Mainsheet forces and mainsheet arrangement.

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Mainsheet forces and mainsheet arrangement.

Postby 109jb » Thu Aug 09, 2018 12:11 pm

Working on getting my DS II ready to go. I bought this boat last year and have only had it out once so far. The boat has a 3:1 mid-boom mainsheet. Just wondering how much force there is on the sheets/blocks under full sail in a good wind? Reason is because my upper block looks a little weak and doesn't match the Schaefer block mounted to the CB trunk. I looked it up and it is a block from a laser sailboat and has a working load rating of 180 pounds, which I don't think is enough, but to replace it I would like to know what load rating I should consider as minimum?
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Re: Mainsheet forces and mainsheet arrangement.

Postby 109jb » Thu Aug 09, 2018 9:11 pm

Doing more examination tonight I found that my boom is bent to port and the bend is right at the mid-boom sheet attachment. The bend is slight and I suppose it could have happened sometime in the past by being stepped on or something, but it sure makes me wonder if the forces could be too high the way it is currently set up. One thing I have thought about is if the eye is too far forward. Currently it is 4-1/2 feet forward of the aft end of the boom. I've been looking at pictures of other daysailers and it looks like that is further forward that what I see online for a mid-boom sheet setup. Does anyone have any opinions?
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Re: Mainsheet forces and mainsheet arrangement.

Postby GreenLake » Fri Aug 10, 2018 9:11 pm

The maximum working force on rigging can be estimated under the assumption that the boat capsizes with all crew hiked fully.

772

This assumption gives you the maximum heeling force before the boat goes over. If you assume that not more than this force can act at the tip of the boom, you can figure out the maximum loads for your sheeting arrangement.

In usual operation, you can adjust the main in all conditions, meaning that your maximum "pull" on the sheet is strong enough that, with the 3:1 purchase, it can overcome the wind force on the sail. That estimate will give you another, smaller number.

Finally, an uncontrolled gybe will put shock loads on your rigging which are not covered by these two estimates. If your boom is bent to port, rather than up, chances are a crash gybe in high winds was arrested by the mainsheet. Looks like it didn't tear out your blocks, so that would argue that they are strong enough.

However, we don't know the actual history, including whether someone put the boom on the lawn (or other soft ground) and accidentally drove over it. :shock:

If worried about center boom loads you could always switch to end-boom sheeting.
~ green ~ lake ~ ~
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Re: Mainsheet forces and mainsheet arrangement.

Postby 109jb » Fri Aug 10, 2018 10:42 pm

Thanks. I am contemplating going back to the end boom sheeting, but the accidental gybe sounds like a likely culprit. I went out and looked at the eye on the boom and it does appear to be bent slightly to starboard, so it fits with a boom swinging being halted by the sheet. For now I will just straighten the boom and keep the mid-boom sheet and sail it. I can always switch later as it looks pretty straightforward. Thanks.
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