Saturday, October 24, 2015

Tanzer 16 Seat Repair -- a bad idea poorly executed

Tanzer 16 Seat Repair  -- a bad idea poorly executed -- October 24, 2015.  Back in the spring of this year an fiber optic camera revealed the cause of sound and excessive oil-can movement in the seat.  It had a crack.  It seemed like someone must have dropped something heavy and hard in that spot--but actually it was the combination of a less-than-swell design that was executed badly.  Maybe the workers had smoked a lot of weed at lunch--it was after all Arlington Washington in the 70's.  But this crack started back at the plant with the construction of seats.

They built the seats and forward air chamber  as one piece.  The mold was laying top down.  They sprayed on gelcoat.  Then they put in a layer of chopper glass--a thin layer--a thin layer without enough resin to thoroughly wet the surface.  Then they laid a number of small blocks of plywood on top of the wet resin.  The resin was supposed to be the glue that held the small blocks in place.  Then they put more chopper glass on the backs of the small blocks.  You can see all this when you cut out a section of the seat.

An oscillating saw cut a small hole in the seat.  As the damage revealed
 itself, the hole was enlarged to remove all the cracked pieces.  The builders
dumped in a load of small foam blocks for flotation.





















With section of seat removed, you can clearly see the construction details and why it failed.  Mickey mouse as it was, this design would have worked if the builders used enough glass and enough resin between the gel coat and plywood blocks.

Here you see the three blocks that were never glued to the
top layer of chopper gun.  At the first stress they just let
go and pulled the surrounding blocks with them.



























The total damage was about twice as big as the hole in the seat.  



















The small blocks were probably used so that the plywood reinforcing material would conform to the curve of the seat without  a lot of fussy fabrication.  Nowadays it would probably be laid up with some fancy foam for reinforcing.  If they had built the seats for strength, they probably would have skipped the plywood and hand laid-up two layers of roving on top of the first layer of gelcoat and chopper glass. But that would have added substantial cost and weight to the boat.  Goodness knows the boat is heavy enough already.  At the very least they should have staggered the placement of the blocks so the seams didn't line up so neatly.

Next--putting it back together.

4 comments:

  1. Many boats have cored composite decks and other horizontal pieces. The core material is not exactly reinforcing, it is just there to separate and hold firm the two skins. In laminates like this the core material (plywood blocks, balsa blocks, foam, or whatever) is simply there to keep the two skins, top and bottom, firmly placed and separated. The strength is in the skins, one in compression and one in tension, the distance apart they are, and how well they are held in place. A two inch thick section of paper honeycomb would be stiffer (and lighter) than 1/2 inch of sold plywood. No reasonable thickness of built up lamination of roving would produce a solid seat without weighing a ton, so the cored approach is the way to go. But if either skin becomes detached, or the core loses its integrity, the panel becomes no stronger than either of the two skins. In your case it seems the skins were not saturated with resin and not completely attached to the core, but the approach was sound.

    I just finished repairing the cockpit sole on my boat; similar construction except that the core material was balsa, not plywood and it failed due to water ingression which caused the balsa to rot. In my case a three square foot area became soft and all of the core had to be replaced. I cut the top skin with a right angle grinder, removed the section of skin, dug out the old core and cleaned up the area, put in new core (pieces of solid board, pine does nicely) with a thick paste of epoxy resin and high density filler. Then glued the top skin back down with more epoxy paste and repaired the cut with fiberglass mat. (Well ,I am doing that, it isn't finished). Next I will fair it and paint it.
    This is not the first repair like this I have done to the boat. It is messy and time consuming, but it is simple and it works.
    Good luck with however you proceed to fix this.

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  2. Thanks for the information, Fred. I kind of was working my away around to discovering/inventing what you are suggesting. The big thing to figure out is how to clamp it together since I can't get inside the seat. I think I have come up with a way to do this.

    ReplyDelete
  3. Well, it looks like you have a hole into the area under the seat, so empty out all those foam blocks and build some temporary support to hold the seat up (so it can't sag) then do your work on the top and use sand bags to press the top down. The trickiest part will be to restore the seat surface to original pattern.

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