Might have had a little win in the junk design stakes.

My original calculations for Centre of effort came from some old partial plans for the Hartley 21.

The scale drawing I had was the Fixed Keel version which carries a 26 foot mast and 25.6 square metres of sail.

Just received a new set of plans.

There is a scale drawing of the 3/4 rig for a swing keel Hartley 21 (my boat, less ballast) which carries a 24 foot mast and 17 sq m of sail.

The CE of this rig is further forward at 2781mm from the bow.

My proposed mast position was on the foredeck.

The centre of the new mast would be 1390mm from the bow.

So working backwards

Given 10% of the junk rig forward of the mast.

40% of the chord is 2781-1390 = 1391 mm

40% of the Boom length at 10 degrees= 1410 mm

Total length of Boom for junk sail to match the old sail CE = 3525 mm

Plugging into the Junk rig calculator, An 18 sq metre Arne sail needs a boom of 3550 mm

So 18-20 sq metre should be in the sweet spot.

Will check this then draw it up

 

Peter,

there is a possible way around the CE problem which does not involve moving the mast tabernacle aft into the cabin:

If the boat were mine, I would just add a little mizzen, 10-15% of the mainsail. Its un-stayed mast could be fitted in a step, outside the transom (a little offset). The sail could be 3-sided with a sprit boom and a retractable boomkin.  It should be a quick job to raise and lower the mizzen mast (with the sail rolled up on it), and then fit the boomkin and sheet with the boat still on the trailer.

This mizzen will bring the the total CE far enough aft, and it will make a fine self-steering much as the time, as well as being a good anchor-riding sail.

Arne

Still working on a junk rig for a 6.5 metre Hartley.

This boat will be trailered to the coast so aiming for a quick mast up. 

The easiest place for the mast is on the front deck approx. 1450mm from the bow

Mast Travel Position

Mast Raise Position

The target mast length of 7 to 7.5 metre carries nicely an AR 1.85 with a boom 

Boom 3.8

Sail area 21 sq metres

OR

Boom 4 m

Sail area of 23 sq metres

4 metre Boom

The centre of effort of both sails is about 5-7 % of the water line length forward of the calculated C of E of the Bermuda sail on the scaled plans. Used both cardboard balance and geometry to arrive at the same position.

Am I stuck with putting the mast further aft through the roof? 

Hi All . Just wondered what a reasonable stub height would be above the partners to collect the bundle (parrells etc) before I crack the mast for lowering. Just working on some options to raise the mast and drop it in a hole.  Playing with a idea where once raised the 7 metre mast can be loosened to drop / ease home about 700mm. Will worry about lifting it up once the final mast weight is established. Regards PA

Peter Armstrong wrote:

Arne

Out of interest, the slide in 3m stub option? Were you thinking solid round timber or another smaller aluminium tube.

Thanks PA

I was thinking of inserting a smaller aluminium tube, If one with a close fit were not available, I would just add 3-4 waistbelts of grp tape on the stub until these became a close fit.

Arne