Hybrid mast making

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  • 16 Oct 2021 14:10
    Reply # 11524126 on 11448992
    Anonymous wrote:

    A bit late in the game for that? I thought you had already made the partners and mast step based on wood masts (hollow).


    Yes, Len, but it's never too late to explore ideas ;-)


    Therefore, you should be able to take the plans size and wood type and work the formula backwards to see what numbers the designer used for wood and then use those numbers with aluminum.


    Yes, reverse engineer! Thanks! Obvious really, but that is indeed the way to proceed :-)



  • 13 Oct 2021 15:30
    Reply # 11448992 on 11381352
    Anonymous wrote:

    I wonder if I might ask a (what may be a quite basic) question of the experts here?

    Experts? I probably don't quite qualify on that count...

    When I do the calculations for mast breaking strength to calculate the diameter and wall thickness of an aluminium tube for a hybrid mast, does the fact that the boat has 2 masts come into play? Do both masts have to be strong enough to cover the Mb x 2.5 (for example), or can I halve the required mast strength as there are 2 of them, or does some other factor come into play?

    A bit late in the game for that? I thought you had already made the partners and mast step based on wood masts (hollow). So I am guessing that the  current partners and steps are based on the plans and plans sail area. Therefore, you should be able to take the plans size and wood type and work the formula backwards to see what numbers the designer used for wood and then use those numbers with aluminum.

    That's all very well and good but the questions still remain. The moment of the hull is what it is. While I would not suggest going with one mast, it may be worth while calculating out one mast with the whole sail area of both sails and the highest AR out of the two (I think they are the same AR actually). This will give you the largest possible mast diameter, one that is too big. If nothing else, this will make schooner rig masts at least seem small by comparison.

    So anyway, back to the question. As above the hull moment "is what it is" and each mast should  probably be calculated as if it was the only mast. The stiffness of the hull that the mast has to deal with on a schooner rigged vessel is going to be much higher than on a vessel using a single sail of the same area/AR as one of yours. While it is true that the second sail will start to  push the hull over at the same time as the first, there by relieving some of the force on  the partner, that does not take into account any resonances or cases where one sail is up and the other is not. Also, any gust may hit one sail before the other.

    Having said all that, I will note that these calculations (that you mentioned above) are about the loads on the mast when the wind is on the beam. They do not take into account the hull moment when running down wind when the hull is much stiffer and yet it still seems to work fine even when mast size is calculated for a vessel with lower beam on moment.

    My understanding is that you are not planning day sails but rather offshore voyaging. This is partly the reason for a two mast rig, it gives redundancy as well as adjustable balance for easy steering auto or manual. I think this also means being conservative with mast sizing.

    I am not sure I have been much help...


  • 13 Oct 2021 10:26
    Reply # 11439453 on 3500760
    Anonymous member (Administrator)

    I only see two-three reasons for using a ketch or schooner rig on a moderate size boat:

    • ·         If the boat has an inferior rudder, when two sails let one steer the boat with the sails.
    • ·         If the boat has low stability for its length (e.g. sharpies) so the 2-stick rig can bring the centre of SA (plus CG) down a little.
    • ·         A third argument may be in case it is easy to find the right mast section for a schooner, but difficult to find or make the bigger sloop mast.

    If Tony’s Benford dory is to be given a long shallow keel with a low ballast ratio and an un-balanced rudder on it, that would be my only argument for the schooner. It is not enough to say that “it looks right on a Benford dory”.

    Our practical experience with sloop rigs up here in Stavanger encourage me to speak for the sloop JR with sails up to 60sqm, at least. One must then be aware of a few things, like not making the sail too broad, and having a good, balanced rudder. The area of a sloop rig may well be a little smaller than on a schooner rig. It will still be faster.

    Have another look at this piece from the Stavanger rally in 2010. Most of the boats were sloops, and I don’t think any of their owners wished they were schooners.

    Arne


  • 13 Oct 2021 08:31
    Reply # 11435720 on 11412459
    Arne Kverneland wrote:

    …Why on earth bother with two masts on that little boat?

    as soon as a boat weighs a couple of tons i see good reasons for a fragmented rig. really small boats may be trimmed by shifting your body ballast. but with a 31.8' benford dory two separate sails may be the easier way.

    …but the sloop rig has advantages as well, as arne mentioned…

    utnik

  • 12 Oct 2021 22:48
    Reply # 11412459 on 3500760
    Anonymous member (Administrator)

    One simple question:
    Why on earth bother with two masts on that little boat? A sloop JR with a 50sqm sail with 20-25% balance in it would be cheaper, faster and easier to handle.

    Arne

  • 12 Oct 2021 17:24
    Reply # 11399516 on 3500760

    hi tony

    Tony Brown wrote:

    …I do , however, get the distinct impression that there are a couple of different formula being used to calculate the mast dimensions. Could that be so?

    you're right. there are different calculation methods around. some of them based on a strange understanding of the forces involved. (even those in the 'jr-bible' from hasler and mcLeod)

    it doesn't really matter how big your sail area is. what you need is a set of spars (steps, partners) capable of a walk of your heeled ship across any seas as turbulent as you are willing so sail on.
    when jumping heavy heeled over crests, the inertia of the rig will play its part in the equation…

    you have to guess (or measure) the righting moment of your ship. there are rules of thumb based on the width and weight of the ship. (you will find a set of calculation tools from jra member oscar fröberg here.)

    decide about your use of the ship. a day sailor may avoid stormy conditions, while an offshore sailor will have to cope with them.

    if i had a strong boat with two masts, i would like to be able to sail free from a lee shore, even with one sail broken and in strong weather. this would need two really strong spars.

    depending on your plans you may not need exactly this strength. i was just writing down what sprouts from my mindset when loaded with questions about the bending forces on a mast…

    utnik

  • 12 Oct 2021 12:32
    Reply # 11386121 on 11381352
    Anonymous wrote:

    I wonder if I might ask a (what may be a quite basic) question of the experts here?

    I'm currently exploring mast ideas for my 31,8ft Benford dory schooner. I've read Arne's excellent chapter on hybrid masts, and also read through this thread and various other writings from people here. What I'm not clear about is this;

    When I do the calculations for mast breaking strength to calculate the diameter and wall thickness of an aluminium tube for a hybrid mast, does the fact that the boat has 2 masts come into play? Do both masts have to be strong enough to cover the Mb x 2.5 (for example), or can I halve the required mast strength as there are 2 of them, or does some other factor come into play?

    The tube dimensions that are coming out of my calculations don't seem to align with the dimensions that I'm reading that people have actually used on their projects.

    I'd be very thankful if somebody could set me straight here


    Frederik has very kindly phoned me to point me very much in the right direction here. A big thank you to him. I think I'm getting there now.... I do , however, get the distinct impression that there are a couple of different formula being used to calculate the mast dimensions. Could that be so?

    Also, in chapter 6b, Arne says "With two or more
    masts, I guess one will have to rethink about the strength factor for each mast".

    This rather leads me back to my original question...

    Last modified: 12 Oct 2021 13:56 | Anonymous member
  • 12 Oct 2021 10:49
    Reply # 11381352 on 3500760

    I wonder if I might ask a (what may be a quite basic) question of the experts here?

    I'm currently exploring mast ideas for my 31,8ft Benford dory schooner. I've read Arne's excellent chapter on hybrid masts, and also read through this thread and various other writings from people here. What I'm not clear about is this;

    When I do the calculations for mast breaking strength to calculate the diameter and wall thickness of an aluminium tube for a hybrid mast, does the fact that the boat has 2 masts come into play? Do both masts have to be strong enough to cover the Mb x 2.5 (for example), or can I halve the required mast strength as there are 2 of them, or does some other factor come into play?

    The tube dimensions that are coming out of my calculations don't seem to align with the dimensions that I'm reading that people have actually used on their projects.

    I'd be very thankful if somebody could set me straight here

  • 19 Nov 2015 19:18
    Reply # 3649648 on 3500760

    The mast was finally stepped yesterday - a half-hour job with a small truck-mounted crane. All went well, though it rained heavily just afterwards, and I had to quickly tape the mast coat in place. 

    Later, the rain stopped, and I was able to pour the polyurethane at the partners and step - that went well, too - and connect up the VHF antenna to my AIS. I just have to connect the all round white light, and the job will be complete.

  • 21 Oct 2015 20:05
    Reply # 3593707 on 3500760
    Pragmatism and economics. It would be relatively hard and expensive to get a tapered alloy pole bought and delivered to Whangarei. It was easy and inexpensive to get a 6 metre tube from Ullrich Aluminium, already anodised and delivered to the boatyard.


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