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Discussion Starter · #1 ·
Forgive me if this has been asked, answered, or is just common knowledge but I am trying to determine the height of my boom as based on the luff length of my mainsails. I have all the measurements I need but am trying to figure in a dimension for how much distance should remain after fully hoisting the main between the halyard sheave and the head of the sail. My boom is on a track so compensating for luff stretch over time will not be an issue, there is also the Cunningham. I just need a "minimum" spec so I can measure for a stern arch and also place the booms track is a centered location. Also, I need to install a sheave at the masthead, when locating one of these is there a distance from the actual masthead you want to maintain? This seems so simple but I am starting from scratch so I don't want to assume anything. THANKS in advance for any help.
 

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Sounds like you might be rigged for racing. Many of these boats have a band on the mast (obviously you don't or you wouldn't be asking)... but my point is there might be an association that sets forth the location for the black band, and that should be pretty close to "max hoist".
 

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First of all and assuming that you have a woven dacron mainsail with a boltrope (three strand rope inside of a sleeve at the luff), that mainsail luff dimension will progressively get smaller and smaller (shrink) over time, and not 'get longer'. The boltrope will become 'fatter and shorter' due to constant loading of the boltrope not 'longer' - called hysterisis of the rope lay. Such progressive shrinkage of the boltrope is the chief cause of why dacron mainsails become 'baggy'.

How to properly measure will include the length of any eye splice (taper) and any shackle between the top sheave and the headboard of the sail.

The most common way to do this measuring is to look at the designers drawings and look for precisely where (if you had one) a fixed gooseneck would be positioned, then measure the distance between the gooseneck and the masthead sheave (allowing for the halyard terminal eyesplice, etc.). If you dont 'stretch out' the luff of a dacron mainsail that has a 'boltope' (by approx 1 inch for every 10-11 ft. of luff length) the aft end of the boom will usually always be LOWER (into the cockpit) by this amount !!!!! Failure to 'stretch out' will result in a boat that heels over aggressively, 'increased weather helm', is cranky, is slow ... and has its boom aft end TOO LOW into the cockpit
When measuring (and using a boltroped mainsail) ADD the normal 'overtension' that one normally applies to extra boltrope strain ... that boltrope normally should be additionally stretched out by 1 inch for every 10 feet of luff length. Note: when this extra halyard strain is applied, the AFT END of the sail will also RISE by this approximate length, thus raising the boom at its aft end by this amount.

Most folks simply do not correctly raise a boltroped dacron mainsail and thus the boom (aft end) is usually always carried too low and with the aft end of the boom 'drooping' into the cockpit, etc.
Here's an article which more fully explains how to correctly RAISE a 'boltroped' woven dacron mainsail. Suggest you START your measuring based on a PROPERLY RAISED mainsail FIRST: How to properly RAISE a woven dacron mainsail - SailboatOwners.com
 

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Rich's right, fersure!
I followed his instructions in a previous post and it made a world of difference in sail shaping,handling and .pointing.
Thanks :D
 

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Discussion Starter · #6 ·
RichH, excellent info. This was the stretch I was referring to. This is a cruising vessel not a racer so just looking for a generic setup. We have a brand new main and an older one, once we get on the water it will be interesting to see the tuning difference in the two. Concerning the masthead halyard sheave; basically place it a reasonable height say 2" from the top of the mast? If I can get that locked in I can use your stretch ratio to determine the approx. tension for each sail and my calculations will be complete. Since I have a sliding gooseneck none of this will really be too critical it's just useful in determining a few cockpit designs at this point. Thanks
 

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Id make the assumption that the old sail has a severely shrunken boltrope, and the new main has a correct dimension boltrope.
Id suggest: to carefully measure and record the luff dimension of the new dacron sail so you can 'follow' the inevitable luff shrinkage and then have the sail's luff dimension 'eased' by a sailmaker when the shrinkage becomes significant. Doing so will lead to a much longer service life of the new sail AND much better sailing performance as the new sail 'ages' (boltrope begins to shrink). Having the precise original (as made) luff dimension will make it MUCH easier for a sailmaker (or yourself) to affect the proper and minor corrections later on.

I usually recommend that for anyone purchasing a new woven dacron mainsail with boltrope to have an excess amount of boltrope extend and be attached to the headboard .... all ready to go to be slipped into the boltrope sleeve when the boltrope does shrink. With an amount of boltrope already stored along the top of the headboard, all one has to do is cut the 'sail twine binding' on the luff sleeve, pull the luff until the precise distance is restored, and resew by hand the sail twine binding - a 30 minute job for a DIYer --- but you NEED the OEM luff dimensions to do this.
 

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Discussion Starter · #8 ·
Great suggestion, measurements of both sails are already in our folder. Following your line of thinking would it not be a wise idea to ease the luff length of the older sail. I am not sure that it shrank 4", i don't think it was used that much, perhaps when the previous owner had this new sail made up the tech who measured for it just calculated things differently. Don't really know. It also possible that after being stored for months or years now that the boltrope just needs to be tensioned out. I measured both of these sails by tying the tack to a tree and pulled the head with a short lengths of line while my girlfriend recorded the measurement. They could both realistically be a little longer,I just needed a rough idea for boom height. I'm not an expert, if we pass any sail shops along the way down the ICW I'll certainly have the older sail looked over.
 

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Many variables can account for radically different dimensions on sails. This is especially so if the boat was not personally measured by the sailmaker who instead used 'catalogue' dimensions. Another possibility is that the old sail was originally from an entirely different boat, etc. -- eg. a 'used' sail, etc.
 
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