Skid Shoes

Now that I have my mind right concerning plywood, what about bottom protection? Greg Tatman advocates the use of UHMW plastic and Roger Fletcher and others seem to prefer using 1/4 ply with fiberglass and graphite. What are the thoughts on this.

Also, what about the use of adheasive between the frames and side panels? I would think that it would go all the way but in the book it says below the water line.

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Permalink Reply by Dave Z on November 25, 2010 at 7:30am
Here goes... there is always alot of talk on this subject. Take a search through old posts. I'll be short with what I do, I have been long winded before...

I use Sika 291 or #M 5200 on the bottom frames only. I do not use adhesives on the side panels to frames. You end up getting much too much on stuff that isn't supposed to get caulked and the cleanup is a bear. I'm not so sure adhesive on the sides does much anyway. But man, do I over-use it on the chine area and bottom.

Skid shoes- I've done two boats with 1/2" bottoms and three layers of 8oz glass, with graphite mixed in. seems to work ok. however terribly rocky rivers where you will surely hit unseen rocks will cause pretty nasty interior impact fractures. The whole energy transfer thing. Both of those boats have some serious plywood cracks right under the front seat.

The last boat and likely all future boats- 1/4" sacrifical plywood shoe. I just turned my boat over for the last time this fall as I hosed it out for storage. The two year old skid shoe is holding up wonderfully. there is nothing wrong with it. I have ZERO impact fractures on my interior as well. I have not experienced any "sticking' issues to rocks either. A 2 year old $26 dollar bottom treatment and I haven't done any interior or exterior repairs. works for me.

UHMW has its issues and benefits. I haven't used this method, so I'll let others comment.
Permalink Reply by Greg Hatten on November 25, 2010 at 9:44am
I'll "bite"... I have long been an advocate for the UHMW bottom because it slides so nicely over river rocks, driveways, and boat ramps... it's slick, strong, and has worked well on my boat (we have pretty moderate temps in the Willamette Valley and THAT is a huge factor). However - I've had my boat on the water for 5 seasons and this year, I experienced "first hand" the drawbacks of UHMW. Since you have to apply the plastic shoe to the bottom with screws, the plastic eventually starts working the screws loose because it expands and contracts with every trip - cold water, hot summer storage, cold water etc.... When the screws loosen, water seeps into those holes and the wood is compromised. I'm going to remove the bottom this winter, dry the holes out, reapply the UHMW and bed the screws with 5200 caulk and see if I can get another season or two out of it... after that, I'll probably replace the bottom and be "kinder" to my boat by not running such boney water...... or not. GH
Permalink Reply by mitchell on November 25, 2010 at 10:31am
I'll check with the previous owner of my db about when the plywood skid shoe was put on. He owned the boat from when it was built in 1968 until he sold it to me in 2001. I believe the plywood shoe was put on in 68 and is the same one that is sitting on the bottom today. I've taken some hits that have take a bite or two out of the shoe but have made some epoxy fillings to correct the dings. When patch has dried and sanded, I use coat-it on the shoe and she is smooth as silk.

hijack....Greg, could not sleep last nite. Wondered into the front room to grab an old issue of STS to read in bed. I get about 1/2 the way through the mag and see an article of you playing hooky on your lunch time. Nice article. I love Bi-Mart.
Permalink Reply by Greg Hatten on November 25, 2010 at 1:31pm
Ha Ha... so you read "Steelhead Nooner"!! Thanks for the fond memory - I hadn't thought about that article in a long time. My obsession with steelhead lead me to an obsession with wood boats to chase em. Glad you are a Bi-Mart fan - very Northwest company!! GH
Permalink Reply by mlwebb on November 27, 2010 at 11:26pm
I did a lot of reading on this subject, and some thinking (I have a background in architecture, and the structural issues interested me). I looked at as many damage photos as I could find, most the interior cracks mentioned by Dave. My solution, as yet untested as the boat is waiting for its maiden voyage, was to put approximately equal strength fiberglass interior and exterior on the bottom (the interior obviously applied before installing the bottom, then adding a couple coats of "Coat-It" epoxy/graphite/kevlar coating for abrasion resistance. I guess I will find out whether theory is reflected in experience. The structural picture depends somewhat on the relative strengths of plywood vs epoxy/glass, but in most cases where the plywood fractured, the fiberglass was in compression (its better in tension), and the wood it tension - too much. The stitch and glue guys talk about interior reinforcing. Putting fairly equal amounts inside and out is structurally more efficient, because you get to add the thickness of the plywood to the effective structural depth (section modulus), -think I beam or honeycomb panel.
I like the wood skid shoe idea too, probably the most practical and cost efficient, and obviously suited to a boat where you want to oil the whole interior.
Interior fiberglass has its caveats - painting the floor between the frames (to protect interior epoxy fiberglass from UV damage may not be everyones idea of a good time), and (as the stitch and glue guys would point out) there is a kind of slippery slope here. Depends on how traditional you want to be, I guess.
Keep in mind I'm just tinkering, there is a huge body of experience in the back pages of this site, and montana-boatbuilders.
I didn't seal the sides, the bottom was painful enough to clean up. I used 3M 5200 for the bottom (didn't post those pics!), then ordered some mahogany sikaflex 291-LOT, which I liked a lot better.
Michael
Permalink Reply by Rick Newman on November 28, 2010 at 7:48am
Micheal and all, while perusing the web for some information I ran across a document assembled by VTT a Finish materials supplier. It was paid for by the Finnish government, presumably to improve the quality of Finnish fishing boats. The report compares the strength of a variety of materials both alone and with additional laminations. The report provides documentation to the added strength of fiberglass laminated to both sides of plywood, reinforcing your comments and beliefs with proper materials testing.

Some time ago Ukalady shared some photos of her beautiful boat after it tried to crush a rock in the Colorado River. The damage was apparently much less than it could have been. If I remember right she reinforced both sides of her plywood hull material with fiberglass and epoxy. The report "reinforces" why her boat had less damage than plywood hulls without the laminations.

While boats destined for the Colorado and other rocky, whitewater rivers obviously need to be stronger than those meant for Class II streams there are certainly some lessons to be learned from the information presented in the attached document. I have also shared this document with the folks on the stitch and glue drift boat forum; http://montana-riverboats.com/phorum/list.php?3
I have attached the pdf of that report. Thanks to Sandy for formatting it.
Attachments:
Permalink Reply by mlwebb on November 28, 2010 at 1:26pm
Thanks Rick for posting that, I spent some time looking for some objective test data. A couple take-aways:
"Coating the outer face with a thin FRP laminate (one layer of glass-epoxy) increases the specific impact strength by a factor of 2.7 and 2.9 for aircraft and combi plywood respectively, whereas coating both faces of the combi plywood with FRP increases the specific impact strength by a factor of 5.5."
Here is a simplified illustration of why:

And (speaking of FRP):
"It is not surprising that both a high fibre content and a high amount of continuous fibres produce higher specific impact strength values"
Fiberglass fabric choice can also have an impact on strength, I found some data poking around a fabric manufacturers site. Here is a comparison of a plain weave and a satin weave:
Plain - style 7500, 9.64 oz
and 7781 satin weave, 8.8 oz
Biaxial Fabrics are stronger yet, due to a higher percentage of the fibers orientated to the strain - a plain fabric normal to the ribs has half the fibers running chine to chine, half frame to frame. A 45 deg biaxial cloth puts roughly 40% more fibers into play (square root of 2, hypotenuse of a right triangle). In theory one should be able to make a stronger bottom on a framed boat than a stitch&glue because you have the shorter span (frame to frame) rather than chine to chine.
Michael
Permalink Reply by Randy Dersham on November 28, 2010 at 2:49pm
This is excellent and I think you have it exactly right.

One comment. The Plywood would be much more likely to break at the lever created by the frame. The bend will occur exactly as you have illustrated but the lever created by the rise in the plywood to the frame will cause the plywood to fail at the most stiff or least flexible portion of the plywood. Most often that happens right next to the frame or along a bulkhead joint.

Once in a a while I see a break like this in between the frames. That usually happens if the rock is pointed which focuses the energy at the point of impact.

Just FYI, we use a 19oz tri-axel cloth for the bottom. I order mine at Fiberglass Supply Then we paint the inside with Durabak. Using 1/2 inch fir for the bottom, this has been very successful.

For our boats with 1/4in. fir sides we coat both sides with 9oz bi-axle +45/-45 exactly as you have shown.
Permalink Reply by mlwebb on November 28, 2010 at 9:19pm
Randy,
That would be a failure in shear, rather than bending (as illustrated above) , like so?

I can see how that would be a common "grounding" injury. Particularly with a loaded boat.
Frame spacing, plywood thickness and amount of load would seem to be the major factors - quality of plywood, adhesion of epoxy/glass to the wood (peel strength) also. I suppose a cheap test of how much shear strength your fiberglass fabric would add (relatively) would be how easy it is to cut with a scissors. On a thicker cloth like your 19oz, a quality epoxy resin would add a significant amount of shear resistance, in addition to the cloth. On my little Trapper - since I was rearranging the frames I spaced them closer on the downriver (wider) end, less close towards the stem (upriver).
Michael
Permalink Reply by Scott Hepler on November 28, 2010 at 5:15pm
There is a parallel discussion over at Montana river boats. Some of the guys are now building boats for white water use that suspends the inside compartments over the floor, just the opposite to a framed boat. The reasoning is in the illustration above, in that when the impact happens next to an inside support the weaker material fails. The main reason is that the load isnt able to radiate to a larger area.
Jason at Montana boatbuilders has been using plascore bottoms with a layer of fiberglass on each side plus a layer of Kevlar on both sides for several years now and he has repaired many of the boats since. In all the boat repairs he has done there has never been a puncture go through one of his hulls. There has been penetration through the bottom layer and into the plascore but never all the way through the hull
Clackacraft has been doing this for years with there bottoms but they have so much flex in there that they arent as responsive and are inefficient to row because the floor also flexes when you get into the oars.

Here is part of the thread from the discussion:

By adding some flex to the material and using a material which is compressible we change the force of the impact. We can't do this with plywood or aluminum. Plywood and aluminum can flex but not compress. In a wood boat with frames, the thick frame is much stronger then the plywood it supports. A hard impact will just punch through the plywood if it's not thick/strong' enough. Just think about how much force would be involved to brake through 1/2 inch plywood and brake a 2 inch thick oak frame. Just like a floor in a house. Drop a 500 pound rock on the floor and it will punch through the plywood. Drop it on the floor over a joist and it could brake the plywood and joist. How can we prevent this? By cross bridging the floor joist and sending the impact to all the other joist in the floor. That's what we can do with the new super fibers and they are so strong we can get rid of the joists all together.
Permalink Reply by Dave Z on November 28, 2010 at 6:08pm
Scott,

You are referring to the "oil canning effect" that S&G boats experience with hull flex. There is I believe a balance as you say between hull stiffness, bottom flex and laminate of materials to make it all work.

On S&G boats it works nicely because there are no frames. I think the frames complicates things when glassing on the inside. Anyone ever do this? do you glass the sheet first, then install, or do you glass in place? I'm also concerned with water finding its way between the glass and plywood and we all know what that entails. I know Ray heater glasses his interior floors.........Hello Ray?!!!?!??!?!

That said... My oiled 1/2" and 1/4" wood skid shoe screwed to it with no other means of adhesion or treatment has proven to be what works for me and my waters.

This has been great discussion.
Permalink Reply by Frank on February 26, 2013 at 6:57pm

Hello, Micheal

             Im on the fence of "best way for bottom treatment' Im building a 12 ft drift boat,Ken Hankinson plans. it does not have any ribs" traditional style". It has for & aft compartments for structural integrity & bottom has 3/8 ply marranti with three 3"x 3/4 white oak keel & battens followed by 2"x 1" inner chines. Want to omit outer chine lumber due to intrusion of water due to screw holes, want to protect outer chine with multiple fiberglass/epoxy/coat it, up to water line, with same oz fiberglass/ epoxy on inside up past chine painted. Or one layer of 10 oz  s cloth/polyuarea sprayed on outside to waterline & 10 oz s cloth/epoxy inside up past waterline painted. Or polyuera on outside, heavy on chine area up to waterline and polyurea on inside past waterline.  Any help would be greatly appreciated

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