fibreglassing to steel.

Hi everyone.
I am building a 24ft cruiser and am looking for opinions and advice in regard to fitting a fibreblass cabin. Is it possible to successfully glass to steel. I have had mixed feedback thus far. This is not a structual exercise (as in assisting in hull integrity) I want to build a ply cab then lay the glass from the sheer over the ply onto the other sheer. Any advice would be greatly appreciated.

 

...build your glass cabin and then attach it with a mastic compound to the raised deck section overlay, you can use 3M 5200 if you like. Secure with only a few bolts and the cabin will be going nowhere, it is only a 24 footer, you can delete the mechanical fastenings if you so desire.The steel deck and raised section must be sandblasted and painted with a good epoxy paint system before considering and attachments

 

I agree with that

 

Thanks for the help, that was the direction i was starting to head. Just wanted to make sure.

 

I suggest to omit the Epoxy paint after sandblasting. Sandblast and bond immediately on a warm not too humid day. Use mechanical fasteners to hold in place, remove later if you want.

Fred

 

Fred, I respectfully do not agree with bonding to raw steel......it is simply asking for trouble. This is the area where water will gather, maybe only from condensation, maybe from bad installation and sea water comes into contact, but either way, protective coating of the steel is required, and is standard procedure in the real world.

 

Not to be disagreeable Fred, but why would you not protect the ferrous steel from the elements? It's foolishly optimistic to think that the sealant/adhesive between the deck and cabin would be adequate for that purpose. I'd bet my last boat nail that if that cabin were ever removed some of the deck (in the form of scale/rust) would come with it. Ensuring the sealant covers all unprotected surfaces could be challenging at best. If properly applied an epoxy or any other high quality metal paint can provide an excellent substrate to bond to. No reason not to paint, in my opinion. I also agree that no fasteners are necessary, provided there's adequate bonding area between the deck and house. 5200 is tenacious stuff, and less holes is always better.
To answer the question directly, I don't know of any long term successful glass to steel (or most other metals for that matter) applications, but I sure know of many, many unsuccessful ones. Polyester resin I'm sure is a no-go, but perhaps there are other resin chemistries that are suitable for FRP layup which also bind to metal. Might ask the guys over at Gougeon Brothers if they can help. Maybe one of West's specialty resins would work.

 

I disagree in the fasteners department. I've seen and once had a cabin knocked off it's underpinnings by a wave, so fasteners are a must. I also am a firm believer in repair and replacement planning, so a bolt down cabin shell makes a lot more sense then bonding on to the deck.

Steel will 'glass very well with epoxy. The usual approach is the blast the steel and apply unthickened epoxy right away.

In this application, I'd use a base coat of epoxy, which then is over coated with paint, though under the cabin to deck flange, there doesn't have to be paint, just the epoxy (3 coats minimum) will suffice. Make sure you coat the fastener holes with epoxy as well. Use a high elongation bedding compound. Polyurethane and polysulfide will both work in this role, though I'd avoid 3M-5200 (too rigid) in favor of 4200, 4000 or Sikflex 291. The adhesive aspect of a bedding compound on a mechanically fastened element is moot. The bottom line is you wan to hold this thing down really well, but the ability to remove it if necessary without much damage when the time comes. Aggressive adhesive/sealants will do more harm then good in this regard.

 

If you do go with fasteners (still think they may not be ABSOLUTELY necessary, but a good insurance policy just the same...it's just situation dependent, of course) then do plan ahead and dry fit the structure, clamp or otherwise immobilize it, and through drill both materials simultaneously. Remove the cabin and de-burr and bevel the steel holes and pay careful attention to epoxying the inside of the holes. Don't forget, before you epoxy you may have to oversize the holes in the steel after you've through drilled to accommodate the build-up of epoxy. If you're relying on the fasteners then you want to make sure to protect the steel that they're relying on.

 

On our steel boat we have chosen to have removable cabin tops. Reasons are divers and include keeping open the possibility to replace the steel tops by wooden ones. The boat is inspired by fishing vessels which would traditionally have a cargo hold with huge opening hatch, we have replaced the cargo hatch by a cabin top.
Our boat is much heavier than the one Bal is working with but the bolted/glued cabin top idea is comparable.
On our boat both the cabin top and deck have a L shape flange opened towards the interior so the fasteners are on the inside of the boat. The steel is coated as the rest of the boat will be, high build epoxy primer and polyurethane top coat. Sandwiched between the two flanges is a 2in wide EPDM foam gasket band. This gasket is positioned over the bolt holes on the interior 2 inch of the flange. Flange is 3in wide. On the exterior side of the flange we also beaded/sealed with sikaflex 291.

Murielle

 

I respect your opinion and must admit that my advice is based on aircraft experience. The bond between the steel and the 'PAINT" would be inferior to the bond directly to the 5200. However absolute cover and no pin holes would be a requirement. Thanks for the reply

 

A steel hull and fiberglass top has been successfully mated in UAE. The deck has a vertical flange to mate to the top. It's big. Someting like a 130 footer.

In my personal experience for steel and glass mating (small scale) cracks appears in the fglass holes due to different coefficicient of thermal expansion. I always use elastic sealant and oversize the holes on fglass then use fender washers.