Motor sailing catamaran rig design

Hi, I have just found this site, very informative and interesting too! Bearing in mind I am not a tecno wizz on calculations etc, I have noticed that in all the discussions of displacement/length, re catamarans, there does not appear to be any mention of the relevance of the weight of the vessel, in regard to the rig construction and configuration. IE, should the rig be designed with the weight of the vessel in mind, or is it only the length of the vessel that matters?
Look forward to some discussion on this topic.

 

One of the accepted factors relating to weight and sail area for multihulls is the Bruce Number, which is the square root of the sail area (sq.ft.) divided by the cube root of the displacement (pounds). According to Chris White in his book "The Cruising Multihull", the range of BN is from about 1.0 to over 2.0, with the higher the number, the higher the "power-to-weight" ratio, i.e. more potential speed.

Eric

 

Thanks for that Eric, but that is relating to sail area, I am more looking ideas on rig design, construction, configuration etc and how that is affected by the boats inertia when sailing into wave faces (in an off shore environment)

 

Can a vessel be under-rigged for its weight?
Will a mast designed for a lighter vessel, be adequate for a heaveir vessel?

 

Depends what you want to do with it. A too-small rig will be a real pain in light air. I like the idea of having a rather large sailplan- you can always reef a big mainsail, but it's hard to put bigger sails on an under-rigged boat when the wind goes down.

Generally no. The key is the boat's righting moment, and thus its power to carry sail. The more righting moment the boat can produce, the stronger its mast and rigging have to be to hold up the sail without breaking. More RM means the boat will stay more upright for the same wind and sail area, thus increasing the force on the sail, thus giving better speed but also putting more stress on the rig. When re-rigging a boat, NAs will look at not only the length and displacement, but how much the boat pushes back when heeled (ie. its RM), to determine how much stress the rig has to stand up to.
Putting the rig of a 50,000 lb boat on a 75,000 lb boat, all other things being equal, will probably cause either the mast or shrouds to fail.

 

Thankyou for your input MarshMatt.

 

Can someone explain to me how 'righting moment' is calculated?
So for a 60' catamaran weighing say 55,000lb, beam of 28', what would the righting moment be and what size size mast and sail area is correct, or within cooee for that righting moment.

 

A relatively crude method to find the righting moment is to:

1.) Take the maximum beam of the boat,
2.) Subtract the width of one hull from that,
3.) Divide the result of 2.) by 2.0, and
4.) multiply the result from 3.) by the displacement of th boat.

I hope this helps.

Bob

 

Why not use the good old fashioned S/D? Then you can compare not only multihulls with each other but with monohulls as well.

Bob

 

You can. Sail Area/Displacement ratios tend to run a little higher for multihulls than for ballasted monohulls, so just be aware of that.

Eric

 

That's the art of sailboat engineering right there! In a cat, you essentially have half the weight of the boat exerting a moment about the leeward hull. The moment arm on such a boat as long as the boat is wide- hence why cats can sail so fast and still remain upright. Essentially, RM is a measure of how hard a boat pushes back when the wind tries to heel it. Once RM is known, along with estimates of sail shape and size, rough estimates of the forces on the mast and shrouds (if it has them) can be found with a bit of basic physics. The tricky part (and this is not my field of expertise, btw) is in first figuring out what assumptions about the situation are valid, and then once the forces are known, figuring out just how strong each link in the system has to be, and thus how big and of what materials.

 

hmmm, thanks for all these brain teasers, I will need to cogitate with a rum in hand and see if I can make sense of it all. I am trying to get into my head the 'moment', I think I understand the 'arm' but honestly, being a bit blonde at the moment. :-/ .
So the 'moment' is the force or resistance the boat exerts, so the heavier the boat, the bigger the moment?
Bob, I did the sums as you explained, came up with a figure, what does that figure represent? (sorry, being very blonde now I suppose)
Really appreciate all your input too, thanks.

 

A moment is the rotational equivalent of a force, so to speak. Picture something (like a boat, maybe) free to rotate about one axis. Apply a force F on the object at a distance D from that axis. The moment created by that force is the product of D with the component of F that is perpendicular to D.
An introductory physics or naval architecture textbook might be of help here.

 

For the non-technical: It's harder to turn a boat over if it's wider, and its harder to turn it over if it's heavier. If it's both, it's harder still to turn it over. A bigger sail has more ability to turn a boat over than a smaller one, and a longer mast has more ability to turn a boat over, and if you have both together, then you have even more force trying to turn a boat over. Unless the mast isn't strong enough, (smaller diameter and smaller wall thickness,) in which case the mast might break before the boat turns over. Or, maybe, if the wires that hold the mast up are too eensy-weensy, then they might go sproinggg-kapowie first, and then the mast may break, or maybe just fall over, if you're lucky.

 

Motorsailing Subject

I assume you have discovered the 'search function' as well??

For instance:
http://boatdesign.net/forums/showthread.php?t=501&highlight=motorsailer

http://boatdesign.net/forums/showpost.php?p=43251&postcount=25

http://boatdesign.net/forums/showthread.php?t=4499