Boat extension at transom, stringer design

Discussion in 'Fiberglass and Composite Boat Building' started by XJ9, Mar 4, 2012.

  1. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    Hi everyone,

    Hopefully I have posted this question to the right forum. This is the first time I have posted here, so please forgive my ignorance.

    My father and I have been modifying a Hobson Solocraft under the guidance of the designer (Henry Hobson). Solocraft are well respected in Tasmania by cray fishermen, abalone divers and others that expect a lot from their boat. Anyway, because of some health issues in the past, the project has been delayed for a couple of years and unfortunately Henry Hobson passed away recently, leaving us with some missing parts of this design. We have no idea of what has happened to his drawings etc (if any) as we just followed Henry's direction, step-by-step in continuing the modifications that he had begun.

    The original 16' boat has been extended at the transom by another 5' using polyester resin, mat and woven rovings, making it approximately 21' overall (6.4m). The glass is 1/4" thick generally, with some areas a bit thicker (keel, chines...). It is almost ready for paint on the bottom and then we will roll the hull over and set about adding some stringers and a bulkhead to the aft section. Henry suggested that he was intending to add a top-hat section stringer. He showed me a rough outline drawn on the side of the boat, but no dimensions as we weren't up to that part yet.

    My thoughts now are to add perhaps two top-hat section stringers to effectively divide the aft section into three - roughly 2' wide each. These stringers would be about 8" tall with some laminated timber in the top and ply shear webs on the sides. The intention is to tie the new section of the boat into the old with as much gluing area as practical, hence the choice of top-hat sections, and then add a 3/4" ply bulkhead half way to match the rest of the boat (which only has bulkheads as the box keel serves at an inverted top-hat section stringer). The sole of 1/2" ply can then sit on top of that structure. I am not sure of how thick the ply webs should be, but I would think 1/2" plus. Stringers and bulkheads, corners of the transom will be taped in with 12oz biaxial tape and R180 Epoxy. Any advice or suggested alternatives would be greatly appreciated.

    Hopefully the pictures I attempted to attach worked so that the above ramblings will make some sense. The first picture is of the preliminary framework for the extension - male molded like the rest of the boat - and the second picture is of the inside of the extension with the mold removed, well most of it anyway. the rest will come off when we get in there to wash it out.
     

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  2. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    This is the sort of thing I had in mind for the stringers. I'm still not sure if the ply web on the sides will be stiff enough at 1/2" thick. Has anyone done this sort of thing before?
     

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  3. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    If anyone is interested, here's the progress so far. There's still a bit more sanding etc to do before the primer can go on.
     

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  4. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    Progress Update

    The hull has been primed with Jotun Penguard HB. Some Jotafair has been added to fill a few pinholes and a couple of low spots since this picture was taken. The finish will be far from perfect, but hopefully acceptable considering it is a male molded boat. No doubt the epoxy paint will add a lot to the waterproofness and life of the polyester resin fibreglass.
     

    Attached Files:

  5. tunnels

    tunnels Previous Member

    Hi !!
    Why didnt to extend the centre part ?? what were you thinking :eek:?? it could be a real speed killer as the back end of what you have will cause drag and suck the boat down a little !! into the water:?: But you never know !! might be wrong !! :p
     
  6. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    The designer (Henry Hobson), after testing the initial hull section decided that the box keel added a bit too much buoyancy for the beam and that made it slightly unstable at rest (well, he said "rolly"). When we bought the hull, Henry had already started modifications to extend the boat to around 6m. He did not wish to continue the keel right to the stern and said that the drag would be insignificant. Beleive me, I was pretty sceptical about this my self and we had some discussions about how the existing keel should be faired into the extension. Far be it from me to question a guy that has built 80 odd boats for use (and are still being used) around Tasmania including the West Coast which is an extreme testing ground. The hull also has a significant hook as the bottom of the transom is in the same plane as the keel. The placement of the transom was determined by Henry and he seemed pleased with the result. How will it handle now? Good question ...it might be horrible, but we will have to wait and see how it goes when it finally hits the water. If it has bow steering issues, I suppose that we will have to extend the box keel, effectively removing the hook.
     
  7. m3mm0s rib
    Joined: Aug 2011
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    Location: GREECE

    m3mm0s rib Senior Member

    I think that everything is on
     
  8. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    After receiving some advice from some other members of this forum, we spent some time adding some structural epoxy filler to the chines to give them a better/sharper edge . We used some 50mm PVC strips covered with packing tape (just in case epoxy does stick to PVC) and held to hull with double sided tape as form-work for the chine rails. The double sided tape held pretty well since there was minimal wieght in the formwork and there was only a small amount of filler behind it. The epoxy thickener was a mixture of sawdust, wood flour, fine fibreglass and a small amount of talc. Hopefully it will prove fairly chip resistant.
     

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  9. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    The new build has had some Jotafair added and has been primed ready for the Jotun Imperite 300 topcoat. There are still a few slight undulations in the hull, but it would take an enourmous amount of filler and time to get it much better. Frankly, the added weight is not required. We will try to get a couple of coats of the top coat on next weekend and then let it cure for a few days before framing it up ready for roll over. Can't wait to get it on the water!
     

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  10. XJ9
    Joined: Mar 2012
    Posts: 37
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    Location: Tasmania

    XJ9 Junior Member

    Here's a couple of update pictures. Not much has been done over winter because of the cold weather, but the boat has been roller over ready for some stringers etc. Since these photos were taken, another layer of ply has been added to the transom and some cutouts made ready for the outboard. The next job will be to tab the inside of the transom and figure out exactly what to do about some stringers for the last 5 foot.
     

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  11. groper
    Joined: Jun 2011
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    Location: australia

    groper Senior Member

    Nice work your doing... However, i also do not the like box keel idea... it will create a ventilated hole behind it at speed which will suck the transom down and increase your fuel consumption. And as the designer said, less stability at rest...

    As for your stringers, their purpose is to stiffen the hull skins... so they are stiffeners... they do not need to be like giant beams you were proposing earlier... 1 peice of 13-16mm marine ply stood on edge 150mm deep and glassed in is plenty if your going to run 3 along the bottom... stand your bodyweight on a piece of ply on edge and see how stiff they are!

    The sides of the boat are the shear webs, they take the longitudinal bending shear loads, not the stringers...
     
  12. tunnels

    tunnels Previous Member

    There a few things in there that you need to seriously consider and dont just go charging off and doin it and wonder whay you have problems at sometime in the future . Ply in edge is one such problem !! the way the beam is used and what its made from and whats under the beam is a major consideration that has some serious implications !!! need to do some drawings . Skinny beams under sever loading are as usless as **** on a bull !! :eek::confused:
     
  13. XJ9
    Joined: Mar 2012
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    Location: Tasmania

    XJ9 Junior Member

    Thanks for your suggestions. Every boat builder and engineer that I have consulted has a slightly different opinion about stringer structure. One of Mr Hobson's employees said that they would use something like some 6"X2" hardwood timber for the stringers (4 of) and although that would be heavier than 12-16mm ply on it's edge, the weight is down low and much stronger. An engineer involved in building and repairing composite boats was quite happy with my hat-section design. It has to be strong enough to provide some stiffening to the bottom of the boat at the stern as it is basically two flat panels butted together at the keel with fairly plumb sides. There is also the issue of transfering the transom loads to the rest of the hull and tieing in the extension with the original structure, especially with a 115HP outboard hanging on the back. Not a good look for the boat to fall in half in any sort of conditions.

    The front 16' of the boat has no stringers (other than some 1.5" square longitudinal sole support beams, but they aren't attached directly to the bottom of the hull) as the box keel takes on the role of stringers. It does have 1/2" ply webs every 800mm (about 2' 6") that support the longitudinal beams and the sole beamwise - you can see that structure in a couple of the pictures of the central well. Much of this structure was not evident, or at least easy to see with the boat upside-down. Perhaps my original stringer design is overbuilt but complies with the standards enough for the engineer not to freak out (the other boat we have of similar length has/had about 8 2"X1" stringers tabbed in to the very thin hull every 2'). After having a good look at the hull now that it has been rolled over, I'm inclined to use solid or laminated timber "beams" instead of the hat-section to simplify things a bit given the angles involved, but I think that they will need to be thicker than 12-16mm unless I add a lot of glass and epoxy or a heap of stringers and that woud use up valuable space for buoyancy foam.

    Many of Henry's boats had fibreglass hat-section deck beams etc and that's the sort of thing he had planned for this boat, so it's hard to not go down that track. For anyone interested, here's a couple of this boats that are still going strong after 20-30 years http://www.boatsales.com.au/used/hobson/solocraft/
     

  14. XJ9
    Joined: Mar 2012
    Posts: 37
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    Location: Tasmania

    XJ9 Junior Member

    Slow progress

    This project stalled for a while because of some family health problems, but we have been doing a bit to it lately.

    Finally got around to tabbing in the transom with 3 layers of 12oz Biax tape and 1 layer of 936gm (33oz??) biax over the top. The transom is 4 layers of 1/2" ply laminated together with epoxy plus the 1/4" outside fibreglass skin. Next step will be to glass the inside skin - I'm thinking about 2 layers of the 936 gram biax. I know that this is pretty heavy stuff, but I already have it and it wets out nicely with the FGI epoxy we are using.

    The join in the hull now has three layers of 12oz biax tape on the inside (as well as the 12" or so overlap on the outside) and I will probably give it a good sand and add another layer before the stringers get added.
     

    Attached Files:

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