Towing a spare boat??

Discussion in 'Boat Design' started by ted655, Mar 9, 2007.

  1. ted655
    Joined: May 2003
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    Location: Butte La Rose, LA.

    ted655 Senior Member located some 5/8" shock cord, rated @ minimum break strength of 500#
    Anyone know of any bigger?
  2. TerryKing
    Joined: Feb 2007
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    Location: Topsham, Vermont

    TerryKing On The Water SOON

    Wow.. that should do it.. Thinking about the forces, the fore-aft forces shouldn't be anywhere near 500 pounds unless you suddenly go WOT or one boat or the other hits something. The Side forces from a large wake hitting from the side still don't seem too bad if the compliant shock-absorbing mount you make has some travel to it. I sort of envision a 12 to 18 inch square or circle as the outer part attached to your tow bar system, and the center attachment point on the towed boat. Maybe. You can experiment with the shock cord configuration, from one strand every 120 degrees (3 cords) to a double strand every 90 degrees or more...

    But it seems, from my unscientific testing of recalling fending off medium size boats with a good hard foot-shove as they were about to the hit the dock, that something like 100 pounds (Which a good shove from a large Mariner might provide) had quite an influence on the Bow end of a small boat. And a lot less on the stern, where there's a lot more mass and resistance presented.

    Heck, I don't know how to calibrate this... I'd prototype it with some lashed-together 2x4's (That's a US inches thing...) and the shock cord, first.

    Make sure you let us all know how this works out, because One Of These Days I'm gonna run a boat big enough to want to tow another boat :)
  3. ted655
    Joined: May 2003
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    Location: Butte La Rose, LA.

    ted655 Senior Member

    As for mass, the bow is as square as the stern (well... slightly tapered). There is a 5' rake on the bow. The boat draws 10"-12" deprnding on the load. There is the OB shaft & skeg at the stern. I'm thinking of making 2-4 circle bands per side. Thats around 3,000# breaking strength per side.Thje spec I couldn't get was the "pounds of resistance per inch" This value is really nore important than breaking strength I think. It's akin to arm wrestling. More important to resist than the breaking point of the fore arm bone.
    Talking to my aluminum welder, he suggested we purposely engineer a weak point in the A-frame. He reasoned it better to fail there than rip holes out of the transom, at the mounting points.
  4. Laursen
    Joined: Feb 2004
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    Location: Minnesota

    Laursen Junior Member

    I have a few concerns one tow a 24 ft boat stern first is going to be a huge resistance
    two. the amount of movement of two vessels at plane is quite incredible even thou it appears that it is not (inquire with the water taxi from Australia the slinky one ) if one were to theorize I would suppose if one constructed a fifth wheel kind of mechanism like they do on a animal trailer not the RV kind you may get a enough movement if I remember right there was a fish farm tug out on the west coast that had a concept like that. might of even been in Canada never mind the stresses of a large vessel passing anything that is ridged will not hold and if it does it will not be for long
    I have seen alot of ideas come and go as both a builder and as Tug Skipper for many years The forces in a lock system is unbelievable and are not evident until you are running double or triples tied to each other

    I have seen in the panama system during a storm that the front stanchion was ripped right off the barge and it had four inches of steel hanging from it and the rest of the mount area was intact on the barge :eek:

    Frank Laursen

  5. ted655
    Joined: May 2003
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    Location: Butte La Rose, LA.

    ted655 Senior Member

    :) Thanks. I'm not towing anything stern first. It was a proposel by another, thinking out loud.
    At best, a towbar/ball is the most weight & complication I want fastened to the stern of the HB. It wasn't stated but, neither boat will plane. 3-5 knots is tops.
    All any of us can do on this planet is to design & engineer for the "reasonable".
    Anytime we try to best Mother Nature, we lose, BUT nothing would be built if absolute success was the criteria. We've been thru 2 major hurricanes. I know what can happen, I choose to focus on what is reasonable prudence. If caught in a storm as you have experienced, the first thing I:) 'd do would be to unhook from my tow and spend my efforts on just surviving.
    Anything can be underbuilt OR overbuilt. I'm looking for that spot inbetween.
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