Mast "A" frame construction ....

[Guest]

Mast \"A\" frame construction ....

25ft motor sailer ..... about 8m mast ... moderately heavy, deck-stepped on "Blade". Can be erected by 4 people without extra aids. J figure is near 3m

I want to construct an A frame to assist in this act.

So I have 2 lengths of 6m long 22mm diam. galvanized pipe. Wall thickness is 2mm.

Now I am thinking that 3 - 3.5m length pipe - 2 lengths formed into an inverted V by bolting together tops .... bottom of legs slotted and drilled to take bolts to slip over U bolt chain-plate fittings on deck.
Halyard from mast top to top of inverted V and then main-sheets and blocks from there to stem. Hauling on main-sheets to raise mast already connected and pivoting on deck-blade.

Now the question is - Is the pipe strong enough to do this job ? If not what size pipe is suitable ?

There will be at least 2 people steadying the mast...

 

[sarabande]

Re: Mast \"A\" frame construction ....

Without the tech spec for the modulus of elasticity, yield strength, and a few other bits of info, it would be difficult to perform a finite stress analysis of the A frame and the pipework. I guess you could show to yourself how relatively weak a 2mm wall on a 22mm dia pipe is by placing the ends on two chairs and sitting in the middle. It's guaranteed to buckle.

Why not use a couple of old ally spinnaker poles ? They are designed for tension and compression, and the shape resists bending.

 

[Lakesailor]

Re: Mast \"A\" frame construction ....

Interestingly I have just built a launch trolley for my trimaran from 1" steel pipe with 2mm wall thicknes (I know, mixed units) It isn't as strong as I would have supposed. The boat is balanced slightly forward of it's pivot point and the two lengths of tube which go from the handle to the axle support the bow. It is slightly springy when I put the trolley to the floor. The boat weighs 65Kgs But that is a test of beam strength and you will be exerting mainly compression loads.
I would imagine it would do the job. A cross piece halfway up would help prevent bowing or pinching of the frame.

 

[graham]

Re: Mast \"A\" frame construction ....

I think the pipe you have is 3/4 id which could be a bit light even though all the load is in compression. You could stiffen it by cutting some timber to hammer up it or probably 1 inch id would be better.

I have been gathering the bits and bobs needed to do something similar for my boat. I am thinking of some poles to provide sideways support as well ,

This will require a collar around the mast that can slide up and down as the mast raises /lowers.

Im thinking of a large steel pipe bracket for this ,the type that are made like two semicircles that bolt together. The bolt flanges could become the fixing points for the top of the poles and it needs to be loose enough to wrap the mast in carpet first. Hope this makes sense?

 

[Guest]

Re: Mast \"A\" frame construction ....

The problem is getting hold of two old spinny poles ........ I only have one new one and any other such items - being an ex Soviet state - get repaired / scrounged / taken before yours truly even gets a look in !!

I could of course cut each 6m length in 2 and double up each side ? But then it starts to become awkward to handle .. moreso than a larger bore piping ...

Mmmmmmm maybe it's back to the 4x4 joists I have ... that take a lot of lifting !!

 

[Stemar]

Re: Mast \"A\" frame construction ....

I'm no engineer, but a back of envelope triangle of forces and rusty A level physics suggests that the poles sound a bit light to me.

Could you lay your hands on some ally scaffolding pole? You'd have no worries about strength then/

 

[Guest]

Re: Mast \"A\" frame construction ....

It's why I posted actually .... over the phone all sounded good... when you actually see them .... you start to wonder.

 

[VicS]

Re: Mast \"A\" frame construction ....

They sound a bit light to me as well. They are only going to be subject to compression loads but if they bow a bit failure will follow quickly. Lakey's suggestion to fit a cross brace would only prevent bowing laterally they could still bow the other way, fore and aft, with the same results.

FWIW worth I have a wooden A frame. The legs are less than 2X2 (not sure exactly what without going and measuring) but for a much lighter mast.

I am fortunate in having a beam across the cabin top that the feet of the frame rest against. It is sized so that the apex reaches just short of the stem head fitting. In use I attactch all the shrouds and backstays and stand the bottle screws up with lengths of shock cord tied to the guard rails (otherwise they can snag under the U bolt fittings). I attach the forestay (complete with furler) to a eye one side of the apex of the frame and the main sheet between another eye, on the other side, and the stem head and haul it up with that. A spare halyard holds it up while the forestay is transferred to the stem head.

It does need a bit of steadying too keep it all straight although with such a small rig I find I can do that at the same time as hauling in the main sheet. Another helper is useful because the shrouds and especially the back stays will catch on things you would not imagine they could. if necessary the operation can be puased and the mainsheet cleated.

It is very important to realise that in the inital stages of the lift that there is a tremendous shearing load on the bolts/screws securing the mast step to the deck. I cant remeber if you have a simple pivot like mine or a proper tabernacle but either way you must be absolutely certain of the integrity of those fastenings. It helps of course to support the mast as high as possible before starting the lift.

One day I must photograph my system in use. Other small boat owners might like to copy it.

 

[Blueboatman]

Re: Mast \"A\" frame construction ....

Nigel,having done this a few times using whatever is around ,I would suggest you need 2 poles 4metres long(actually 3m would do with someone hanging on the mast foot to counter its top heavyness as you swing it vertical..... can you get a couple of 4x4 posts?Lash/bolt em together at the top,rig a block and tackle to a strop around the mast just BELOW the spreaders,and tie the post feet to the shroud points on the deck.
We once pulled the mast out of a 35 footoer by sandwiching it between 2 boats and using both of the outlying boats' main halliards,again led to a strop below the spreaders and tied at its tail down to the mast winch to reduce the load on the spreader feet.Worked a treat

 

[Mudplugger]

Re: Mast \"A\" frame construction ....

Nigel, whilst undoubtably an A frame is a useful piece of kit, it shouldn't be essential for the size of mast that you refer too. In the past regularly lifted and dropped my 33' mast on my 28' Atlanta. done with just using Spinnaker pole guyed in the vertical position to the chain plates either side,and attached to the mast in the horizontal posn. Cap shrouds & lowers loosely attached, Jib Halliard to spinny pole end, Main Sheet tackle from Stem Head to spinny pole end same as jib halliard, and haul away! don't forget to put the Backstay on tho!! Provided spinny pole is FIRMLY attached to the mast and centred, all is straightforward, and Robert is your mothers brother. hth

 

[Lakesailor]

Re: Mast \"A\" frame construction ....

I seem to remember he doesn't have a tabernacle as when I showed my pics on a previous thread he said that.
I agree that the tube Nigel has will be very marginal.
I bought some 51mm alloy tube (with 4mm walls) for my cross beams on my trimaran. I would suggest that it would be far too hefty for the A frame but something around 35-40mm would be ideal (thinking about the weight of the frame when upright - 51 mm pipe is 1.6 kg per metre)
I bet Nigel can get his hands on all sorts of stuff from the process sites he deals with.

 

[VicS]

Re: Mast \"A\" frame construction ....

[ QUOTE ]
I cant remeber if you have a simple pivot like mine or a proper tabernacle

[/ QUOTE ] I just re-read your initial question and see that the mast is stepped on a "blade" frankly I would not like to use this system with that type of mast step.

Perhaps you should look at the system that Compass24.com sell and consider copying that.

 

[Habebty]

Re: Mast \"A\" frame construction ....

I used to have a masthead rigged Pegasus 800. Find one and copy the A frame that is normally permanently fitted to the deck. Note that there are dedicated bolt holes in two fittings either side of the coachroof to fit brace wires that attach to a D ring on the mast to stop the blade type tabernacle from twisting as it is raised lowered. Also the coachroof fittings are level and in line with the pivot point of the tabernacle - very important. A 6-1 pulley similar to a mainsheet was fitted between the foredeck and the apex of the A frame for raising/lowering the mast.
I took my pegasus up to the Norfolk Broads one year and was able to lower and lift the mast on my own for Haddiscoe Bridge.

 

[VicS]

Re: Mast \"A\" frame construction ....

[ QUOTE ]
I used to have a masthead rigged Pegasus 800

[/ QUOTE ] There is a Pegasus 800 called "Mad Moment" sitting on the bottom somewhere between the south coast and the Channel islands (Sail no 1 IIRC) Been down there for quite a few years now. I don't remeber it having any permanent tackle for lifting the mast so perhaps not worth looking for that one.

Should add that the crew, a family of 4, were picked up safely before it went down. An orange smoke was spotted by a passing ship.

 

[Guest]

Re: Mast \"A\" frame construction ....

We did the lowering with spinny pole and even with guys - it gave us a real fright as we got lower ... and then one guy gave way .. the pole slipped round to side ... I thought we'd lost the mast !

My chain plates are a lot lower than the mast pivot and that's the major problem......... they are actually eyes that come up through the toe-rail ... so difficult to extend up.

I've trawled the net and seen all sorts of standing A frames - using apparently what appears similar pipe !!

I'm wary of this and will be using a few tough guys along with me to hand the thing up again this weekend ... but would like to have some gear made up to do this job easier ...

I have a pal who has offered me drawings - but haven't rec'd them yet ... so now have no time to get it made ready for the regatta ! Reason I grabbed the pipe mentioned above.......

It's crazy - my son and I used to do my Snapdragon 23 without any additions ..... and he was only early teens ... but we did have a tabernacle stepped job. This boat has the deck blade and slot mast base .... not ideal for this.

 

[boguing]

Re: Mast \"A\" frame construction ....

Firstly, I'd say make the struts closer to 5m (their length as the hypotenuse of the right angled triangle with a more than 4m vertical side, so that the mast is hanging from them) (strop under the spreaders), and you won't need to worry about restraining the heel.

Next. If you can ensure that there is only compression applied - no lateral loads (or significant dents in the tube) the 2mm wall tube it will be fine. The mast's probably only 3mm wall alloy.

 

[graham]

Re: Mast \"A\" frame construction ....

I watched someone lower then raise their mast with a pair of sheerlegs from compass marine. It was very easy,no strain on anything. I think it would be a bit more awkward with a roller headsail but still less strain.

 

[Lakesailor]

Re: Mast \"A\" frame construction ....

That was going to be my other suggestion. I made shearlegs from wooden 2x1 roofing battens (I happened to have some handy) and they were just strong enough to lift my 22ft mast. As long as I was smooth and steady with the lift.
But alloy pipe would be a lod better. However remember that you are lifting some alloy tube up to avertical position above the deck to start with. Not a lot different from lifting a mast really.

 

[Spyro]

Re: Mast \"A\" frame construction ....

I used 2x3m lenghts of steel fixed at the toe rail and one slightly shorter fixed on to a deck fitting to form a tri-pod. Much more stable. I used this method to lower my 30ft mast.

 

[Lakesailor]

Re: Mast \"A\" frame construction ....

Would that be do-able on the water? I found even wooden struts unwieldy to raise whilst afloat.