Keel abrasive protection, HDPE

Another option is mica. I have used it in epoxy, it makes it very hard and abrasion resistant. Some aerospace resins are mica filled. It will destroy a drill bit. I have no experience with it in salt water.

http://www.micaworld.in/micapowderandflakes.html

 

You cant use fasteners on the bottom off a plywood boat.

Metal is a poor choice..paint wont stick, electrolysis issues, fastening issues and it dents when loaded. You want something that is adhered to the bottom , something that accepts paint and something that spreads the load when the boat is grounded out.

Use eglass. adhere it to the bottom glass skin with sika or 5200. Use several layers of glass so that grounding point loads are spread out.

 

Yes, you can use fasteners on the bottom of a plywood boat (why can't you Michael?). Metal is often my first choice, mostly because it's so tough, but also because half oval is easily available, predrilled and socially acceptable. Folks see a length or two of half oval and they know precisely what it's for. Electrolysis issues aren't much of a problem if you match materials, which is the prudent course. Why would you want to paint a sacrificial strip anyway? 'Glass is a poor abrasion strip, unless quite thick and at that, still not as good as a plain old solid back length of stainless half oval or a nice hunk of live oak. If you're going to use a goo and fabric, use polyester (Xynole), which is several times more abrasion resistant than 'glass cloth.

 

> Aluminum oxide isn't the best "hardening" filler material you can employ. It's typical use is for as a thermal break and temperature control, not as abrasion resistant filler. Harder materials (aluminum is very soft) ...

The hardness of aluminum oxide is far beyond anything mentioned above, especially aluminum. That's why they use it in industrial epoxy floors (for abrasion and impact resistance).

 

I've create some drawings of my hull shape to give some solid picture to comment on.

I think the half-round idea has merit to give thickness for embeddeding the screws. It would make it a bit harder to keep clean, but maybe not.
What I'm thinking is to use the half-rounds to hold the sheet in place,
something like this perhaps: The material could be hdpe or copper.
I was thinking of adding another thin layer under the sacrificial hdpe
to hold the screws, perhaps a hardwood bounded with expoxy.



Another idea that I find very appealing is to make this sacrificial material
an actual battery. Use aluminum as the base cathode material,
and then a zinc anode outer layer. The zinc will have to be replaced (how often?), but it creates current to power things on the boat while it is being depleted. A two-fer, hull protection and battery in one.

 

I've got what you need -- I think it's diamond dust mixed with epoxy, i'll even pay you to remove them. The problem will be gluing the pieces back together after. . Getting serious, these epoxy based rubbing strakes are actually epoxy glued to the hull and from the photos you can see there is no fear of them falling off. Using an industrial quality air chizel they come off in chunks taking the gel coat and raising the fibers of the glass matt under it. Their surface has the roughness of say 80grit sandpaper. ---

 

HDPE is very wear resistant and it is very slippery. If you need to drag a boat up and down a beach a few strips on the bottom can reduce the effort by a lot.

C.O.

 

Just as a public service announcement...Correct me if I'm wrong...for slides & ultimate durability you want ----

Ultra-high-molecular-weight polyethylene UHMW-PE - This is the good stuff - more expensive of course.

Very-high-molecular-weight polyethylene VHMW-PE - This is an in-between grade King Starboard sells - I've never used it.

High-molecular-weight polyethylene HMW-PE or HDPE - This is the stuff called Starboard - very useful but not slippery enough for slides nor as tough.

I'd use UHMV-PE fastened with countersunk lightish screws into epoxy/mix "plugs" the idea being if the screws broke or pulled out the epoxy "plug" would stay & preserve the seal on the bottom structure. You might research how the river drift boat guys attach UHMW-PE to their hulls - it's a pretty common application.Moe

 

They weld studs to the aluminum bottom, then use countersunk machine screws set in 5200 to fasten the plastic.

Something to consider is that these river boats dont live in the water.

 

How about coating the bottom with Polyurea? Hundreds of Alaskan fishing boats have been to avoid ice damage. I'm considering coating my build with it instead of fiberglass. Will post a cost comparison when I get the figures

 

They make UMHWPE in molded shapes that can be held in place by slipping them on so no screws needed, would just need to epoxy in the part to slip onto to.

 

That seems like the best option to me. I want to do similar with a rub rail on my new catamaran build. How would you do it? the stuff comes in a roll and so probably would need some "help" to keep its shape in a straight line, no? Seems like you could make up some structural bog and press the piece in place till it cures. Then it could always be slipped off and replaced if needed. That's cool, but, seems like the difficulty would be in keeping it in line while it cures. temp screws might cause "dimples".

http://www.interstateplastics.com/search.php?searchtext=uhmw profile

 

HDPE Jet Boat has a thread that shows how HDPE can be worked. Have you seen it? http://www.boatdesign.net/forums/boat-design/3-5m-hdpe-jetboat-44992.html

 

Using the approach of sliding them on, I think one should take advantage of the big difference between thermal expansion of different materials.
So having steel or aluminum be the component the uhmw slides onto, you could chill the metal and warm the UMHW, and it should slide in/on easily, and then tighten up when it cools. I will have to get some pieces and do some testing.

For a rub rail, UMHW will protect your boat, but might do damage to what it rubs against.

JackB

 

HDPE is commonly used in plastic lumber, typical Patio Decks. On one deck we built the full length plastic deck boards expanded approx. 1in on 18ft. between temperatures of 10 to 30 deg. C. Not uncommon temp. variations between North American Atlantic sea water and a dried out hot sandy beach. This expansion/contraction rate will be a challenge regardless of what fastening system you use. My choice would be FRP rubbing strakes. Spot fasten these every foot or so with temp screws and epoxy bog sitting on say 1/8in. spacers. Once the epoxy has set up run a bead of 5200 or equivalent down along each side between the strake and the hull. I'd be temped to use a good quality exterior plastic window house caulking,(Quad) at 1/4 the price of 5200. Removal for replacement will just be a matter of cutting away the 5200 and using a sawsall cut thru the spot glued epoxy bog . Sand off the epoxy left on the hull, using a sharp chizel remove the remaining 5200 and you're ready with a cleaned surface to attach the replacement. Antifouling will adhere to the FRP but from my experience not well to most solid plastics. ( At least not to my keels teflon shoe). ---

A yacht is not defined by the vessel but by the care and love of her owner ---