gin-pole geometry

[Mike2309]

The problem will be familiar to anyone with a coachroof-stepped mast, the foot being higher than the bow roller and shroud u-bolts. How do you prevent sideways movement when raising the mast? The aft lower can be attached before raising but is slack - while the other lower won't reach it's u bolt - until the mast is vertical.That sideways movement is not just a matter of the wind "or something" wafting the thing one way or the other if you're unlucky,gravity actually pulls it out and down as soon as it's winched up off the crutch, and I would have known this before breaking the mast foot if I could have pictured more clearly the intersection of the two hemispheres centred on the mast foot and bow roller , described by a point representing the masthead, and radii of length equal to the mast height and masthead-to bow-roller distance respectively (and how that locus changes as the latter radius decreases) - if you see what I mean .
Anyway, a few experiments with rulers, string and sellotape suggest that a gin pole pivoted at the foot reduces the sideways pull (as well as making the hoist easier) So, has anyone done the maths or fiddled with the sticks and strings long enough, (or broken enough masts) to know if there is actually a ratio of ginpole to mast length which reduces it to zero? Or perhaps is the effect I'm seeing down to the width of the rulers I'm using for mast and pole giving the sideways stability? And is it sellotaping them hinge fashion that does it? Would using a turning block on the pulpit (higher than the mast foot) help? Should I think about creating attachment points (in mid-air, using a bridle arrangement) level with the mastfoot for guys that would not need tending as the mast goes up?
I'm finding that I simply cannot see the 3D geometry any longer.

Of course extra hands manning guys would help but I'm thinking about raising (and lowering ) the mast single-handed on the water .

 

[Corribee Boy]

You're thinking too hard!

The gin pole needs to be long enough to reach the bow roller when the mast's up, assuming that's where you're attaching your mainsheet to pull the end of the pole down and lift the mast.

My gin pole is attached rigidly to the mast using a strap, not to prevent sideways movement but to prevent mast rotation.

As you say toward the end of the post, making up a harness to support rings on the same axis as the mast pivot, and then rigging temporary shrouds ( I double them over the opposite crosstrees then back down to the ring) will prevent the mast from sagging sideways as you lift it from the crutch. They need no adjustment as the mast rises.

Have fun

 

[prv]

Afraid I don't fully understand your post.

I've not been able to rig satisfactory guys to stop the mast swinging sideways, but since I can generally arrange to raise or lower it either in the yard or at low tide on the mud, and don't try it in any wind, the boat is fixed in a level attitude and there's nothing to make it swing off-line. I don't understand your idea that something in the arrangement is forcing the mast off-centre.

For what it's worth, I use the bowsprit as a gin pole. It's pivoted in the middle of the foredeck, and I raise it to roughly a 60º - 70º*angle from horizontal. The jib halyard that I haul on goes to the end of the bowsprit; I stand just aft of the bowsprit facing the mast, so the end of the halyard is passing above my head. I don't haul the bowsprit down as part of the lift, I secure the mast and then lower it. Someone to start the lift by pushing upwards from the cockpit is helpful but probably not necessary.

Pete

 

[VicS]

The problem will be familiar to anyone with a coachroof-stepped mast, the foot being higher than the bow roller and shroud u-bolts. How do you prevent sideways movement when raising the mast? The aft lower can be attached before raising but is slack - while the other lower won't reach it's u bolt - until the mast is vertical.That sideways movement is not just a matter of the wind "or something" wafting the thing one way or the other if you're unlucky,gravity actually pulls it out and down as soon as it's winched up off the crutch, and I would have known this before breaking the mast foot if I could have pictured more clearly the intersection of the two hemispheres centred on the mast foot and bow roller , described by a point representing the masthead, and radii of length equal to the mast height and masthead-to bow-roller distance respectively (and how that locus changes as the latter radius decreases) - if you see what I mean .
Anyway, a few experiments with rulers, string and sellotape suggest that a gin pole pivoted at the foot reduces the sideways pull (as well as making the hoist easier) So, has anyone done the maths or fiddled with the sticks and strings long enough, (or broken enough masts) to know if there is actually a ratio of ginpole to mast length which reduces it to zero? Or perhaps is the effect I'm seeing down to the width of the rulers I'm using for mast and pole giving the sideways stability? And is it sellotaping them hinge fashion that does it? Would using a turning block on the pulpit (higher than the mast foot) help? Should I think about creating attachment points (in mid-air, using a bridle arrangement) level with the mastfoot for guys that would not need tending as the mast goes up?
I'm finding that I simply cannot see the 3D geometry any longer.

Of course extra hands manning guys would help but I'm thinking about raising (and lowering ) the mast single-handed on the water .

Personally I am not at all happy about the gin pole method although I realse people do use it.

I have an A frame. Its feet rest aginst the ends of the beam across the cabin top on which the mast is stepped. Its apex just reaches the stem head fitting. It is perhaps marginally more stable than the gin pole. Theapex of the A frame cannot go "off-centre" as gin pole perhaps could unless very rigidly fastened to the mast

I shackle the main sheet tackle bewteen the A frame apex and the stem head and the forestay also to the apex of the A frame.

I would not attempt to raise (or lower) the mast afloat without a couple of helpers.
I do not attempt to raise the mast ashore in high winds.

I prefer to have someone around even if not actively helping when raising the mast ( Shrouds get hooked up on things you'd not think they could)

When raising the mast I stand the bottle screws up with bits of shock cord tied to the guardrails so that they cannot snag.

You are thinking along the right lines when you say "Should I think about creating attachment points (in mid-air, using a bridle arrangement) level with the mastfoot for guys that would not need tending as the mast goes up?"

 

[Mike2309]

""I don't understand your idea that something in the arrangement is forcing the mast off-centre.""

Yes I was pretty wordy and confused there . But there is something forcing the mast sideways - gravity! The arc that the masthead is free to swing in if you don't stop it going sideways is not horizontal, it is downwards and outwards. Not downwards very much, it's true - and when I've raised the mast before I've had an assistant (like yours) who pushed it up and held it central until it was out of reach . By then the lower shrouds were tight enough to hold it straight (enough) not to break stuff.

 

[Mike2309]

Personally I am not at all happy about the gin pole method although I realse people do use it.

I have an A frame. Its feet rest aginst the ends of the beam across the cabin top on which the mast is stepped. Its apex just reaches the stem head fitting. It is perhaps marginally more stable than the gin pole. Theapex of the A frame cannot go "off-centre" as gin pole perhaps could unless very rigidly fastened to the mast

I shackle the main sheet tackle bewteen the A frame apex and the stem head and the forestay also to the apex of the A frame.

I would not attempt to raise (or lower) the mast afloat without a couple of helpers.
I do not attempt to raise the mast ashore in high winds.

I prefer to have someone around even if not actively helping when raising the mast ( Shrouds get hooked up on things you'd not think they could)

When raising the mast I stand the bottle screws up with bits of shock cord tied to the guardrails so that they cannot snag.

You are thinking along the right lines when you say "Should I think about creating attachment points (in mid-air, using a bridle arrangement) level with the mastfoot for guys that would not need tending as the mast goes up?"

The A frame idea won't work for me - I don't have a beam on top of the coachroof for it to bear against. And besides, the mid-air rings would be spaced wider apart than the A frame legs could be, and the temporary shrouds would be longer( both good things I would think). Or perhaps (probably)you're thinking A frame plus temporary shrouds.

 

[Mike2309]

You're thinking too hard!

The gin pole needs to be long enough to reach the bow roller when the mast's up, assuming that's where you're attaching your mainsheet to pull the end of the pole down and lift the mast.

My gin pole is attached rigidly to the mast using a strap, not to prevent sideways movement but to prevent mast rotation.

As you say toward the end of the post, making up a harness to support rings on the same axis as the mast pivot, and then rigging temporary shrouds ( I double them over the opposite crosstrees then back down to the ring) will prevent the mast from sagging sideways as you lift it from the crutch. They need no adjustment as the mast rises.

Have fun

Yes I shall give that a try tomorrow assuming it's not (still) blowing a hooley.
And yes I have been known to overthink a problem (I once skipped a lecture and had to re-invent the Binomial Distribution in order to do the "homework" that week).
Still - I wouldn't mind knowing if the gin pole does change the geometry.

 

[theoldsalt]

Mike,

Have a look at this video. There are temporary lines each side to hold the mast amidships arranged such that the tension seems to remain constant as the mast goes up.

http://www.youtube.com/watch?v=7AQ-jmoMOFg


These lines have the affect of shrouds attached at the same height as the mast pivot point.

He calls them mast stabalising lines

 

[john_morris_uk]

If I understand the problem properly, you would solve it by arranging for the shrouds to be pivoted at EXACTLY the same pivot point as the foot of the mast. I have been involved in raising masts rather taller than your mast almost single handedly with a gin pole and with such a method. You need to contrive to have the pivot points of the shrouds in mid air - and this requires some ingenuity.

For those who say that you need an A-frame, my response would be to make sure that the gin pole is also stayed sideways as the mast is hoisted again to the same pivot points as described in the previous paragraph.

You may have to be imaginative as to how to achieve this 'pivot point for the shrouds in mid air', but if you get it right there is no swaying around and the whole mast goes up under perfect control.

 

[Mike2309]

Wrabness: 'The feeling after having tried to dry oneself with a damp towel'

PS : Do I recognise that as coming from the dictionary of place names that also features "Kettering - the effect seen on the backside after using a cane bottomed chair." ?

 

[VicS]

The A frame idea won't work for me - I don't have a beam on top of the coachroof for it to bear against. And besides, the mid-air rings would be spaced wider apart than the A frame legs could be, and the temporary shrouds would be longer( both good things I would think). Or perhaps (probably)you're thinking A frame plus temporary shrouds.

I thought it might not work for you but was not sure.


I was thinking of the mid air rings/ temporary stays possibly to supplement the a frame but more so to give decent lateral stability when using a gin pole.

My boat has lower shrouds in line with the mast ( The spreaders and caps are swept aft dinghy style) I have sometimes wondered whether to shorten them and attach them to plates on the cabin sides so that fixing points are exactly in line with the pivot bolt through the mast step. That would give perfect lateral stability throughout raising and lowering.


( To be honest I now get the yard to raise the mast, only lowering myself. Until this year there was no additional charge and even now its not much.)

 

[Mike2309]

Both mast and gin-pole stayed sounds like belt and braces. Surely if the mast is stayed during the raising and the stemhead roller isn't going to shift, the pole has to stay in the plane of the mast/centreline of boat ?

 

[john_morris_uk]

Both mast and gin-pole stayed sounds like belt and braces. Surely if the mast is stayed during the raising and the stemhead roller isn't going to shift, the pole has to stay in the plane of the mast/centreline of boat ?

It might be belt and braces, but if its all braced up I have done it single handed and you can stop and make a cup of tea midway if you want - the whole thing is totally under control.

 

[Mike2309]

It might be belt and braces, but if its all braced up I have done it single handed and you can stop and make a cup of tea midway if you want - the whole thing is totally under control.

Ah - you've really played an ace there ! (you did say a cup of tea and a cake didn't you?)

 

[VicS]

Both mast and gin-pole stayed sounds like belt and braces. Surely if the mast is stayed during the raising and the stemhead roller isn't going to shift, the pole has to stay in the plane of the mast/centreline of boat ?

I was thinking in terms of stays to give lateral stability to the mast rather than to the gin pole.

The only time the pole is at risk of "capsizing" is in the initial stages of the lift before the mast head is raised above a line through the stem head and the mast heel. Even that an be avoided by raising the mast on a sufficiently high support first.

 

[William_H]

Mast raising

I thought it might not work for you but was not sure.


I was thinking of the mid air rings/ temporary stays possibly to supplement the a frame but more so to give decent lateral stability when using a gin pole.

My boat has lower shrouds in line with the mast ( The spreaders and caps are swept aft dinghy style) I have sometimes wondered whether to shorten them and attach them to plates on the cabin sides so that fixing points are exactly in line with the pivot bolt through the mast step. That would give perfect lateral stability throughout raising and lowering.


( To be honest I now get the yard to raise the mast, only lowering myself. Until this year there was no additional charge and even now its not much.)

Vic a lot of people around here do just that. Providing you have a mast head rig with cap shrouds abeam the mast then the cap shrouds can be shortened and a piece of stainless steel often tubing is put in line the top of which is exactly in line with mast pivot. This tube is then stayed forward to the gunwhale. (permanently).
Almost all bigger sail boats around here are set up for mast lowering due to bridges between estuary and ocean. So it is not unusual to see 70ft masts being lowered while under way.
Most find it more convenient to use 2 spin poles. Mine attach to purpose fitted rings on the side decks. The position being dictated by the spin pole length.
I have a cleaver little arrangement of a piece of 3/16 SS rod which is clamped to the bottom of the forestay. It has a loop on the end onto which I attach the tackle 3 purchase. This keeps the end of the forestay with a spliced thimble and eye free to be attached to the bow fitting when mast is up. I actually have a high field lever at the bottom of the forestay to enable the forestay to be attached then quickly tightened. Currently my tackle has an SS ring shackled between it and the forestay. The spin poles snap onto the ring.
Usually being only 27ft and having a helper it is easiest for me to stand on the cabin top and guide the mast to stop it swinging sideways. Do it up or down fairly smartly.
However mine is a fractional rig so supporting the mast sideways with the stays is not so easy. For a while I used to attach a clamp to the cap shroud about level with mast base. I attached a rope to this which ran through a pulley on the gunwhale forward of the mast (one each side.) this rope ran back to cleats so that I could bend the cap shroud forward and provide tension as the mast traversed down thus stopping swing. It worked OK.
The Mc Gregor 26 with fractional rig uses baby side stays in line with mast witha chain plate on the cabin top raised to match the mast pivot and going up to about half way up the mast. These are permanent and stop the mast swaying when in traverse..
Another drastic solution is used by some big fractional rig boats. 2 poles (spin poles maybe) are attached to a car which slides up the main sail track. This is controlled by main halyard and a down haul line. The poles are pivotted at a point on the gunwhale aft of the mast about mid way along the cockpit. This pair of poles can be used to raise the mast by pulling on the down haul so forcing the car down the track and raising the mast. You need a tackle on the forestay for final tensioning though I would have a gin pole or poles as well. The two poles at the back also become a crutch for the mast when it is down. (low enough for bridges etc.) and very stable.
Finally a few of my friends have the dreaded roller reefing. This makes mast lowering more difficult. The usual trick is to use a jib or spin halyard to actually lower the mast using gin poles. The halyard must be tightened/ pulled down until the tension is off the forestay so that clevis pin can be removed from under the drum. The forestay with rolled jib attached is quite heavy and once detached must be lashed to the mast. When raising it is a bit tricky to get the pin back in being under the drum. You might try a long pin with a tapered point .
One friend used to tie a line around the sail (rolling hitch?) and used that to a tackle to tighten or take tension to release the forestay attachment then fit a tackle at the bottom of the forestay to actually lower the mast.
So it is all doable. I found a very high crutch at the stern makes it all a lot easier. especially the part where you attach or remove the pivot pin at the base of the mast. good luck olewill

 

[Fantasie 19]

I was thinking of the mid air rings/ temporary stays possibly to supplement the a frame but more so to give decent lateral stability when using a gin pole.

I think that's the answer - bridles - on a video I saw ages ago, the chap had a bridle on either side of the mast... three pieces of rope all attached to a single ring - one rope to a forward cleat, one rope to a rear cleat, tightened so that when lifted the ring was in line with the pivot point of the mast.... the third bit of rope goes up the mast (he used main halyard to pull them up) but about a couple of yard or so up the mast he puts a loop of rope around them so that it kind of acts like a set of side stays... put some grunt on the main halyard to apply a bit of pressure, make sure the ring is in line with the mast foot, and in effect the mast is secured in a fore/aft arc - it can't swing sideways...... as the mast goes up and down, the rope attached to the mast slides on the ring maintaining side ways pressure whatever angle....

edit: it's not the same video I saw, but the video in theoldsalt's post shows the same thing about 7 minutes in....

 

[John the kiwi]

Turning on the pulpit.

26 ft Trailer Yacht, 32ft mast.
I did the geometry too when devising my current system.
I have a 1:1 ratio small boat trailer winch with about 10metres of 6mm braid on it. The winch has a slot in the base that fits over the spinnaker pole becket on the mast and is retained with a stainless peg. The rope goes from winch to turning block on the top of the pulpit then back to the (cleated) jib halyard. Standing beside the mast and winding winch handle with one hand i have no problem retaining the mast from sideways movement with the other hand and mostly raise and lower mast by myself with no assistance. If i did have a big cross wind, i would grab an assistant just to help centralise the mast, but have not actually needed this yet. Once up, the ratchet on the winch holds it while i fasten and tension the forestay at my leisure.
I was initially worried about the load on the pulpit which is at a max. when mast is horizontal, but it really doesn't seem excessive. If you really wanted to reduce it, you could use a telescopic mast rear support to give the mast a bit of a head start.
I have to say my method of mast raising is simpler, more controlled and less stressful than anyone elses in the trailer yacht squadron i sail with.
Good luck.

 

[Wunja]

Providing you have a mast head rig with cap shrouds abeam the mast then the cap shrouds can be shortened and a piece of stainless steel often tubing is put in line the top of which is exactly in line with mast pivot. This tube is then stayed forward to the gunwhale. (permanently).

I have exactly this set up, together with an A frame and have never had an issue with the mast going out of line. I use a 3:1 tackle from the A-frame to the bow roller, with the working end lead back to the sheet winch, which is operated from the pontoon. I wouldn't want the mast falling on anyone in the boat.
The mast usually needs a helping hand to get it started going down, and controlling the last bit via the back stay when going up, but otherwise could do it single handed.