Extending fiberclass cabin aft

Discussion in 'Fiberglass and Composite Boat Building' started by Mait, Oct 12, 2022.

  1. Mait
    Joined: Oct 2022
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    Mait Junior Member

    Hello and thank you for taking me on board!
    I hope to get some good advice from more experienced boat builders regarding fibreglass techniques.

    I am rebuilding and refitting an OE36 sailboat from 1974, by Swedish designer Olle Enderlein.
    During the rebuild I would like to extend the cabin 20cms (8 inches) aftwards on both sides of the companionway hatch.

    This is how it looks before modification:
    20210807_162319.jpeg 20210806_120114.jpeg

    And this is how it should look after the mod:
    IMG_3610.jpeg
    IMG_3599.jpeg
    IMG_3597.jpeg IMG_3595.jpeg
    IMG_3601.jpeg

    I would appreciate any ideas and suggestions how to carry out the job. I've thought of different options myself.

    Like cutting the left and right side vertical walls off, then extending the cabin top with fiberglass sandwich using wooden battens and board under the new laminate as a base for new lamination (similar to extending the transom some have done).

    Or to build a female mold over the existing cabin top before cutting anyting off. Making the new part in the mold, Then cutting the vertical walls off, put the new part in place and laminate it from under side to required thickness.

    I am not completely new to fibreglass but my experience of kind of limited.

    There is also a video showing the boat and idea about the modification (at 08:59)


    Thank you!
     

    Attached Files:

  2. wet feet
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    wet feet Senior Member

    It is a lot of work for such a small increase in internal space and by not moving the central portion with the hatch by something like the same distance,you won't feel much difference inside the boat.It also looks as though it would intrude into the space the mainsheet occupies when offwind.

    If you are quite sure the gain is worth the considerable effort,it needs to be done to a good standard or you will damage the boat's value.If I had to do the job I would build a plug on the existing deck moulding and take a local mould that had a generous overlap on the part that will remain after the addition is made.The mould would need to be braced to hold the correct shape when released but doesn't really need to be of the quality one would use for a production tool.It may help to use a few lines on the surface to indicate the extent of the component that will be grafted on to the existing deck.A new piece will need to be laminated in the usual way and of a sufficiently strong laminate to function correctly.At which point the old deck has the section removed and the new piece can be cut to a close approximation of the correct size.The mould that it was made in can be used as an assembly jig by temporarily fixing it in place and the new deck section can be propped into the correct location for bonding in.I would expect anybody doing this kind of thing to understand the need to bevel all edges for the bonding strips and to have some appreciation of the need to take great care to line up the new section correctly before bonding as the finishing of a mis-aligned piece could be very tedious.If you went with my suggestion of moving the hatch along,there would be no real need to do more than create a longer sliding hatch as it wouldn't have to move all the way to the position the current hatch occupies when open.What you do with the mainsheet is another thing entirely.
     
  3. Mait
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    Mait Junior Member

    Thank you for thinking along!
    Building a plug of the entire extension and then making the mold on top of it is probably the best way to do it.
    The gain in interior space is not big but worth it in my mind. At the moment it is impossible to use the navigation table and sit behind it.
    This 20cms would give me the needed "headroom"

    The taveller will be moved aft towards the center of the cockpit anyway, no matter if I will modify the cabin or not. So no worries here. Also the winches will be moved aft to help singlehanding the boat.
     
  4. fallguy
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    fallguy Senior Member

    What do you intend to do with the structural bulkhead?

    I'm not certain removal is arbitrary, so probably the biggest sticky bit is how to leave enough of it..

    ...discussion...

    I would build all that with a light plywood frame and vac bag or infused relieved panels off a table. Or hand laid if the vac is too much.
     
  5. Mait
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    Mait Junior Member

    The cabin aft wall is separate from the structural bulkhead, its not connected to it. No worries about it, as long as the new build will be as strong as the old one.

    To build it on the frame is the simplest method to build it, but the most difficult to make it smooth and match the curves, isn't it?

     
  6. wet feet
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    wet feet Senior Member

    I would guess that moving the aft face of the deck moulding to a point nearer the forward end of the well would slightly improve the structural aspect of the deck moulding.What isn't obvious to me is what kind of support there is for the winch pedestal legs in their revised location.From the look of the packing pieces beneath their legs in the pictures I can't really decide if a teak deck has been removed or if there is a desire to raise the pedestals for aesthetic reasons.It will be important to consider the effects of these changes on the boat as a whole and not treat each aspect in isolation from the others.I hope the other areas of the boat don't have balsa in the same condition as that inside the sliding hatch and I would make it a priority to find out.
     
  7. Mait
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    Mait Junior Member

    The winch pedestals will have the same structure to stand on along the whole side-deck. Does not matter much if they are moved 15-20cms towards the stern, they need backing plates and reinforcing under the deck anyway.
    The backing pieces beneath the winch pedestal legs are remnants of removed teak deck (removed by PO). The teak pieces were there to raise the pedestals in order to match screw holes to sail the boat home (a temporary fix).
    The balsa needs replacing in some places and this will be taken care of.

    That's a good point that moving the aft face of cabin towards the stern might compromise the strength of the deck at the cabin top.


     
  8. wet feet
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    wet feet Senior Member

    Would I be correct in believing that teak will not be going back on the deck? It would allow you to move the winch pedestals down to sit on the deck and wouldn't affect their operation as the working zone will still be comfortably clear of the coamings.
    I actually think you might be improving the rigidity of the deck moulding by moving the aft face of the cabin to be more closely in line with well but you rightly mention that some attention will need to be paid to the overall stiffness.Some will be recovered by the new outboard face increasing in size,once the extension is in place.Another way of linking things together is to use a post from the edge of the nav station bulkhead to the coachroof and mirror it on the opposite side of the boat.The handholds are always good to have.I'm assuming that you have no concerns about replacing the existing headlining to cover the increased area.
     
  9. Mait
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    Mait Junior Member

    You are right, there will be no teak on the deck, except caprails and some trim maybe. So the winch pedestals will sit directly on the deck and align flush with the upper edge of the coamings, when moved 20cms towards the stern.
    I was planning to use full length posts from nav station / galley bulkhead as you suggest. I forgot to count them in as supports for the deck.
    As a conclusion - I do not see any structural issues with the planned mod. The only question for me is which way is the most efficient to carry it out and achieve satisfying results.
     
  10. fallguy
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    fallguy Senior Member

    For a one off build, making the thing on a mould is fairly ridiculous, in my opinion. And I just finished building an entire boat.

    I would build it with Gurit pvc or corecell M. If there are any hardpoints or places you wish to bolt hardware; you can decore.

    If you use 12mm or thicker, it will support itself and you can mock it all up. It forms easily with a sureform, you can relieve the edges for tabbing on the outside, once you have it all roughed in. The notion of forming a male plug, then a female mould here and then building a part is unwise and will really increase the work, not to mention the difficulties laminating.

    I would rough it all in with corecell. Then laminate the inside, then put it back together and tab it. Then laminate the outside over the tabbing. Or laminate both sides on the table if flat.

    For non-flat surfaces, I would use corecell strips about 1" wide over a former and glue them together, then laminate the inside. Then glass in place over formers.

    This is how I built certain parts of the boat, the deck shoe the largest. For my deck shoe, I bonded the core pieces together on the former which was on the boat. Then removed it and glassed the inside and flipped it over, prefaired and glassed the outside. For you, same process, but if you want to, you can make the flat panels to completion and put them in place and just do any non-flat surface as a single piece. Clear as mud?
     
  11. fallguy
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    fallguy Senior Member

    Otherwise, if you want to use plywood, that is also possible. Looks like the pictures were plywood.

    The nice thing about core is that it is easy to build reliefs and eliminate fairing the entire thing to bury the tabbing.

    For my cabin roof, I used corecell scrim over 6mm okume. This eliminates the need to strip plank the roof piece. You'd have to prefill the edges of the scrim to avoid resin runouts, but a few pieces of scrim would work well for you over a stay in place piece of okume. If it needs to be walkable, then you'll need to laminate the inside of the plywood, so be ready to remove it after glassing the top.

    If you don't understand the concept of relieving the edges for tabbing, let me know. I have a video of one way it can be done.

    Also, hot glue is your friend here. You spot glue things to hold them where you want when mocking it up and hot glue can be left in the thickened resin bonds for final fit as well. Cuts easy enough with a box cutters for removal..etc.
     
  12. fallguy
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    fallguy Senior Member

    I have pictures of the deck shoe work if you want to see them as well.
     
  13. Rumars
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    Rumars Senior Member

    The easiest way is to not have to laminate overhead from inside the boat. This means building over a male plug. To build it I would do as follows:
    1. Cut the aft cabin wall away.
    2. Take some thin ply, mdf, etc. approx. 500mm longer then the extension, put on mold release (important) and screw it from inside to the cabin top, this will give you the necessary curve. Mark where the sidewalls intersect the bridgedeck, then cut trough the top laminate and remove the core about 50-100mm to the outside. Do the same to the cabin top. Continue to build the plug with thin ply, place it on the inside glass layer. If the area is not cored, cut or grind a groove in the glass. Tape the ply to the bridgedeck and coaming on the inside. The corners where two ply pieces meet get reinforced on the inside with a 25x25mm batten so you can round over the corners.
    3. Apply mold release over everything (can be brown parcel tape).

    Your plug is now ready and attached solidly to existing glass. To laminate fillet all corners with thickened resin, then lay on the inner skin by overlapping it onto the decored area. After curing attach core with thickend resin (weights, screws), fair, then laminate the outside skin. The previously decored areas also get recored, then you have to feather the edge of the existing top laminate. Now you can laminate the outside skin over the entire area. Do some preliminary grinding and fairing, the final part will be when you refinish the whole deck.
    Go inside the boat, remove the plug and cut away the bridgedeck then grind the edges smooth.
     
  14. wet feet
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    wet feet Senior Member

    And your opinion is based on altering how many deck mouldings?I have actually altered at least a dozen and like Rumars,wouldn't choose to laminate overhead.A basic mould with only a moderate level of finish is sufficient and can be made in a couple of days.Rumars outlines the process and I broadly agree with him.I would incorporate core in the new extension where it won't intrude on the area required for a bond on the outer skin and once the outer skin is bonded its time to add the core and then laminate the last strip.
     

  15. fallguy
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    fallguy Senior Member

    I never said I'd laminate overhead.

    I said I'd glass the top exterior and take it off and glass it off the boat on the table.

    Making a male plug, female mould, and a part off the boat to take to the boat is what I find to be silly.

    Rumars is also suggesting building on the plug unless I have misread.
     
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