Wavelength and towing

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

  1. Deering
    Joined: Feb 2005
    Posts: 477
    Likes: 23, Points: 18, Legacy Rep: 44
    Location: Juneau, Alaska

    Deering Senior Member

    I’m exploring the option of towing an 18 to 20 ft skiff behind my 51 ft catamaran. Typical cruise speed would be about 11 to 12 kts. I’d like to determine the length of my tow line. There’s a pretty steep transverse wave about 50 feet off my stern that appears to be unaffected by rough water, but that strikes me as too close. The next wave isn’t as well developed and experiences disruption from other waves, but would probably work. Thoughts/advice?

    I’d also like to calculate the position of those waves. I assume that depends on the tow-boat’s velocity as well as it’s length. Can you point me towards the equations that defines that?
     
  2. BlueBell
    Joined: May 2017
    Posts: 1,259
    Likes: 241, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    The equation is the length of rope that it tows best with.
    Try different lengths.
    Then let us know what length towed best.
    Okay?
     
  3. Deering
    Joined: Feb 2005
    Posts: 477
    Likes: 23, Points: 18, Legacy Rep: 44
    Location: Juneau, Alaska

    Deering Senior Member

    Thanks for your insightful response. I was trying to approach this as a design question (boatdesign.net, right?). My bad.

    Now about that wavelength question - any pearls of wisdom there?
     
    BlueBell likes this.
  4. BlueBell
    Joined: May 2017
    Posts: 1,259
    Likes: 241, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Okay, we've established you have a sense of humour, perhaps the most important variable.
    No idea how one calculates stern waves, sorry.
    But don't you really want to know the best spot in one of those waves to tow in?
    You could use a spring scale in the tow line to measure drag and "tune" accordingly.
    Cheers
     
    Barry likes this.
  5. Rurudyne
    Joined: Mar 2014
    Posts: 1,163
    Likes: 36, Points: 48, Legacy Rep: 155
    Location: North Texas

    Rurudyne Senior Member

    The length of the rope used should be determined by the following formula:

    Length = ( yacht freeboard to tie off + 2 x dingy LWL x MIL fudge factor ) / AW fudge factor

    MIL: determined by how annoying your mother-in-Law in the dingy is

    AW: determined by how annoyed your wife is that her mother is in the dingy
     
    Deering and BlueBell like this.
  6. JSL
    Joined: Nov 2012
    Posts: 793
    Likes: 54, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    Not sure about a cat but when we towed a 15' skiff (planing hull) behind our 46' power cruiser at about 12 knots we had the 'tow' running down (just below the crest... less resistance) of the 2nd wave (first one after the 'rooster tail') from the stern. In open water you can have it on the 3 wave (a better MIL rate) but the ocean state my influence the 'tow'.
    And... Watch out for 'twerps' than might want to scoot past your stern and foul the towline.
    Hope this makes sense.
     
    Rurudyne likes this.
  7. Deering
    Joined: Feb 2005
    Posts: 477
    Likes: 23, Points: 18, Legacy Rep: 44
    Location: Juneau, Alaska

    Deering Senior Member

    Totally understand the fudge factor, though it varies considerably. At times my wife is even happier than I am that her mother is in the tender. I’ll have to think about the math on that one.

    If the tender were 15 ft that’d have it at less than 35 ft behind my boat. Would I run the risk of being rammed if I slowed down suddenly?I’ve heard from others that the rule of thumb is 2x the LWL of the mother ship, but don’t know how that was derived.
     
  8. Deering
    Joined: Feb 2005
    Posts: 477
    Likes: 23, Points: 18, Legacy Rep: 44
    Location: Juneau, Alaska

    Deering Senior Member

    So if I’m understanding you, the tow would have been located something like 40’ behind you...does that sound about right? Or was it the next wave after that?

    What’s a ‘twerp’?
     
  9. philSweet
    Joined: May 2008
    Posts: 2,337
    Likes: 199, Points: 63, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    It's an idiot on a pwc that gets mad at you after they clothesline themselves.
     
    Deering likes this.
  10. Squidly-Diddly
    Joined: Sep 2007
    Posts: 1,661
    Likes: 101, Points: 63, Legacy Rep: 304
    Location: SF bay

    Squidly-Diddly Senior Member

    Don't real ships towing other real ships put several tons of weight on the tow line as cushion? IIRC some WW2 destroyer (2500 tones?) put 6 tones of weight on a tow line.

    Bonus: when you come to stop the boat being towed will naturally snuggle up to the mothership as the weight drops.....and/or anchor both boats as weight finds bottom.

    I'm thinking about 30lbs would be right for 17ft boat in tow.
     
  11. JSL
    Joined: Nov 2012
    Posts: 793
    Likes: 54, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    yes..Twerp = idiot, cad,
    As to tow length, I guess one boat length (mothership) sounds about right. As a kid I was told to make sure there was some curvature/sag ( catenary) in the towline. We never fitted weight (the 'tow' was never close enough) but did keep a knife handy in case the 'tow' got out of hand like in some rapids* and had to be cut free. Our rapids in BC can run up to 17 knots - quite a ride in a 12 knot boat.
     
  12. Deering
    Joined: Feb 2005
    Posts: 477
    Likes: 23, Points: 18, Legacy Rep: 44
    Location: Juneau, Alaska

    Deering Senior Member

    Seems to me that would increase the chances of your towline being sucked into the props.
     
  13. Rurudyne
    Joined: Mar 2014
    Posts: 1,163
    Likes: 36, Points: 48, Legacy Rep: 155
    Location: North Texas

    Rurudyne Senior Member

    It was kinda off the cuff. I may have easily read the LWL mothership and misremembered it.

    Really choosing the rope length based on your boat's wave system at the time sounds like a great idea.
     
  14. JSL
    Joined: Nov 2012
    Posts: 793
    Likes: 54, Points: 28, Legacy Rep: 41
    Location: Delta BC

    JSL Senior Member

    Deering_ yes, it can get sucked into the props... especially if the weight is not at 'mid-length' of the towline. The one time I used 'weights' it was a 2L plastic milk container filler with water. The 5 lbs did the trick and was neutrally buoyant when immersed so did not sink.
     

  15. BlueBell
    Joined: May 2017
    Posts: 1,259
    Likes: 241, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Deering,
    You're an accomplished guy.
    You're a hands on guy, I see that.
    I have some tough news for you:
    There is no equation for tow line length.
    But the good news is, you don't need one!
    If it's an academic question then it's for you to figure out.
    But why, when you just adjust the length according to sea conditions, loading, weather, etc, etc, etc.
    No formula for that.
    Is there?
     
    Last edited: Jun 19, 2019
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.