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Old 07-25-2016, 04:00 PM   #1
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Towing Dinghys

Has anyone calculated the difference of an increase fuel burn when towing a dinghy compared to not towing? (Perhaps a 10 ft RIB with a 8 to 15 hp ob motor or something similar) I'd like to get an idea of what it might cost me by being lazy and not hoisting the dinghy out of the water. I know that there will be many variables as a bigger boat with larger engines will feel the drag less, but there may be enough experience among us to get an idea of some averages. Thanks in advance for any data.
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Old 07-25-2016, 04:56 PM   #2
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It won't be much at moderate speeds. While towing a 9' RIB with no motor on at 6-7 kts, the tow bridle pulls at 30-40 lbs or so. At the same rpm as when going 6 kts in forward, when backing down on an anchor, it probably pulls ten times that much and probably more if I were in forward as the prop is more efficient in forward.


So at worst you lose about 10% in fuel efficiency while towing a dinghy at 6 kts. That ratio would probably get lower as you move up in speed. The trawler will require much more fuel, but because the dinghy is planning it won't require proportionally as much more hp.


Maybe someone with a Flowscan can tell you more precisely.


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Old 07-25-2016, 06:19 PM   #3
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Actually i doubt you could even measure the difference. You are pushing a 24000 pound boat through the water. A 300 pound load is not going to make any noticeable difference. I have a fully electronic QSB 5.9 230 HO. It is probably the same block you have in your Monk. We can can see a difference in fuel consumption between 0 wind and 10-15 on the nose, but that also includes an increase in sea state.
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Old 07-25-2016, 07:33 PM   #4
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:My new dingy might up my fuel bill....

Probably it won't be towing it though that causes the problem fuel bill.....
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Old 07-25-2016, 08:50 PM   #5
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:My new dingy might up my fuel bill....

Probably it won't be towing it though that causes the problem fuel bill.....
I can't speak as to that boat but a 39' Contender with triple Yamaha 300's gets 1.26 nmpg at 33 knots, 1.03 nmpg at 43 knots. WOT is 0.73 nmpg at 57 knots. I was shocked that center consoles got such good mileage and had the range they did.
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Old 07-25-2016, 10:31 PM   #6
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when we tow our 20ft AL. boat the fuel consumption stays about the same as when we aren't towing it... but we it does slow us down by about 1 knot... I can't imagine towing a little rib would add to fuel consumption or noticeably slow you down..
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Old 07-26-2016, 12:44 AM   #7
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I tow a West Marine 310 with a 15hp Yamaha 2 stroke on plane. At 17 knots, I do not know it is even back there. There is no noticeable difference in performance/economy.
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Old 07-26-2016, 08:17 AM   #8
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Thanks for the input. From anecdotal evidence it seems that it will not cost much to be lazy and leave the dink in the water for most short hops.
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Old 07-26-2016, 09:04 AM   #9
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I have had horrible experiences towing my dinghy and for that reason I don't. There were times with the dinghy ... an 8' Ely fiberglass, no engine almost filled with water. At that time I had my sailboat so we could not have been going over 5K.

Another time towing my 10RIB Avon with an 8HP soon loaded with water. The heavy load caused one of the side patches where I attached the tow line, to rip off. Yes, I know that was a mistake and should have attached the painter to the tow eye. I almost lost the dinghy along with its engine.


I have had OK luck pulling the bow of the dinghy as close to the stern as possible. I find it is just easier and safer to lift the dinghy and outboard onto the forward deck and secure it into its chocks. Then I can see it at all times and know that it is safe.
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Old 07-26-2016, 09:16 AM   #10
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I can't speak as to that boat but a 39' Contender with triple Yamaha 300's gets 1.26 nmpg at 33 knots, 1.03 nmpg at 43 knots. WOT is 0.73 nmpg at 57 knots. I was shocked that center consoles got such good mileage and had the range they did.
OK, thanks. Something new to add to my bucket list.
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Old 07-26-2016, 10:46 AM   #11
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Many rowing dinks are actually displacement hulled. so yes.. there will be drag.
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Old 07-26-2016, 11:30 AM   #12
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It all depends on the relative sizes of the boat and dinghy. Towing a 10-12 foot dinghy behind a 40-50 footer won't be noticed. In contrast towing an 8-10 foot dinghy behind a 25-30 footer will have a noticeable impact, perhaps as large at 5-7%.
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Old 07-26-2016, 11:59 AM   #13
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I towed my 10.5 ft dinghy with my 40 ft single engine sunday. I had to put on an additional 100 rpm to cruise the speed I normally do.
I would say that is noticeable, but perhaps not significant for fuel consumption.
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Old 07-26-2016, 12:07 PM   #14
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I towed my 10.5 ft dinghy with my 40 ft single engine sunday. I had to put on an additional 100 rpm to cruise the speed I normally do.
I would say that is noticeable, but perhaps not significant for fuel consumption.
Useful Jay....

Whether a small dingy is shaped more like a displacement hull or a flat bottomed skiff, it only matters how full it is or how fast you tow it.

Then there is the matter of tow boat size and dingy size/resistance if a noticeable effect will even be noticed.

Without specifics....it's all conjecture.

Dingy size, how loaded, how towed, what speed and what's pulling it is needed to even remotely compare.

Not sure...but 13 years assistance towing did teach me something.
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Old 07-26-2016, 12:07 PM   #15
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We had a 9ft plastic rowing dink. rowed very well. motored awful. It Did track well but really did put drag on my 30ft sailboat
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Old 07-26-2016, 12:13 PM   #16
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If talking about drag on a sailing vessel under sail...certainly a bit different....maybe noticeable in light airs but on my 23 foot sail boat in over 10 knots of wind, dragging the dink was never noticed.

A 9 foot, lightly loaded, plastic sailing dink behind a 40 foot trawler doing 8 knots would hardly be noticeable on a flowscan compared to an increase of a knot of current or 10 knots of wind.

The better it rowed should mean the less power it needed to be towed.

Non issue.

20 something, low powered cruiser...much like a sailboat sailing in light airs...maybe a bit noticeable.
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Old 07-26-2016, 12:38 PM   #17
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when motoring...
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Old 07-26-2016, 01:39 PM   #18
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Quote:
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It all depends on the relative sizes of the boat and dinghy. Towing a 10-12 foot dinghy behind a 40-50 footer won't be noticed. In contrast towing an 8-10 foot dinghy behind a 25-30 footer will have a noticeable impact, perhaps as large at 5-7%.

THAT IS SOOOO TRUE!!!

In fact, if I had not looked back when I did at my dinghy I was towing with my 40' Silverton in Vineyard Sound, I would have lost it.
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Old 07-27-2016, 11:31 PM   #19
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I tried towing my dinghy today and was not happy with it. 43' single engine boat pulling a 10' Walker Bay Genesis RIB with a heavy 8hp four stroke. I used the tow eye on the bow but the the dinghy bow rode so high that I was concerned that it would take on water over the stern.
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Old 07-28-2016, 12:32 AM   #20
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So many factors at play!
Size of the dinghy relative to the towing boat.
Whether the dinghy is loaded properly, ie the above Walker Bay with a 4-stroke 8 on the transom is heavy by the stern, while most Hypalon RIB with the same engine will not be and will be better behaved under tow.

When I had a 30' sail, I towed a sport yak, a Galleon 9', and an Achilles soft bottom with a 15 on the transom. The smallest was the worst, the largest was the best, for towing management, but I wasn't able to detect the presence of any of them in the fuel consumption. The bigger dinghy did cost up to 1/2 a knot when sailing.
With my trawler, I now carry or tow a 12' RIB with a 40hp 4 stroke, at least 750# total weight. I am certain it costs some fuel to tow, but I have never been able to tow consistently between fills, to see how much. I tow when I am lazy but so far, I have always carried on long runs, like crossing the gulf.
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