PJR Mast diameter formula a bit high?

  • 12 Sep 2018 16:38
    Reply # 6666185 on 6659511
    Deleted user

    Thanks David, that should help a lot :)

    Bill F

  • 08 Sep 2018 23:55
    Reply # 6660506 on 6659511

    Hi Bill,

    look at the boat of the month and the June 2017 issue of the JRA Magazine for Little Gypsy Girl, most of the information is there. The wishbones were made from 1" square pine (actual size not nominal which is usually close to 3/4") and are probably a bit heavy for the sail size of 110 sq ft. 


    Last modified: 09 Sep 2018 00:15 | Anonymous member
  • 08 Sep 2018 17:58
    Reply # 6660228 on 6659511
    Deleted user

    Thanks Guys, those are a couple of lovely boats.

    I hadn't considered displacement, my boat is pretty lightly built , probably not much more than 150 kilos/340lbs including crew so I guess there's not as much need for a heavy mast, actually, there's no need for 100sq ft/10sq m of sail either I suppose, it's more like 75 sq ft plus a 25 sq ft ghoster that'll be reefed at force 2 or sooner.

    I'll take it out with my dinghy rig in the next day or two and see how it goes with 70 sq ft on a 2"/50mm with 17.5% walls 12'/3.7m hollow mast, if that doesn't break I'll have an idea of how heavy to make the new mast :)

    Do you have any info you'd be willing to share on your sail build David? I'm planning on a rectangular aero junk myself and have no idea what size the battens should be.

    Bill F

  • 08 Sep 2018 08:00
    Reply # 6659843 on 6659511
    Anonymous member (Administrator)


    when I rigged my 5.5m/200kg dinghy , Broremann, I first used a borrowed mast with 70mm diameter(spruce). This was on the light side, so I had to reef early.

    Then I had one built with 90mm diameter, and this proved to be perfect. Maybe 80mm would be fine for your 16-footer? It depends how heavy that dinghy is and how hard you sail it. If you plan to sail with two blokes on the rail, it may be better with 90mm. Remember, the bending strength of a mast varies with the cube of its diameter. Therefore, a 90mm mast is 2.13 times as strong as one of 70mm.


  • 08 Sep 2018 00:16
    Reply # 6659535 on 6659511

    Hi Bill,

    in using the PJR formula you must remember that it is based on fairly heavy cruising , ballasted keel boats. I have found that a dinghy can use a much lighter section and still be more than adequate structurally. On Little Gypsy Girl I used the sail and masts from the original dinghy on the revised larger ballasted hull and found that the masts bent alarmingly, they had been fine on the dinghy. The new mast is a solid spar 4" in diameter and 20 ft long and it has proved structurally adequate even in a full knock down under full sail (approx 110 sq ft).

    I hope that is of some help in judging what section to use on your boat.


  • 07 Sep 2018 23:55
    Message # 6659511
    Deleted user

    Hi All

    I could be reading the formula wrong, but with the PJR figures a mast for 100 sq ft sail 16 feet above the partners would be 51 feet in diameter (D in inches = (16 + 2 x 10)17.1) works out to 615 inches...

    the metric formula is better at 4 1/2 inches in diameter solid mast, 5 inches diameter hollow, still waaaaaaaaaay too big for me to handle.

    Are the formulas in the PJR a bit on the conservative side?

    Bill F

    Last modified: 07 Sep 2018 23:56 | Deleted user
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