Ok to go up mast whilst in cradle ?

[Graham_Wright]

"swinging" ?

How?

Swinging=moving from side to side.

 

[Lakesailor]

Well I said

To get the boat moving and overcome the strength and stability of the cradle you'll need the mast top man to get his weight outside of the cradle base (ie: the mast tip well over). Even then he will be trying to lift the ballasted keel to get the cradle to tip over.
It's just not going to happen.

and JDC said

The wind exerts a force on the mast, and thus a heeling moment (moment only - it doen't heel becuase it's shored up)

The human suspended from the top of the mast exerts a heeling force if the mast isn't excactly vertical.

Let's compare the two:

1. wind
Force per unit area, F/A = 1/2 rho * v^2 * Cdrag
Heeling moment integrated up the mast is 1/2 w * L^2 * F/A

Cdrag = 1.2 (super aerodynamic car = 0.3 or so, radio mast typically 1.2)
rho = 1.2kg/m^3
v, wind velocity = 20m/s (40kts - a good gale but not exceptional)
w, width of mast and rest of rigging, so 20cm?
L, length of mast = 10m?

Moment due to wind = 1/2 * 1/2 * 1.2 * 1.2 * 0.2 * 10^2 * 20^2 = 4100Nm

2. Human up mast
Moment = L * T * sin(theta),
where T is weight of human (in N) = 1000 say
theta = list, or angle from vertical = 2.5 degrees say (much less usually I think)

Moment due to weight up mast = 520Nm, a factor of 8 less than that due to wind.

So I agree with the others saying that one does not have to worry about the boat falling over if you go up the mast when ashore: if your boat is well enough supported to survive the winter winds it can take you up the mast.

But a rigger said it's dangerous.

Take your pick.

I think the rigger is a numpty who has never tried to work it out.
If he is talking about shored up boats, then it doesn't apply as the OP wasn't.

 

[Avocet]

My only concern (sorry if it's been mentioned already - haven't read the whole thread in detail) is that if the yacht is shored-up rather than in a cradle, some (perhaps too many!) of the props could drop out.

I can understand the calculation, and I wouldn't actually be worried about the weight of the person swinging about slightly at the masthead in itself. Clearly, that's not going to be anything like the heeling moment on a blustery day. If, however, the combined weight of the person, plus whatever small heeling moment they contribute, is enough the flex the hull minutely around the top of one prop, then when the person swings the other way, the prop might have been dislodged slightly (e.g. pushed minutely into the ground a bit further) and could then drop away.

 

[guernseyman]

Swinging=moving from side to side.

At the top of the mast? On a short length?

If you mean swinging in a huge arc when just above the deck, now you're talking: that could upset a badly cradled boat.

 

[prv]

If, however, the combined weight of the person, plus whatever small heeling moment they contribute, is enough the flex the hull minutely around the top of one prop, then when the person swings the other way, the prop might have been dislodged slightly (e.g. pushed minutely into the ground a bit further) and could then drop away.

Surely if the shoring is that fragile, walking around on deck will have the same effect? We've already seen that standing on the side-deck heels a boat in the water more than hanging from the masthead does.

Pete

 

[Conachair]

2. Human up mast
Moment = L * T * sin(theta),
where T is weight of human (in N) = 1000 say
theta = list, or angle from vertical = 2.5 degrees say (much less usually I think)

Moment due to weight up mast = 520Nm,

Which is the same moment as the same bloke standing 0.5m to the side of the mast. Makes no difference if he's up the mast or standing on the deck. Assuming the mast is vertical and ignoring dynamic loads.

 

[Avocet]

Except that if your boat IS going to shift, you can do a lot more about it on the deck than you can at the top of the mast! People tell tales of disaster caused by tying over-boom covers or tarpaulins to their shores, so I imagine that they could be dislodged fairly easily?

 

[KREW2]

If it is not safe for a person to go up the mast while a boat is in it's cradle, for fear of it being made top heavy, then the boat itself is not safe in the cradle.

 

[Lakesailor]

Quite. The practice of shoring up boats is antiquated and not necessary these days. Insurance companies, who like to poke their noses in everywhere, should have sorted that a while ago.
I'm serious, the YM Crash Test Boat should do this one as there is so much scaremongering and woolly thinking on the matter.

 

[alan_d]

The practice of shoring up boats is antiquated and not necessary these days.

Why, do they stand up on their own now?

Agree about the scaremongering ...

 

[Scotty_Tradewind]

Not many cradles that I've seen have 4 supports each side criss-crossed together with batten and also have a support under the bow and poss stern.
Through the winter my yards owner checks each wedge in every shore each working day.
I slept aboard during the week for a while this winter and I was woken by a loud hammering right by my ear so I know it's done.
Some cradles I've seen with 30' boats only have 4 supports in all and they are not usually crossbraced in an impressive way from what I've seen.
I'll stick with props on a firm ground with no hesitation.
I've seen the fairly new metal shores with a threaded support face and I'm really not impressed with those as they are not easily cross-braced together.
A group of them also blew over a few weeks ago and the metal face landed on the wing of a car parked too close. Change is not always for the better I feel.