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Polymer type and Installation

papdavis

Member
Hi all,
My buddy and I have a 16X8 fiberglass Dixie airboat. Years ago, when we were strong and poor, we put white polymer 3/8 sheets on the bottom of the boat using 1/4 " flathead screws, countersinked. I don't remember the year we did this, but it's now cracking due to age. (maybe 1990s)

I have looked at this site for info...still confused but have questions...

We are not poor, but no longer strong due to age...but we want to do this project by ourselves, reliving past triumphs!

So, the questions are:
1. What type of polymer to use? Red, blue, white?? Still 3/8"?

2. Where is the best place to purchase the best polymer- we live in Houston, Texas.

2. Since we have through bolted the old polymer, should we continue to use the flathead screws , countersinked? Or is there a better type of bolting?

Thanks to all for your help!

Don
 
White does not have any UV stabilizer, typically no other additives. Get 3/8 UV stabilized in whatever color you like, black is the most available right now and availability is the key right now. If you can't find UHMW sheet in Houston, you can't find it anywhere right now.

I prefer riveted, but you already have the holes for screws, stick with that. Lay the boat upside down in the hot summer Houston sun, lay out the sheet and hang weights off the sides with vise grips to keep it tight. Start drilling and screwing from the middle of the transom and work your way out and forward. Only work in the heat, UHMW expands a lot when hot and will be tight as a banjo string in the water. Don't try to pre-drill, just do one hole at a time. Absolutely countersink the flat heads, one at a time.

If you want to go full in old guy anal, add SST sealing washers on the inside.

McMaster-Carr
 
Always wondered how to do it, (I had DB replace it on my aluminum hull).
Thanks Slidin' Gator!
 

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The key is to get it hotter during installation than it will get in service. If it heats up beyond the installation temperature, it grows larger than the screw restraints and start to bubble off the hull, creating much more drag. It's not so much of an issue for a hull seeing water, but put that hull on the bone dry ground in sticky grass/mud on a hot afternoon and it doesn't take too long before the poly is smoking hot and you run out of accelerator.

Think about an open hull vs. deckover in the hot sun, that open hull is baking the poly all day while the deckover keeps the hull shaded. White paint on the inside of an open hull runs cooler than black.
 
The key is to get it hotter during installation than it will get in service. If it heats up beyond the installation temperature, it grows larger than the screw restraints and start to bubble off the hull, creating much more drag. It's not so much of an issue for a hull seeing water, but put that hull on the bone dry ground in sticky grass/mud on a hot afternoon and it doesn't take too long before the poly is smoking hot and you run out of accelerator.

Think about an open hull vs. deckover in the hot sun, that open hull is baking the poly all day while the deckover keeps the hull shaded. White paint on the inside of an open hull runs cooler than black.
Yes sir, Thanks for your explanation! Great to have an expert to explain the science!
 
Hi all,
My buddy and I have a 16X8 fiberglass Dixie airboat. Years ago, when we were strong and poor, we put white polymer 3/8 sheets on the bottom of the boat using 1/4 " flathead screws, countersinked. I don't remember the year we did this, but it's now cracking due to age. (maybe 1990s)

I have looked at this site for info...still confused but have questions...

We are not poor, but no longer strong due to age...but we want to do this project by ourselves, reliving past triumphs!

So, the questions are:
1. What type of polymer to use? Red, blue, white?? Still 3/8"?

2. Where is the best place to purchase the best polymer- we live in Houston, Texas.

2. Since we have through bolted the old polymer, should we continue to use the flathead screws , countersinked? Or is there a better type of bolting?

Thanks to all for your help!

Don
Check out my Bondable UHMW polymer called OceaPoly. It last 4 times longer than mechanical fastened HDPE polymer according to the US Coast Guard. No fasteners no holes, no expansion or contraction. For more information go to www.oceapoly.com
 
Check out my Bondable UHMW polymer called OceaPoly. It last 4 times longer than mechanical fastened HDPE polymer according to the US Coast Guard. No fasteners no holes, no expansion or contraction. For more information go to www.oceapoly.com
Well, let's get one thing cleared up for everyone here.

Almost no one uses HDPE on their airboat to begin with. High Density polyethylene.

What everyone uses is UHMWPE. Ultra High Molecular Weight Polyethylene.

So, while your product may be quite nice, it will in no way last 4x as long as what is typically used by everyone.

No reason to try and be deceptive and rely on the ignorance of people here. You generally won't get away with it here anyway given the experienced old farts we have...wait, guess I am getting old too. Anyway....

Bonded installs have been tried here before and some have had success and others not. Oceapoly isn't a new product on the global market and wasn't developed for airboat use. What's unique to airboat use is that it will get very hot vs when used on a real boat like it is typically used for. That heat really stresses the adhesive and if it delaminates its a bad day for the boat owner for sure....

So, while very attractive to not have to drill holes in our ever more expensive hulls, you have a pretty steep trust hill to climb reliability and safety wise.

I wish you all the best in making it work and we will have an alternate to a thousand rivets....
 
The key is to get it hotter during installation than it will get in service. If it heats up beyond the installation temperature, it grows larger than the screw restraints and start to bubble off the hull, creating much more drag. It's not so much of an issue for a hull seeing water, but put that hull on the bone dry ground in sticky grass/mud on a hot afternoon and it doesn't take too long before the poly is smoking hot and you run out of accelerator.

Think about an open hull vs. deckover in the hot sun, that open hull is baking the poly all day while the deckover keeps the hull shaded. White paint on the inside of an open hull runs cooler than black.
Slidin gator I'm digging through old forums trying to get my memory refreshed for poly install. Dad and I did a 12' aluminum boat a few years back. We ordered rivets from jc rivets and I remember them asking me if I wanted harder or softer and thinking back on it now I can't remember what we bought. Also seemed like we ordered a 100 of them a little longer ( for when you run into a not so flat spot on your boat and the shorter ones won't reach ) someone had told me once how much stick out you wanted before hammering the rivets ( a particular length of stick out will keep from excess rivet inside hull) and I don't remember that either, I bought 3/8 poly and assuming my boat hull bottom is 1/8" to 3/16" thick. Seems like best I remember you wanted your material thickness of hull and poly plus a 1/4" ? Does that sounds right? We rounded off a flat chisel for our back up to give the excess a nice rounded head. Also Seems like our local guys had said you don't want the poly too hot but not too cold? I like the weight idea you speak of for we just started on one end and too off to the other. I'm excited because this Hull has no holes and I can lay out some straight lines somewhere straight 😅 compared to us re using out old holes on the other boat ( they were crazy crooked and drove me insane but knew it would be on bottom and go unnoticed once flipped over ) I'm in Oklahoma and wanted to do this in next week or two weve been hitting some mid 80s low 90s. Any info is greatly appreciated. Thanks.
 

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Slidin gator I'm digging through old forums trying to get my memory refreshed for poly install. Dad and I did a 12' aluminum boat a few years back. We ordered rivets from jc rivets and I remember them asking me if I wanted harder or softer and thinking back on it now I can't remember what we bought. Also seemed like we ordered a 100 of them a little longer ( for when you run into a not so flat spot on your boat and the shorter ones won't reach ) someone had told me once how much stick out you wanted before hammering the rivets ( a particular length of stick out will keep from excess rivet inside hull) and I don't remember that either, I bought 3/8 poly and assuming my boat hull bottom is 1/8" to 3/16" thick. Seems like best I remember you wanted your material thickness of hull and poly plus a 1/4" ? Does that sounds right? We rounded off a flat chisel for our back up to give the excess a nice rounded head. Also Seems like our local guys had said you don't want the poly too hot but not too cold? I like the weight idea you speak of for we just started on one end and too off to the other. I'm excited because this Hull has no holes and I can lay out some straight lines somewhere straight 😅 compared to us re using out old holes on the other boat ( they were crazy crooked and drove me insane but knew it would be on bottom and go unnoticed once flipped over ) I'm in Oklahoma and wanted to do this in next week or two weve been hitting some mid 80s low 90s. Any info is greatly appreciated. Thanks.

12ft,

This first link answers your riveting questions and more, basically we us Medium Hard (AD) rivets and select length for 3/8" of head to smash during bucking. Too much rivet and you end up bending it over, too little and it doesn't grip well.
Stossel Palm Beach Hull Riveted Rake Cap Replacement

Some more riveting threads, I hope the pics come back soon!
Shadoobie, Shattered Shattered
Hull Riveting Tools and Supplies

Your local guys may be right on the "Just Right" install temp for what yall do and the hulls you run. It is certainly possible for the poly to buckle the hull if put on too hot and then subject to cold (and the poly shrinks more than hull). For my purposes in S. Fla. my hull doesn't see much for freezing temps but damnit it get's hot in the afternoon and so does the tall grass. Keep in mind I'm building to run miles of dry and sticky, big difference vs. hitting a sand bar or two for a cold drink. I am planning a poly job myself, for a few July/August mid-day sessions, direct sun.

From the first link above.
There are basically 2 different rivet styles, and several materials. Primary rivet styles are Flat Head (AN426) and Round or Button Head (AN470). This picture shows a 3/16" diameter Flat Head and a 5/32" Button Head.

IMG_3524-600x800.jpg



The AN numbers are the most common, but you might also find them called out by Mill Spec number:

Flat Head = AN426 = MS20426
Button = AN 470 = MS20470

These numbers call out the configuration of the rivet.

Rivet-Table.jpg



There are multiple materials available for rivets, and they cover the range of useful materials, from soft to very very hard (Secs say vary vary, I guess for emphasis).

Stossel hulls normally use Medium Hard rivets specified as "AD" (AN426AD, AN470AD). "A" rivets are soft and "AD" are medium hard. "D" "DD" and "DDD" rivets are progressively harder and higher strength, but also more likely to fracture from impacts etc. So the 2117-T4 rivets are 38,000 PSI tensile strength vs. 72,000 PSI for the 7075-T6 hull material and 36,000 PSI for the stainless cap. That's correct, the hull material can handle 2X the load of the SST rake cap, but the cap is stiffer.

Note that they list shear strength values too. Shear strength is just what you think it is, the metals sliced in half because the mating parts moved. Shear strength is always less than tensile strength, but if a fastener fails in shear, the connection has already failed. The purpose of any fastener, rivet, bolt etc. is to compress two or more surfaces together. The compression results in friction between parts that resists movement, adhesives improve on this (see next post). If the mating parts move, the friction drops so all the sudden the fastener sees a shear load and will eventually fail.

The point is that the mating parts are not supposed to move, the connection tensile strength and proper tightening are the critical values. The rivets are supposed to pull two or three pieces of metal together nice and tight and keep it that way.

It is also worth noting that 7075 aluminum gets it's strength from a high copper content. Beyond making welding impossible, the copper makes this the most corrosive grade of aluminum and is the anode ("-"), which corrodes away just like the negative terminal on the start battery. The 304 SST Rake cap is the most noble ("+") material and is the cathode, which actually gains material in the corrosion loop. 2117 Aluminum rivets are a good bit less corrosive than the 7075 hull sheeting ( and a little better than 6061). Ideally the rivets would be the most noble, but we settle for less than the hull, which has way more area to corrode/consume over time vs. the small area of the rivets, which are in direct contact with the SST. If we went harder on the rivets they become more anodic ("-", corrosive) and can become the focal point of corrosion.

Rivets are a pretty basic Aviation commodity, they are not outrageously priced. Monroe in Melbourne is a local source, Aircraft Spruce is a national source.

Aerospace Rivets | Monroe

Rivets | Aircraft Spruce

The last part of the part numbering is the size. Rivets are specified in 32's of an inch on diameter and 16's of an inch on length. So these bags hold AN426AD (Flat Head, Medium Hard) rivets, 3/16" diameter (-6), 1/2" and 5/8" long (-8 & -10). Note that flat heads are measured from the top of the head as they are intended to be countersunk below the surface of the mating (SST) surface.

For the -8, 1/2" rivets, we are joining 0.100 thick hull sheet to 0.062" SST cap. So that is 0.16" grip length and 0.34" (~3/8") length to be compressed during riveting. For the -10, 5/8" rivets we have the added thickness of the 1/8" patch plates used in the last rebuild, keeping the exposed head length the same. General rule of thumb, thickness of the materials to join plus 2x rivet diameter, +/-

IMG_3543.jpg
 
Thank you much for the feed back. Hate to over analyze but want it to be right best that I can. Thanks
 
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