Towing a tender

[Turner]

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?

 

[C lectric]

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.

 

[lipets]

You also can hold the tow line at different lengths to find the least resistance and tied it off

 

[DavidM]

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.

David

 

[Larry M]

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.

 

[DavidM]

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.


David

 

[Larry M]

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.

 

[Fletcher500]

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.

 

[Turner]

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?

 

[psneeld]

Something to be concerned about, 14 years as an assistance tower and many a time wakes and boaters not paying attention threatened tows.

 

[smitty477]

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.

 

[Fletcher500]

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.

 

[gotfw]

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.

 

[EngNate]

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.

 

[Pete Meisinger]

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.

pete

 

[psneeld]

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.

 

[caltexflanc]

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.

 

[psneeld]

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.

 

[Art]

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!!

 

[psneeld]

Mostly a name, not a fact.

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

 

[Art]

Mostly a name, not a fact.

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

If you're referring to my post #19; wherein the 1962 13'3" Boston Whaler I owned back in the 1960s was somewhat compared to the 1975 14'8" Crestliner we currently have... Your post's inclination is partly correct in my view.

In that: Both boats are built very well to last and last!

However, having piloted both runabouts in some fairly nasty stuff... I'll take the Whaler's seakeeping capability heads and tails over the Crestliner.

That said: Due to completely different hull shapes/configurations... comparing the two little boats against one another for sea keeping... is like apples and oranges.

Back in the day - The super stabil imitation tri hull bottom configuration of the Whaler did not bother my back too much when sitting on its hard wood bench as it slammed down onto waves/wakes [solo - after being airborne].

Now: My wife and I really enjoy the thick cushioned seats in the mono hull configured Crestliner whose bottom's forward V allows a more gentle "push" [entry] through waves/wakes.

I really dug my Whaler! We really dig our Crestliner!! Both boat brands are basically bullet-poof; so long as well cared for and not beat to death during use.

 

[Lepke]

Make sure what you fasten the towline to on both ends is strong enough. When you're towing, try pulling the towline and you'll get an idea of the force.
People on any size boat throwing a big wake are a PIA. Wish I could shoot 'em.

 

[AusCan]

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.

pete

My 30 foot boat has 42 hp which is more than enough to tow a tender if I ever wanted to. I almost never use more than about 30 hp.

The sea conditions are usually too rough around here to tow a tender except on unusually calm days.

The last time I towed my inflatable dinghy, the tow line snapped in heavy seas, and it took a month to find it again. Now it rides on the roof, and there's no problems.

 

[Turner]

We contacted a Beta Marine dealer to price a 75hp - 100hp engine. After their engineer did the math on our displacement hull and prop size the said that both those up engines were to big for our boat and would be very inefficient. He said he didn't want to waste our money, he recommended a 50hp, said it would run 24 hrs a day at 1/2 throttle running 7.5 knts at 2,000 rpm. I was sceptical but impressed with his honesty. We bought the 50hp and it has done exactly what he said it would do! 1/2 throttle, 7.5 knts, 2,000 rpm using 1 1/4 gal an hr! He mentioned that many people think bigger is better which maybe true in some situations but not for a full displacement 30' trawler. I thank him repeated for saving us a few thousand dollars!

 

[caltexflanc]

Mostly a name, not a fact.

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

Well, as I learned in reality, a 2005 Boston Whaler 130 Sport tows beautifully in a wide variety of sea conditions, including beam seas at the margins of what was bearable on a stabilized Hatteras 56MY.

 

[psneeld]

I guess beauty and reality is in the eye of the beholder.


I venture to say all small skiffs have their issues....depending on what one likes and dislikes about them.


I'll just restate my point in post #18..... "I would much rather tow an inflatable under normal conditions than a small hard skiff."


In conditions approaching uncomfortable in the mother-ship, I rather not be towing at all.

 

[EngNate]

If you're going to tow a skiff like this regularly its best to have a towing eye installed (on the skiff) as low as possible, just above the waterline. The trailering eyes are too high and this causes the bow to dig in and the boat to track poorly. If this is impractical to do, get the attachment on the towing boat high, and the weight in the towed boat aft for better handling. My tow line is just 65' long and I've found that to work best with my rig. I've dragged the 17' around with the 31 for years. One does have to observe that sea conditions will impose limitations, and long open crossings may not be a good idea. I don't tow my small boat through any conditions that I couldn't drive it through.

 

[Ski in NC]

I towed an 18' skiff with 70hp OB behind a 33' snapper boat. Towed it about 150nm at 7.5kts. At that speed the skiff was on the hump and tow line was way over 100lbf of tension. Bow high and plowing. I ended up reducing speed to get its bow down. Also adjusted tow line length to get it in a good spot between stern wave humps. Skiff weight maybe 1400lb. I did not check fuel burn but at 7.5kts engines were definitely working harder. At 6.5 rig settled down and tension in tow line went WAY down.

That whole hull speed thingy. Short boats behind longer boats and the hull speeds don't match.

 

[psneeld]

Good points Ski.... but I will add a light boat like a RHIB may plane easily when unloaded and towed at some reasonable speed. Even hard dinks if the speed gets high enough and the dink is on it's flat spots early.


Also true that some trailer eyes are too high to tow from...especially at higher speeds...but I will say very few boats I towed from trailer eyes for 14 years as an assistance tower didn't work for what I needed. Usually it's a matter of either tow hawser length .....short and if the tow point is higher than the eye no worries...long tow with catenary pulling downward and depending on bow buoyancy, the eye may or may not need adjusting/moving.

 

[Turner]

I was shocked when the salesman recommended a 50hp and almost ignored him, I'm glad I didn't. In fact he said he wouldn't sell me a larger motor and I would have to find another dealer if I wanted to waste my money! He took the measurements and pitch of our prop, studied the contours of our boat and said we would do fine with a 50hp. Said we could run 24hrs at 2500 rpm (7 knots) at half throttle. He was right. Anything larger the extra horsepower would never be used and when higher rpm attempted the stern would squat in the water, speed wouldn't increase, fuel consumption would, and it wouldn't be practical as some owners on this site attested to with the regret of going with more hp. With a semi displacement hill more is not better. Hats off to the honest salesman who was more interested in doing the right thing than making more money!

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.

pete