Mark and Ueli,

I use a combination of what you use. I aim for maximum 30° halyard angle, that is, with the sail moved to maximum aft position. These days I have settled on having the slingpoint 55% aft of the throat on the yard. This prevents the sail from getting tail-heavy when raising or lowering, and also offloads the yard hauling parrel, YHP.

A couple of days ago, a German mailed me about the possibility of fitting a JR to a Spaekhugger. I imported his (Bermuda) sailplan to my QCAD program and suggested a JR for it. On the shown sailplan, the sail rides near the max forward position (15-16% balance). In case this results in a lee helm, I planned for shifting the sail as much as 25cm aft (to 10-11% balance).  From this aft position did I draw the 30° halyard line to establish the mast height, as Mark describes. In the shown forward position, the angle is only 14°.

The resulting, generous mast height, gives plenty of halyard drift and also lets one attach the YHP further up (about 2/3) the yard. This again ensures lighter forces in both the YHP and throat hauling parrel, THP.

Cheers,
Arne

PS: Note that the JR mast is still 1.81m  -  almost 6'  -  shorter than the Bermuda mast, while the top of the JR sail is only about 20cm lower than the Bm sail.

(Arne's album, sketch section 3, photo 42)

At last a topic i can contribute to. I have the folios from Jock Mcleod and in  designing my current rig came across an instruction to draw a line at 30 degrees from the middle of the yard.  Where this cuts the mast line is the top of your mast.  I added a fudge factor of one foot.

Planform is only about defining the shape of a sail. It cannot include the mast line, as this can vary widely for some sail shapes. I'm going to stick with my view that the minimum mast height is given by an answer to the question "how close, in terms of potential damage to blocks and halyard caused by twisting as the sail is squared off, can the yard and mast attachment points be?"

I would suggest that the question should be "what is the minimum permissible halyard drift?" and that the answer is "it depends on several factors".

It is independent of the type of planform, but is dependent on the type of block used, the type of rope used, and the type of attachment to mast and yard.

PJR gives a drift of 0.3B, which for a batten length of, say, 4m would be 1.2m. This is generous to the point of being excessive, but allows for indecision on yard sling position, sail AR, need to hoist the sail higher for convenience at deck level and suchlike factors.

As a rule of thumb, I would suggest that modern yacht blocks are comfortable with a distance between them of twice their length, provided that each is free to rotate on its mounting by about +/- 45˚, this being achieved easily enough with a shackle through a hoop or eye, or a lashing, or a soft shackle. That would indicate a minimum permissible drift of four times the length of one of the blocks, or about 500mm, where two blocks are used. The use of a becket and its associated knot or splice would make it sensible to increase the drift to allow for extra distance between the blocks.

The way to reduce that is to use a single part halyard with a self tailing winch, or to use my preferred form of halyard, two parts, with two attachment points at the masthead, again with a self tailing winch. The end of the halyard is made fast to an attachment point on the masthead that is above the sail (at 7 o'clock, if the sail is on the port side), and passes through a single block on the yard and a single block at the masthead (at 5'oclock), and then to the deck. If the yard block has a little rotational freedom (but not complete freedom), it is permissible to hoist almost to the "two-blocks" state, or at least to a drift of three times the length of one of the blocks, that drift being measured between the attachment points on yard and mast.

But if possible, it's better to go for more than the absolute minimum drift that the blocks can tolerate. Another factor that might be considered is that a tricolour light really ought to be above the top of the fully hoisted sail.

Let me put it another way.

Once the sail area, plan form and aspect ratio have been decided, it is possible then to draw the sail with its CE where we want. We can then draw the mast in its correct position, and we can see where the mast crosses the yard.

At that point in the process, however, minimum mast height is not yet obvious.

The top of the mast will have to be higher than the point where the mast crosses the yard, in order to give sufficient drift to the halyard. My question is: how much higher? Perhaps my question should have been “how do you calculate the preferred minimum amount of drift” because that is, I suppose, what ultimately gives the minimum Mast height.

I realise the answer to this question is different according to whether the planform is of a high peak or moderate peak type – which is partly the reason I am asking the question on this forum. For each planform there must be some principle on which minimum halyard drift (and therefore mimimum height of mast) is determined.

In a way that David Th did not intend, it is indeed a question of “how long is a piece of string?”, but it is surely not arbitrary.

It may be a dumb question, but how is minimum mast height determined once all the other important parameters are in place?

(As a matter of fact, it is a real question for me. Like Scott, I can see the trade-off between mast height and sail area and, in general terms, where the various different planforms score. I am limited by mast height.)