Hull extension approach options

Discussion in 'Boat Design' started by Bigtalljv, Sep 30, 2021.

  1. Bigtalljv
    Joined: Aug 2019
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    Location: California

    Bigtalljv Junior Member

    Hi,

    I have posted a while back about the hull I want to modify. After a several year delay I’m getting back to it. I’m in California and it’s a local built hull out of Santa Barbara, Radon. They are tanks but popular here. I want to extend it 2.5’ to put a swim step and outboards in it, it was originally an IO. The design and dimensions are not an issue, I know what I propose has been done a lot and they perform great.
    My question is how to go about it. Originally I was going to lop off the bottom of the existing transom and build new hull and transom out from there. Now I am considering moving forward several feet and cutting the hull in half there to extend it. Then it’s just laying up large straight flat sections instead of having to mold up a new transom. Then I can modify the existing transom to be the swim step and outboard mount.

    anything I might be missing there?

    apparently I can’t type on an iPhone, is there a way to change the title?

    Thanks
    Jason
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome back Bigtalljv.

    Here is a link for reference to your thread 2 years ago re the mods you were proposing then.
    Outboard bracket Vs hull extension https://www.boatdesign.net/threads/outboard-bracket-vs-hull-extension.62704/

    And a thread that you started a few days ago about epoxies -
    2:1 epoxies in quantity https://www.boatdesign.net/threads/2-1-epoxies-in-quantity.66100/

    However I am a bit baffled here - you mention that you will need 50 gallons of epoxy - but you note that you are simply extending your Radon 24 by 2'6".
    I don't think you will need 50 gallons for this?
    Or are you building another / new boat?

    Even for hull extensions, so long as you do the preparation work very assiduously, it is possible to use polyester instead of epoxy - a pal of mine here has a Luhrs sportsfisherman and a small boat building / repair business, and last year he extended the Luhrs by about 4' or 5' to give a bigger fishing cockpit.
    He cut the transom off and moved it aft the required amount, and then built a temporary mould between the two sections to laminate in.
    And he used polyester.
    This would be much easier if the hull is constant width aft of amidships - but if it tapers to a stern that is narrower than amidships, then you might get a slight 'kink' (for want of a better word) in the shape when you do your 'cut and shut' job with the 2'6" extension.
    Although my pal's Luhrs has a slight kink, it is not noticeable at all, and you can really only see it if you look for it deliberately.

    My pal also ordered new longer propeller shafts, so that the propellers could be in the same relative position in front of the rudders, built new 'A' brackets to support them, and glassed in new stern tubes (as the angle had changed, re the longer shafts).
    And he has been very happy with the boat ever since.
     
    Last edited: Sep 30, 2021
  3. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Probably that the idea of chop and fill is a lot of work, I think it would be advisable not to cut the existing hull, just well reinforce the transom that is there, such that your extension can be bolted on to it, seeing it is only 30" you intend to add, if it was 5' maybe not such a good idea. Then of course there is the matter of whether you want this pod to have interaction with the water underway, have you settled on that as a yea or a nay ?
     
  4. Bigtalljv
    Joined: Aug 2019
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    Bigtalljv Junior Member

    Yes, it will be a hull extension not a bracket, that is what I know works and have a model for it.
    there is a (good) chance that my math goofed on the quantity of epoxy needed. Either way I have ordered a smaller quantity to start with so that’s no longer an issue if it ever was.
    Perhaps I could use a different resin but I am most familiar with the epoxy I have ordered. I have read many of the mountains of arguments for and against different resins and I don’t want to start that here, I am using epoxy, it’s the only thing I have any experience with and generally the biggest negative is price.

    the boat is all straight flat planes and lines, like I said, a tank, or as someone posted before a dump truck, there is nothing particularly elegant about it. That’s why I seem to think the cut and fill would actually be easier. There would be no inside corners to start with, were as doing an extension off the back would require me to layer up all the inside corners of a box. At the very least that is more awkward positioning to get your arms and body in there.

    so unless there is a specific argument against it I think I have talked myself into the cut in half and fill…..

    sorry for the very erratic discussions.

    jason
     
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  5. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    I don't think I'd touch the cut and fill idea with a barge-pole, it is way, way more work, and not worth it for a mere 30 inches. You cut and fill, and what about the transom ? Is it already converted to outboards, from the original I/O ?
     
  6. Bigtalljv
    Joined: Aug 2019
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    Bigtalljv Junior Member

    Hmm, you sounds serious about that. a smart man would listen to you, the jury is still out in what I am.

    No the existing transom is not ready for outboards but it’s 1/2 of solid fiberglass so I feel like I have something stout to work with. I’ll have to add a few layers of plywood, and a bulkhead, and the new stringers into there…..
     
  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    It is generally easier to cut somewhere aft, but forward of the transom, but...

    Only if the hull is not changing much/straight, or it'll look off and you can't make it right.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    In that case, (still the I/O transom), I would go to work inside to properly strengthen that transom, sufficient for, and tailored to, a bolt-on extension. This cut and fill method also involves stringers, deck, sole, bottom, topsides etc, a lot of work, and as mentioned by bajansailor, if everything isn't parallel to the centreline, where you cut, it won't meet properly
     
  9. Bigtalljv
    Joined: Aug 2019
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    Bigtalljv Junior Member

    Ok thanks. I gonna have to ponder this. It’s a full rebuild so I already am going to be replacing stringer, deck/sole, bulkheads, everything else, etc.

    I really don’t want a bolt on but I’m gonna think about your suggestions for a while. It wasn’t the answer I was hoping for so it takes some time to absorb the advice.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    OK so a complete rebuild, it is enough trouble supporting the hull properly when you replace internal structures in a boat, without the added complication of sawing it in half, and then have everything connect together satisfactorily with your in-fill section, this sounds like a whole lot of work to me. Others may see it differently, I would want to avoid that cut altogether, especially for a 30" increase.
     
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  11. ondarvr
    Joined: Dec 2005
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    ondarvr Senior Member

    For thirty inches I'd just add on, not cut and fill, much easier and the same outcome.
     
    Last edited: Oct 1, 2021
  12. Bigtalljv
    Joined: Aug 2019
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    Bigtalljv Junior Member

    I heard from the expert that i can’t do the cut and extend, it doesn’t work and kinks. I will need to go out the back. It’s looks all straight and flat but isn’t….

    jason
     
  13. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    For 30", I'd just add a bracket for ob and ditch the inboard.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Better to find out before you did that cut ! That's one detail you have settled.
     

  15. comfisherman
    Joined: Apr 2009
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    comfisherman Senior Member

    Not familiar with the shorter radon boats just the 32ish foot one. Does it have the strakes and the multi stage chine like the larger ones?

    We do a lot of stretched up here as our fisheries are just profitable enough for incremental improvements but not enough for new boats. Certain boats are entirely different but a general rule of thumb is under 4 feet goes on the stern over 4 feet goes mid body. Usually under 4 feet and it can be fairly easy to fair in to the shape of the hull.

    I would think if the 25 is as boxy as the other radon boats just build a jig off the stern cut off the old transom and feather in for the new glass, then get to laying it up. You could design your jig to accommodate the outboards transom angle and lay an appropriate core.

    First time cutting a boat in half or the stern off of it is a little unnerving, with a decent saw it goes faster than you think. Afterwards it's just a lot of work.
     
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