flatpanel construction

I have an old racing runabout that was originally built with all recorcinol joints and the joiner work was excellent. A number of the joints have "let go" due to flexing of the framework due to the pounding that these small boats take. The boat is a D mod runabout, so we are talking about running close to 70 mph and bouncing around the course.

Recorcinol is a fairly brittle glue and if you have flexing going on between the two pieces, it can crack. This is a 30 year old boat and a lot of the joints are good as new and some couldn't be taken apart without ripping the wood, but others were cleanly broken with glue on both parts. Again, I'm pretty sure it was a flexibility issue, but this is a limitation of recorcinol glues.

 

I would not use HDPE. It is not very strong, fiberglass and epoxy will not adhere to it, it is very expensive. I would use wood covered with epoxy or aluminum. Much stronger, easier to get, easy to work with, paintable, an cheaper to boot if you know how to do it.

 

HDPE or UHMWPE would be first recommendations, as they don't rot, are easily mechanically attached with light screws or a bedding compound, they can take considerable abuse, self lubricating in abrasion situations, they're inert, easier to work then wood or metal and generally the best way to go.

Hardwood is also a choice, but not epoxied, as this will just seal in moisture once it's met it's first rock or gravel bottom. Wood will eventually rot, isn't as easy as plastics in regard to dealing with dimensional instability and will not hold up like these high density plastics. Metal is an option, though cost for a good solid back half oval, in stainless or bronze will make you cringe. Hollow back half oval couldn't be considered for obvious reasons and brass really doesn't like the marine environment very much.

You don't have to paint rub strips, which is in the favor of plastics and lastly why would you want to 'glass in sacrificial rub strips? "glass the hull fit your rub strips, bond the fastener holes and bed the strip.

 

PAR, could you go into the details of your statement ""glass the hull fit your rub strips, bond the fastener holes and bed the strip. "

Where I live we are constantly dragging our boats up and down a rocky beach. We normally use HDPE pipe "rollers" under the boats, but I have been thinking that rub strips on the bottom would be easier.

My boat is a 21' long with a glass sheathed 1/2" plywood bottom. What is the best way to connect the plastic strips? If I screw them to the bottom I am worried about moisture getting in the screw holes and leading to rot. Will a good bedding compound job prevent this? What should a guy use for bedding compound?

When through bolting something to plywood I usually drill an oversize hole that I then fill with thickened epoxy, when cured I redrill it for the bolt. Can the same thing be done for screws?

C.O.
http://whatsintheshop.blogspot.com/

 

A good bedding will prevent moisture from getting at fasteners initially, but in time the bedding fails and you don't know until you have a leak or rot. I prefer polysulfide under the waterline on wooden boats, polyurethane on other materials. I always bond underwater fastener holes, regardless of type, ditto weather deck or other very wet location fasteners.

This is precisely what "bonding" a fastener is and yes, it works for screws. The epoxy filled hole prevents moisture from getting at the wood, so no rot. If using HDPE or other high density plastic, use well over size holes on trailered boats. These plastics can expand quite a bit with temperature change and need room in the fastener holes. Elongated slots are sometimes necessary, depending on environment and usage condition swings.

 

Consider the following posts, "Polyester on wood. Forget it."
"Would never consider using Polyester over plywood, never."
"Thin the un-waxed resin and saturate the timber/ply. -Really the same as using epoxy."

Who is right?

They all are for different reasons.
1) Polyester is not a glue. It shrinks as it cures compromising the bond.
2) It's not waterproof. Even the small amount of moisture absorbed by the resin can be wicked deeper into the wood causing problems. Not the least of which is the weakening of the cells forming the mechanical bond.
3) The heat and pressure used in the manufacture of ply tends to "caseharden" the surface veneer. This makes a mechanical bond more difficult to achieve. Even sanding doesn't entirely solve this problem.
4) Even thinning the resin with styrene and saturating the wood surface doesn't improve the bond.

That all sounds pretty hopeless, not so? Yet DaEdster seems to have solved those problems. I don't know his secret but I suspect it is how the resin is thinned. Styrene dilutes and weakens the resin. I use acetone. This thins the resin without changing it's characteristics and when the acetone evaporates a thin layer of resin is left on the cell walls. It is important to make sure the surface cells are fully saturated as this blocks the natural wicking action that draws moisture into the heart of the wood. As unwaxed laminating resin doesn't dry when exposed to air it forms a good chemical bond with the laminate while the hardened resin on the cell walls gives a good mechanical grip.

 

I can understand and generally support the first two statements, as they're easily backed with testing and empirical evidence. The last statement is by someone who isn't familiar with the results of polyester (doesn't matter which resin system) on wood and is simply not even close to reality, let alone truth. Elongation and other modulus issues aside, polyester just can't preform in this role, particularly compared to epoxy.

I would suggest you need a better grip on the chemical and physical characteristics, not to mention the mechanical properties involved here, before making a statement as obviously flawed as this.

 

Already PAR gave a good answer. For the 4th time at least I'll repeat:
wood, plywood and other wood derivates contain phenols and phenol compounds that inhibit the curing (ie polymerization) of the polyester resin. The resin won't harden on the very surface of the wood
Acetone is the worst thing to use with polyester (and epoxy) it leaves thousands of pinholes while evaporating. Probably also it will also affect the polymerisation. I suspect also that it will free the phenols from the wood and thus worsening the inhibition problem.
Thirty years ago we made very precise trials of composites wood/polyester resins at the French Navy for mine hunters and protection of frigates.
A lot of systems were tried in a scientific way, with a lot of substrates, interfaces, resins and fibers. Plus artificial ageing, salt water chamber etc...
It doesn't work, except using an a very special interface between the wood and the polyester.
This interface is so expensive and asks so much work that it's not worth compared with a common epoxy resin, also the results under stress were not brilliant.
No, it's not a PPU or polyurethane monocomponent as was proposed by VOSSCHEMIE 40 years ago. This system systematically failed after a short time by delamination.
The polyurethanes hardened with isocyanates do not work also.
Trying to inhibit the phenols with accelerators like the cobalt naphtenate help a bit on short term but finally do not work.

If you accept a no-value small boat like a dinghy made with inferior materials like exterior plywood and poyester go with that. Do not try to finsh it nicely; it's not worth. Paint it with the cheapest paint. The boat must not stay in water and must be kept dry under roof. Just use it a few hours/year. Hide the fact you are the owner because it will be ugly and it will be probably more expensive than a plastic thing.
It will last a few years and you'll make one again....and I hope you'll have learnt the lesson and you'll make a good one with the right materials.

 

Has anyone used prefab epoxy sandwich panels like Fibrelam on flat hulls? Besides airliner flooring it has been used in primary aircraft structures. Expensive stuff but in some cases may be justified.

http://www.hexcel.com/Resources/DataSheets/Panel-Data-Sheets/Fibrelam_global.pdf

Dino

 

I'm just now reading thru this subject thread, and I find it just might have some parallels with my future Thai cottage/houseboat plans.
http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987.html

http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987-4.html#post531819

http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987-3.html#post424272


So tell me what kind of craft you are seeking 'Boat Fan' ??

 

Those UHMW polyethylene floats are a great idea.

But I think Mark Twain might have traveled on something that looked like this
http://www.boatdesign.net/forums/boat-design/retirement-houseboat-floating-home-23987-3.html#post424272

 

I've got a design like that in the works...but can't release it right now.

 

I was going to suggest relatively cheap polypropylene honeycomb cored panels by Nida-core, but when I went to look them up I see they have been bought out by 3M,....so I don't know now what has happened to their variety of selection, nor the pricing. But I would be willing to bet selection has decreased dramatically, and prices have gone UP.
http://nida-core.com/english/

I attended an International HomeBuilders Show down in Orlando last year, and the products they had available were amazing. Unfortunately I could not get thru even 1/3 of the show before I injured myself and couldn't finish the show. This past year the show was held in Miami, but I was out of the country.

There are some fascinating materials available for adaptation to 'floating home' projects

 

Extruded Polystyrene (XPS)

You might want to have a look at this material as well,....EXTRUDED POLYSTYRENE,...not to be confused with expanded polystyrene

http://www.boatdesign.net/forums/fiberglass-composite-boat-building/extruded-polystyrene-xps-24213.html

Do a search on this forum as there are mentions made of this material.

 

James,
What is the cost of the FRP you use?

I don't know the sizes/thickness, but what would be the cost of a 3 meter x 1.5 meter panel, 2 cm. thick? (or whatever size is realistic)

How do you bond them together? Do you need a frame, or can they be bonded together to form a self-supporting structure, like plywood?

Thanks!