• 24 Jun 2018 19:58
    Reply # 6338713 on 6316327
    Deleted user

    Thank you for the assistance guys. I will look closely at those calculations soon.

    The H28 is 3500kg (perhaps 4500 with all my stuff in it...) so as an estimation 200mm should be enough.

    Right now I am excavating the forward cabin and insulating it. Putting in the mast is the next priority.

  • 22 Jun 2018 14:26
    Reply # 6332630 on 6316327


    I covered the engineer's way of sizing a mast tube here, but if you can't do these calcs, then I think you're somewhere near right with a 200mm tube. Annies's right, the 152mm tube is just enough for her boats, but not enough for yours.

  • 22 Jun 2018 12:26
    Reply # 6332221 on 6316327

    A quick rule of thumb that I have found works for me for aluminium tube masts is as follows.

    Take a quarter of the circumference of the mast and multiply by the wall thickness of the tube, multiply by 0.8 of the diameter of the mast and then by 12000 pounds per square inch (approximately half of the ultimate strength of 6063 T6 aluminium alloy, this gives a two to one safety factor) This is a rough estimate of the working strength of the mast in foot pounds.

    Multiply the sail area in square feet by the distance from the waterline to the center of effort of the sail, multiply by one (the pressure of a breeze of 20 miles per hour, about 16 knots, this being the usual point to start reefing sail). This gives the pressure on the mast from the sail plan.  The two should be approximately equal for design of a mast to suit a sail. For extra heavy boats or long distance cruising boats be a bit more conservative on the strength of the mast by reducing the strength factor to 8000 pounds per square inch. 

    I hope this is of some help.


  • 22 Jun 2018 09:42
    Reply # 6332099 on 6316327
    Anonymous member (Administrator)

    You may be right about that Annie.  

    The Ullrich catalogue which is on line shows  these 6m sections:

    (1) 152.4mm O.D. x 5.00mm wall thickness  6.251 kg/m

    (2) 162.00mm O.D. x 4.00mm wall thickness  5.361 kg/m

    (3) 177.80mm O.D. x 6.35mm wall thickness 9.235 kg/m

    (4) 203.20mm O.D. x 6.35mm wall thickness 10.603 kg/m

    Not sure if all of these are available, I know they had quite a few of the 152.4 in stock a couple of months ago.

  • 22 Jun 2018 08:31
    Reply # 6332050 on 6316327
    I would reckon the H28 probably displaces about 5 tons - a couple more than the Raven - and you are talking about putting quite a lot of sail area on it.  My feeling is that 152mm diameter is about right up to about 35 sq m.  Above that I'd go for a larger section and maybe with the extra displacement of the H28 it might be prudent to go a bit bigger.  I confess that I'm no expert on alloy and our mast database isn't as much use as it might be for sails of this sort of size.  However, I feel that I'm putting the absolute maximum on my section with a 35 sq m sail.  For peace of mind, it might be worth going up a size.  My section has a 5.2mm wall, from memory.

    Your 250mm diameter is more the size you'd choose for a wooden mast.

    As Graeme says, someone like David Tyler would have a better idea; however, as you are by no means the first person to broach this subject, a serious rummage through the fora might yield more information to help you in your decision making.

  • 21 Jun 2018 23:32
    Reply # 6331442 on 6316327
    Anonymous member (Administrator)

    On my boat which is a Pelorus I am using the 152mm OD and 5mm wall thickness standard Ullrich tube for the bottom 6m. Also that's what Annie used successfully in the Raven conversion (Fantail) if you look at Annie's blog you will see all about that. As your boat is in approximately the same category as a Pelorus or a Raven, roughly speaking, probably that size will be Ok for you also.

    If you need a larger section perhaps one of the more qualified people will chime in here.

    Ullrich have that section in stock, you can order it through the Henderson branch, and if you want they will get it anodised for you at a small extra cost - I took advice from others and got the anodising done, and am pleased I did. They delivered the anodised tube to my address.

    Last modified: 21 Jun 2018 23:38 | Anonymous member (Administrator)
  • 21 Jun 2018 20:15
    Reply # 6330238 on 6316327
    Deleted user

    So the 200mm diameter alloy tube will be fine for the mast base? I read that 250 was necessary for a boat/sail the size of mine, but that could be for a wooden mast.

    If that is the case I can start work on my mast in the next month or so!

  • 21 Jun 2018 08:48
    Reply # 6328706 on 6328420
    Anonymous wrote:An added bonus is that the mast only needs to be 9m long. I can get 6m of 200mm dia 5mm alu tube from Ullrich which is basically the necessary diameter for the masthead (i think) so I can build the base out of wood. If the masthead needs to be thinner I can always go wood, alu, wood I guess.
    Most hybrid masts use alloy at the bottom and wood (or another material) at the top,so that you get a tapered topmast which looks better and has less windage.  Moreover, diameter for diameter, the alloy is smaller, so you have a smaller mast in the boat.  Of course, you could go wood-alloy-wood, which is what I'm doing with my tabernacle, but it is an extra stage of work.  On the other hand, you don't have a metal mast right next to your bed!
  • 21 Jun 2018 08:39
    Reply # 6328619 on 6328118
    Anonymous wrote:

    How did you find the upwind performance on Fantail? I ask mainly because I live at Herald Island, which is even further up the Auckland harbour than most people have to travel. Especially with the prevailing easterlies it can be a menace even with a bermudan rig. 

    I notice that SibLim has a more regular HM style sail. Was that a design consideration or personal preference?

    I would say that Fantail wind to windward with her cambered junk rig as well as she did with her fully-battened mainsail and roller furling genoa.  And with a lot less grief.  With an ebb tide, it's fun sailing down from Herald Is as I know from first hand experience.  However, these long sails just to get out to sea do pall after a while - the locals here in Whangarei have to sail 13 miles to get past out of the harbour.

    Actually, the sail I'm putting on SibLim is quite different from an HM sail, having much less of a triangle at the top.  I fancied trying a different type of sail and feel that the high-peaked yard on a cambered sail stresses the sail too much.  This is a personal opinion and is just as likely to be wrong as correct.  I wanted a more HAR than I'd had on Fantail, but without too tall a mast.  I also rather like the look of the flatter top.

  • 21 Jun 2018 05:54
    Reply # 6328420 on 6316327
    Deleted user

    A big thank you to Arne Kverneland for producing a cut and stick sailplan. Wow! So helpful.

    Much appreciated sir!

    Below is the sailplan. The mast is in a good position even without rake.

    I could go for a bigger sail, and I will certainly have a look at the others, but this seems really nice for the boat. An added bonus is that the mast only needs to be 9m long. I can get 6m of 200mm dia 5mm alu tube from Ullrich which is basically the necessary diameter for the masthead (i think) so I can build the base out of wood. If the masthead needs to be thinner I can always go wood, alu, wood I guess.

    Anyone know anyone who needs a 10m bermudan rig? ;) Probably asking in the wrong place...

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
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