Bimini design ideas

Kelpie

Kelpie

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Putting together a bimini frame on my Moody 39. Being a centre cockpit, it's wider than it is long. Very wide- about 3.5m in fact.
I'm using 25mm stainless, as I have a stash of it already and the fittings are readily available.
For simplicity the frame will be permanent, no folding or hinging required. Canopy will be made from rigid solar panels and rigid sheeting (not yet decided, possibly foam cored grp, or ply as a temporary solution). The panels will be port and starboard to get them away from the shade of the boom.

Rough dimensions are 1.7m fore-aft, and 3.5m across. Wondering if that width is likely to sag? There will be a slight upwards bow to it, and in each corner a triangle made with a short (~1ft) length of tubing as a brace. I'm sure it will be fine holding its own weight but the aft cross piece is going to become a great handhold and see a lot of abuse. I'm considering adding two vertical posts to help support it, but I don't want to clutter up that space unless I absolutely have to. Just looking for a show of hands about whether it's likely to be necessary.
 
The canvas would have to be really well tensioned if it isn't to sag, but frame tents manage that sort of distance. I'd also be concerned about what would happen if you have the best part of 100kg, plus shock load hanging from the middle of a 3.5m 25mm pole. I'm no engineer, so the calculations are well above my 50 year old A level, but take a bit of pipe and hang from it. I think it will fail, but would be delighted to be proved wrong, as I will have a similar project for our new cat this winter.

One thought: Could you make the bimini high enough to be out of reach, and make shorter hand holds for people to grab on the front of the aft cabin?
 
Kelpie said:
Putting together a bimini frame on my Moody 39. Being a centre cockpit, it's wider than it is long. Very wide- about 3.5m in fact.
I'm using 25mm stainless, as I have a stash of it already and the fittings are readily available.
For simplicity the frame will be permanent, no folding or hinging required. Canopy will be made from rigid solar panels and rigid sheeting (not yet decided, possibly foam cored grp, or ply as a temporary solution). The panels will be port and starboard to get them away from the shade of the boom.

Rough dimensions are 1.7m fore-aft, and 3.5m across. Wondering if that width is likely to sag? There will be a slight upwards bow to it, and in each corner a triangle made with a short (~1ft) length of tubing as a brace. I'm sure it will be fine holding its own weight but the aft cross piece is going to become a great handhold and see a lot of abuse. I'm considering adding two vertical posts to help support it, but I don't want to clutter up that space unless I absolutely have to. Just looking for a show of hands about whether it's likely to be necessary.
Click to expand...
Just been through a similar process which gave rise to the following:

If you are using the hinge or slide-on fittings the little grub screws aren't up to much. Welding would add a lot of stiffness and strength; drilling bolts all the way through would be next best.

Could you(if I understand correctly?) move the triangulation points further inboard at the vulnerable point, and down at the vertical struts? In my imaginary picture that would be a slight visual intrusion but still leave the gap free for passage.

Could you consider sleeving - internally or externally - the entire piece that you worry about being grabbed? Or invest in one piece that's thicker wall - which adds little weight but lots of strength.

Finally, you could electrify the bit you don't want people grabbing on to. There are lots of bits on boats that aren't structural and shouldn't be relied on - sprayhoods, Biminis guardrails stanchions in general. Some boats have them designed to take a red-blooded well-nourished man, most don't. Crew should know the difference between using their core muscles and keeping weight through the legs (while using boat bits for balance only) versus heaving on bits that shouldn't be heaved on.
 
Thanks all for your thoughts.
I'm not concerned about canvas sagging- there won't be any. Just to reiterate, the canopy itself will be rigid solar panels and sheeting.
I'll see if I can upload a drawing to make it easier to explain. Moving the bracing struts is certainly one answer, but there are winches aft of the cockpit and they would become obstructed if the struts were much lower.
I can't raise the bimini out of reach, because the boom is above it (traveller is aft of the cockpit). And actually I think it will make a great handhold.

I've trawled through google images (lots of TecSew pictures on there) and you do seem to get full width biminis without intermediate supports even on wide twin-wheel AWBs. Annoyingly most pictures featuring centre cockpit boats tend to be advertising enclosures, which makes it a bit of a guessing game working out what's going on with the frame.
I think on larger boats they must be using bigger tubing. I'm looking in to that option, it will add cost and, crucially, time. I really want to have this project finished by the end of the month.
I've also got various lengths of aluminium tubing, including sections of dinghy mast, booms, etc. Mostly just a little too short, inevitably. But maybe I can figure out a way of doubling up on just the middle of the span.
 
For the 3.5m long transverse sections of the bimini, would it be feasible to build a sort of 'I' beam with your 25 mm diameter tubing by using two tubes, perhaps 50 mm apart to form the 'flanges', and welding in short lengths of the same tube as spacers at a regular spacing to form the 'web' ?
 
Bajansailor said:
For the 3.5m long transverse sections of the bimini, would it be feasible to build a sort of 'I' beam with your 25 mm diameter tubing by using two tubes, perhaps 50 mm apart to form the 'flanges', and welding in short lengths of the same tube as spacers at a regular spacing to form the 'web' ?
Click to expand...
Yes I've been considering that. It's a lot of extra fabrication though. Going to visit the boat again today and mess about with bits of pole to see just how much I can drop the corner supports without affecting access to the winches.
 
Having taken a second look, I've decided that a single central support will be acceptable, and in fact provide a good handhold. It will have to be angled aft somewhat as it meets the deck, but that's ok and should provide extra stiffness.

I can obtain thicker tubing but then run in to problems with fittings to work with the two different diameters, which means more welding.
 
The thickness (gauge) of your 25mm tubing will make a big difference to the answer.

My gut feel is that with well braced corners and heavy gauge tubing you could probably hang on a 3.5m span. For lighter tubing you wouldn't want to do that, but could rely on it as a handhold (especially if there is some lateral bracing from the rigid infill panels).

Happy to do some back of the envelope calculations if you have a little more detail.
 
I think I've narrowed this down to two options.
Firstly, with the tubing I've already got I need to make a joint anyway in that big span. So how about sleeving over a length of 28mm tube, if I can source any. That would double the wall thickness and marginally increase the diameter too, so I presume a minimum of twice the stiffness?
Secondly, make the joint using an internal sleeve or a butt-weld (if I can get someone to do that), and use a tee to add a mid-span support. I don't think it's all that much of a compromise, it would be a great handhold, would help guide the sheet around the bimini, and sounds like a nice easy option.
 
The slight upwards bow will prevent sagging as long as you have it tight like a drum. A couple of webbing straps at the stern usually tightens the frame work and canopy well.
Just dont leave it out in snow or ice and you should be grand. Sagging usually occurs if its not tight enough, the arch is not curved enough or when snow and ice arrive.
Good luck with your project.
 
The issue with centre cockpit boats with the mail sheet aft is clearing the main sheets with the Bimini frame when out in a reach, the wider the Bimini the further forward it has to finish.
Tube size, it’s not all about the O/D, use thicker walled tube, we use 1.5mm, sometimes thicker, most use 1.2, you need 1.5 with your span as a minimum or go to 32 mm.
However, I would question at the width you state how far forward the Bimini needs to finish to clear Main sheets is these are aft of the cockpit?
Lastly, curvature is important, it adds strength in the same way as an arch, make sure you have a decent camber/curvature

John Bland
Tecsew Ltd
 
jclarkemarine said:
The slight upwards bow will prevent sagging as long as you have it tight like a drum. A couple of webbing straps at the stern usually tightens the frame work and canopy well.
Just dont leave it out in snow or ice and you should be grand. Sagging usually occurs if its not tight enough, the arch is not curved enough or when snow and ice arrive.
Good luck with your project.
Click to expand...
Problem is I'm using solar panels as the top, not canvas.

The contraption is now mostly assembled on the boat. The two large solar panels are in place, the middle is currently not covered. Not a big problem as we're still in Scotland, although rain protection would certainly be nice!
I've braced each corner with diagonals which definitely helps, but especially at the forward end I couldn't make these as long as I would have liked or it would have made access to the side decks impossible.

One feature which I did not originally plan on was to add bracing tubes running aft from the rear support legs, just above the guardwires, to the pushpit. This has really stiffened everything up, and will give an excellent mounting point for the solar panels which currently just hang on the wires.
Next step is to beef up the supports at the bottom of each leg- these just sit in a socket on the coaming side, but I'm going to add a second, higher, bracket on each leg.

So far I've been less concerned about sag and span, and more concerned about how wobbly the whole thing is...
 
This chap built a hard top for his cat:

Using a foam cored fibreglass panel it came in at 50kg weight plus the frame. More than a month of labour. No small endeavour.

I’d definitely go for 32mm tube with your current plan.
 
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