Lengthening a sst60 F3 tunnel boat

Discussion in 'Boat Design' started by Foiltech, May 12, 2016.

  1. Foiltech
    Joined: May 2016
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    Foiltech New Member

    Hi all first time poster. I have a F3 tunnel boat my daughter races here in Australia. The boat was originally built to be 5 metres long and was cut back to 4.3 metres a couple of years ago to run as a F3. The boat is timber
    I now want to extend it back to its original 5 m length I'm looking for some advice pictures help on how to do it properly. I have built several tunnels before but I have never extended one. The previous owner basically cut it at the second to last frame and capped it all and put in a new transom. Here's a couple of pics
    Thanks in advance
    Cheers
     

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  2. Foiltech
    Joined: May 2016
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    Foiltech New Member

    Another
     

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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Have you tried stretching it, with a tractor pulling on each end ? No ? You'd have to say doing it properly would not be much easier than just building another boat, considering that structural failure could be very nasty. You could cover some of that cost selling the short one.
     
  4. TheChillPrince
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    TheChillPrince Junior Member

    I agree with Mr Efficiency,I also say you may save the life of your daughter by selling it and either building her or buying her a new one. If you do sell it tell the buyer it was shortened.

    I never trust stretched or shortened boats and never will, to me that just adds a weak point, because you just don't know how good it was done, especially if you do it yourself when you have never done this type of thing before.

    But whatever you do i wish you luck, Also welcome to the forum.
     
  5. boatenthusiast
    Joined: Jul 2015
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    boatenthusiast Junior Member

    I agree with the above posters, Either sell it or build a new one.
     
  6. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    I have extend my boat with 80 cm, but I have ADDED the extra 80 cm, NOT extend it. If you extend it, you may weaken the boat. I made an extra 40mm Transom to the existing transom and have bolted it with proper 1/1/4 inch stainless steel bolts and stainless steel plates to the old transom + walls. If it is properly engineered to the existing boat hull, in such a way that it becomes a No risk for your daughter. If the stainless steel plates are mounted in such a way that it becomes a solid unit with the old part of the boat and transom. Also I have filled the new added part with foam, to give extra save buoyance. You may need to fill the maybe small gap between the two sections with a mixture of not more than 30 % special lime and fibre glass. The shape is up to you what it should look like when a new section is added. That is left over to you. Good luck and if you do it well, no risk for your kids. Bert
     
  7. BertKu
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi Foiltech,

    Sadly I have to withdraw my suggestion to you in the previous thread. I made some calculations and enlarged + rotated your picture and I do not think that with the speed and wave bashing, your girls may lose the engine and get stranded when the added portion tears off from he transom. It is different in my case, whereby at my speed the forces are very low. I agree thus with the previous repliers and sell the boat, or ask a NA and fibreglass boat builders whether they have a solution for you. Sorry for the bad news. Bert
     
  8. BertKu
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi Foiltech,
    Some photo's in how I did it.
    Photo 1, I needed to shape the two plywood pieces to the same shape of the transom. I used 6 tons hydraulic carjacks.
    Photo 2, I made the new transom in my workshop, did the fibre glassing there and bolted the new transom to the old transom with a layer of epoxy glue.

    The difference between you and my boat is. My electric engine I have built, is not mounted to the new transom extension, but still I left it to the old transom. Also my bolting is above the waterline.

    You should speak to a structural engineer. I think it can be done, but not without some experienced people looking at your boat and give idea's like extending with fibre glass layers horizontal between the two section and probably you have to open up the top area and see how you could mount a solid construction. Afterwards you have to seal it op up again and spray paint it. (or white fibreglass a layer)
    Bert
     

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  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    This isn't rocket science, nor especially difficult, though several precautions need to be made.

    You can add the two feet you want, but the longitudinals will need to be scarfed well forward of the current transom. The bottom, side, deck and cockpit flank panels will also need to have their new pieces staggered considerably. Simply adding the length you need with out "reaching" up into the existing structure, will just cause one huge stress riser along the joint of this extension. Most of these boats get their longitudinal stiffness from the cockpit flank pieces, which run from the transom to up under the foredeck. These could be scabbed onto, but a deep scarf or Payson butt joint will be necessary, to insure rigidity. The bottom panels will also need 12:1 scarfs. The side could live with 8:1 scarfs as can the deck. In the end, you'll be cutting and scabbing so much of the existing structure, it might be worth just making a new one, but you could save this one for a few seasons with some wood butchery and epoxy. It would be wise to look over the original plans to identify the longitudinals and plan out where to stagger the scabs.
     

  10. Foiltech
    Joined: May 2016
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    Foiltech New Member

    PAR
    Thanks for your response. I had it in the back of my mind on how to do it but I wanted to get a second opinion. The way you say to do it is exactly the way I want to do it apart from I'm going to ad a few cap plates and some double up stringers. I've always scarfed 14:1 on the ply and stringer repairs 6:1 with 1/4 ply strips either side of joint. I've built 4 tunnel boats in the past and repaired many but have never added length back to. Every stringer will be staggered and the cockpit side I will start again 2 1/2' forward From the existing last frame and carry back to the new transom I will scarf and laminate extra pieces on the inside for extra strength. Existing flanks are 3/8 forward to 3/4 over the last 3 1/2 feet Weight I don't have to worry about as the boat will be about 80 pound underweight for its class
    Cheers


     
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