Designing a stern gantry

[Coaster]

I have decided to add a stern gantry to a 34' sloop rigged sailing boat. Having mocked-up the approximate shape and location using plastic pipe I now need to decide on an exact specification.

The gantry is to support a Garmin GMR 18 radome (7.2kg), 2 gps antennae, a navtex antenna, some photovoltaic panels and (probably) various other lightweight items. I intend to have twin tubes, parallel with and about 200mm from each other.

The gantry width, at the top, will be about 2100mm. At the base the width will be about 2700mm, with the tubes rising vertically from base plates just inboard of the existing quarter rails. From the level of the top quarter rails the tubes will slope inboard, hence the difference in width between bottom and top of the gantry. Short horizontal pieces will link the main tubes at about 500mm centres.

I intend to fasten the plates to the deck using M10 through bolts with substantial backing pads of 18mm marine ply. In addition the forward bolt through each plate will be attached by "L" section s/s brackets to an adjacent bulkhead.

That's the plan.

I would welcome comments on the most appropriate tube size for 316 grade stainless steel. (The quarter rails are 25mm diameter). I'm concerned about the top of the gantry sagging, especially if I mount the radome centrally. Should I choose a more complicated design, to stiffen the top?

Help, please.

 

[mack3737]

Your design sounds good, I am planning something similar myself. If you use a 40 or 50 mm tube it can be thin wall and is very stiff. Finding somebody with the tools to bend your tube without kinking/ flattening the bends may be difficult! Aalco sell forged bends but only in 45 and 90 degree. I would tack weld it together and trial fit before fully welding it out.

 

[Roberto]

similar

I have a similar design, apart from a couple of antennas it holds the dinghy (2.6m inflatable)with the 4hp outboard without any problems (even when the rib is half full of rain water).
The stainless tubes have a 28mm diametre.
I will also fit two solar panels which weigh about 20kg.

I'll see if a can find a picture

 

[oldsaltoz]

The gantry is to support a Garmin GMR 18 radome (7.2kg), 2 gps antennae, a navtex antenna, some photovoltaic panels and (probably) various other lightweight items. I intent to have twin tubes, parallel with and about 200mm from each other.

The gantry width, at the top, will be about 2100mm. At the base the width will be about 2700mm, with the tubes rising vertically from base plates just inboard of the existing quarter rails. From the level of the top quarter rails the tubes will slope inboard, hence the difference in width between bottom and top of the gantry. Short horizontal pieces will link the main tubes at about 500mm centres.

I intend to fasten the plates to the deck using M10 through bolts with substantial backing pads of 18mm marine ply. In addition the forward bolt through each plate will be attached by "L" section s/s brackets to an adjacent bulkhead.

Having repaired or modified a few of these over the years, the common fault made by clients would be the rear tube mounting, they tend to cause cracking due to load movement, the front mount can suffer from high lift loads, so a large backing plate is critical.

See if you can arrange the rear bar sit on an 'L' bracket that covers the topside and transom.

Also consider a section of square tube across the top to imptove load bearing as well as gussets or braces at both ends. Also think about adding some extra tangs to securing other items, life ring, danbuoy, flags and tent covers.

Andavagoodweekend......

 

[vyv_cox]

Here's one that I liked, taken a few years ago. All seems to be 25 mm tubing but only mounted onto the pushpit, not to the deck as in your proposal. No radar but plenty of other weighty items.

If intending to visit warmer places it's a good idea to incorporate bimini or awning facilities from the start.

 

[geoffatstanpit]

Sorry but I think that's a dreadful design. Unless the top of the awning is a permanent, load-bearing feature (?!) the stress concentration of the back leg on the top pushpit rail would be horrendous.

 

[Billjratt]

Just imagine what a gale would do to that!
In fact, a poorly driven boat with davits fitted could carry the lot away if caught by a sidewind at the wrong time..
Definately deck/transom mount, proper 'space-frame' design for stiffness, and build for lots more strength than the weight of your existing components.
Next time there's a decent wind, go and stand in the back garden holding a board the size of your solarcell panel....

 

[Orla]

Here's a few pics of ours, it was built out of 38mm, the bends were welded in, very hard to bend the SS to that angle.
We had fairly robust deck plates made up, also decided to go for flat plate to brace the two poles together. There are three plates on the top and two on each side which are going to be used for a wind generator and other bits and bobs.
We are not going to use this for dinghy davits, we are going to install a Hydrovane which would prevent this.

And maybe not a good idea to build it in Scotland in winter

Hope this helps
Doug

 

[Roberto]

picture

here is a picture of ours

as to self steering gear, we have fitted a windpilot: it can be removed by just unscrewing two bolts, so during coastal cruising the windpilot goes inside a locker and the stern arch is used as dinghy davits, during longer hops the dinghy is deflated and put in a locker anyway so we can install the windpilot (it was not fitted yet when the picture was taken)

 

[Pinetops]

Designing a Stern Gantry

It takes an engineer to design something which is just strong enough and it should also be aesthetically pleasing! Many of the gantries one sees have redundant members and detract from the look of the yacht. I prefer a gantry to incline aft (but not beyond the stern) and to be as low as practical. It should not restrict crew movement, especially when berthing Med style and handling the mainsail.
25mm tubing should be OK subject to the following. The fore and aft stability will be improved by widenning above 200mm, possibly wider at the base than at the top. Side to side stability is more of a problem. Our bimini uses 25mm tubing, is 2.5m wide and free standing to allow movement outside it along the side decks. Its limited side to side flexibility in a seaway would not be acceptable in a gantry which would need to have some form of bracing, perhaps by connection to the pushpit. Possible sagging of the top tubes would be reduced by curving them slightly, like a sprayhood.

 

[Orla]

"as to self steering gear, we have fitted a windpilot: it can be removed by just unscrewing two bolts, so during coastal cruising the windpilot goes inside a locker and the stern arch is used as dinghy davits, during longer hops the dinghy is deflated and put in a locker anyway so we can install the windpilot"

That sounds like a good idea, like you say i like the idea of keeping the dinghy there during coastal sailing, will look into it...
many thanks

Doug

 

[Roberto]

"as to self steering gear, we have fitted a windpilot: it can be removed by just unscrewing two bolts, so during coastal cruising the windpilot goes inside a locker and the stern arch is used as dinghy davits, during longer hops the dinghy is deflated and put in a locker anyway so we can install the windpilot"

That sounds like a good idea, like you say i like the idea of keeping the dinghy there during coastal sailing, will look into it...
many thanks

Doug

I have found this picture, if you undo the two bolts and take off the pilot, the only thing which remains on the transom are the two circular shapes (the one on the transom and the one having the red piece of string passing in front of it)

It will depend on the shape or the transom, when I take off the pilot in my boat I can use the bathing ladder and platform, I can lift the dinghy and the outboard, and the passarelle/gangway can be conveniently adapted to the remaining windpilot support

it does not seem to accept the link to the picture, you can find it here, you may click to have it enlarged

http://sybrancaleone.blogspot.com/2009/08/passerella-allopera.html

 

[Coaster]

Roberto. Thanks for the details and the photo. Can I ask what the span is? And is there any significant load on the centre of the span?

Oldsaltoz. On each side the front and rear poles will be on one plate. I did wonder about using alternate diagonal tubing between the front and rear poles (i.e. a zigzag arrangement) to increase fore-and-aft stiffness.

LordMacdonald. How does the 38mm gantry tubing compare with your quarter railing diameter? Is the latter 25mm?

Pinetops. Thanks for your comments. I agree very much with the points about appearance. I would have a problem increasing width between the front and rear pole bases due to existing deck fittings etc. but I’ll look again at that. I already intend to connect the rear poles to the original quarter rails, to improve lateral stability.

I am particularly grateful for your suggestion about curving the top section. I need to get the centre as high as possible, to ensure the radome is above boom level. (We have a centre cockpit boat with a high boom). It occurs to me that we can form the top section in three parts i.e. moderately sloping outer thirds and a horizontal centre third. That would give the effect of an arch and may also be aesthetically pleasing.

 

[Roberto]

Roberto. Thanks for the details and the photo. Can I ask what the span is? And is there any significant load on the centre of the span?

The width at the base of the stern arch is 250cm (transom width), at the top it reduces to 150cm/170cm (depending whether it is measured outside the vertical tubes or inside the load bearing top side).

The load in the middle of the arch is the stern navigation light :smile:

 

[Coaster]

Thank you Roberto.

 

[Orla]

LordMacdonald. How does the 38mm gantry tubing compare with your quarter railing diameter? Is the latter 25mm?

Yes quite right the quarter rail is 25mm, I wanted a very robust and strong arch so 38mm seemed the right dia for the size of our arch.
It is also an independant unit with no links to the existing steelwork

 

[HinewaisMan]

Here’s a few pics of ours – sorry about the clutter. Click to get bigger images.

We too went with 38mm tube as the primary arch with single tubing - the engineering didn’t warrant double 38mm tubes, with a bracing strut forward to take the tension of the overhang over the stern. We needed to lean back to allow clearance for the mizzen boom and height for the Aires Wind Vane.

We also decided to loose the old push-pit and integrate it into the new frame - the continuation of the top push-pit horizontal on the post quarter also gives us somewhere to store the outboard. This steel is 25mm.

The frame supports 2 wind generators on posts, two solar panels, the GPS receiver and braces the spare HF whip. Plus, spare anchor etc etc.

We’re steel so the 4 feet are bolted through our 4mm deck, with teak pads topsides and an additional 3mm plate below, bolted with, I think, 5/8th bolts.

The solar panels are fitted with single bolts each end to allow us to tip them to follow the sun.

One other thought have loads of tangs (loops of stainless rod you can clip blocks, pascereles etc to ) welded on while your doing the job – they come in so useful.

P
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[Bobobolinsky]

In South Africa, where sailing boats are often proof that stainless floats, they use 304 "Hygenic tube". It comes ready polished. It is slightly differing size to 316 and thinner. 0.75" tube is 25mm OD. It bends easily enough up to 0.75" with a plumbers bending machine. Joints are easy to complete with a TIG welder. The advantage is that it's 50% the cost of 316 Schd tube.