Can a ship tow a sail boat?

[chrisr]

When towing one needs to do so in a seamanlike manner. there is a host of things to consider...sea state...speed...construction of tow-er and tow-ee...towline length & material...etc

There is no one size fits all, however obviously a ship could tow a yacht IF you can convince the master to go slow enough. why would he prolong his voyage by weeks with all the expected 'please explains' from owners ?

the answer is of course SALVAGE. if you (or your insurers) is willing to pay the many $000ks involved, then the rest will fall into place

incidentally, we (12m 7.7mt catamaran) recently had to be towed in by the water police (long story for another day). they decided that 10k was a 'safe' speed to tow at. considering pretty flat water and 100m of nylon tow line, it worked, but @#$% ! very stressful on the poor helmsman...extreme concentration for the 2hrs it took to get in. almost as bad as a big down hill in my racing days (but then we would change helm every 30mins or so)

cheers,

[a64pilot]

Originally Posted by lord at sea

what gives you the impression that the mast would break, considering the amount of pressure that it has to endure while under a full press of canvas?

Several things
First we are talking about much higher forces than seen under sail.
Secondly when sailing the mast is not really under that much load and it’s under compression load at that, the rigging is what takes the load and of course dumps it into the hull through the chain plates. But tie to the mast, the mast takes all of the load, and worse it’s a concentrated load, all of it is pulling at one spot on the mast. I don’t know if the mast would fail first or if it would be pulled off of the base, either amounts to mast failure though.
The mast is just an aluminum tube, and your putting it in a shear load, and it’s not likely to resist that all that well.

[Auspicious]

Originally Posted by Lodesman

If you want to call it pressure reduction in very shallow water, then go ahead - most mariners call it "squat". What you refer to as 'stern squat' has little to do with Bernoulli-effect downward "lift", but is rather a result of the vessel building a large bow wave and pitching up on that - if you say that is hydrodynamic lift, then I defer to your expertise. Squat, as mariners know it, certainly does not happen to an appreciable amount in deep water. If you have the book, then please direct us to the section that supports this assertion that a sailboat exceeding hull speed will "lift" itself underwater. IME, a displacement hull above hull speed (I know you do not like this term, but it has its purpose) makes a massive wake, and is noticeably trimmed by the stern, but does not 'submarine'.

Ultimately all lift is Bernoulli effect isn't it? You're correct that bow rise (change in trim) can result in stern squat. So can lift. Large ships nearing capacity of locks and canals bumping the bottom is pure Bernoulli.

I'm sorry that I don't have the SNAME books anymore. They were lost when I moved from a house to a boat in the consequence of my divorce.

I'm okay with the use of the term "hull speed" by those who understand it's limitations and that it is a generalization to describe a characteristic of the speed-power curve that has no deterministic value. Anyone who comes up with Bernoulli gets the benefit of the doubt. *grin*

Nothing is simple.

[a64pilot]

Originally Posted by lord at sea

given what you think , How would you be able to use an anchor for mooring or a sea anchor in a storm, if they were no able to withstand enormous pressure.

That is no where near the forces it would take to tow the boat in this case at 14 kts, not even close.

[lord at sea]

Originally Posted by Don C L

Welcome aboard lord at sea!
I am pretty sure the speed must be a good deal more than hull speed to start to threaten the boat. I've gone a good bit over my hull speed and though there was a lot of bow wave and liquid commotion the boat did not take on water. However there was enough commotion that I am inclined to believe reports of displacement hulls being pulled down low enough to take on water... but like Lodesman, I'd like to find that reference.

Your both right , I was not thinking straight last night, the term sailing under shouldn't have gone in that remark, I was thinking more about the shape of the hull and keel as to it's ability to tow in a manner that would not go askew.

[lord at sea]

Originally Posted by a64pilot

That is no where near the forces it would take to tow the boat in this case at 14 kts, not even close.

We're talking about a free floating vessel that has momentum, and is shaped to move with the least resistance possible, and it's not as if the towing ship will go form a stand still to 14 kts in mire seconds.

[a64pilot]

Originally Posted by lord at sea

We're talking about a free floating vessel that has momentum, and is shaped to move with the least resistance possible, and it's not as if the towing ship will go form a stand still to 14 kts in mire seconds.

Your confusing drag with inertia. I’m saying the steady state forces required to pull that boat at 14 kts are way more than you think.
I’d say the rope parted well before the ship got to full speed I bet.

[StuM]

Originally Posted by a64pilot

Your confusing drag with inertia. I’m saying the steady state forces required to pull that boat at 14 kts are way more than you think.
I’d say the rope parted well before the ship got to full speed I bet.

Not just steady state forces involved either.



05:31. Yacht yaws to starboard, huge extra drag (note the turbulence on the port bow) and the towrope separates.

[Montanan]

Reference https://www.cbc.ca/news/canada/newfo...scue-1.4836238

A distressed sailboat off the coast of Cape Breton has been successfully towed to Sydney, where everyone was found to be OK including the captain, who refused to leave his boat, even during the tow.

A Marine Atlantic ferry helped assist with the rescue of the man and his dog.

At 5:15 p.m. AT the coast guard ship Spindrift was able to reach the sailboat and successfully tow it to safety.

The man refused to leave the struggling vessel, according to Darrell Mercer, a Marine Atlantic spokesperson.

The MV Highlanders set sail Monday at 11:45 a.m. AT from Sydney, N.S., on its way to Port aux Basques, N.L., when the captain of the ferry got a call from the coast guard about a sailboat in distress about 20 nautical miles north of Sydney.

The ferry then shifted course to assist the craft, he said.

Maj. Amber Bineau of the Canadian Armed Forces said they received a signal from a personal locator beacon on the sailboat at about 2:15 p.m.

The 30-metre boat is a personal craft, she said, and its engine is disabled. There is also a hole in the hull just above the water line, she said.


A Cormorant helicopter flew out to the vessel, but the man insisted on staying aboard his boat, Bineau said.

"It is typical for captains to want to remain on their vessels," she said.

"Jurisdictionally, we don't have any authority to force an individual off their property."

The Cormorant landed in Sydney in case the man changed his mind or the state of the sea forced an emergency evacuation, she said.

As of Monday evening, waves in the area were about three metres high, she said.

"It is a bit rough."

Bineau thanked Marine Atlantic in being able to assist with the rescue effort.

[StuM]

Originally Posted by Montanan

Reference https://www.cbc.ca/news/canada/newfo...scue-1.4836238

...
At 5:15 p.m. AT the coast guard ship Spindrift was able to reach the sailboat and successfully tow it to safety.
..
The 30-metre boat is a personal craft, she said, and its engine is disabled. There is also a hole in the hull just above the water line, she said.

So a 15.8m Arun Class SAR vessel, equipped to do that job:


https://inter-j01.dfo-mpo.gc.ca/fdat...el-details/104




was able to tow a sailboat (and no way is it a 30m vessel based on the pictures in the link) at an undisclosed speed.


Hardly relevant to the OP.


And not surprising when it routinely does this:

[Lodesman]

Originally Posted by Auspicious

Ultimately all lift is Bernoulli effect isn't it?

I used to think so, but I heard a rather convincing argument otherwise - the best example of that is a vessel on plane; its lift is derived kinetically from colliding with the surface of the water - no Bernoulli.

[Lodesman]

Originally Posted by StuM

Nor do they "plane", they "skip".

Skipping and planing are the same thing - at least as far as the physical process is concerned.

[Auspicious]

Originally Posted by Lodesman

I used to think so, but I heard a rather convincing argument otherwise - the best example of that is a vessel on plane; its lift is derived kinetically from colliding with the surface of the water - no Bernoulli.

That's correct. When I was in school we didn't call that lift, we called it planing. Incidental vocabulary.

[billknny]

Originally Posted by roverhi

Modern ships apparently can't slow down enough to tow a sailboat at a survivable speed. Don't know what their lowest speed is but they have to have a tug pulling backwards against the forward thrust of the ship to slow down enough to meet speed requirements in SF Bay.

The speed requirement in SF Bay is 15 knots. Every ship can motor--and maneuver--at that speed.

Every ship in SF Bay carrying hazardous cargo is required to have a tug escort. Towing the tug behind lets them help with the maneuvering. They are NOT need to slow the boat down.

How do these stories get started? Does somebody just make them up? I guess if you say it with enough authority people believe you.

[Don C L]

In the case where I was in a boat being towed, we did lash the tow line to the mast. It's a pretty common practice I believe. But the mast was not taking the strain so much as it turned out, in our case it was the hawseholes where the chain jammed there. They held quite well as it turned out.
Our boat, being something of a "maxi" of that day, had a pretty flat bottom and though it was missing its rudder, remained pretty stable, there was no yawing. The skipper had us move as much weight aft as possible. The yawing, from both too much weight in the bow, and the design, and too much speed, will certainly aggravate the stress, probably in exponential ways, on the fittings and line. The weight must be all aft to keep the bow up. Even still, the weight and drag of the towline in the water tends to pull the bow down. Of course the sea state is crucial too. Being pulled down into a trough, catching the bow, it will likely try to veer off.