Size of support needed

[CapnCrunch]

Our boat is a 44' Lancer Motorsailer with a large flybridge. I have mounted dinghy davits on the aft end of the flybridge and the Admiral has her hammock stretched athwartships between the railings. Total additional weight (including the Admiral) is about 350 pounds. I have noticed that, when the dinghy (without motor) is hoisted and she is in her hammock, that there is some flexing in the aft, thinner section when the stern tops a wave. I think the easiest way to do this is to run a tube from the top of the transom coaming to the aft underside of the flybridge near each corner (red line in pic, below). The question is how "heavy" the tube needs to be. Could I use something like the standard 1" OD stainless tubing used in railings and biminis or would I need larger O.D. and/or thicker wall?

P.S. please excuse the poor image quality

[psk125]

1” might be enough, but you will want to use something with a hefty wall thickness. Whatever you put there is going to get dinged - by the dinghy, by crew grabbing for a handhold, by lines tying the boat.. If it isn’t pretty tough, it will bend, and possibly dimple. Then you’ll have to replace it. If the aft upper “thin” section is flexing, building a solid base for the end of the support to land on will be important. Simply bolting it to what looks like the flange of the fiberglass flybridge deck will end up with it all flexing and eventually cracking the fiberglass.

[Eigenvector]

I can help you out. I'll assume you want 2 struts. Post here or via pm the length of the red line. Buckling is going to drive the design and not stress.
I'll send you back some sizes that will work.


I may take me a few days.



Regards,

[AndyEss]

Both excellent responses.

You are trying to change what is now a cantilevered beam(s) assembly to a simply supported span.

The supports will act as long compression elements (columns). The strength of long columns is sensitive to geometry (radius of gyration).

1” thick wall tubing might work, but going to 1-1/2 to 2” will probably perform much better and will certainly look much better.

[GordMay]

Originally Posted by AndyEss

... 1” thick wall tubing might work, but going to 1-1/2 to 2” will probably perform much better and will certainly look much better.

Indeed.

[waterman46]

Originally Posted by CapnCrunch

Our boat is a 44' Lancer Motorsailer with a large flybridge. I have mounted dinghy davits on the aft end of the flybridge and the Admiral has her hammock stretched athwartships between the railings. Total additional weight (including the Admiral) is about 350 pounds. I have noticed that, when the dinghy (without motor) is hoisted and she is in her hammock, that there is some flexing in the aft, thinner section when the stern tops a wave. I think the easiest way to do this is to run a tube from the top of the transom coaming to the aft underside of the flybridge near each corner (red line in pic, below). The question is how "heavy" the tube needs to be. Could I use something like the standard 1" OD stainless tubing used in railings and biminis or would I need larger O.D. and/or thicker wall?

P.S. please excuse the poor image quality

Normal 1" stainless tube of the kind used for lifeline stanchions is very strong. What is more important is how you plan to attach it. I would put a 4"-5" square SS/Aluminum plate for it to butt up against, at each end, and you can use regular stanchion bases on top of the plate to hold the SS tube.

[Djarraluda]

Strength is a word with differing comprehensions.
Buckling is the ‘strength’ issue you have and because you are supporting a cantilever, you have to prevent excessive stiffness there.
So, I would look at 50mm , 2 inch as a start and have a sliding fit socket at one end. That way, compression is stopped but extension allowed, if the boat wants it. Whilst at the fabrication, consider attachment points that will be useful for things in the future.

[CapnCrunch]

Great responses, all. Thank you. To elaborate and answer a few questions:

1. The distance from the inner edge of the top of the coaming to the underside of the flybridge is 46" (the red line in the pic)
2. Yes. I had planned on two supports
3. I had planned on having the tubes welded to SS angles mounted to the top/inside edge of the coaming and the bottom/outside edge of the flybridge.
4. Because of the horrific cost of thick-walled SS tubing, I was thinking of using standard thickness 1" tube (0.065 inch (16 Gauge)) with a length of 7/8" tube (which I have on hand) inside of it.
5. I have no worries about the strength of the aft coaming. It is a double wall with each wall being 1-1/2" balsa-cored glass.
6. FWIW the bot carries a boomless rig and the mainsail traveller runs across the aft end end of the flybridge.

Hope this helps.

[AndyEss]

Stainless steel Sch 40 pipe isn’t that expensive. It’s not shiny and polished.
Your location is on fresh water - how about aluminum pipe?

[Djarraluda]

I would still worry about bending, but I see your point and yes that would form a pretty thick composite tube.
Perhaps you use two tubes, top plate on the 1 inch, bottom plate on the 7/8ths and slide them together. This still allows the extension if needed but prevents any compression.
Must admit though, I believe it will look too thin and weedy. 32 mmtube fits the hand better than 25mm and gives a more solid feel.
An alternative could be to use 50 x 50 marine grade aluminium ( it has rounded corners in lieu of the sharp square corners of domestic stuff). Powder coat white and it will blend in with fibreglass.
Roger

[CapnCrunch]

Originally Posted by AndyEss

Stainless steel Sch 40 pipe isn’t that expensive. It’s not shiny and polished.
Your location is on fresh water - how about aluminum pipe?

We are refitting the boat for salt water. If we were staying in the river, I wouldn't even worry about it because we don't bounce over waves like we would in the ocean.

[Cheechako]

It's a compression load, so 1" standard wall is theoretically enough, but what matters in a column load in compression is straightness, I'd feel better at 1.5-2". But if it works now, the 1" is going to do a lot for you. ... just dont side load it. The diameter will do more for you than a thicker wall.

[CapnCrunch]

Originally Posted by Cheechako

what matters in a column load in compression is straightness

Yeah. That's actually my main concern. The flybridge ends about 6" inboard of the coaming, so the support will have to run at about a 10-12 degree angle.

[Cheechako]

Originally Posted by CapnCrunch

Yeah. That's actually my main concern. The flybridge ends about 6" inboard of the coaming, so the support will have to run at about a 10-12 degree angle.

Probably better with a little larger diameter then, if the davits are cantilevered out much that makes a far heavier load than just the weight, plus the load is trying to pivot putting side load on the supports... What size are the davit "arms"?

[CapnCrunch]

Originally Posted by Djarraluda

Perhaps you use two tubes, top plate on the 1 inch, bottom plate on the 7/8ths and slide them together. This still allows the extension if needed but prevents any compression.

I had the same thought after someone suggested that the relationship between the coaming and the flybridge would need to be somewhat "flexible." My only worry about that configuration would be the inner tube rattling against the outer. I hate rattles!