Calling All Engineers

[talbot]

I just put my boat in. Wondering what forces I'm actually dealing with when I raise the mast. I have no idea how to calculate that. My gin pole system may be overbuilt, or I may be a few ounces away from the thing falling on my head. Can anyone help?

Here is the diagram of my system, shown with the mast fully down and resting in its crutch. Blue indicates metal supports, red indicates lifting lines. The weight is calculated as 24' of Dwyer extrusion, plus 8mm Endura halyards, 5mm Dyneema rigging, airfoil spreaders, triplex AWG 16 boat cable, and miscellaneous shackles.

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[Riggerdood]

I'm no engineer talbot, but I do know that those forces would be much lower if point A were higher on the mast, i.e. using/connecting to the headstay or jib halyard. I'm sure you have a good reason for A to be where it is tho ...

[Tom G P 21]

Agree, us the jib halyard. Also is your stern mast support adjustable to raise the mast above the horizontal when you start?

[talbot]

. . .forces would be much lower if point A were higher on the mast, i.e. using/connecting to the headstay or jib halyard. I'm sure you have a good reason for A to be where it is tho ...

Maybe. But as the angle of pull moves higher on the mast, the force is mainly directed to driving the base of the mast forward. I used to pull from the spreaders, and it was no easier. I'm trying to get the most upward pressure on the mast as early in the lift as possible. That is where it's needed. After the mast reaches 60 degrees or so, it takes little force to tip it the rest of the way. FYI, MacGregors lift their masts much faster than I can with a tiny derrick that attaches to a special fitting on their mast steps. So I'm still waiting for an actual calculation of the force. Where are those engineers?
And, yes, the Mast-Up can extend to give you a couple of extra feet at the start of the lift.

[Roland of Macatawa]

talbot, a couple of questions:

Which boat model is this? if it is a P18, i have an altogether alternate suggestion which a friend of mine uses on his P18.

Do i assume correctly that:
1. Your gin-pole is a pivoting gin-pole and not a vertically fixed gin-pole with a pulley at the top?
2. And that line AB is discontinuous with line BC, with each attached to the gin-pole separately a point B?
3. And that there is enough forward clearance for the gin-pole to rotate 90 degrees forward, bringing point B to just above the pulley at point C.
4. Aren't you concerned about the tension in line BC rather than that in line AB?
Afterall, the tension in BC transmitted through the pulley is the force you, by pulling on the line, need to provide to lift the mast.

Do you have adequate lateral stability?

Regards, Roland

ps. I sail a Com-Pac SunDayCat with a built-in, and never removed, pivoting gin-pole.

[Riggerdood]

Maybe. But as the angle of pull moves higher on the mast, the force is mainly directed to driving the base of the mast forward. I used to pull from the spreaders, and it was no easier. I'm trying to get the most upward pressure on the mast as early in the lift as possible. That is where it's needed. After the mast reaches 60 degrees or so, it takes little force to tip it the rest of the way. FYI, MacGregors lift their masts much faster than I can with a tiny derrick that attaches to a special fitting on their mast steps. So I'm still waiting for an actual calculation of the force. Where are those engineers?
And, yes, the Mast-Up can extend to give you a couple of extra feet at the start of the lift.

See, you're already the better engineer than I!

[talbot]

talbot, a couple of questions:

Which boat model is this?
--P21
Do i assume correctly that:
1. Your gin-pole is a pivoting gin-pole
--Yep
2. And that line AB is discontinuous with line BC
--Yep
3. And that there is enough forward clearance for the gin-pole to rotate 90 degrees forward
--Yep
4. Aren't you concerned about the tension in line BC rather than that in line AB?
--No. I'm mainly concerned with getting the lightest possible gin pole that won't collapse on me. Current one is a perforated steel A-frame. So it's actually the max downward force on the A-frame that I would like to know.
Do you have adequate lateral stability?
--Yes. Not a problem.

[Roland of Macatawa]

talbot:

I made some analysis and calculations about your gin-pole arrangement.
To do so, I made some presumptions.
First, that the center of mass of the mass is centered along its length, 12 ft from the base.
Second, that the pivot point of the mast base and the gin-pole are on coincident axes 6.7 ft below the top of the gin-pole.
(This number preserves the consistency of lengths of the sides of the right triangle 6 x 6.7 x 9, by the Pythagorean Theorem.)

Approximate results then, for the initial starting position as you had shown, are:
Tension in Line AB: 86 lbf
Tension in Line BC: 81 lbf
Compression in Gin-pole: 121 lbf
Apply a safety factor to each of these.

Changing the attachment point on the mast, point A, to a higher position is helpful.
If you move point A to the top end of the mast, the results become:
Tension in Line AB: 59 lbf
Tension in Line BC: 81 lbf
Compression in Gin-pole: 73 lbf

Changing to an intermediate position yields intermediate results.

Note: Changing the attachment point does not change the tension in Line BC, which is the force necessary to lift the mast.
But moving the attachment point upward reduces the tension in Line AB and reduces the compression on the gin-pole.

Moving the attachment point upwards, gives greater leverage to the attachment point and the vertical force needed to lift there is reduced.
This, in turn, reduces the vertical component of the tension in Line AB:
Which in turn, reduces its vertical reaction force downward in compression onto the gin-pole.

You are correct in saying that lifting the mast upwards becomes easier as the mast goes up.
This is due to two helpful effects.
First the horizontal distance from the pivot to the center of mass is being foreshortened.
And, as it rotates, Line BC becomes more perpendicular to the gin-pole making the tension in Line BC more 'efficient'.

Hope that's helpful.

Regards, Roland

[talbot]

That's exactly what I wanted to know. Your assumptions are more or less correct (close enough for Precision work). It's particularly good to know that the higher lifting point does indeed reduce the vertical load on the A-frame. Also, that the loads are in a range where I might be able to benchtest the strength of alternative materials without risking breaking my mast or noggin.

[Riggerdood]

I think you found your engineer talbot! Don't discount your own analytical abilities tho ... ;~P