Installing UHMW bottom

I am about to re-install the UHMW bottom that came with the tatman used boat I just picked up.    What size stainless steel screws should I run down to Home Depot and buy to get this job done?  Should I screw them in along with any sealant?

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Permalink Reply by Adam "Dutch" Gottschling on February 11, 2011 at 7:09am

Hey Terry,  

 

Living in Wyoming and UHMW doesn't work out so well in the long run. We've seen that stuff shrink up to a half inch and actually pull screws out in the process. Wetter climates it seems to be the way to go. Would recommend fiberglass and epoxy if you want the 10 plus yr. bottom.

 

I have a friend in Lander Wy. that has had a UHMW bottom. stored inside. and the damn thing warped, cracked, and seperated from the bottom. He's been wanting to get that stuff off for awhile.

Permalink Reply by Terry E Walsh on February 16, 2011 at 8:40pm
Thank you for the info!
Permalink Reply by Randy Dersham on February 11, 2011 at 10:47am

Michael, because you live in Bend I would also not recommend the UHMW.  I stored a boat at an indoor storage facility in Bend for two years.  When I returned to Oregon the bottom was almost falling off because of the extreme daily temp shift that Dutch describes.

 

The very easiest solution would be to sand the bottom and seal it with epoxy, then cover with Coat It or Gluv-it.

Without fiberglass those epoxy finishes will get knocked off and/or chip which will require a day of sand and re-coat each year but it is only a day of work. 

 

If you want to do it right then remove the chine battens, put down fiberglass cloth with flow coats, then replace the chine battens with the bottom edges just proud of the bottom to protect the fiberglass edge. Obviously a bigger job but one that will protect the boat much longer.

Permalink Reply by Greg Max on February 11, 2011 at 11:36am
I'm not really sure if this is recommended or not, but when I attached my UHMW shoe, I only screwed it in around the edges of the boat (into the chine).  I did not put any fasteners into the frames at all.  I attached it while the plastic was cold (about 50 degrees), figuring that it was about as small as it would get.  I live in CO so we have some pretty big temperature swings, but the lack of fasteners in the middle of the shoe allows it to sort of "bubble" out when it gets hot, allowing vertical movement rather than forcing the expansion and contraction to happen in the horizontal plane, placing less strain on the screws.  As far as I can tell, the shoe returns to its flat, flush state when it hits the cold water.  I've had it on for 3 years now without any trouble yet, so we'll see how it continues to age.
Permalink Reply by Greg Hatten on February 11, 2011 at 12:38pm

Watch for "trapped" water between the plastic and the boat bottom when you take it out of the river.  With no support in the middle, that "trapped" water adds a lot of extra weight and strain on those outside screws.  I have my UHMW shoe screwed to the frames and was bugged by the trapped water between frames after trips.  As a "remedy", when I took the shoe off last year for a boat bottom "check up", I drilled "drain holes" in the plastic between frames with a counter sink "bit"... I feel better knowing that water isn't puddled up between trips.

GH

Permalink Reply by Greg Max on February 11, 2011 at 1:27pm
Great idea Greg, I like your style.  I'll drill those before I get her back on the water in March.
Permalink Reply by Les Kirk on February 11, 2011 at 1:39pm
In the boiler design business we deal with differential thermal expansion constantly - up to 12 inches of differential between adjacent parts sometimes.  You cannot fight it you must allow for it- hold on to it but let it move.  I'm not sure this idea will work in practice because of the protrusions but I'll throw it out anyway.  Drill and screw one hole through the UHMD into the frame closest to the center of the bottom (equal from side to side and from stem to stern) - all expansion/contraction will be driven from this point.  Mark each additional screw hole.  Using a straight edge from the center screw draw a short (1"?) line through each marked location.  Create a slotted hole in the UHMD for the screw along that line - that is the direction you are going to allow the UHMD to expand/contract.  The more distant the hole from the center, the longer the slot (you can estimate the slot length from the coefficient of thermal expansion of UHMD times the distance from the fixed point times the degrees of temperature difference).  The slot must extend away from the center for the greatest expansion expected from the center point and towards it for the most contraction expected.  Make some round "washers" from UHMD and drive the screws through the UHMD washer into the frames through the slot (thus the protrusions).  Just the idle thoughts of an idle engineer.
Permalink Reply by Randy Dersham on February 11, 2011 at 2:42pm

Kirk, this does indeed work.  Your suggested solution is very close to what the aluminum boat builders in Oregon use.

Sanderson and I have argued that we could use this for wooden boats.  I believe that it would be possible to embed an anchor into the boat bottom and seal it well.  Then drill an over sized hole for into the UHWM along with a counter sink for a washer.  The bolt/washer combo fits into the counter sink and the plastic is allowed to expand in any direction because the hole and the counter sink are over sized. 

 

Sanderson's argument is that the continuous thread of the anchor will allow water no mater how you try to seal it.  He might be right.

 

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