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.

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

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.

:My new dingy might up my fuel bill....

Probably it won't be towing it though that causes the problem fuel bill.....

psneeld said:

: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.

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..

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.

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.

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.

BandB said:

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.

Many rowing dinks are actually displacement hulled. so yes.. there will be drag.

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%.

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.

jleonard said:

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.

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

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.

when motoring...

TDunn said:

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.

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.

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.

Dave,

How fast were you towing? Did the boat swamp? Did the dinghy yaw back and forth? (I always town with a double bridal with the bitter ends off both stern quarters to dampen the yaw.)

Paul

dhays, same WB Genesis RIB but with a 15hp Honda. A miserable tow! In fact, a miserable drive at low speeds, even with the console. Nose high until it tips down on a plane.

Fun with numbers:

I figure my 8' dink has about 40lbs of tension in the tow line at 7.5kts. Measured with my calibrated right arm.

7.5kts is about 750 ft/min or about 12.5 ft/sec.

40 lb at 12.5 ft/sec is 500 ft lb/sec.

One hp is 550 ft lb/sec, so my tow is taking 0.91hp.

My main engine is buring 1.9gph, and at this low power setting is probably about 15hp/gph, so about 28hp going down the shaft.

At that slow speed the prop slip is high. 12.02kt theoretical vs 7.5 actual. Applying this ratio to the boat power required, 7.5/12.02 x 28hp gives 17.5hp needed to push the main boat. The rest of the hp going down the shaft is wasted in prop slip. Sloppy math I know, but probably close.

So 0.91hp is for dink, 17.46 for total rig. So about 5% of my burn can be attributed to dink pulling.

Bigger effect could come from tweaking rpm by 25.

Now towing a big hardshell dink, those tow lines are TIGHT. It would be interesting for someone to put one of those 1000lb fish scales on a tow and see exactly how much pull is required.

Too hot to work and my tile guys did not show again. And I even got the AC going for them last night. Grrr. Don't ever build a house. Off to Lowes. Again.

DHeckrotte said:

dhays, same WB Genesis RIB but with a 15hp Honda. A miserable tow! In fact, a miserable drive at low speeds, even with the console. Nose high until it tips down on a plane.

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The dinghy and motor combo came with my boat when I bought it. I vowed not to do anything about them until I had lived with them for at least a year. They were bought by the PO when he first bought this boat and I don't think he had much experience. The RIB is a good boat. The Honda 8hp 4-stroke is a great motor, and with power tilt and electric start is much nicer than any outboard I have owned previously. The problem is that they are a terrible combination. The 8hp doesn't have enough power to get the RIB up on a plane. I have the version with a floor but no console, so heavier than some RIBs but lighter than yours. While not being able to plane is fine, the motor is so heavy that, as you said, it is always bow high.

I have considered selling the motor and putting my Torqueedo outboard on. It won't plane the RIB but is a lot lighter, cheaper to maintain, and doesn't require gasoline. I would go any slower than I do now and would do it more quietly and with less wake. It also might tow better since I could easily take the motor off and apart. My wife would like a RIB with a console that will plane, but my crane is only rated for 600lbs so not sure I want to spend the money.

Ski in NC said:

Too hot to work and my tile guys did not show again. And I even got the AC going for them last night. Grrr. Don't ever build a house. Off to Lowes. Again.

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That's where I'm lucky. Wouldn't know how to build a house, so only way I could do it would be a contractor. So far we've never gone that route, although we have done some significant remodeling of houses we purchased. Always contracted.

Dave, I don't think your issue is totally related to weight on the stern. I think it is a function of speed. You are right at the speed where the dinghy wants to plane. If you could go 11 knots or more, your dinghy would probably pop right out of the water and happily glide along on plane.

With that said, going from an 8hp to a 25hp would not increase your weight greatly....at least I don't think. Especially if you could find a used two stroke...which is likely similar in weight to your 8hp four stroke.

dhays- Can you rig a piece of split pvc pipe and hose clamp to make a tiller extension? If you can sit a few feet fwd that dink might plane.

For a while I had a 4hp evinrude two stroke on my dink and it would only plane if I rigged an extension. Even with the extension, a small cooler with six beers, a gallon of bilge water, a full meal in my belly... Any of those would prevent planing, it was that marginal.

BnB... One and done for me and this house crap. About 90% done at this point.

Last summer we towed our 15' Klamath aluminum fishing boat from the Seattle area to the Broughtons and back a little over 800nm with no discernible change in fuel consumption. It towed like a dream even in 5-7' seas we encountered on the way from Comox to Nanaimo.

"I have considered selling the motor and putting my Torqueedo outboard on. It won't plane the RIB but is a lot lighter, cheaper to maintain, and doesn't require gasoline. I would go any slower than I do now and would do it more quietly and with less wake. It also might tow better since I could easily take the motor off and apart. My wife would like a RIB with a console that will plane, but my crane is only rated for 600lbs so not sure I want to spend the money."

Dave:

I bought my present dinghy here on TF from a guy who was downsizing to one like yours, for the simple reason that his (owned) slip wouldn't accommodate his newer, bigger boat with the Caribe 12 on the transom mount he had used previously, and was too heavy for his 600# capacity crane. I would be interested to hear his comparison between the two dinghies after a few years of using the WB.

My swap was a significant upsizing, from a 10' RIB with a 20hp 2 cycle, to this 12' with 40hp and a console. It tows well, cruises very well at 20 knots, and I can lift it onto my davits. If I was in your present shoes, I would listen to your wife, and do whatever that requires, to get a faster, planing dinghy with a console. It is for her after all.

Ski in NC said:

dhays- Can you rig a piece of split pvc pipe and hose clamp to make a tiller extension? If you can sit a few feet fwd that dink might plane.

For a while I had a 4hp evinrude two stroke on my dink and it would only plane if I rigged an extension.

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Tried using an extension and still it would not quite plane. Likely I am just too fat. I will try again.