Small boat mast climb?

Yes Brendan, but surely the shape of the sails means that the forces exerted at the mast head are lower than those exerted lower down.
Put it this way, I have seen someone go up my mast on a very benign day and just a small gust started to tip the boat more than I liked, and the rigger as well. On my boat, which is very close to the dimensions of the poster's, I would not like to go up the mast.

I am used to heights, and I used to teach rock climbing in Scouts, so I am well used to what mast climbing entails.
 
[ QUOTE ]
I am no lightweight!

[/ QUOTE ] An honest comment Steve . . . I of course am a featherweight compared to you, but find the Deffee mast ladder ideal for our Vega. I recently managed to get as far as the crosstrees while hundreds of miles offshore in a 2m swell - not something I could have achieved without the ladder.

- Nick
 
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VicS, why did you exclude the weight of the boat?

[/ QUOTE ] really because I did not know how to use it but on reflection your calculation is probably more valid but you are assuming the distance between the centre of buoyancy and the centre of gravity.
I would not include 1000lbs of gear partly because I don't think a boat that size would have as much as that, partly because some may be above the centre of buoyancy and partly to let what there is be the "safety margin".

Anyway taking 4400 lbs and a 1ft separation between the centres of buoyancy and gravity the righting moment of 4400 lbs feet (or even your larger figure of 5400) is still lower than the capsizing moment of 6012 lbs feet.

Regarding the suggestion(s) that since the weight at the top of the mast is acting downwards perhaps it should be noted that both capsizing AND righting moments should be multiplied by the sine of the angle of heel. Zero when the boat is absolutely dead upright but 1 when the boat is on its beam ends. So no capsizing moment (or righting moment) when dead upright but you cannot preserve that unstable position without some means of restraining the heeling.

BTW The correct units are lbs feet, not foot-lbs as they are units of work in the ft-lbs-sec system of units. 1 foot-pound being the work done in raising a mass of 1 pound vertically through 1 foot against gravity. (Ft lbs are often incorrectly quoted for torque settings when talking about tightening bolts. They too should be lbs feet.)

I agree the practical approach with a rope from the mast head is probably the best solution but it would be interesting to see the theoretical approach calculated properly. We have a resident mathematics teacher on the forums somewhere don't we? Is it Bob Frost?
 
The centre of gravity doesn't have to be below the centre of buoyancy - and often isn't. The shape of the hull makes the biggest difference. The original poster's boat is quite beamy for its length so it might be OK. Ours is 27' long but only 7'6" in the beam (less at the waterline!) and our mast is about 30' tall. As I weigh rather a lot, I wouldn't DARE! That said, I hoisted Mrs Avocet up there once to take some measurements while afloat. Whilst her weight must remain a closely guarded secret (!) I can safely say it is about half of mine. Even so, we took a halyard to the pontoon on one side of us and the topping lift to the other to prevent heel. We tightened them up as much as possible and then hoisted her on the main halyard using the spinnaker halyard as a backup and the boat was rock steady.

If the worse were to happen, the boat would start to heel. It's anyone's guess whether the righting moment would increase faster than the overturning moment as the mast canted further over but most boats seem to reach maximum righting moment between 50 and 80 degrees of heel.

If you can get hold of a "G-Z curve" for your make of boat, you can complete Vic's calculation using your weight times the distance you are horizontally from the centreline of the boat (rough approximation) acting to roll the boat and the righting moment from the G-Z curve acting to correct that.

I think the worst that will happen is that you'll (a) get wet, and (b) might do some stanchion / guardrail damage depending on how close you are to the pontoon.

The only problem I can foresee with the bosun's chair experiment is that the boat might not heel if you're close to the centreline. Whilst it's true that this will also be the case if you're at the top of the mast, you might find that if you DO start to move sideways once you're up there, (wash from a passing boat, gust of wind etc) you'll rapidly move further and further sideways!

As a complete aside, we have big tides where we are and high harbour walls. We can dry out against a wall and be nearly at the top of the mast anyway when standing on the quayside. Is that an option?
 
Have you ever tried to careen a yacht over using a halyard?
It takes a hell of a lot of doing.
If really worried attach two lines to a halyard and hoist it. Then take each to an adjacent pontoon (if you're on fingers) and make off loosely. They will limit the amount the boat can heel.They don't have to be taught. You could allow 3 or 4 feet of movement at the mast head and still not get your body weight outboard of the hull footprint.

Vic's calculations are in extremis. When the mast is lying horizontally.
 
The design stability figures for the Vega are quoted as turning moments,ie

5,000 ft-lbs at 20 deg heel
8,000 ft-lbs at 40 deg heel

I would reckon that if you were hanging just below the masthead, you would be some 35 ft above the turning point.

At 20 deg heel, you would be 12 ft off the centre line, so 5,000/12 = 417 lbs would be the equilibrium figure.

At 40m deg heel, you would be 24 ft off the centre line, so 8,000/24 = 333 lbs would be the equilibrium figure.

If you weighed less than the above figures, the righting moment would overcome your weight.

As the boat heels more, it will support less weight, but it would need some other force to get it to heel in the first place.

With some more figures, you could produce a curve, and with a known body weight find the angle at which she would settle.

Try swinging the boom out, and hanging off that to see how much she heels.

BTW - I'm surprised to read of so many peeps climbing the mast when ashore. My surveyer gave me the impression that he would only climb a mast if the boat was afloat.

See http://www.pre-engineering.com/resources/engineeringterms.html for definitions of units.

"Moment - A measure of the tendency of a force to cause rotation about an axis. The graphical representation of a moment acting on an object is called a curl. A curl is an arc shaped arrow drawn near and about the axis of rotation. Typical units are in-lbs, ft-lbs and ft-kips, N-m ( some professions use lb-ft and kip-ft, many engineers like the more common ft-lbs and ft-kips - either is correct). Moment(M) = Force(F) times the perpendicular distance to the axis(d). M = F x d. "

Ash
 
I have tried going up the mast of my 21fter once and got only about 3/4 of the 27ft because the motion of the boat became very unpleasant. Any heeling while you are up there is intolerable wheras when you are sailing you can handle a lot of heel.
So I think the original poster is right to be concerned. It may be OK and he may be happy up the top but for my money I would spend the money and more on a mast hinge and gear to permit easy lowering of the mast.
For work on the mast top it is usually easier to lower mast to about 40 degrees while alongside a jetty. Angle the boat so the mast top is over the jetty and climb up a step ladder. Very stable very comfortable. Or remove the whole mast and do all the jobs at sea level. olewill
 
Many thanks for all the replies.

Decided that I will get the Deffee ladder and try out Bajansailor’s idea about first suspending myself from the mast head, but close to the deck to get a feel for what happens as I increase the distance from the centre line.

I’ll also employ Avocets idea of bracing the mast to restrict lateral movement by attaching a line to the pontoon fingers both port and starboard, after mooring tightly in position.

I’ll let the forum know how I get on.

Cheers

Colin
 
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I’ll also employ Avocets idea of bracing the mast to restrict lateral movement by attaching a line to the pontoon fingers both port and starboard, after mooring tightly in position.

[/ QUOTE ]

I wish I'd thought of that. /forums/images/graemlins/wink.gif

I have used a domestic aluminium extending ladder, top hoisted up by a spare halyard and bottom made fast at deck level. That worked fine.
 
Fascinating question, and one I have myself sometimes pondered on. I concluded that with a 21 foot length it might be pushing it a bit.
But for me the over-riding factor is my own head (or lack of) for heights. I tried going up using a bosun's chair with the boat ashore and safely chocked on a trailer. I found at about 15 feet I was so terrified I simply could not make myself go any higher. So even if the boat didn't actually capsize, the swaying around combined with the height would put it totally out of the question when afloat.
I did successfully put up an ordinary aluminium ladder and use that, resting it on the ground and running along the line of the shrouds. I found it a bit scary, like climbing and lopping a tree, but just about OK.
So I suppose ultimately it depends how you feel about heights. If they don't worry you you could just try and stop if the boat started to tip too much. It would be better to get it to sway first as a test, rather than try and hold it upright and then suddenly lose control. Imagine being at about 45 degrees over the water and still swinging over - would you hold on tight and hope, or jump?
 
Not first hand - but I do remember a pic in Y&Y a few years ago of (I think) Chris Law pulling a J24 over by going up the mast to sort out a spi halyard that had stuck up in a bit of a blow. I think there was a bit of help from the wind!

I've been up my Parker 21 mast a few times with a ladder (only way if there's no-one to help) or the usual halyard method. Biggest problem is crew walking across the deck to get a better view. Feels like you're sweeping great arcs across the sky.
 
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