Yacht legs - tubes

[oldbilbo]

I'm mulling over options for construction of a pair of yacht legs, and considering the respective merits of some tubing I have tucked away.....

Each of the tubes in this pic are 2m long. The larger tube, one of a pair, is alloy, 82mm OD and 3mm wall thickness. The smaller is powder-coated steel, 50mm OD and wall thickness 1.6mm including paint.

I have the 'qualitative' expectation that the alloy tube(s) will be well capable of enduring the likely loads as yacht legs on a 3T full-keel boat. They have, however, another duty, and I'd prefer to use the narrower-diameter steel poles..... if they're likely to prove up to the job.

What do the structural engineers among us think?

 

[vyv_cox]

I ran a tape measure over my two-piece legs. The upper section is 80 mm diameter, the lower and the extending piece are 70 mm with an internal socket that is 65 mm. All tubes seem to have a wall thickness of 5-6 mm. Based on that I suspect that your tubes are nowhere near strong enough. It's not the compressive loads that are the problem, as buckling needs a mighty force, it's the bending loads as the boat settles. So diameter is a critical dimension as this is the factor that maximises bending strength. The steel one in particular seems well undersized. General types of aluminium have roughly half the tensile strength of equivalent steel, so it's probably safe to assume a steel tube needs half the wall thickness of aluminium for the same diameter.

Can you get hold of a steel scaffolding pole? 48 mm diameter, 4 mm wall thickness. Aluminium dimensions the same but perhaps not man enough in that material.

 

[Skylark]

I bought a 6 leg cradle several years ago for a 36 ft LOA, 1.6m draft AWB. I now have a 41ft AWB with 2.1m draft and had to buy new, longer poles (and an additional centre section). The original poles are 117 cm long, 60mm dia with 5mm wall thickness.

 

[doug748]

I am not an engineer but I have made my own alloy legs and used them.

As vyv has said, the steel section looks far too feeble, even before you consider the possible effects of corrosion.

I think you stand a chance with the stouter pole, particularly if you support much of the upper section with your wood (?) stand off/block to the hull. Drive a c 3ft+ wood dowel into the section to straddle the each side of the point at which the leg becomes unsupported and I think you may be covered.

 

[oldbilbo]

I suspect that your tubes are nowhere near strong enough. Can you get hold of a steel scaffolding pole? 48 mm diameter, 4 mm wall thickness.

Right. You're 'the man'. That's settled....

I certainly can find some scaffolding poles. I can also find some spare alloy mast section 65mm x 100mm x 2.45mm which is growing weeds at present.....

Back to the drawing board. :disgust:

 

[Kelpie]

This looks useful...
http://www.metals4u.co.uk/aluminium-tube/3od-x-14-wall-aluminium-tube/22/1989/detail.asp

 

[viago]

I'm mulling over options for construction of a pair of yacht legs, and considering the respective merits of some tubing I have tucked away.....

Each of the tubes in this pic are 2m long. The larger tube, one of a pair, is alloy, 82mm OD and 3mm wall thickness. The smaller is powder-coated steel, 50mm OD and wall thickness 1.6mm including paint.

I have the 'qualitative' expectation that the alloy tube(s) will be well capable of enduring the likely loads as yacht legs on a 3T full-keel boat. They have, however, another duty, and I'd prefer to use the narrower-diameter steel poles..... if they're likely to prove up to the job.

What do the structural engineers among us think?

i made a non telescopic pair using these: http://www.aluminiumwarehouse.co.uk...32</sub></small>_in_x_7_swg/product_info.html


good strength and used the off cuts to construct the arm.

i used these people to supply all the necessary fixings.
http://www.scaffoldingsupplies.co.uk/products/tube_clamps.html

 

[oldbilbo]

Interesting sources. Thanks, guys.

 

[vyv_cox]

i made a non telescopic pair using these: http://www.aluminiumwarehouse.co.uk...32</sub></small>_in_x_7_swg/product_info.html


good strength and used the off cuts to construct the arm.

i used these people to supply all the necessary fixings.
http://www.scaffoldingsupplies.co.uk/products/tube_clamps.html

That looks like the tubes and fittings in my tripod, used for lifting engines from cars. They have always seemed plenty strong enough for my job and I guess would be good for yacht legs if not too long.

 

[Blueboatman]

Couple of elegant 6x4 stout timbers, inner face with added material scalped to the yachts topside section?

 

[LadyInBed]

What size boat are they going to support?

 

[oldbilbo]

What size boat are they going to support?

A 27' x 7'8" Cutlass 27 with a designer-specified displacement of 2.9T. Keel to toerail is about 2m. Of course, it may have put on some weight in middle-age....

 

[fisherman]

An engineer friend has pointed out that square section is better than round, as you have flat faces resisting any bending moment. Square section steel may be a good option, and easy to fit wood cheeks. Come to that, square ally is available.

http://www.non-ferrous-metal-southwest.com/plymouth-metal-centre-products.html

 

[Kelpie]

An aluminium version of the Acrow Prop is available, called the Super Prop. Chunky but rated at 12t per leg.

 

[fisherman]

My legs are 5"x5" douglas fir, but you have me thinking about ally box section now. In re the OP, I would say that although the materials presented may be OK, just, in essence, it's the fixings that will create weak spots, and abnormal stresses on uneven ground that will find them.

 

[oldbilbo]

An engineer friend has pointed out that square section is better than round, as you have flat faces resisting any bending moment. Square section steel may be a good option, and easy to fit wood cheeks. Come to that, square ally is available.

http://www.non-ferrous-metal-southwest.com/plymouth-metal-centre-products.html

Righty-o! I've spoken now to 'Paul' at Plymouth Metal Centre aka Avon Steel, thanks, and he's going to kick around the numbers on what might be an appropriate section vis-vis bending moments, etc.

Oh, BTW, I'm also visualising what something made up of epoxy-laminated multiple ply might look like. 'Twould probably take a nice coat of varnish better than some angular hollow section aly tube..... Just musing....

 

[sarabande]

I like the idea of an engineered wooden structure, partly for lightness without excessive cost, but also for DIY potential. If De Havilland can build Mosquitoes of thin plywood....

And if you design in to the legs a ladder, a passerelle, a mast A frame, plus a Man Overboard recovery crane/ramp...


What we need is some balsawood, an Xcraft knife, a bit of glue, several pints of beers, and a forum team to have a brianstorming session

 

[fisherman]

Righty-o! I've spoken now to 'Paul' at Plymouth Metal Centre aka Avon Steel, thanks, and he's going to kick around the numbers on what might be an appropriate section vis-vis bending moments, etc.

Oh, BTW, I'm also visualising what something made up of epoxy-laminated multiple ply might look like. 'Twould probably take a nice coat of varnish better than some angular hollow section aly tube..... Just musing....

Laminated ply could also be hollow.

 

[alahol2]

I needed a 'quick and dirty' set of legs to allow me to dry out prior to craning some years ago.
I made a set out of 4x4 fence posts and a few offcuts. The top end of the legs was lashed to the centre cleats (after ensuring they were adequately backed). The lower end of the legs had the usual fore/aft lines. The boat is 28ft and fairly long keeled but only drawing about 3ft, weighs just short of 4 tonnes.
There was a fair amount of sucking of teeth among the observers but nothing untoward happened. In fact they worked very well, the lashings allowed a small amount of flexibility as the boat settled.
I'm not totally convinced I'd use them on a softer surface than the gravel slipway.

 

[Avocet]

I'm following this thread with interest - partly because I have a Cutlass, and partly because I'm curious as to how the loadings are likely to be determined. Obviously, when drying on a good, firm and perfectly flat and level bottom, I can see the loads being very small indeed (and almost evenly shared between the two legs and the keel). You could do it with a good pair of broom handles!

What I can't for the life of me work out, is how much of a "worst case" would it be reasonable to plan for? As soon as the boat heels even a fraction of a degree, pretty much all the load will then be shared by the keel and the loaded leg. As the angle of heel increases, the compressive load on the loaded leg will go up - quite rapidly, I think, as the Cutlass is so narrow in the beam. I've no idea how high the centre of gravity is on a Cutlass, but they draw 4'6" and have a 50% ballast ratio. If I guess at it being bout 5' above the bottom of the keel, that means it can heel about 35 degrees before the centre of gravity passes over the point of contact between the loaded leg and the mud. Just before that point, I guess nearly all the weight of the boat will be on the leg and beyond that point, it will fall over anyway. Is it reasonable to say that each leg should therefore be capable of supporting 3 tons in compression without buckling? The other thing to remember, of course, is that it won't be pure buckling, because as the boat heels, there will be (I guess?) a fair bit of friction between the foot and the sea bed. Also, depending on how well fastened to the side of the boat they are, there will be a combination of buckling and bending to withstand. That's the killer! I guess about 5 feet of the leg won't be in contact with the side of the hull, so we've got a cantilever beam loaded in bending that is "built-in" to the side of the hull at it's top end and has a compressive load superimposed down it's length...

Lastly, this won't be a "2-D" problem anyway, as I wouldn't mind betting it will try to corkscrew one way or the other if it starts to lean.