster I go, the coefficient of drag is going down as the boat rises out of the water.

The only "good" advice in that article is don't run around with 2 tons of stuff you don't need. Other than that, the guy has his head up his ass.
 
Four Suns said:
That article has some correct points but the drag calculations and fuel savings are total BS. For a planing hull, the TOTAL coefficient of drag (CD) increases as the boat slows down and sinks in the water more. His assumption is that skin friction drag and wave drag (something for displacement hulls) is the same for planing hulls regardless of speed and it is not. The pushing of water dominates the drag calculations. As an example, my boat get the same "MPG" if I go 23 knots or 25 knots or 21 knots... it's because the faster I go, the coefficient of drag is going down as the boat rises out of the water.

The only "good" advice in that article is don't run around with 2 tons of stuff you don't need. Other than that, the guy has his head up his ass.
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Gary, that's what I said!
 
The calculations he shows:

fueldrag.jpg


Assume a constant CD... A planing hull does not have a constant CD (and nowhere near it) so he fails the test.
 
Hampton said:
Gary, that's what I said!
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I saw that... I wasn't pounding on you...

I assume pilots know what coefficient of drag is... ;-) And when the flaps are down the Cd changes from when the flaps are up....
 
Depends on what you are trying to accomplish. You extend your range quite a lot by running at slower speeds. My range is a little over 200 miles at normal cruise and about twice that at displacement speeds. Obviously you take a lot more time to arrive at your destination at slower speeds. But if you are doing the Loop for example, speed and fuel management come into play because of the distance between fuel stops.
 
sbw1 said:
Depends on what you are trying to accomplish. You extend your range quite a lot by running at slower speeds. My range is a little over 200 miles at normal cruise and about twice that at displacement speeds. Obviously you take a lot more time to arrive at your destination at slower speeds. But if you are doing the Loop for example, speed and fuel management come into play because of the distance between fuel stops.
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I'm not referring to displacement speeds but for speeds on plane. Slow speeds on plane are bad because the drag is so high. The author refers to a figure of 20 knots to 18 knots and that's not near displacement speeds for a recreational boat. I can post my fuel burn numbers and slowing down does not increase range all the time as he states. This is very boat/engine dependent and his figures, again, assume something that is not true... a constant coefficient of drag across the speed range...
 
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Four Suns said:
I saw that... I wasn't pounding on you...

I assume pilots know what coefficient of drag is... ;-) And when the flaps are down the Cd changes from when the flaps are up....
Click to expand...

No, no. I was just joking. I disagree'd with it slightly, but gave him credit for other aspects of it (my standard type of response).

You slammed him for generalizing (not inappropriate, just more your style).

I thought it was a funny comparison.
 
Knowing that I can go through 160 gallons a day (easy) if I am running one end of lake powell too the other. I try to keep my long runs down to minimum. Normally I use as mush in my genset due to my favorite spot on the lake is only 5 miles from the launch. Having added a fuel flow sensor and recently made a offshore trip to catalina island in California I was able to monitor my use and I did find myself backing off a little. :thumbsup:
 
I thought it was kind of pointless when they noted the fuel change from 2000 rpm down to 1800 rpms. What "normal" hull designed for planing is even close to efficient at those speeds? Maybe some diesels, but that is the "hump" he refers to as no no land. So why is that even a valid comparison?

It's funny how many people you see pushing that hump around creating a big wake. I guess they are trying to race at no wake speeds or something?
 
So this whole problem got me wondering what the real relationship between velocity and drag is on a planning hull because it sure doesn't vary by the square of the velocity (while on plane) as that author suggests.

I found this very elegant article written called the "Waterskier's Paradox." What a great way to look at this! If you think about it, when you are on a wakeboard or waterski, the rope tension (i.e. drag or power required) sure doesn't increase as the boat goes faster. In fact, when you slow down while on a ski, the rope tension INCREASES as you sink into the water (angle of attack of the ski increases and more area of ski in the water)

Here's the article:

http://www.hullform.com/skier-paradox.html

The article explains how the dependency on velocity is removed from the drag (i.e. power required).

The point is, the design of the hull (weight/balance/shape) along with engine efficiency curves are going to determine the "sweet spot" where the MPG is maximized. The analysis the original author made is bogus as slowing down may, in fact, give you worse fuel economy.

I have, as well as maintain, fuel burn vs. RPM vs. speed curves for my boat. They are generally for calm conditions (I do have a few for not-so-calm conditions) and they are programmed into my Coastal Explorer chartplotting application. When I create a route, I can put in various speeds from no-wake to cruise, etc. and the fuel burn estimates the thing spits out are pretty accurate. Just an FYI.

I know I've said this before but MPG rarely determines my speed anyway... it's sea conditions...
 
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