Calculating Block Load

[Charlie]

Hello All:

I am looking at replacing the blocks on my main sheet. The system that I would like to use is mid boom sheeting :I'll try to explain it with words b/c I can't get a picture till May. the dead ended side of the main sheet will attach to a becket on a single block. The sheet would then go through a fiddle block on the traveller car, back up to a second single block on the boom then back to the fiddle block and back up to the single block with becket and from there along the boom to a block that is toward the mast and then to a block on the deck and finally to a winch under the dodger.It is system 6 in the Harken Catalogue.

My main sail is > 450 sq ft and I am planning on outfitting for a bluewater cruise. The blocks on the boat now are lewmars and look to be 20 yrs old. I just think it is time to replace them. I am planning on using Garhuer. Gar recommended that I use 40 series blocks with a 60 series at the boom. He is probably right but for my own comfort I'd like to know how to calculate the load. All the blocks I've choosen have a 2800lb working load.

[delmarrey]

Why not find out what your old blocks were rated at and duplicate them, load wise. They seem to have held up all these years!

[Charlie]

Thought of that but I don't know what the model # was on the blocks. Harken catalog used to have calcs but I think that people would use their calcs to buy from other companies.

[delmarrey]

I think I seen a formula some where and may have in my Favorites. I'll look around tomorrow. It's on the other computer. That's unless someone else comes up with it.
You'll need to know the distance from the gooseneck to the attachment of the main upper block. The stresses get higher as it moves towards the gooseneck. 450 sq. ft. is a fair amount of sail and can get pretty tough pulling that baby in with little purchase............................._/)

[Weyalan]

Load = (E^2 x P^2 x 0.00431 x V^2) / [(P^2 + E^2)^0.5 x (E - X)]

Where
E = lenght of main foot (feet)
P = lenght of main luff (feet)
V = wind speed (knots)
X = distance from end of boom to mainshet attacHment (ft)

N.B. Gives load in lbs

[delmarrey]

Yeah! I found it too......................._/)

http://www.harken.com/charts/mainsheetsysloading.php

http://harken.com/calculators/MainsheetLoading.aspx

http://harken.com/rigtips/main6.gif

If this is your system it seems a little on the light side for a 450Sq. ft. sail. Being mid boom I think I would add double blocks at A,B, & C and save the winch for the last little bit on a closehaul.

[Charlie]

Weylan and Del:

Thnaks. If I had thought before I typed I would have looked at the Harken site. I'll let you guys know what I come up with. Currently there is a 3 to 1 on the boat. If I change the amount of purchase I will have to change the length of the Mainsheet and it seems to be in decent shape.

I was mistaken on the size of the main it is actually 306 sq ft.

[hellosailor]

Charlie-
Along the calculations, consider what part you want to have fail during an extreme load. i.e. offshore the worst possible load would be rolling the boat, or putting the sail under and having green water loading on it. Either the sail, the boom, or the rigging might be the weak point, somewhere along the line that might be something to think about.
If you are just looking at normal sailing loads, there are standards for how many pounds of load per square foot of sail area, that would be the starting place. And of course, the means by which the blocks are fixed into the deck/boom are also something to look at, whether you have 20-year old pop rivets or nuts with back plates, they still can become the "dotted line" where things come apart.

All good reasons why sometimes we just look to replace what's been there and working.<G>

[Charlie]

HS:

Thanks for the advice. I didn't think of havng the equivalent of a shear pin on my Mainsheet. Starts to get beyond my understanding of engineering though. I'll have to put more thought into that one.

[hellosailor]

Charlie-
Most of us ignore these things (designing in shear pins, so to speak). I was reading an interview with an industrial designer some years ago, and the interviewer asked him about the way cell phones and similar devices always "flew apart" when you dropped them. It turns out the heavy mass of the battery breaks things--unless you design it to fly apart, and then the battery takes all the force away, leaving the gizmo to just bounce like the "feather" it is.

As Bob Heinlein said, sufficiently advanced technology is indistinguishable from magic. Sometimes we get lucky--we're in an age when things are reaching that point.<G>

[delmarrey]

306 s/f, that's a little EZer to work with but still a lot of stress for a mid boom traveler. The purchase might be OK but the travel car and track would need to be for a 40'-45' boat IMO.

To add to what hellosailor said about considerations, accidental jibes can be damaging for large mainsails. With a mid boom system it doesn't have so far to travel but the stress is increased. That's one reason I installed a boom brake, one of the best investments I ever made.

One of the things I did to reduce the dipping of the boom was to go to a loose footed main. As well, I installed a traveller on the boom, with an outhaul, so that I can ease off on the clew allowing the sail to fill before-the-wind BUT with the boom at a 45º angle only. I don't allow my boom to go out pass 45º any more. I'll be adding running back stays for a cutter rig and this also prevents the boom from hitting those stays.

When setting up the rigging for offshore it's always better to go over kill, remembering the weakest link will go first............................_/)

[delmarrey]

When you get it set up take'r out accross the Straits' on a 25 kt day with the 6' wind waves. That'll prove your system for the most part..........._/)

[Charlie]

HS:

At some point you have to say "enough" I remember taking a class in siesmic retrofits. As it turned out the real answer to siesmic strength was to allow the building to move not to make it stronger. I grew up in a row housewhere the buildings might have from 1" to nothing between the houses. The idea of engineering the solution so that the house would not move during an earthquake was impossible and economically impractical. On the other had if the house could be designed to allow movement then there was good chance that the structure would survive. I think this is well illustrated in the use of stretchy materials for anchor rodes (and snubbers on all chain rodes). Don't know how to design in that safety margin. First thought is that a stronger block would be better for offshore work but now I'll have to think about that one.