Installing self bailing decks

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Goingfishing
Posts: 15
Joined: Tue Jul 28, 2015 8:02 am
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Installing self bailing decks

#1

Post by Goingfishing »

Thoughts on welded in vs sikaflex and riveted in decks.
I have always welded in the deck/floor in my boats. I have been asked to build a number of 16ft hulls, I have looked through their design and have found in the specification they have the self bailing deck to be sikaflex sealed and riveted to the transverse frames and upper longitudinal frames as opposed to traditional weld in and plug welding.
In my mind this is not a bad idea. Deck will be easy to install without all the distortion created by welding.
Anyone installed decks this way before?
kmorin
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Location: Kenai, Alaska

Re: Installing self bailing decks

#2

Post by kmorin »

goingfishing,
Goingfishing wrote:I have been asked to build a number of 16ft hulls,
now that's a nice offer for a builder! And you'd want to look closely at the relationship you, as the builder, would have long term as the idea of riveting/screwing/non-welded fastening has some serious questionable issues in my view.

First of all, rivets would leave some path for moisture into the bilge area, and leave moisture in the rivet body and rivet body's hole in the deck and frame... so unless the rivets were 50series alloy like a deck (or 60 series like extrusions or decking) then.. there will be galvanic corrosion guaranteed.

Next, the surface between the rivets bodies and framing will be crevice corrosion sites, and that will include the frame tops since they will not seal (pressure tested "seal") and will be another crevice site for corrosion.

Not seeing the plans, I can't remark on the hull to deck 'seal' seam but any elastomeric compound will flex- has too. That means the structural gains you'd get from the bottom/deck "truss as a beam" is seriously compromised and the overall structure has to made of heavier scantlings to compensate for this lost of 'final' metal to metal values at the deck to topsides seams.

If you have any influence on the design? or input on the design? since the boats are that small(?) I'd ask them to consider building in a 'hollow surf board' manner then adding the topsides, using a full rotisserie like the Davis Jig, shown below.

Image

Using a rotating fixture/rotisserie like this example of one built mainly of scrap; a full weld out and air test can be done in extremely short time, compared to sitting the boat in floor mounted jig or standing the frames on their deck ends and the below decks work using this method of building would lend itself to a series of small boats.

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Goin'Fishn', this picture of a 25'er's bottom is to show the idea- not to suggest your build will be the same size!! The Davis Jig allowed the work to move pretty rapidly compared to the keel down (or even keel up framing) and the 'deck down' or of build allowed total access into the skiff while standing on the shop floor- all the time. (I'm a little bit old and stiff compared to my "crawling in the bilge days" and as shown I had a very young and strong helper who made the entire build possible!)

[ing]http://i383.photobucket.com/albums/oo27 ... ams_3a.jpg[/img]
Above is a photo showing the various stages of framing but clearly illustrates the easy of access by leaving the topsides off until the entire lower hull is done. The added time of a 2nd or upper chine seam at the deck line is completely offset by the reduced time to put in the deck using this method.

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This picture (above) shows the deck going on- while still framing the deck transverse angle fits forward. By cambering the deck very slightly a little tension was put on the surface and the keyholes were put into tapered holes so the TIG arc would not 'wander' to the top edges and the holes could be filled from the framing (top surface) below up to the top without distortion.

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As I mentioned in a previous picture the idea was to build as a "hollow surf board" having the entire bottom done and welded, and I air tested to bout 3psi and found a bunch of MIG leaks all in one deck cross seam where I'd failed to clean out the deck plate butt welds in an area about 14" long. But the outstanding weld access to the deck to lower topsides was a very nice outside corner (shown below)

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The weld shown above is the deck to lower topsides outside corner. I TIG welded it because I prefer that method (its more fun than MIG and I use cold wire feed gun) and this allowed me to carry a full keyhole on this weld while still joining 1/4"(0.25"- 6mm) to 5/32" (0.160"- 4mm) 5086 in a single pass.

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Just above is a photo of the outside seam of the topsides to deck. The seam is not welded in this photo, and the black soot of the inside MIG tack shows in the seam. This nicely V'd seam allowed the topsides to be welded to the deck seam by sitting in an office chair- rolling along the chines, in about 45 minutes including moving the chair and the heavy displacement welder.

The deck weld shown in the previous photo was air belt sanded to a 45deg angle then air tested, and that left a 45 chamfer at the deck edge, (shown in this photo just above with the topsides tacked in place) previously sealed @ 3PSI test, on which to mount the upper topsides lower edge. (2nd chine so to speak) The upper and lower topsides were in a plane with one another- but as note this would make a great location for a 2nd chine flat, that would put more water deflection in the hull shape with no extra labor- given the adoption of the 'surf board' step in building.

So, while not aware of the plans, my suggestion is to consider if the design could be adapted to allow you to:

#1 make a Davis Jig fixture to rotate by hand. Note: that if the end beams (verticals at 90 to the long support beam) are lifted and lowered the boat's wt is at the neutral axis of the mass being rotated- my shop mate above is not using much of any force and no 'effort' to turn the boat- it is balanced - in the photo of the bottom skin tacked up.

#2 Your very much increased access to fits/tacks/welds and weld prep is so much increased that the fixture allows substantial reduction in effort- at the same time improving the quality of the work.

#3 By building deck-down-to-keel first; 90% of the climbing time that is lost in skiff building/welding is eliminated and the weld quality goes up because of improved position of all welds.

#4 The reason to consider gluing in decks is gone because all the 'problems' in a full-topsides-then-deck-insert have gone from the build. No kneeling to weld, keyholes done in any position, outside deck to topsides seams are like a nice tank weld, and the heat of contraction is not in the field on the topsides- the main seams are along the edges of hull panels.

Just some food for thought, and of course you'd have to adapt all this to your boat and your work techniques but it can be done; and IMO save money while improving the final boat.

Cheers,
Kevin Morin
Kenai, AK
kmorin
Goingfishing
Posts: 15
Joined: Tue Jul 28, 2015 8:02 am
8

Re: Installing self bailing decks

#3

Post by Goingfishing »

Kevin as always thank you for you expert and detailed reply.
I have pondered this over the Christmas period, I have done a bit of research into the designer/ architect apparently he retired about 20years ago at age 75. From what I can find out is the customer ordering these boats is a environmental water testing company out to save the world from global warming, so I will assume they have some office based environmental scientist in charge of purchasing and he was sold on some old design found on the Internet.
kmorin
Donator 08, 09, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24
Posts: 1735
Joined: Mon Aug 18, 2008 1:37 am
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Location: Kenai, Alaska

Re: Installing self bailing decks

#4

Post by kmorin »

goingfishing,
all the more reason to appeal to a 'modern update' to the poor concept of a glued in panel for a deck! Although, I'm not saying that non-boat building office types will necessarily understand the problems!

You can always try the "why doesn't any leading manufacturer do this?" lead in... attempting to appeal to any common sense, but sometimes that faculty is not present?

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Above, tacked up, but on the Davis Jig, is a little 14'er I did years ago. All except the well (not cut in here) was 0.100" (7/32"-2.5mm) mostly 5052 and I did the skiff as one piece topsides, but the fixture still allowed the work to be relatively fast compared to climbing in and out?

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Above photo of the interior deck's framing show how easily the parts were fit, prepped, tacked and welded- the position of the skiff could be rolled so rapidly that all points of access were immediate, pretty handy.

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All painted and dolled up.. (canopy and frame not shown) the skiff turned out fair and smooth, decks and all. So the rotisserie fixture can be a great help to any builder - IMO.

Cheers,
Kevin Morin
Kenai, AK
kmorin
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