Standing cabin build on 19’ aluminum runabout

Discussion in 'Fiberglass and Composite Boat Building' started by Seasideinvermont, Aug 22, 2024.

?

Do you think this is going to work:

  1. Well

  2. Poorly

  3. I hate this boat

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  1. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Zip tie the plywood to the outside of the framing, glass the outside, cut the zip ties and pull the framing out!
    Then flip the cabin over and glass the inside, adding a few gussets at critical points, and some pvc or cardboard half tubes to stiffen larger flat areas.
    This cabin requires very little in the way of framing, especially considering the layup schedule you have mentioned.
     
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  2. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    May I ask why exactly you don't want any form of diagonal bracing?
    I would install small braces made out of metal pipe with flattened and bent ends inletted into the beams, they will also provide convenient handholds for the crew. Alternatively, a few small knees would tremendously ease the fiberglassing of the joints, they don't have to be as big as your existing one, just make a smooth transition in the joint. Glass doesn't like right angle intersections anyway, so instead of filleting the joint install a small wood filler piece in the corner.
    That's how a true fiberglass post structure would look anyway, the inside of the corner would be a big half circle not a sharp corner with a small fillet.
     
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  3. Seasideinvermont
    Joined: Aug 2024
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    Location: Albany, Vermont

    Seasideinvermont Junior Member

    Head banging things annoy me :)

    it will have some small knees however

    I’d be disinclined to use a metal bracket as I think a glued and screwed wood knee - while perhaps slightly flexible- will be just as strong as a screw and perhaps more durable over time.
    Good suggestion, though
    the window openings will be a fillet, every other place will have a concave knee or other angle to broaden the leverage angle.

    I thought the windows out a lot and decided I could aesthetically tolerate slightly rounded corners to appease the fillets, but vanity-mirror aka ‘camper shell window’ effects would require mental health care I couldn’t afford and don’t have insurance coverage for… I grew up in NH and I have a 12-step-program-worthy dependence on traditional or colonial and shaker shapes.
     
  4. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    Make the knees out of thick plywood, cut them from the sheet on the diagonal. Glue them in with epoxy in small inlets, no screws at all, and there will be no flex.

    If the goal is perfectly square corners you need different joinery. It can still be achieved despite your existing lap joints. What you need to do is embedd a steel connector into the middle of the joints. You have enough wood there to make a strong enough bracket dissappear into it even with reinforced radiused corners.
    They can be made by welding from flat bar or you can have them cut in one piece from thicker stock. Plain steel would be my choice, with a good primer and paint system, or if you prefer, galvanized.
     
  5. Seasideinvermont
    Joined: Aug 2024
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    Location: Albany, Vermont

    Seasideinvermont Junior Member

    I have the roof beams screwed and epoxied and the forward hump installed and it’s stiffened up nicely.
    I could plasma cut a bracket of some sort and bury it easy enough.
    The windows will have hinges on the side of the sash. Like a casement window. They will lay against D weatherstripping, and the cabin will be painted a slightly darker color to give the appearance of being smaller than it is; so the window sash can be made with just a smidge over an 8th inch radius on the facing/exterior of the sash. That’s square enough.
    The sashes will probably be meranti finished in varnish. So they will come off as square, visually

    it will be three-sides enclosed. Soft ‘curtains’ with snaps will eventually enclose the cabin aft, but 95%+ of the time soft sides will not be used
     
  6. Seasideinvermont
    Joined: Aug 2024
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    Location: Albany, Vermont

    Seasideinvermont Junior Member

    Thank you - and others- again for your thoughts!

    I may forego the lateral knees after all… the posts/pillars are glued up 3/4” boards. Hollow on the inside. Probably could’ve/should’ve resawn them to ~3/8” as they are wicked strong but I was concerned about the near right-angles.

    As it turned out, once the cabin sides had 1/4 plywood glued on it stiffened immensely but I took some time to work on it today and temporarily applied the 1/4 plywood to exterior overhead. The torsional rigidity of screwed roof sheathing transfers any potential movement laterally at the “B” posts to the lower cabin sides and then 90* to forward hump. It appears to distribute any rocking/twisting so thoroughly downwards due to the 69x70” roof plywood’s resistance and the ‘posts’ are so substantial that I must confess I was overthinking things...

    It’s going to be fine.

    Additionally, if I were to do again it’s fairly likely I could make adequate posts with half the weight by using thinner stock. That would reduce the Zmore elevated COG. However, I think it’s a non-point: that would only save a few pounds as white cedar is fairly light. Further, and ignoring COG for a moment, I haven’t exceed the oem’s dry weight with what I’ve added because I removed so much oem stuff (all gone) and 1/8” non-yellowing acrylic is like one fourth the weight of 1/4” real glass.

    If I didn’t have to work I potentially could be in paint Sunday afternoon!
    it wouldn’t be complete, but once the structure is done I can do front windows and install the helm and some seats and get it on the water in this fabulous Vermont early fall weather. That’s my current target/hope: Memphramagog with the grandkids
     
  7. Seasideinvermont
    Joined: Aug 2024
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    Location: Albany, Vermont

    Seasideinvermont Junior Member

    Updates; flotation questions

    The cabin assembly has been removed to ‘glass the bottom of the cabin sides and do some fairing that would be more difficult right-side up, it’s now permanently reinstalled with bolts with the intersection bedded in polyurethane caulking.

    Two important features came to light.
    The most important factor that came to light is that the cabin weight (currently without windows or overhead installed) is roughly half of what I anticipated; I can easily lift it myself and its weight I estimate to be around 75-80lbs! I was surprised, actually. While I didn’t think it would be very heavy, I didn’t expect it to be so lightweight. Bonus! It’s currently less than the oem weight I removed.

    Additionally, the fore/aft weight distribution is roughly 10” aft of the what the oem components exhibited. This also is a good thing since the underfloor gas tank shifts some of the latitudinal weight bias forward- ‘guessing’ that those two factors wash when the tank is full.

    So wind issues will still be a negative, but other than a bit of rise (literally!) in cog the weight is manageable by the hull, and using 3/4” fir ply for the sole does increase the floor weight ~33% (roughly 50# down low).

    Which brings me to FLOTATION.

    I installed ~738.2 pounds of dry-weight flotation underfloor. The oem weight in air not including outboard is ~900lbs. So by coast guard factors those equate to some positive buoyancy in water of the hull weight.

    I can fill the the voids in the overhead for an additional ~250# weight in air offset, plus under the bow, the gunwales, the shears, etc. and I sum roughly 1400#.
    So if submerged that is roughly 2100lbs of dry weight supportable.
    That's going to be about right I think! Plus, the wood materials above the gunwales should provide enough flotation themselves (not factored for) that it shouldn’t capsize if the unthinkable happens. The 90hp, batteries, trolling motor, and fixed gear should make weight bias low enough to keep it upright although perhaps bow-high.

    Any thoughts on flotation?
     
  8. fallguy
    Joined: Dec 2016
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    fallguy Boat Builder

    In general, you want a boat full of water to not tip over. This means, in general, flotation foams are best placed in a fashion where a sinking boat sinks evenly. And a vessel with a high vcg that lays over sideways makes it more difficult to achieve.

    Consider flooding the boat; would it stay upright? Is there a place to foam aft sides and aft transom? Or would it lay over to port? Etc
     
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  9. Seasideinvermont
    Joined: Aug 2024
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    Location: Albany, Vermont

    Seasideinvermont Junior Member

    Actually have thought about that :D

    The oem only had foam below the sole. And I suspect not enough to float the boat without a motor attached.

    The reconfiguration in my mind seems as though it ‘could’ possibly lay on its side if flooded. Or the aft end (due to motor) could sink ~6’ and float with the cabin roof not submerged but at 35-40-degrees, bow up.
    Neither is ideal but current USCG guidelines aside if it doesn’t sink my underlying goal would be met. With limited space in a 19’ runabout hull the under floor foam is a lot of buoyancy in my mind, but then again motor would be what? 500#, and the hull empty was ~900# by oem specs. But then again 50%+ of the flotation would be 4” under the gunwales and up to the cabin top. Being that the cabin is forward this leads me to suspect that the transom would sink with a pivot point 6-7’ from the bow/stem.

    Realistically it should never be swamped- but the flotation- any flotation- may save a life or two if some unimaginable circumstance transpires.

    Nevertheless, there’s limited ability to do flotation otherwise, but the two questions for those more knowledgeable than I remain:
    -is the foam buoyancy adequate
    -is there something I missed?

    I know enough to know what I don’t know, and I know I don’t know that.
     
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