Can a ship tow a sail boat?

[GrowleyMonster]

As for the sailing under business, I have never heard this first hand from anyone who has actually experienced this. In fact, I will offer this. I owned an old sand lighter in Belize and once accepted a tow from a skiff up the Sibun River to a landing near my farm. Displacement sailboat, long keel. Speed about 16 knots on the straight stretches. WLL was probably 25 feet. You do the math. The boat was extremely tender and wanted to yaw a lot but at no time did it ever seem to be underwater LOL. Oh, and this was of course an open boat, only a little piece of a foredeck to make for some relatively protected storage.

[Souzag818]

Originally Posted by Jdege

Saw this, yesterday.

https://youtu.be/uJHRzqB-TZI

I couldn't even watch the whole video. When I saw them pushing on the lifelines in an "attempt" to get the boat floating again, I'd had enough...

[Sojourner]

Originally Posted by roland stockham

The issue is not how fast the boat can go it is the turbulence generated behind the ship. With something like a large rib you can go twice as fast.

^^THIS^^ would seem relevant behind the kind of ships the OP was talking about.... Just the other day I cut behind a rather large Greek ferry leaving port in my (quite beefy) 3.3m metal Rib dinghy, maybe 100 meters back. The prop wash turned me sideways and it was a bit of work getting out of it. I admit I was having a bit of fun testing it as the day before that, I passed 500 or more meters behind a carnival cruise line leaving the same port and the wash pushed me sideways a good few dozen meters even at that distance....

Being towed behind that thing would be a rough ride no matter what. And no, my Amel could not survive anything like 14 knots, I'm sure of it. When you pull a displacement hull faster than it can go, it'll yaw side to side with increasing frequency and intensity until something gives.

[Patrigo]

Reading all of the above, I am amazed that so few people seem to know anything about displacement hulls and hull speed. It is something I became aware of many years ago from reading and listening.
When a displacement hull reaches its hull speed it has created a bow wave and a stern wave with a trough between. Exceeding hull speed deepens the trough and the bow and stern waves engulf the vessel. Hull speed can be calculated by the formula 1.5(square root of waterline length).[Cannot remember the exact multiplier but 1.5 is close] This happened a few times to square riggers rushing to UK with a cargo of wheat from Australia, trying to complete the journey ahead of others to get a higher price. Sometimes sails could not be doused quickly enough to avert a catastrophe! I met a Yank in Fiji years ago who told me he had taken a tow from a USCG vessel and they took off like a bat out of hell. He took an axe to the towline just as the bow wave was coming over the foredeck. I don't know enough about the science of it to appease the many sceptics above, but before you give me a hard time, just google the above formula and check it out.

[GrowleyMonster]

Originally Posted by Patrigo

Reading all of the above, I am amazed that so few people seem to know anything about displacement hulls and hull speed. It is something I became aware of many years ago from reading and listening.
When a displacement hull reaches its hull speed it has created a bow wave and a stern wave with a trough between. Exceeding hull speed deepens the trough and the bow and stern waves engulf the vessel. Hull speed can be calculated by the formula 1.5(square root of waterline length).[Cannot remember the exact multiplier but 1.5 is close] This happened a few times to square riggers rushing to UK with a cargo of wheat from Australia, trying to complete the journey ahead of others to get a higher price. Sometimes sails could not be doused quickly enough to avert a catastrophe! I met a Yank in Fiji years ago who told me he had taken a tow from a USCG vessel and they took off like a bat out of hell. He took an axe to the towline just as the bow wave was coming over the foredeck. I don't know enough about the science of it to appease the many sceptics above, but before you give me a hard time, just google the above formula and check it out.

The "official" constant is 1.34 but in actual practice, first of all this will vary with different hull forms and ballasting, and second, it is not a line in the sand, but a very very wide zone.


The lost grain ships probably went down mostly because they surfed into the next wave in a following sea, particularly when rounding the capes. Hogging and sagging with the seas may have been a factor, as well, causing hull failure in some cases. Cargo shifting was sometimes a problem, and pressing down (filling a hold to capacity) was not always the answer, because wet grain expands and sometimes grain got wet. The bow and stern wave effect contributed a little but was not the major factor, I believe. Oh yeah, and mast height. These ships carried a LOT of sail pretty high up, certainly enough to rotate the hull forward and down about its transverse axis when other conditions allowed this. Shortening sail was like pulling teeth with some of those very competitive captains.



A bow wave coming over the foredeck is not going to sink a well found boat. Bouyancy does not simply disappear. It is understandable to be alarmed at this occurrence but the boat can take it. And most modern sailboats will have a tendency to lift a bit at speed, as well, though this is not necessarily a good thing, as the boat will then be very tender. I think I already related my experience of being towed up the Sibun river at 16kts. The scary part was the yawing, and the bow's tendency to not really pay much attention to where I was pushing the stern with the tiller. Given the same opportunity again, I would have stuck with the plan and poled/rowed/sailed up to my landing. Broaching in an open boat is kind of a bad thing. Of course my neighbor didn't think anything was wrong and I was just frightened by his blazing speed which he was so proud of, and expertly executed turns around big rocks and sunken forest giants. We had a discussion about that when we arrived. I may have used some foul language before I thanked him. But I don't believe I was ever in any danger of being engulfed by my own bow wave or stern wave or being sucked down to the bottom of the river.

[Lodesman]

Originally Posted by Patrigo

Reading all of the above, I am amazed that so few people seem to know anything about displacement hulls and hull speed.

Growley was a lot more diplomatic - I'm not sure I'll be able to pull off that level of tactfulness. All I can say about the line I quoted is that it is the very definition of irony.

The water in the trough has the same buoyancy as all the water around it. You are correct that at hull-speed the wavelength of the bow-wave equals the waterline length of the vessel - that defines hull speed. This means the water is above the boot-top at the bow and the stern, but the vessel is still fully supported by the water, just the same as it would be if it was driving through a wind-driven swell of that wavelength. Pushing beyond "hull-speed" makes the wavelength of the bow-wave more that WLL, so the vessel is no longer supported in a more or less horizontal orientation. With the bow up on the bow-wave and the stern in the trough, the vessel is literally driving uphill - this is why it needs exponential increases in power to continue accelerating. Few vessels have the horsepower to fight both drag and gravity, so that's why we have hull-speed. Put in that extra HP, and the vessel will continue riding that bow-wave in a bow-up orientation; there being less water supporting at the bow, will cause the stern to sink a bit to compensate, with the decrease in deadwood making the directional stability a bit skittish.

Unless someone can provide video evidence to prove it, let's consider this nonsense about vessels "sailing under" to be utter twaddle, along with siren song and sea-monsters.

[Dave_S]

I will bet it is possible to tow almost any displacement mono at 14kn with a bit of trial and error but what ships captain is going to put up with this. Any details about ships responding to Mayday calls have surprised me about how accommodating they are but they wont give you 5 test runs to sort out the lines.

[Pizzazz]

Interesting thread from a hydrodynamic perspective. Could this work, referenced to my boat, a H31, 10,000 lb:

- Lose the mast (easy as it is deck stepped)

- Drop the keel (remove the nuts, hammer down the bolts, until the keel separates). Plug the holes. Boat is 30% lighter.

- Drop the rudder

- Attach the boat to towing line via four lines, one piece to the front keel bolt opening, one to the front cleats, one each to the back cleats. Adjust the lines, so that the boat would be bow high when towing (basically, to force it to climb the bow wave at speed).

So, could this work? I know that we are not saving much once we get rid of half of the boat but this is just a thought exercise, I agree that practically salvage is the only answer.

[boattransportpr]

Yes, but before going to go further check the size and specification of the sailboat

The more reduced the size of the boat, the less problem to tow a sailboat

[HeinSdL]

Originally Posted by Lodesman

Where does this belief come from? Have any of you actually seen a sailboat "sail under"? Rumours of square riggers going under - are you serious? I have seen displacement ships above hull speed, and can say that I've not observed any bodily sinkage and definitely no hole opens up below them. Even in the OP's video, the sailboat likely capped its hull-speed and wildly slewed over on (imho) its way towards tipping over and skipping across the surface on its side when the tow-rope mercifully parted - didn't see it start to sink though.

It's not really a belief, but in any case the theory. As a yacht speeds up its bow wave gets longer until hull speed is reached at which point the wave is of the same length as the boat and it is hard to go much faster. Hard because the wave will not get longer but will increase in amplitude, which costs a lot of energy.

Towing a boat much faster than hull speed will result in a variety of outcomes. 1. Forces required too large and the attachment point fails, 2. A suitable hull form will start to plane, 3. A heavy displacement vessel will produce a wave of sufficient amplitude that it will break on itself over the stern of the boat 4. Given that outcomes 2 and 3 require very good steering (unlikely and not a role for the faint hearted) the yacht will lose its identity as a performing hydrodynamic shape and will simply be dragged along as a lump of plastic/metal/wood. Until something 'gives'. Regardless, being towed at speeds much greater than hull speed I would probably not attempt.

[GrowleyMonster]

Regarding the grain ships. Rogue waves could well have been responsible for some losses. This is a documented phenomenon with empirical hard evidence from satellite and bouy data, not just sea stories. Even modern ships are not designed to withstand a 90 foot breaking wave, and would have no guarantee of surviving.

[MicHughV]

When I first started sailing I was very gung-ho..but also very broke.

To this end, my dink was a fiberglass rowing/sailboat, about 10' long or so. ie, a non-planing hull. On it's transom, I had a 2 hp outboard that provided me adequate zing, generating about 1-2 mph or whatever the "hull speed" of my dink was.

Being bored at anchorage one day, I decided to experiment with a friend's 9.9 hp outboard. This was strapped to my dink, together with fuel tank and me.

First attempt at full throttle nearly had me somersaulting backwards. Re-arranging my body weight further forward, I tried again. The folly of youth at work here.

The bow wave was impressive as the stern dug a hole in the water. At this point, I would say my dink was moving forward at a 45 degree up angle with the transom scant millimeters above the water and steering was a joke, but I was still only moving forward at 1-2 mph.

Try as I might, my dink was simply not going to plane, being restricted by it's underbody design...lapstrake.

The same principle applies to a displacement sailing hull, which pretty much restricts hull speed based on LWL.

Towing my fiberglass dink behind my boat at speed also resulted in the same pattern as described above, ie, bow up, transom down and enormous strain on the tow rope.

While it's true that sliding down a wave face can generate a " momentary" burst of extra speed, there are different physics involved there.

Examples may exist of sailboats being towed at speeds faster than hull speed, but the strain on the towing warp and fasteners would be substantial and would require a person at the helm at all times. One veer to one side or another would likely double the force on any tow rope.

When all is said and done, trying to tow a sailboat faster than it's own hull speed, is going to be an exercise in failure.

And it's really just as simple as that.

[a64pilot]

Funny I tried a 10 HP long ago on a double ended canoe, not one made for an engine, they have a different name.
Anyway I flipped it over I think twice before I gave up.

However on the sailing under thing, I’m relatively certain many tug boats can add enough power so that the stern goes under water, and that is something that limits their speed. Of course they do have a low stern.

[MicHughV]

yes, but a tug generally pulls from a location near it's center, so as to allow the tug steerage, from there, this is simple math, given enough throttle and tied to a towing warp attached to towing post located near the middle of the boat, the forward thrust could push the tug's transom down.

but, it's an interesting thought. I have seen many tug boats out of the water, and their underwater body is simple massive to behold. Enormous displacement. I've also had many large tugs come by me at speed.

They throw out a prodigious bow wave, that is for sure, and the aft deck does appear to be quite low to the water, but I think they are designed for this, as the railing around the deck is pretty much open at deck level, allowing any water or wave to simply run off.

[MicHughV]

A64 Pilot, I went online at Youtube to see if I could find any video testing of tug doing a bollard pull.

I attach a couple of links. Surprisingly, the stern of the tug does not squat down as much as one would think, though I'm sure it does squat some.

You can see the tow posts location though, quite a bit forward from the stern.

I am familiar with our local ocean tugs, these sport twin diesel engines of around 6,000 hp each, so no shortage of power there. These tugs are nothing more than engines and enormous fuel tanks.
I'm not exactly sure of their bollard pull stats, but it must be around 45 tons

https://www.youtube.com/watch?v=nItiyJ7wyrg

https://youtu.be/jzYC-VDas1A