Help with (engineering) advice on size of pipe required to support mast stepping

[Northeaster]

Hi Folks,

I will be adding some 8"x8"x 10' hemlock beams around my existing crib dock (current dimensions are 8' wide x 8' deep x 6' high) before attaching the gangway and floating docks. I would like bolt into position, a steel, or aluminum pipe, that will be high enough, and strong enough to step and unstep the mast on my 30' sailboat. I can easily notch out the existing crib beams and the new ones, to allow something like a 4" or 5" pipe to be sandwiched in between, and therefore fixed to the crib. I would also bolt it to the beams, etc.

the deck stepped mast likely weighs 150 - 200 lbs (30 year old boat with heavy single spreader rig) and is about 34' long, and the deck could be floating about even with the top of the crib, at low tide.
However, the mast step would be about 15 - 20' out from the crib, as it rests in the water, on the flating dock.
Or, at High tide, I could likely bring it in closer to the crib, perhaps temporarily tied inside the floating dock, adjacent the gangway.

I have seen some small marinas where their mast crane is a fairly tall, heavy pipe, with a block and tackle attached to the top, and a rope runs from it to the top of a secondary pipe, which is attached 10 or more ft up the main pipe. This secondary "arm" pipe does the lifting, and is supported by the main pipe.

I would think that the system would have to be about 20' high to be able to lift the mast from above it's halfway point, and be able to swing, or reach out to position the mast for stepping / unstepping each season.

If anyone would care to provide some advice on what size or design of system would be best for this application, I would appreciate it.

If necessary, I could run supporting lines from the top of the pipe / crane, to the base or midway point of a couple of large pine trees which sit 50' back from the crib dock.

****I do not need to rig up the whole system now, but want to put an adequately sized pipe in the crib, to be used this fall to unstep!****I am thinking of something like this ship /barge derreck image:

http://myweb.tiscali.co.uk/gansg/9-loads/c-type.jpg

Thanks in advance for all responses!

[shogan50]

I wasn't able to follow the whole crib description, but if you sketch what you want and send it to me, I'd be happy to do a little math steer you in the right direction. In general, if you are using steel or aluminum pipe in the derek configuration, without doing the math, 4 to 6 inch, .13 wall is probably in the ball park. Aluminum may need to be larger dia, all else being equal. I'd start by pricing your pipe and then seeing if it is strong enough. Buckling will be your failure mode. Diameter (section) is what resists buckling. Wall thickness, is less dominant.

[Northeaster]

Shogan - Thanks for the reply.

re: the Crib, the idea is merely to secure the pipe in between the old and new beams of the crib (there are about 5 beams on each side that form a crib, that is filled with rocks), as I am beefing it up a bit, by adding another layer of 8"x8" x10' beams around the perimeter, to strengthen and protect the ones that are already there.
I figured that instead of just bolting the pipe to the outside if the crib (which is completely out of water at low tide), I would notch the outside of the old beams, and the inside of the new beams that are placed against them, creating a vertical notch in each beam, so the pipe can be secured against the side of the crib, in between the old and new beams.

Thanks for the info on diameter resisting buckling, as opposed to thickness.

[tommays]

It depends on how you support the pipe

On my lifting rig we have cables going to the top much like standing rigging which gets the job done with 2"

If you went with just pipe i would WAG it would need to be at least 4"

[MastUndSchotbruch]

Make sure your weight estimate (150 - 200 lbs) is in the ballpark.

I have a 35 year old boat, single spreader rig, and when they pulled the spar, the rigger estimated the mast weight at ~700 lbs! Of course that is with spreaders, running and standing rigging etc all included but you would need the same.

My mast is keel-stepped so likely a bit heavier than your deck-stepped mast but 200lbs seems very light to me.

[JimsCAL]

Our club has a rig similar to what you are describing. When not in use it sits in chocks along the side of our main dock. It's aluminum, about 5 inches in diameter and supported by two SS wires led to the other side of the dock when up. A block and tackle is attached to the top of the pole for raising and lowering the mast. Don't forget to take into account if your mast is deck or keel stepped. A keel stepped mast will require a longer pole since it must raise the mast a longer distance.

[Northeaster]

Thanks for all of the good advice so far. Much appreciated!

- I would think the mast is less than 200lbs, as 3 people can lift and move it - heavy, but I don't think anyone is lifting more that 70 lbs..or 80 lbs.. It could go as high as 300, I guess, but definitely not near 700.

If anyone has, or could take a picture of their (club or private) setup, especially if it is not very large, I would appreciate it!
I have looked at a couple, but I know they are built heavy enough to take a mush biiger mast off than mine, and I don't want too much overkill.

I realistically need to lift a perhaps 250 weight from a distance of 20ft out from the base of the crane, at a height of 20 ft.

I can certainly tie a couple of temporary lines or cables, from the top, back to a couple of strong trees, each at perhaps a 15 - 30 degree angle from the load.

[MastUndSchotbruch]

Originally Posted by Northeaster

Thanks for all of the good advice so far. Much appreciated!

- I would think the mast is less than 200lbs, as 3 people can lift and move it - heavy, but I don't think anyone is lifting more that 70 lbs..or 80 lbs.. It could go as high as 300, I guess, but definitely not near 700.

Guess it depends how beefy your friends are. I surely can lift more than 80 pounds and my first name is not Arnold... Try it in the gym where the weights are conveniently labeled.

Point it, I know you don't want to overengineer it but the reverse is worse. Also, this is not necessarily a static load. You need some reserve for shock loads, wind forces etc. And things may bind. Etc. Wouldn't be fun if the thing buckles with the mast in the sling...

[Northeaster]

I agree about erring on the safe side, and not wanting suprises.

I think it was just my father-in-law and wife who helped me lift/ move it last time, and neither are beefy! Your point is taken, but I still don't think it was much over a couple of hundred pounds.

[MastUndSchotbruch]

Originally Posted by Northeaster

I agree about erring on the safe side, and not wanting suprises.

I think it was just my father-in-law and wife who helped me lift/ move it last time, and neither are beefy! Your point is taken, but I still don't think it was much over a couple of hundred pounds.

Just want to make sure there is no problem. Keep us posted...