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Old 11-19-2019, 11:33 AM   #1
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Towing a tender

Question, we have a 30 ft Sundowner trawler with a new 50 hp Beta Marine engine, we are planning some 8hr - 12hr cruises towing a 16 ft Key West tender, approx weight 900 lbs. What concerns should we have regarding fuel consumption and speed. Will a tender that size creatate significant negative effects?
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Old 11-19-2019, 12:13 PM   #2
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I do know your boat but not your dinghy.
I doubt you will get a definitive answer on increased fuel use since the only ones who will know are those already doing EXACTLY the same using exactly the same setup.

Fuel use will go up and your boat speed down a bit at the same engine rpm, towing vs not towing, I would GUESS 10-15%.
The dingbat will add drag.
That increase in fuel use should not be a deal breaker unless you are planning to push the main boats tankage range.

I know many people who tow dinghys successfully with boat/dinghy ratios similar to what you are proposing and although the dinghy can be a problem sometimes they would not be without it.

But I cannot answer your question any more precisely.
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Old 11-19-2019, 12:30 PM   #3
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You also can hold the tow line at different lengths to find the least resistance and tied it off
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Old 11-19-2019, 01:38 PM   #4
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As noted above about adjusting the tow line length, dinghy towing can have very little effect on fuel usage of the towing boat. Even in the worse configuration, with the dinghy always pointed bow up into the boat's stern wave, the effect on fuel consumption is mostly unmeasurable.

Here is some physics and math: Worst I have seen is a tension of about 100 lbs on the tow rope at 7 kts. 7 kts is about 12 ft per second. So that is 100*12= 1200 ft lbs/second or about 2.2 hp. Since it takes about a gallon per hour to make 18 hp for most diesels that is 2.2/18=.12 gph of diesel. You won't begin to notice that little diesel usage and it will be much less than that if you adjust the tow rope length correctly.

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Old 11-19-2019, 02:11 PM   #5
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We towed a 18.5’ Key West, center console two years ago from the Berry's to Vero Beach via Lake Worth. No problems towing. The tow line was ~75 feet plus the length of the bridle that probably added another 20'. Once in the ICW, we shortened the tow line to ~50’.

I’d think about doing it again. We’re full displacement and couldn't go fast enough to get the Key West on/out of the water and to maintain 6.5-7 knots our fuel burn went up 25-50%. It was like pulling a plow. Docking with just Lena and I was also a bit of a challenge.

It was nice in the Berry's though, the additional mobility/access to other (skinny) waters without having to move Hobo was pretty cool.
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Old 11-19-2019, 02:39 PM   #6
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An 18-1/2' center console is more than a dinghy. It will weigh 1,500-2,000 lbs which is 5-10 times more than a 10' RIB with a 10 hp O/B on it. That will increase its towing force to as much as 1,000 lbs which will definitely result in a measurable fuel consumption increase.


But I agree, having such a tender would be nice in the Bahamas and may be worth the extra fuel consumed.


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Old 11-19-2019, 03:31 PM   #7
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Joe: Can you try towing the Key West locally to give it a try? What didn’t help my situation was the Key West was half full of fuel plus fishing rods, anchor, life jackets, boat toys, lines, tools, etc. The weight adds up quick.
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Old 11-19-2019, 05:33 PM   #8
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I spent the Summer towing a hard shell Bullfrog 11 ft with a 30Hp. I used a bridal for the longer runs, and a single line on a side cleat when in protected waters. I would make minor adjustments to the tow bridal length as needed, but come night fall didn’t touch it and just checked it was still back there. Largest swell we hit while towing was 4 to 5 ft on 6 seconds. I have always carried a small Dinghy before, so towing this past Summer was a new gig. Learned a lot, and got a nice cut on my finger during the process...another story. This is smaller than a center console, but my unscientific feeling was that I was losing about a .5 kn of speed when towing offshore, especially with any chop or swell.
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Old 12-08-2019, 09:18 AM   #9
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An additional question on towing a tender which I asked before but posted it on a wrong thread. Anyone have issues with the inconsiderate people on large boats fly past creating huge wakes not thinking a thing about it! I'm curious if any of you have had your tender sink or damaged by large wakes? I'm thinking the shorter the line the more control we will have over the situation. Any thoughts?
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Old 12-08-2019, 09:22 AM   #10
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Something to be concerned about, 14 years as an assistance tower and many a time wakes and boaters not paying attention threatened tows.
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Old 12-08-2019, 10:23 AM   #11
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Quote:
Originally Posted by Turner View Post
An additional question on towing a tender which I asked before but posted it on a wrong thread. Anyone have issues with the inconsiderate people on large boats fly past creating huge wakes not thinking a thing about it! I'm curious if any of you have had your tender sink or damaged by large wakes? I'm thinking the shorter the line the more control we will have over the situation. Any thoughts?
A good question....
This is why we never towed dinghies less than about 16'.
And the ones we did tow were RIBS that had self bailing hulls and were towed with a bow up attitude.
In the same category of concerns was heavier seas and longer heavy rains.
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Old 12-08-2019, 10:35 AM   #12
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A large boat wake was how I got the cut on my finger as noted in my earlier post. I was in the middle of the bay, close tied the tender to get inside to make an adjustment. A big boat came by and I went on a bucking bull ride. The Dink slammed on top of the swim step and took a chunk of FG out. The next wake I was aiming for the side of the boat, and thought I would take a piece of FG out off the side of the boat. Lesson, secure everything properly and don’t get inside the Dink when in the Harbor, and not in the ocean unless absolutely necessary. Pay attention to your tow line and lengths when running in congested areas in mooring fields etc. I put a little reminder note next to the helm: “Towing, close haul when needed”. Don’t rely on your Nephew to tie it off the Dinghy prior to dropping the mooring can hawsers on the bigger boat without checking he did it properly. I could go on.
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Old 12-08-2019, 12:07 PM   #13
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We towed a 19 Grady behind our Mainship 390 while replacing our dinghy. There was no noticeable effect and it was nice to have it once we reached our destination.
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Old 12-08-2019, 01:21 PM   #14
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15-20% will be right in there. That's what I observe with my rig, more like 15 I'm inclined to say. 31 towing a 17' center console, big difference is I'm 210 hp. At my standard cruising rpm it slows me down 15% with about the same fuel burn.
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Old 12-08-2019, 01:45 PM   #15
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I'm not a big fan of towing a tender. I had a couple poor experiences, one from trying to go to quickly, one from a poor towing bridle and once from a combination of things. Much rather go through a hassle and get the dink out of the water (Not perfect either).

But.. towing a 900 # 16 foot tender with a 50 hp main engine ? I don't think you should do it. How did you end up with a 30 foot boat with such low hp? I think the combined weight of your vessel and the tender exceeds the capacity of that engine. Not just weight but combined "drag".

I suspect some of our math experts will crunch the numbers and share the results with us.

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Old 12-08-2019, 02:33 PM   #16
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50hp is plenty of hp to tow...as long speed isn't required....or sea conditions aren't survival conditions.

Everyone should be prepared to tow as sometimes ones davit (s) may not be working as advertised.
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Old 12-08-2019, 02:53 PM   #17
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I'd be very wary about towing a lightweight inflatable boat. Besides, isn't a main point of having a lightweight dinghy to have something easy to carry on board?

We towed our 13' Boston Whaler in all kinds of natural and man-made sea conditions. It took them all with aplomb.
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Old 12-08-2019, 03:32 PM   #18
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I would much rather tow an inflatable under normal conditions than a small hard skiff.

High winds and all bets are off, probably including small skiffs anyway.
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Old 12-08-2019, 04:37 PM   #19
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We towed our 13' Boston Whaler in all kinds of natural and man-made sea conditions. It took them all with aplomb.[/QUOTE]

Summers throughout 1960's my family often traveled off shore NY to Maine, and in between. Had 13'3" Boston Whaler with 40 hp johnson either in tow aback our 38' SD sport fisher sedan... or... I'd take off in the Whaler and meet family at predetermined next port of call. During both types of phases of tender included travel, we have been in some pretty nasty seas [close duration big rollers from off shore storm with strong wind chop]; easily successful results occurred always of tender accompaniment.

Currently, in SF Delta fresh waters [usually very calm conditions], I tow a 14'8" Crestliner "Stinger" with 50 hp Johnson. The secret is to know how long the tow line should be in coordination with tow-boat's wake configuration as well as sea conditions. Always best to have fairly easy way to modify tow line length. For inner harbor tow; close to transom or side tie works well. If you go close to transom be sure you have appropriate fender [or rub rail] protections in place - so the tender does not put dents into tow boat transom and tow boat's swimstep does not damage [mark up] the tender's hull.

I would not like to do the 14'8" Crestliner in similar tow conditions as mentioned above during the 60's. Boston Whaler 13'3" was a joy to do it in all tow conditions. Off Jones beach LI, NY I had that Whaler in some really rough breakers. Yeah... I was young and a little too brave back then in my teens!

GO Whalers!!
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Old 12-08-2019, 04:47 PM   #20
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Mostly a name, not a fact.

They do have some strong points, but brand allegiance is way beyond reality in my professional boating experience.
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