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...

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

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

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

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.