Hull steel - superstructure fiberglass

Dear fellows,

I am working in a project where it will be required the installation of a fiberglass deck and superstructure on a steel hull.

For the connection between these two different materials I am considering to fit a (horizontal) flat bar all around the bulwark where it can be laid the fiberglass panels on and then a bolted joint will complete the connection.

Although using a flexible gasket in between the fiberglass and steel to reduce the effects of the different expansion properties of these materials I am still fearing that the fiberglass panels can crack around the bolts.

Could you guys give me some feedback about this idea please?

 

A rubber gasket would only work if the mating surfaces were perfectly true...one hundred percent surface contact along the entire joint..

Mechanical fasteners need to be made one hundred percent waterproof for the life of the vessel or corrosion will form under the sheer clamp.

I would investigate an adhisive bond.

You may be able to fabricate the steel part of the sheer clamp out of Stainless steel to eliminate corrosion problems if you decide mechanical fasteners are needed.

I find fastenerers tapped into steel are easier to keep watertight than fasteners bored thru with a nut and washer

 

Thank you Michael Pierzga for your opinion.

In fact the watertightness of the connection is also a issue I will have to sort out. But I reckon if the gap between the bolt and the fiberglass panel is filled up with Sika or another silicone of this kind and by using a proper washer it will be prevented the infiltration of water through the connection.

My main concern rests on the mechanical side of the problem: will the hotspots around the bolts be enough to crack the fiberglass panel?

The adhesives will for sure eliminate this problem. Thanks for the tip.

 

Stress concentration at the fasteners. I dont know.

Whenever you have a long line of fasteners on a flexible structure like a boat some will be loaded, some will be slack.

The only fiberglass to metal joints that I come across are superstructure components on metal yachts. These were bonded with a Sika type product.

Ive never seen a sheer clamp , metal to fiberglass, joint

Perhaps others have more info

 

I would look at the coefficients of linear thermal expansion for both and calculate how much differential movement (or stress) you need to accommodate. You could have a .25" difference that might very well cause cracks.

I would guess that a steel bar on top of the fiberglass would better distribute the force. Ie, steel bar, fiberglass, steel instead of just washers.

If a rubber gasket is compressed, it provides a good seal, even on imperfect surfaces. Thin lines of glue or caulk can't move very far before they break.

 

You might have a look thru this subject thread as well
http://www.boatdesign.net/forums/boat-building/steel-hulls-composite-superstructure-topsides-47349.html

I'm working on something similar.
Brian