Structural Support Topside '23 Tsunami

[abl1111]

Heres a picture of my boat with a newly added hardtop:

http://www.classicseacraft.com/commu...1&d=1407789441

I'd like to fortify under the gunnel where the hardtop mounts aft. All is mounted securely to the boat ( the aft mounts each have 3/4", 8"x8" plywood backing plates ). I am finding that the hardtop has a little 'shimmy' in it, side to side due to where it mounts aft flexes ever so slightly. This gunnel area is not designed for this extra stress. There are no cracks, but I would like to get some advice on how I can add support to this area. I was thinking mini bulkheads under the gunnel to minimize any flex. But, I want to make sure that this is the correct way to attack the problem - without creating another. And then, how do Iactually do it.

Thanks.

[McGillicuddy]

Appearance is very nice, but is there some diagonal pipe or plate braces from the top rear crossbar to the rear vertical pipe. I think that would help.

[Bushwacker]

I think Gillie's right, you have some diagonal braces at the front but none at the back, so I suspect you have a lot of bending going on at the rear where the side supports meet the top. The aft support/top joints aren't nearly as stiff as they are at the front. A 90 degree joint isn't very stiff, so diagonal braces between the vertical and horizontal tubing at the aft top corners would stiffen it a lot. Gel coat is very brittle, so I suspect you would have already seen cracks in the coaming/side deck radius if your problem was too much deflection in the glass.

I'd be careful about adding braces or gussets under the gunnels, because if you create local hard spots in an otherwise soft panel, you'll get stress concentrations at the edge of the hard spots and CREATE cracks that you might not get otherwise! It sounds like you've already got fairly robust pads under the mounts, so I would suggest surrounding those pads with something like 3/4" balsa core wrapped around under the gunnels and tapered at the edges, and then put several layers of 1708 over the whole works to create a cored panel that will be very stiff. And I'd use epoxy resin for maximum bond strength and flexibility.

[abl1111]

Here's the aft shot. I agree with your synopsis. Thx.

Where would you recommend that I have either pipe or aluminum plate welded ? Is that something that is an easy retrofit for a qualified pipe welder ?

I also like the idea of using balsa core under the gunnel - great idea. I've re-cored all my hatches this way, but with standard resin and not upside down. I assume the layup would be glass/balsa/glass ( do you recommend the first layer to be thickened to hold the balsa in place ? )


http://www.classicseacraft.com/commu...1&d=1407813279

[FishStretcher]

You might be able to triangulate the rear span with bolt or clamp on tubes going from approximately where the upper rod holder is on the uprights to where the outer rod holders are on the top crossbar. That won't be quite as good as a weld, but maybe easier to try.

Watch the 3/4 ply pads. They will slowly rot. That is what wood does. I would prefer solid glass there. Tapered out gradually in thickness to reduce the stress raiser and chance of a crack. It isn't a lot of weight and will never rot. Second to that would be a high compressive strength cored composite. Like balsa. I love foam but it probably isn't correct in that location. But it hardly seems worth the effort for a small area like a pad. That's just me. I'd do solid glass with a lot of taper.

[Bushwacker]

Originally Posted by abl1111

Here's the aft shot. I agree with your synopsis. Thx.

Where would you recommend that I have either pipe or aluminum plate welded ? Is that something that is an easy retrofit for a qualified pipe welder ?

I also like the idea of using balsa core under the gunnel - great idea. I've re-cored all my hatches this way, but with standard resin and not upside down. I assume the layup would be glass/balsa/glass ( do you recommend the first layer to be thickened to hold the balsa in place ? )

Suggest you send Don (PipeDreamsMarine) a PM about installing braces, as I believe he builds towers and is probably quite familiar with your type of problem! Some bolted or clamp on braces would be a good way to quickly determine if that will solve your problem without getting into glass work. In general the braces would like to intersect the vertical and horizontal tubes at about a 45 degree angle.

It's a good idea to use some thickened resin when installing any sort of core to insure a good bond. This is a good application for the vacuum bagging technique, although maybe not a large enough project to justify the added expense. The plywood pads will be ok if you encapsulate them with glass and drill the bolt holes oversize, refill with cabosil and then redrill. That way, when the bedding compound around the mounting bolt holes fails (it always does, eventually!), water still won't be able to contact the wood.

[Water Rat]

abl1111,
I have a similar half-tower on my 1978 23' Sceptre. The issue you are having is addressed on my pipework with an inner truss. Unfortunately, I cannot post a picture of it here, but if you go to the Gallery section of the website, look-up Chris Downey's 23' and there should be several pictures of the hardtop/pipework showing what I am talking about. Good luck.

[abl1111]

Sent Pipe Dreams a PM a few days ago - waiting to hear from him.

I am up for anything. It's probably a combination of things, but my gut tells me it's the flex of the topside gunnel - but I can't be sure as I'm the one who is usually driving boat and have not been able to get 'under there' while under way.

[kmoose]

Yep, missing some pretty critical support on the rear. Here is a pic of my Rear supports. No shake or rattle at all.

[DonV]

Hey Moose, that's a good looking SeaCraft in the background!!