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Old 08-14-2016, 08:32 AM   #1
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Fuel consumption at sea versus calculated

What significant differences are you seeing in fuel use at sea versus in smooth water (or calculated), let's say averaged over a long period to account for various sea states?
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Old 08-14-2016, 08:41 AM   #2
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In conditions where my boat pitches a lot, I often see a reduction in boat speed from 6.3 knots down into the 3 knot area. That seems to be in a chop more than swell, so I am guessing a 50 percent reduction is possible for periods...you would have to adjust range based on the type of conditions expected. Which is difficult.

One option is, when boat speed is reduced due to sea state, reduce your throttles accordingly to lessen fuel burn. Reduced fuel without much more loss in speed I would guess in most cases would increase your range again.

A wild guess for average sea states and my boat along the US Atlantic seaboard, maybe 10 %. Basing that on not all wave patterns will be on the nose or slow me by half. Occasionally the wind and waves will be good enough on the stern to increase efficiency a tad. And durations of favorable versus unfavorable events.....which should be in my favor if I use the weather forecasts well enough.

For a true blue water passage, that is why you want a boat that can maintain a fairly predictable speed in varied conditions. Mine can't and neither can a lot of other coastal cruisers due to other design considerations.
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Old 08-14-2016, 08:55 AM   #3
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Great deal will have to do with whether you're planing or displacement. My trawler (displacement) is impacted more in 3'+ seas with either a reduction in speed or an increase in fuel burn. My charter (planing) boat seams to have about the same speed and burn rate going into seas, maybe do to less hull in the water. Probably don't see a 10% change in speed or fuel burn for my trawler.

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Old 08-14-2016, 09:19 AM   #4
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Following seas? on the nose seas? with the prevailing current or against ? speed through water or over ground?
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Old 08-14-2016, 09:31 AM   #5
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Guys have been talking about this for years and it could'nt help but happen .. one could say "obviously".

I never ever have increased rpm to maintain speed. Of course I don't have a mental fixation on a need to maintain speed. So I go a bit slower .. no need to think about that much less worry about it. Slowing down in 6' seas I do to reduce pitching anyway. I usually drop 300 rpm when I feel like slowing down. The wind and seas (up to 7') have never required me to increase rpm to make acceptable headway. Must be the lack of FB .. haha. But there's just not enough windage and the easy lines of the FD hull dos'nt smash and bash along ... just easily slides along. The Willard looks (and probably acts) more like a sailboat than a powerboat. I suspect that the 30' Willards were designed by a NA that usually designs sailboats. The shape of the hull and the life lines (cable like sailboats) instead of steel tubing like most trawlers suggests that .. and probably other features not noticed or recalled.

But more power in heavy going I've never needed or wanted.
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Old 08-14-2016, 11:44 AM   #6
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I don't know of any drop in speed while punching through five-foot, wind-driven waves not uncommon in some of my local waters. In such sees, however, I lower RPMs by 200-300 to reduce speed by about 20 percent to aid in boat control and comfort.

Possibly, not having propellers near the water's surface helps preserve speed when pitching.
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Old 08-14-2016, 05:31 PM   #7
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Same amount per hour burn. Just less speed the worse the waves are.

You may do the 200 mile voyage in calm water using less fuel than when rough. But it isn't that you burn more fuel per hour when rough but that it takes more hours to burn (gph) to get somewhere when it's rough.
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Old 08-14-2016, 05:41 PM   #8
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Originally Posted by markpierce View Post
I don't know of any drop in speed while punching through five-foot, wind-driven waves not uncommon in some of my local waters. In such sees, however, I lower RPMs by 200-300 to reduce speed by about 20 percent to aid in boat control and comfort.

Possibly, not having propellers near the water's surface helps preserve speed when pitching.
5 footers on the bays? Really?
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Old 08-14-2016, 05:44 PM   #9
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Originally Posted by cappy208 View Post
Same amount per hour burn. Just less speed the worse the waves are.

You may do the 200 mile voyage in calm water using less fuel than when rough. But it isn't that you burn more fuel per hour when rough but that it takes more hours to burn (gph) to get somewhere when it's rough.
In severe enough conditions, I wonder how loading the prop more and less effects overall efficiency, would have to think there is some effect, even if imperceptible to the user.
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Old 08-14-2016, 05:48 PM   #10
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Originally Posted by caltexflanc View Post
5 footers on the bays? Really?
Definitely, especially when westerly winds of 15 knots or more oppose an opposite 2.5 knot counter-current, often happening in eastern San Pablo and Suisun bays (both shallow except for the dredged shipping channel).
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Old 08-14-2016, 09:54 PM   #11
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Originally Posted by makobuilders View Post
What significant differences are you seeing in fuel use at sea versus in smooth water (or calculated), let's say averaged over a long period to account for various sea states?
We monitored our fuel usage from Alaska to Panama, Panama to Trinidad to Florida and on the AICW. The AICW was about 5% lower in fuel usage.
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Old 08-15-2016, 06:00 AM   #12
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We monitored our fuel usage from Alaska to Panama, Panama to Trinidad to Florida and on the AICW. The AICW was about 5% lower in fuel usage.
That's good useful info - so basically you burned about 5% more in open seas versus smooth water.

With my heavy boat I would lose approx 1/4 knot out in the ocean in average seas (say 4'-6'). My light center console in wind and seas (3'-4') I would drop from 20 knots down to 14 knots. Very light boat.
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Old 08-15-2016, 06:50 AM   #13
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Climbing and descending waves adds incrementally to linear distance traveled, so even though you burn say 6 gph and your stw doesn't show a significant change you are traveling a longer path and it will ultimately show in the log. There are numerous variables that can affect a vessels maximum range if that is ones objective. Diesels for the most part don't vary their fuel use unless something goes wrong. They will burn the same amount of fuel hour after hour at a rate the user controls with the throttle. The amount of ground covered will vary according to the weight of the vessel, the cleanliness of the prop and hull, currents, weather and most important of all is the way you hold your mouth.


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