Mark, I think there might be an error here.

Have you calculated the centre of area correctly for each of the junk sails?  doesn't look quite right. In each case, the centre of area should be on a vertical line which goes through the midpoint of the boom. (That's the quick and easy way we do it for junk sails). It doesn't matter about where it is on the sail, up or down, all that matters is the position of that vertical line. The fore-sail centre looks incorrect to me.

I have marked the two lines (A and B) where they should be on your diagram ( but only roughly, I am eye-balling off the screen, just to show you the idea, You will need to check the accuracy.

Now, the next thing you must do is calculate the resultant of these two centres. It looks to me as though you have just chosen the midpoint of the two, but that would be incorrect. The position on a line between the two centres is not the mid point, but a point which takes into account, in a proportional way, the AREA of each of the two sails.

I don't know the two areas - the fore sail is smaller in area than the aft sail, I can see that, so straight away I know that the combined centre will not be at the midpoint of the two component vertical lines.

Here's how you do it. Let us say the aft sail is 120 sq m and the fore sail is 80 sq m (just to give a simple example, with simple numbers.) Add the two areas together and we get a total of 200 sq m.  The aft sail is thus 120/200 = 3/5 of the total area. And the fore sail is thus 80/200 = 2/5 of the total area.

Suppose the distance between those two vertical lines (representing the two centres) is 4.5m, just to use an easy number.  Then the resultant centre will need to be at a point which divides the 4.5m into proportions 3/5 and 2/5  (not half way). It will be biased towards the larger of the two areas, so it will be 2/5 of the distance from the aft centre.

(ie 3/5 x 4.5 = 2.7) 

and, just to check, 3/5 of the distance from the fore centre.

(ie 2/5 x 4.5 = 1.8)

So, the resultant line divides the 4.5 into 2.7 and 1.8    -    not half way.

Note, I assume you have the area of the mains'l and genoa of the original rig, and have found their geometric centres correctly. It looks as though you have. But again, unless the two sails happen to have equal areas, the resultant CoE will not be midway between them, but in a position proportional to their respective areas with the bias towards the larger of the two sails, same as above. Check you have done that correctly then do your superposition of the junk rig as above.

Forgive me if you already know all that, but it does look a bit as though your diagram might have been incorrect.

The idea looks good to me. Presumably you have the resultant geomtric centre of the sails in the correct place.

The usual thing for a two-masted SJR seems to be to have the forward sail SJR and the aft sail contiguous (for different reasons than your's, actually, which are soon gong to be put to the test - but your reasons sound good.)

You may also want to look at Paul's post (1st October, in the Tyvek and other building wraps for experimental sails thread) and you will see a two-masted SJR in which both sails are SJR. So far it's looking like a good combination).

Anyway, your proposal looks good too, and it will give an excellent chance to compare the characteristics of the two types of sail, in respect to running rigging, setting and handling. I would be keen to see a report on that later.

Also I am keen to see what Arne thinks about your proposal, as no doubt you will be too.

I have no doubt, size of fore sail will affect the helm balance of the boat - whether helpful or not will depend on your geometric (CoE) balancing correctly with the original designed sail plan. You don't show those centres on your diagram, so no-one can comment. Make sure you have done those calculations before settling on a final sail plan, and post them for comment, or check with Arne if you haven't already done so.

Anonymous wrote:

This is posted on behalf of Mark Shrosbree, a, old JRA member who has just rejoined:

Hello. My name is Mark Shrosbree and I have just become the proud owner of a 40ft Offshore ,that I would like to covert to a junk rigged schooner.

I crossed the Atlantic and have sailed 9000 miles in Cielo a 32ft junk,with a sail i built using Arne's plans off this site.

The problem is that the placement of the masts is not really adjustable without compromising the interior . The forward mast can be placed 1.2m from the bow will have to remain where the mast is now, maybe 200mm aft of this position. Can this work?  Any help with this would be greatly appreciated.

Also,  is there a member who is knowledgeable about using street lamp poles as masts?

No experience, but i found a aluminium street lamp poles . I begin early next month to make alterations

Jacques 

Mark,

I suggest you present the forum with the sailplan. Draw in a horizontal line with the boat’s length, and do also draw in the distance from the bow to the centre of the mast position(s). Without these positions, it is next to impossible to give any qualified help

Good luck!

Cheers from my wheelchair,

Arne

Thank you kindly for your insight. 

I really have my heart set on a schooner. 

I was reading a post where the  mast was offset from center,

if this is possible the main mast could come back about a meter

and would be about 200mm from center. 

I have the possibility of getting street lamp poles, made from 6063 T4

With a 10" 250mm base tapered to approximately 150mm and 8 to 10mm

Thickness.  There's also some broken poles ,that could be inserted to strengthen the bury.

If more information is needed please let me know. 

Thanks