Wavelength and towing

Discussion in 'Boat Design' started by Deering, Jun 16, 2019.

  1. Barry
    Joined: Mar 2002
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    Barry Senior Member

    The purpose is to keep the resistance of the towed boat to the minimum for the cruise speed of your cat and a quick method is to just look at the trim angle
    of the skiff. Bow up and perhaps around hump speed, 10-12 knots, you will find the highest resistance. Depending on the deadrise of the skiff you would want to get to the optimum
    trim angle. For a 12 degree skiff, the optimum trim angle is about 4 - 5 degrees, and the higher the deadrise angle, the higher the trim angle. If I can find the table I will include in another
    post. If the skiff rides quite flat and the tow line will pull the bow down a bit, the skiff may become erratic, darting right to left as the higher deadrise area, bow, of the keel digs in, causing surging in the tow line. As others have said, change the length and if you cannot put some type of scale between the tow line and the cat, just look at the trim angle of the boat

    If there is a defined wave behind the boat, the correct towline length may actually let the standing wave lift the stern a bit, reducing resistance, even at skiff hump speed, ie high bow up attitude
    When ever we towed at higher speeds with a monohull power boat, we tied a bridle to both rear cleats so the load was shared between both cleats. The bridle apex would be maybe 15 feet back and the tow line attached to the apex with a loop. We would tie off one cleat solid, and if our towed boat was not running at a reasonable trim angle,
    we could just untie one cleat and feed out some more of the bridle length until the dinghy towed better.
    Of course with larger monohulls there are distinct waves in the wake where perhaps with a slender hulled cat, they are not so well defined.
     
  2. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    I'd think it would prevent that by keeping the line semi-taut in a vee-shape with any distance between the two boats, or hanging straight down if boats together.
    Wouldn't the line tend to get into the props when NOT under any tension and floating around with slack?
     
  3. fastsailing
    Joined: Sep 2017
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    fastsailing Senior Member

    Assuming the skiff does not have mast rigged or even on it while being towed:
    What would happen if you attach the tow rope to the center of your forward beam (right under the forestay attachment if your 51ft cat is a sailing one) and length such that the bow of the towed skiff is right between the transoms of your cat. And then you would attach another set of ropes tying both the skiff bow and stern separately to each of the transoms of your cat, with just a little slack in those ropes.
    Don't even try this if your are in heavy weather, but for milder conditions, if all the ropes allow the towed skiff moving up & down easily relative to your cat without loading any rope up while that's happening, and yet restrict it moving horizontally anywhere relative to your cat, the results might be satisfactory even if suddenly slowing down, turning or other maneuvers. But it obviously depends also on the vertical clearance under the bridge deck, and the height of the skiff. If there isn't enough it can't work. And I'm assuming that the transoms extend further aft than the bridge deck, thus there is also fore & aft clearance between the boats and the forward set of ropes prevents that clearance reducing to zero.

    I don't claim this works, as I haven't tested it in size of boats described or open ocean conditions in any boat. I'm just asking if it does, and if not why not?
    I don't even know how light or heavy the skiff is. For light weight inflatables I know it's being used when davits are not available or practical for some reason.
     
  4. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I'm guessing this snarky reply nailed it. Small variations in sea-state will make finding a set rope length even at a precisely set speed tricky. I don't see finding any magic numbers that would take advantage of wave/wakes and do something like help the towed boat 'surf'. You might find certain rope length at certain speed and sea state is causing problem or "bucking", but changing tow distance should solve it, so all you can do is keep an eye on it.
    PS-I was under the impression that towing with weighted line involved enough weight to keep the weight and most of the line under water, and having the extra size of the weight in the water would provide lots of damping and shock absorption.
     
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  5. Deering
    Joined: Feb 2005
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    Deering Senior Member

    Under most conditions I agree. But unless there was a lot of weight I could see that as an invitation to suck up the line during any kind of backing maneuver. If the weight was great enough to keep the line taut during backing, wouldn’t it adversely affect towing performance by pulling the bow of the skiff down when you really want it to ride high?
     
  6. Deering
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    Deering Senior Member

    I see what you’re thinking about here and it would be interesting to explore. However mine is a power cat with a bridge deck all the way to the transom. In your concept I think the converging wave between the hulls might prove to be a problem at any but very low speeds.
     

  7. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Hammock Hang Calculator – The Ultimate Hang http://theultimatehang.com/hammock-hang-calculator/
    30lbs at 5deg "Vee" gives 172lbs of tension on the line, so figure about 150lbs of straight pull, for 30/2=15lbs extra weight on the bow.

    30lbs at 15deg gives 58lbs of tension or about 40lbs of straight pull for same 15lbs weight on bow. Sounds like about where you want to be and between 40 and 150lbs "thrust" should pull a 17ft dingy, and 15lbs of weight on each end of the line should keep it away from prop under mild backing, especially since will still be "Vee". Not a dingy towing guy myself, but I'd guess you wouldn't notice even an extra 30lbs on the bow of an "18 to 20 ft skiff".
     
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