Mast raising quandry

[justsomeguy]

Maybe I'm missing something or we are talking about something else, but if the line went from the gin pole to the block up on the mast and was tied off right there I don't see the difference in tying it off lower once it has passed through the block. The mast, the block and the cleat are all moving together and the jib halyard from the block to the gin pole is moving with them and is always the same length and angle.

If you tied it up on the block how would you release it once the mast is up?

You might have to draw me a picture and post it for me to change my mind on this ,

Sum

Our Endeavour 37

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Make it fast at the gin pole head. If you've got a very long gin pole, you could tie a slipknot of sorts. The difference in forces shouldn't be huge, as the angle only changes maybe 15 degrees? But maybe enough to give davenlynn's winch a hard time of it.

 

[Sumner]

Make it fast at the gin pole head. If you've got a very long gin pole, you could tie a slipknot of sorts. The difference in forces shouldn't be huge, as the angle only changes maybe 15 degrees? But maybe enough to give davenlynn's winch a hard time of it.

The jib halyard is fast at the top of the gin pole . I guess I'm still missing something, sorry,

Sum

Our Endeavour 37

Our Trips to Utah, Idaho, Canada, Florida

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[justsomeguy]

Imagine you and I are standing on opposite sides of a small, fast river. We each have an end of a line that is tied to a person floating downstream. If we both haul equally, the floater will be pulled up the middle of the river, showing the resultant force vector. If one of us hauls with somewhat greater force, the floater is slowly pulled closer to that side of the river.
If we're both on the same side of the river, the floater is pulled directly towards all forces acting on him.
The line that extends from your jib block down to your mast cleat is just as taut as the other side of that line that leads to your gin pole. This creates the change (a lower angle) in the resultant forces acting on the load.

 

[Pest UK]

If your man was tied to the rope I would tend to agree with you, but if this man is attached to a pulley block through which the rope is reeved, any extra effort on one mans side would be equalised via the pulleyto the second man keeping him in the middle of the river.
Equally if you tie off your jib sheet to the bottom of the mast or tie a knot to stop the sheet pulling through the block high on the mast will make no difference. Having said that don't tie it to the mast, instead tie it to something at the bows this will give you a mechanical advantage.
PLEASE dont use the electric winch until you've found what's holding the mast down. Two thousand pounds of pull possibly coupled with 3-1 block system will lead to something expensive breaking.

 

[justsomeguy]

Perhaps it's a poor example. Now, picture both of us on the same side of the river...the forces are all parallel. Is there a mechanical engineer in the house?!? I'll try to think of a clearer illustration.

 

[Sumner]

It really comes down to the fact that the way we have ours is what the factory had for the Mac S. It works, is fast and easy to rig, and doesn't really need to be modified as far as I'm concerned as it is an easy one person job raising the mast safely with it. Why fix something that isn't broke as they say.

Now for some reason it isn't working for Dave and why it isn't I don't have a clue at this point. I agree with Pest that it would be unwise just to hook an electric winch on to it at this point and try and overcome what is wrong, but I don't think that is what Dave has in mind.

I also agree with Pest that their isn't an advantage of trying to tie the halyard off up at the block, but disagree about tying it to some other place than the cleat it normally uses on the mast and tying to the bow instead.

While the mast is raising the halyard's length from the block to the gin pole needs to stay a constant length. If it was tied to a fix object down on the boat at the bow then the length of the halyard to keep the gin pole positioned correctly would have to be changing since the distance from the block on the mast to the bow is way longer when it is aft and the mast is down vs. when the mast is vertical.

I guess we can discuss the most optimal way to rig this all day, but I repeat the stock factory system works fine and is one thing on our boat that I didn't mess with ,

Sum

Our Endeavour 37

Our Trips to Utah, Idaho, Canada, Florida

Our MacGregor S Pages

Mac-Venture Links

 

[justsomeguy]

I understand, Sumner. C'mon, davenlynn! Show us what you've got (without the electric winch)!

 

[doehunter]

Perhaps it's a poor example. Now, picture both of us on the same side of the river...the forces are all parallel. Is there a mechanical engineer in the house?!? I'll try to think of a clearer illustration.

How did you make that drawing and post it?
I was thinking of doing one too but wouldn't know how to got it posted.
If I draw on paper and copy it, it makes it a PDF.
What would I do with it after that?

 

[justsomeguy]

How did you make that drawing and post it?
I was thinking of doing one too but wouldn't know how to got it posted.
If I draw on paper and copy it, it makes it a PDF.
What would I do with it after that?

I used Microsoft Paint. When I saved the drawing to my 'puter, I changed it's file extension from .bmp to .jpg Of course, I'm using Windows 98 SE, a relic of the last century, 'cause our newer 'puter (Vista) went tango uniform... I'm sitting at a 19th century desk, typing on a 20th century computer, in the 21st century.

 

[justsomeguy]

As far as posting the pic, Go Advanced.

 

[Pest UK]

Sumner I concede the point of retying the halyard forward, you are absolutely correct.

 

[tkanzler]

Is there a mechanical engineer in the house?!?

Actually, there is. :dance:

Here is a pic of a generic mast with mast raising system. The extra lines with the little box on the end represent the perpendicular distance between that piece of rigging (hoist or halyard) and the hinge. It's theoretical, as there is no actual structure there, but you can generally eyeball it with a tape measure by swinging an arc with the tip of the tape on the hinge bolt and finding the shortest distance to the halyard or to the center of the system of ropes in the case of the hoist. I left the little 'square' indicating box off a couple of them by accident, but it's a pain to edit, then created jpgs, then upload, so I left them as-is.

Variables are as follows:

Wmast - mast weight, taken at the center of gravity
MAmast - moment arm of the mast about the hinge
MAhoist - moment arm of the mast raising hoist about the hinge
MAhalyard - moment arm of the jib halyard about the hinge

The total tension force in the hoisting system is easily calculated by summing moments about the mast hinge, and setting it equal to zero (static equilibrium).

M = 0 = Wmast*MAmast - Fhoist*MAhoist where Fhoist is the tension force in the hoisting system

Using made up numbers below of 100 lb for the mast weight at 120" from the hinge, and locating the hinge (tabernacle) and hoist termination at arbitrary locations that look reasonable to my eye for a typical boat, and using a 5 ft gin pole as an example, you get

0 = 100 lb * 120 in - Fhoist * 35 in, or 100 lb*120 in / 35 in = 343 lb tension load in the hoist (total, regardless of the number of parts).

With the mast at about 20 deg. off the horizontal, the perpendicular distance from the hinge to the hoist has increased, so the moment arm is longer. The gin pole, mast, and halyard geometry hasn't (and won't) changed at all, but the horizontal distance from the CG to the hinge has decreased. The moment at the hinge from the mast has decreased, and the moment arm to the hoist has increased (longer theoretical lever arm), making for better mechanical advantage and somewhat easier lifting, though not hugely so (yet).

I get about 292 lb in the hoist at this position.

With the mast up around 60 degrees or so, the moment about the hinge from the mast weight has gotten pretty small, and the moment arm for the hoist is almost at its largest.

I get about 100 lb in the hoist. Obviously, as the mast is raised, the mast weight moment at the hinge decreases to zero (at vertical), while the hoist geometry, which has already gotten favorable, only improves further as the gin pole to hoist rope angle approaches perpendicular (can't get any better than that).

Anyone can calculate this by simply measuring the perpendicular distance from the hinge to the hoist rigging with a tape measure (with the rigging taught), and the horizontal distance to the mast CG (since gravity always acts downward). The mast weight and CG location has to be known, though the CG is probably pretty close to the center for a mast of constant section with little other stuff on it.

I'll post some diagrams of the halyard termination question shortly. I have to do an errand with the Admiral. :neutral:

 

[tkanzler]

In the last post, I didn't get to the tension in the halyard. The last two diagrams deal with that.

With the same generic mast setup, in the horizontal position (just for convenience), The tension in the halyard is

Fhalyard = 100 lb * 120 in / 58 in = 208 lb

Regardless of the termination method at the mast, it takes about 208 lb of force along that specific line of action to support the mast in that position. Think of the mast, gin pole, and halyard as a single, solid unit. Pull along the red line at any point along that red halyard line in the direction of the red line with 208 lb of force, and it will exactly balance the mast.

It gets interesting when you turn that 208 lb force almost 180 degrees around the block, then along the mast. There is 208 lb of force at any and all points along that halyard, all the way down to the cleat. That means there is an almost 2:1 purchase on the halyard block at the top of the mast (would be exactly 2:1 if the rope turned the full 180 deg.). You end up with nearly 400 lb of force on the little halyard block and fittings and termination. Not necessarily a problem, of course, but it's instructive to know that that's what happens.

Now, if you take the bitter end off the cleat, and put it on the gin pole along with the other end, you still need 208 lb of force along that line of action to balance the mast, but with two lines, each line carries only one half the load. The block also only carries 208 lb to its termination, and there isn't as much compressive load in the mast (not an issue, but worth mentioning).

In either case, the gin pole doesn't feel the difference, nor does the hoisting system. Only the halyard and the block feel it, but that's all internal to the mast/halyard geometry. Rigging either way won't effect the ability of the hoist to rotate the whole mess upward.

I'd suggest the OP tension the system, and measure the perpendicular distance between the hoist rigging and the hinge, and the horizontal distance to the mast center of gravity. With a decent guess at the mast weight, the force required of the hoist is easily calculated.

Of course, there is friction in any rope/sheave system, and with multiple parts and plain (sleeve) bearings, it can be substantial, but that's for another post.

There are, of course, analytical methods for this kind of vector analysis, but for something like this, a graphical solution is quicker, and you can just measure the moment arms and cg location right on the boat in the worst configuration (mast on the crutch, ready to hoist) to get a worst-case estimate of the forces involved.

It should also be noted that you can move the halyard attachment point (sheave, or just a mast hound or other termination), even way down near the base of the mast, and the only thing it changes is the tension in the halyard. The effort at the hoist is no different. You can end up with some very big forces if you get too close to the mast base, to the point where you need large line and a more robust gin pole due to the high compressive load, but the hoist system won't feel it on that side of the gin pole.

 

[Paul Brinkley]

Nice job, Tkanzler. That is exactly what my system is. Gin pole is 9' long, the hoist system is a 3:1, and you can lift the mast with one hand. I do put the mast on a taller crutch than the short one I use for transporting; it is about level for transporting, and up about 15 degrees when I start to raise it. The end of my furler is attached to the gin pole, as is one end of the hoist. The other end of the hoist is attached to the 2nd hole in the bow chainplate. When the mast is vertical, I undo the shackle holding the furler to the gin pole, move the furler to the chainplate, and reinstall the shackle. Without the gin pole and hoist, it is about all a couple of people can do to raise the mast and furler by hand. My homemade system is probably the best $15 or $20 dollars I have spent on the boat! -Paul

 

[Pest UK]

You mentioned "baby stays", to stop the mast going sideways, I assume you know the bottom of the said stays must be level with the bottom of the mast i.e. the same distance from the bow (or stern). Sorry if this is a silly question.

 

[doehunter]

As far as posting the pic, Go Advanced.

It's all moot now.
I had the basic idea in mind but I could have never put it out there like tkanzler did. I'm impressed. Your Da Man!

 

[justsomeguy]

In either case, the gin pole doesn't feel the difference, nor does the hoisting system. Only the halyard and the block feel it, but that's all internal to the mast/halyard geometry. Rigging either way won't effect the ability of the hoist to rotate the whole mess upward.

Wow! Ask and ye shall recieve! I guess what I don't understand is how, when the load at the jib halyard block is essentially doubled, that increase isn't also transmitted to the hoist effort. Thanks for being here. Tom

 

[tkanzler]

It's nearly doubled, but one leg of the two is parallel to the mast centerline, and therefore contributes nothing to raising the mast. The force along the mast from that leg is balanced against the force in the cleat holding the bitter end. All it does is compress the mast, or more accurately, puts it in compression.

For a crazy extreme, put a fiddle block at the top with a becket instead of the halyard block, and one at the cleat, and run 4 parts between the 'halyard' fiddle block and the other fiddle block attached to the cleat. You'll get a tremendous squeezing from the 4 parts between the blocks, parallel to the mast centerline (to the tune of 208 lb x 4 = 832 lb), but just that one part from the top fiddle block to the gin pole will actuall do anything to raise the mast. Total load, including that 5th part (to the gin pole), will be something like 1000 lb. Slightly less than 5 x 208 lb because that last part shoots off at a bit of an angle, but it's that bit of an angle that adds the perpendicular force component to the mast that lifts it. Lots of force to do a little light lifting.

That one part, going to the gin pole, will do the mast raising, and still experience 208 lb of tensile force, while the fiddle blocks will be trying to rip the fittings out of the mast as they're pulled together. Not terribly unlike wrapping a rope around your hand many times, then pulling. You crush your hand with very little pull, and the more turns you put around your hand, the more you crush it for the same amount of pull.

Hope that makes sense.

 

[justsomeguy]

After comparing both images provided by tkanzler's second post, I've got to admit that I was wrong in my assumptions. Does the mast cleat method impart half its load to a compressive force along the mast? (head spinning)

 

[Pest UK]

Thanks Tkansler I'll sleep o.k. tonight now.
Sorry doehunter it is'nt moot now, poor old Davenlyn still can't get the mast up, or did I miss something?