mast length

[yer tiz]

I have a somewhat technical question;  I acquired a 2nd-hand mast which was okay with my old sails, however, I have now acquired new sails necessitating the lowering of the boom.  This was okay but has become a bit of a headache ( literally).
 The luff of my new mainsail is some 5525mm and my mast-foot to the sheerline is some 550mm. My questions are:
1. what length should the mast be?
2, what distance should the boom be from the mast foot?  ( hull mounted not deck) currently this distance is only 800mm from mast foot to boom.  

[MikeDay]

What design boat? What make of mast?

Mike D
N3496

[yer tiz]

Its a Proctor Mk IX ( 1960 clinker)

I belive the mast is a super spars.

[gerry ledger]

The length of the top band is 6100mm from the sheer this is what governs the hight of rig . The luff of main is unmeasrued as the area is derived from a formula with the foot.Hope this helps

[FuzzyDuck]

But Gerry, how do you know where the sear line is if it has worn off your mast?

Simon

[Dave Croft]

The sheer is where the hull intersects with the deck. Roughly it's about 50mm in from the gunwale. You can find it more accurately if you get a piece of card and cut a semi-cirle out then  place the edge on the top plank so the the cut-out goes round the gunwale. You can find the intersection with the deck easily by seeing where the top edge meets the deck.
Some years ago I recued an old Procter Mk11. There were no sails with the boat so I put an old suit I had on. The boom was way lower than on my boat and when I measured the top of the mast it was about 150 mm short! I moved the coloured band up as far as possible and I aaded a block of wood under the mast step  all of which got me about half-way there. The point here is that I belive it was common parctice in the class some years ago to have the mast a little short of maximum allowable, seems crazy and this boat was supplied ready to go from a professional builder. Hope this helps

[FuzzyDuck]

Dave,

Am I right in the following assumption.
When you have found the Shear on the deck you use a straight edge to mark this level onto the mast in its normal position.  This would then give the height to the top band from this point. If the mast is short you can raise the foot?

Simon

[Dave Croft]

Simon,

Yes once you have the sheer on the deck use a straight edge to mark the point on the mast then measure to the lower edge of the top band from that sheer point. It's a good idea to be a few mm inside. If your mast is short then you can do whatever you like to get the height but I would advise against going OTT with packing under the mast heel, use epoxy and some big stainless screws to fix the block. Make sure your top band is as high as it can be as well.
Dave

[yer tiz]

This does seem quite tricky;
I have read from the class measurement manual and:

1. from sheerline to bottom black band 380mm
2. from bottom band to top max 6100mm

with my boat the sheerline is 550mm from mast step plus 380mm from above gives 930mm to boom ( I am assuming the boom gooseneck sits on the bottom black line?)

930mm from bottom of boat to boom seems quite low compared to other boats in my dinghy park.

[Dave Croft]

Don't get bogged down. I suggest you concentrate on the height of the top of the mast from the sheer 6100mm or about 20 feet in old money. If your mast is too short this the prime consideration. the res should fall into place once you have established this. Are you still unsure about how to find the sheer line and mark on the mast? Let me know if you are.

[yer tiz]

I think you are right as:

my sheerline is 550mm from mast step plus 6100, would give a mast length of 6650mm. my current mast is only 6400mm.  the longer mast would mean a foot to boom of 1125 rather than the 800 I have now.  1125 as a max would give a respectable area under the boom, and a lot less head banging!

many thanks
Mark

[Dave Croft]

Sounds exaclty the same problem I had on the Mk11. Maybe you wiould be better off trying to find a second hand mast. There must be lots kicking about the old 12 clubs.