How practical a sailboat to tow a power cruiser for long distance travel?

[rickykngo]

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[fatherchronica]

When my wife's oldest friend visited Mana for the first time we were telling her how much we enjoyed living aboard and that the only thing we missed from living ashore was gardening. She suggested we tow our dinghy full of dirt and vegetable plants. Land lubbers, gotta love em'.

[tkeithlu]

OK, Rickykngo, if you are with us, let's add some theory to the "Have you been smoking dope again" replies you've gotten. You want economy, so here's how you get it. Any single hull boat is efficient going through the water as long as you stay below its "hull speed." Hull speed in knots is calculated as 1.34 times the square root of the length on the water line in feet. For a 44' or so boat with 40' on the waterline, that's 1.34 x 6.32 = 8.47 knots. At under that speed, a trawler with bunches of interior space will get you better than 2 nautical miles per gallon of diesel fuel. My boat meets those specs, despite being much heavier than the average for that length. Ergo, the repeated advice that you get a trawler.

Why is this, you ask? A boat going through the water makes a bow wave. The wave goes up, then down, then up as it travels down the side of the boat. The faster you go, the longer the distance from the first "up" to the second "up. Below hull speed, the transom sits on the second up. Above hull speed, the second up is behind the boat, the bow rises, the stern drops, and the engine needs gobs of fuel because the boat is climbing up a hill. It's called "plowing." On a small boat, plowing preceeds planing, and makes a whopping wake.

So, get a trawler. They are nowhere near as seaworthy as a sloop of similar length, but you are not planning on going up against a gale, anyway. They are the efficient way to get from A to B with lots of interior space.

[a64pilot]

Actually to understand hull speed efficiency is very easy.
Just look behind you, if your rolling a wake, there is tremendous energy in that wake, see how it tosses boats etc around.
The source of all that energy is of course the fuel you burn. No or very little wake? Very little energy wasted making waves. So if you want to maximize fuel efficiency / increase range, slow down until your no longer rolling a wake.

[tkeithlu]

That's beautiful. Gets past my non-artistic number-driven brain in a few sentences.

[tkeithlu]

And if you look behind the transom and you see a big hole in the water......

[HankOnthewater]

By definition, every boat is has a hole in the water, a hole we try to fill with $$$$. To my knowledge no boat owner succeeded with this task.

Still, there are some valid questions in various posts.
First the issue of a relatively small boat, using its sails, whilst towing a much larger boat. I think that question has been answered: Generally not possible and not a good idea.

I must say, I towed, using sails only, a powerboat where both engines failed... the same powerboat twice. Once in my previous (34 ft) sailing boat, and once in a 36 ft yacht. Both yachts were about 10 tonnes, powerboat similar length, but estimated 3-4 tonnes. Both occasions I tried (more to test than anything else) to sail at various points to the wind..... Upwind not possible, found that at best a beam wind was possible but leeway was 20-30 degrees. Broad reach was OK as swell and wind helped.

Second question in post #40.
Size does not matter (in this case), just horses count. A small towing boat with enough horses can tow a much larger boat to its hull speed.
In addition to horses there is another important aspect, already eluded to up-thread: the placement of the towing bollard. If it is right on the stern rail with prop and rudder much further forward..... very difficult towing and even dangerous is the towed boat is much bigger in displacement. Towing point to be over or just in front of rudder or prop.
If bollard is too far forward or too high.... capsize of the towing boat is not uncommon. Towing point to be as low as possible/practical. Lastly, towing line to be fairly long.

I have towed (in wide variety of sea rescue boats) other boats up to approx. 20 (maybe 30?) times the displacement with ease. For longer tows and/or rough seas and/or large boats, we use 100 to 150 metres of towing line.
For close inshore, calmer waters and harbours we shorten towline, and for last part of the trip we come alongside, tie to the disabled boat. We can then manoeuvre that far bigger boat with a relatively small rescue boat. When alongside, horses remain important (to steer and to stop the large boat), but the most important element is the position of where to tie alongside, and lastly the use of correct tie off points on the big boat and the small rescue boat.