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Old 10-16-2013, 11:47 AM   #1
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fuel usage v wave height

How much does your boat speed affect your fuel consumption?

Discounting all of the negatives associated with running a twin boat on one engine, and looking only at fuel consuption v boat speed, here is my $.02 worth.

I spent 38 engine hours this summer running on one engine, while we were too far from home to begin chasing a leak from the oil cooler on the port engine. In order to have that engine available for manouvering, I left chasing the leak till we wouldn't need the engine(s) for a couple of weeks.
I fixed it (cost just over $500 to fix) and on returning to my home marina, fuelled up. I know my usual consumption, which has been consistent over the last 12 years, at 4 gph, running at + or - 8.2 knots. This speed produces a smallish wave. From my vantage point out in front of the wave it is hard to give an accurate measure of the wave height, but I will guess it at 1 ft.

While running on a single engine, I maintained the usual rpm, rather than try to match usual boat speed. This resulted in a speed of 6 to 6.3 knots, and reduced the wave height by about 1/2.

The choice of speed was dictated by the engine itself, as to run at 8 knots produced copious quantities of black smoke, indicating that the single engine was propped wrong for this usage, working far too hard, so I slowed to the point that the smoke disappeared, as observed by the boat following us.

Doing the math on the fuel used over the 75 hours since the last fill, the 38 hours on one engine consumed $500 less fuel than I would normally have used in that time, maintaining the 2 gph per engine normal usage, and improving my mpg from 2.0 to 3.0.

After disclosing this saving to SWMBO, she observed that 8 knots was a better speed for us.
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Old 10-16-2013, 11:58 AM   #2
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Do I have this right - when you run at 6.X knots you use less fuel per mile than when you run at 8.X knots? Makes sense to me, whether on one engine or two.

Your SWMBO is telling you something else though, she needs a go fast so she can look like the babes on the boat covers. Think how happy she'd be at 30 knots on a Bertram! This too makes sense to me, especially with winter approaching in the far North.
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Old 10-16-2013, 01:39 PM   #3
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koliver,
Your 1' wake sounds like my guess for Willy. Explainable because of the large difference in hull length. The wave height is a measurement of energy put into it but also of displacement and beam. If your boat was 6' wide and 80' long going 8 knots we'd be talking about wave height in inches. And a 30' tugboat at 8 knots ... well you get the picture.

But on one boat reducing the wave height from 1' to 6 " should be very close to cutting the energy expended and fuel consumption in half.

Tom are there babes on boat covers? Speaking of boating magazine covers I've always been VERY impressed w the covers on Yachting ... a PNW free mag. Stunning cover shots but no babes that I can recall Tom.
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Old 10-16-2013, 02:21 PM   #4
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Speed costs money , slowing both engines to the 6.3K speed may even better the fuel mileage than running single.

You wont be dragging a 24 ? inch prop dead or freewheeling , and using a bunch of rudder to go straight.

Usually the SQ RT of the LWL is the cheap spot in nm/hr
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Old 10-16-2013, 04:46 PM   #5
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... Hey Koliver.

FF is totaly correct. Performance in Trawler language translates in $$$aving$$$. If 6 saved $500 with only one engine, imagine both engines going at 6.2 knts....That will be $weet
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Old 10-16-2013, 05:23 PM   #6
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Add a 3rd engine and really save some cash!
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Old 10-16-2013, 05:34 PM   #7
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Rick

That was good joke. The mention of the second engine is that he has 2 engines in his boat and, when he sails with only one, he is in jeopardy.
You know exactely what I mean.

That's why I only have one on my boat.
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Old 10-16-2013, 07:38 PM   #8
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Quote:
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Speed costs money , slowing both engines to the 6.3K speed may even better the fuel mileage than running single.

You wont be dragging a 24 ? inch prop dead or freewheeling , and using a bunch of rudder to go straight.

Usually the SQ RT of the LWL is the cheap spot in nm/hr

I think a better general rule is 90% of the sq rt of the LWL is the fastest efficient cruise.
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Old 10-17-2013, 12:23 AM   #9
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Daddyo ....

"fastest efficient cruise"

WUZZATT

What about 80% of the "sq rt of the LWL". Wuzza madder wid tat?

Lets face it. Basically the slower we go the more efficient we are.

No rules formulas or numbers to follow. Just go slower.
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Old 10-17-2013, 02:20 AM   #10
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I agree that 6 knots with both engines would save even more $, although I doubt that I will ever get the experience. I have the capability (again) of running at 8 knots, so that is where you will find me.
You will note that I didn't actually save any $ by running slower, as I simply spent it somewhere else.
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Old 10-17-2013, 06:09 AM   #11
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>I think a better general rule is 90% of the sq rt of the LWL is the fastest efficient cruise.<

Actually boats vary considerably.

The dock Queen beach ball roomaran with vast beam is quite different from a light slim boat with good motion.

The SQ rt of the LWL is known as SL.

The SL times somewhere between .9 and 1.15 will give the lowest fuel burn , at rational speed but all boats do better slower making smaller waves.

Not much difference , but at 6K an extra .6K is getting there 10% faster.

Thats why NA make the Big Bucks!
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Old 10-17-2013, 11:40 AM   #12
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Lets face it. Basically the slower we go the more efficient we are.

No rules formulas or numbers to follow. Just go slower.
It's not rocket science is it? We tend to over think and have a formula for just about every subject on this Forum.I believe we have a lot of folks getting tangled up in their underwear.
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Old 10-17-2013, 12:07 PM   #13
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We tend to over think and have a formula for just about every subject on this Forum.I believe we have a lot of folks getting tangled up in their underwear.



You nailed that one ...
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Old 10-17-2013, 04:03 PM   #14
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Sure we all save $$ going slow, but depending on your motors if you aren't working them hard enough, you end up with soot in the block and bigger expenses. I just got my fluids tested and they pretty much told me I needed to push my Detroit's a bit more. So we took it out on the bay this week and pushed the throttles up. Forgot how nice it was to run at 15-17 knots. We usually live in the 8 knot zone. After about an hour the exhaust cleaned up nice, barely any soot at all.
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