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07-25-2016, 04:00 PM
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#1
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Veteran Member
City: Hampton
Vessel Name: Saltwater Taffy
Vessel Model: Silverton 43
Join Date: Aug 2015
Posts: 64
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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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07-25-2016, 04:56 PM
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#2
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Valued Technical Contributor
City: Litchfield, Ct
Join Date: Aug 2012
Posts: 6,785
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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.
David
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07-25-2016, 06:19 PM
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#3
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Guru
City: New England and Canada to Florida
Vessel Name: Tadhana
Vessel Model: Helmsman 38 Pilothouse
Join Date: Jul 2011
Posts: 596
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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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07-25-2016, 07:33 PM
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#4
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,149
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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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07-25-2016, 08:50 PM
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#5
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Guru
City: Fort Lauderdale. Florida, USA
Join Date: Jan 2014
Posts: 21,449
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Quote:
Originally Posted by psneeld
: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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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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07-25-2016, 10:31 PM
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#6
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Guru
City: Puget Sound
Join Date: Jun 2016
Posts: 631
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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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07-26-2016, 12:44 AM
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#7
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TF Site Team/Forum Founder
City: League City, Tx
Vessel Name: Floatsome & Jetsome
Vessel Model: Meridian 411
Join Date: Oct 2007
Posts: 7,332
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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.
__________________
Prairie 29...Perkins 4236...Sold
Mainship Pilot 30...Yanmar 4LHA-STP...Sold
Carver 356...T-Cummins 330B...Sold
Meridian 411...T-Cummins 450C
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07-26-2016, 08:17 AM
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#8
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Veteran Member
City: Hampton
Vessel Name: Saltwater Taffy
Vessel Model: Silverton 43
Join Date: Aug 2015
Posts: 64
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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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07-26-2016, 09:04 AM
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#9
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Guru
City: Ashland, MA
Vessel Model: 1990 Silverton 40 aftcabin
Join Date: Jul 2016
Posts: 1,208
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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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07-26-2016, 09:16 AM
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#10
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Guru
City: Stuart FL
Vessel Name: Lucky Lucky
Vessel Model: Pacific Mariner 65
Join Date: Aug 2013
Posts: 2,760
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Quote:
Originally Posted by BandB
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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OK, thanks. Something new to add to my bucket list.
__________________
Howard
Lucky Lucky
Stuart, FL
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07-26-2016, 10:46 AM
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#11
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Senior Member
City: Bristol PA
Join Date: Jul 2016
Posts: 137
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Many rowing dinks are actually displacement hulled. so yes.. there will be drag.
__________________
Retired HVAC, wooden boat aficionado & builder of smaller craft. NOT Josie the plumber!
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07-26-2016, 11:30 AM
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#12
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Guru
City: Maine Coast
Vessel Name: Tortuga
Vessel Model: Nunes Brothers Raised Deck Cruiser
Join Date: Feb 2015
Posts: 889
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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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07-26-2016, 11:59 AM
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#13
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Guru
Join Date: Jun 2008
Posts: 5,037
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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.
__________________
Jay Leonard
Ex boats: 1983 40 Albin trunk cabin, 1978 Mainship 34 Model 1
New Port Richey, Fl
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07-26-2016, 12:07 PM
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#14
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,149
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Quote:
Originally Posted by jleonard
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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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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07-26-2016, 12:07 PM
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#15
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Senior Member
City: Bristol PA
Join Date: Jul 2016
Posts: 137
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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
__________________
Retired HVAC, wooden boat aficionado & builder of smaller craft. NOT Josie the plumber!
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07-26-2016, 12:13 PM
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#16
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Guru
City: Ft Pierce
Vessel Name: Sold
Vessel Model: Was an Albin/PSN 40
Join Date: Oct 2011
Posts: 28,149
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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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07-26-2016, 12:38 PM
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#17
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Senior Member
City: Bristol PA
Join Date: Jul 2016
Posts: 137
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when motoring...
__________________
Retired HVAC, wooden boat aficionado & builder of smaller craft. NOT Josie the plumber!
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07-26-2016, 01:39 PM
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#18
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Guru
City: Ashland, MA
Vessel Model: 1990 Silverton 40 aftcabin
Join Date: Jul 2016
Posts: 1,208
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Quote:
Originally Posted by TDunn
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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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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07-27-2016, 11:31 PM
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#19
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Guru
City: Gig Harbor
Vessel Name: Kinship
Vessel Model: North Pacific 43
Join Date: May 2015
Posts: 9,046
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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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07-28-2016, 12:32 AM
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#20
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TF Site Team
City: Saltspring Island
Vessel Name: Retreat
Vessel Model: C&L 44
Join Date: Nov 2007
Posts: 5,663
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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.
__________________
Keith
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