Cabin Roof convexity question

Hi Guys,

I'm working everyday and my old silverton doesn't look very "old" anymore. I'll post some pictures in a week or so but in the mean time I have a question for some of you experienced wood torturers out there.

I've secured my cabin roof. The roof is about 7 feet (2 meters) wide and about 10 feet (3 meters) long. It's constructed of 2 layers of okoume ply
1/2" thick, laminated with epoxy. I made temporary supports and installed them to give the roof some pitch. I designed the supports to provide 3 inches of rise over the 7 foot width of the roof for drainage. The structure is entirely glued (epoxy and/or Sika). It's very solid.

My question is this.

Once I removed the temporary formers we had some bad weather here. As a precaution I installed a brace to hold my boat cover up. I placed the brace in the center of the roof on a pad to spread the load out. Well I think that the stress that the temporary brace placed on the cabin top caused it to deflect down slightly. It's not terrible and it's still very strong, but it's not quite fair in the center of the cabin roof. There is a slight "dip" there maybe 1/4" to 3/8". I'm thinking of doing the following:

Prepping a 2" or 3" five quarter mahogany board. Using a small hydraulic jack I'll raise the unfair center of the cabin roof. I'll then install the five quarter stock so that it fits tight against the roof support beams on the port and starboard sides and forms a convex radius against the bottom of the roof (inside the cabin). Since it is convex the stress should be transferred to the cabin side beams. These beams are about 4 inches wide and bonded to the roof ply with silicon bronze screws and epoxy. The entire roof/beam/windshield/rear bulkhead intersection is one solid assembly.

Is the beam that I describe strong enough to take the load? Is 2" or 3" wide stock sufficient?

How high do you think that I should "jack" the roof to allow for springback once the hydraulic jack is removed?

Any comments are as always appreciated!

Regards,

MIA

 

You really should make a laminated member IF you do what you're saying. Not a 5/4 piece taking a bend. There will be too much stress and the beam will likely either fracture or the ply on the sides will be crushed upward, spread the sides later, or worse.
If I understand you're situation, that is.
If a laminated piece, it should be crowned an inch to an inch and a half MORE than the roof curve on a form beforehand.
You might also try propping with a temp board going fore and aft at CL and humping (higher by a bit for pre-compensation) the middle as far as you dare, and then wetting down the roof top and leaving it for a few days to reset at the higher (correct) height.
The whole point of that laminated ply roof was obviously to eliminate frames---- why add a frame if it's not needed? If it sagged from pressure over time, it can go back by pressure over time.

 

Thanks Alan.....I see what you mean....wetting it out isn't an option as there is a seal coat of epoxy on there already. I see what you mean though about things going back by pressure over time. I hadn't thought of that. The roof was secured all around before the formers were removed so there should indeed be pressure on the roof to force the ply back up. I've a day or two to go before I add a layer of cloth to the roof and start getting her ready to paint. Let's see what happens.....

Thanks again,

MIA

 

73 silverton

Missinginaction, I too, am restoring a 1973 silverton 33 sedan bridge. 5 years and counting. I have had 2 seasons of use on Lake St Clair so this makes my winters working on Blarney 2 not so bad. I have replaced all the decks stem to stearn but as of now have been able to piece in the top with pretty good results. however the day is coming I am sure. As far as your issue with the pitch of the roof I would think that the main support pole in the cabin, the rear cabin wall, the windshield.and the divider wall for the head , cut at the proper pitch or in this case radius should give you enough support to keep the new roof from flexing either way. let me know how things go .

 

73 silverton

Missinginaction, I too, am restoring a 1973 silverton 33 sedan bridge. 5 years and counting. I have had 2 seasons of use on Lake St Clair so this makes my winters working on Blarney 2 not so bad. I have replaced all the decks stem to stearn but as of now have been able to piece in the top with pretty good results. however the day is coming I am sure. As far as your issue with the pitch of the roof I would think that the main support pole in the cabin, the rear cabin wall, the windshield.and the divider wall for the head , cut at the proper pitch or in this case radius should give you enough support to keep the new roof from flexing either way. let me know how things go .

 

Furnitureguy, suddenly I'm not so lonely out here. Thought that I was the only one restoring an old Silverton! Good for you. I'll be taking some photos when I have a minute. Keep in touch.

As for the roof. After sanding the seal coat last night I cleaned the dust off with some damp rags. Poured some water on the roof and the pitch seemed OK. After some additional inspection I think that Alan was right in that the center section looks a little more fair after I took the downward pressure off.

Another thing that I notice is that the divider wall for the head adds some support about 3 feet forward of the rear wall on the starboard side. The plywood "relaxes" slightly in the center of the roof where the divider wall ends. This creates a slight unfair spot as well. I think that I'm being too fussy though as after the flybridge goes up and the rails go on you'll never notice.

Good luck with your boat Furnitureguy and thanks for the encouragement.

MIA