Mast stay

After reading about free turning unstayed mast of some small boats, I'am quite interested to know wether it is applicable to heavy wooden mast . Actually I would like to have our mast unstayed.This mast will be keel step, made of whole tree trunk, about 30 metres height. I yet to discuss with the boat builder(maybe mid of April I will see him). Do really apreciate if you guys can assist me in this.

 

You should calculate the rightening moment of the boat.
I think the mast should be strong enough to capsize the boat
In some traditions the breaking of the mast was a kind of security...

 

I don't want the boat to sink..and I don't want the mast to break.. the height given earlier is wrong ..it is about 70 feet from the deck..the diameter is about 18-20 inches at the butt.Maybe I don't understand at all why the mast must have stays holding it..to prevent breakage ? I thought it is just to hold it down to the ship and transfer the energy from the wind to propel the boat..

 

It depends also on what kind of sails you have. Modern racing sails are set very tight, so the halyards, the forestay, and the sails create very large compression forces on the mast. A djunk rig, for example, is much "friendlier" for the mast and the boat.

 

Agreed Raggi Thor. Much depends on the rig. The old Scots Zulus - working boats had unstayed masts up to 70-80 foot - but were mainly rigged with sprit-sails.Very forgiving.

 

Stays or no stays

Another thing to think about is the Beam of the boat. Wider boats tend to have more initial stability than narrow ones. That usually means they have a quicker, more violent roll in a wave train. This quick roll can be tough on unstayed masts. Convenietly, though, wider boats usually have much better staying bases than narrow boats because their decks are wider, thus making it easier to do the right thing when you need to do it most

A good boat for unstayed masts is one that has very low initial stability and, because of that, has a much gentler roll period. My 20ft design, for example, has a relatively deep 'V' bottom (23 deg.), sharp ends, and internal ballast. It is also quite narrow (5ft), so it is almost a perfect candidate for an unstayed mast.

Another thing to think about is that an unstayed mast is likely to be considerably heavier than a stayed one made out of the same matterial. This greater weight ends up in the worst possible place: well above the Waterline.
This can raise the Center of Gravity (CG) of the boat alarmingly, taking away from ultimate stability, initial stability, or both. This can force you to accept a much smaller sail plan than you may have hoped for.

My design, for example, was originally supposed to have 180 sf. Due to stability reasons, that got sytematically whittled down until it was down to 140 sf. That was a 22% whack. Ouch!

Bob

 

Sharpii2-Another thing to think about is the Beam of the boat. Wider boats tend to have more initial stability than narrow ones. That usually means they have a quicker, more violent roll in a wave train. This quick roll can be tough on unstayed masts.
Thank you guys, frankly I have no experience with sail boat.. all the while what we have or used to is power boat(racing tunnel hull included). Now we are more interested in something that is quiet and able to take us anywhere with minimum fuel cost .Sailing give us that opportunity. The 1st plan is to have 2 mast cloth sail (schooner/ ketch style), our problem started when we plan to reduce man power for the boat, o.k we can do with different rigging, 1 mast and 2 cloth sail (Bermudian style).Brian Eiland write something about mast aft rig..very interesting..:!: will try it on a smaller boat(20 tonnage wooden cargo boat) so at the moment I'am trying to see if there is a possibility to do mast aft rig with minimum stay or unstayyed mast.Sharpii2 your explanation on the roll effect to the mast are fully understood.Thank you.

 

Scots Zulus

Agreed Raggi Thor. Much depends on the rig. The old Scots Zulus - working boats had unstayed masts up to 70-80 foot - but were mainly rigged with sprit-sails.Very forgiving
Bergalia, do you meant the Scots Zulus use jibs only? How big is those boats?
Do they sail good ?

 

The roll inertia of a heavy mast could cancel out some of the stiffer roll of a wider hull

 

Sprit-sails or spriseil in Norwegian is a main sail with a sprit (simmilar to a gaff) going from the mast at the fott of the sail up to the back corner. It is often boom-less. It's still used on small boats here, because it's very quick to roll around the mast when you prefer rowing.

 

Spriseil

Typical spriseil with a fokk (jib).

 

Ari,

You can get a whole lot of information on free-standing rigs at my website, www.sponbergyachtdesign.com, and go to the Free-standing rigs section.

Generally, on sailboats, wires are used to hold the mast up and to keep it straight. This loads the mast in pure compression, and, when supported by multiple sets of spreaders, makes the mast relatively small in cross-section. It is also relatively light in weight.

On a free-standing mast, the mast is loaded in bending. The mast cross-section must be big enough for strength and stiffness, and of course, it is not kept straight but is allowed to bend.

All masts, whether stayed or free-standing, are designed to the maximum righting moment of the boat. In a stayed rig, the factor of safety of load, or of stress in the mast, can be anywhere from 1.1 for an America's Cup boat, to 3 for a very conservative cruising boat. On a free-standing rig with carbon fiber masts, the factor of safety is 3. On occasion I have gone as low as 2.5, and I had one client who wanted to go to 5. But generally, FoS of 3 is sufficient and there is no need to go higher.

In general, I have found that for boats less than 40' LOA, a stayed rig is lighter and cheaper than a free-standing rig. But over 40', a free-standing rig is lighter and cheaper. It does not have all the weight and cost of the standing rigging. Sails should be single ply on a sail track. Rotating masts are much better aerodynamically (more powerful) than non-rotating masts.

Hope that helps.

Eric


Free-standing rigs are made possible with the coming of carbon fiber, which is very strong, stiff, and lightweight. A wood free-standing mast of the size you suggest could possibly be done, but it would have to be such a huge cross-section that it would be way too heavy for the boat. A whole tree-trunk, as you suggest, would be about the worst possible form of construction that you could use. A glued-up hollow section would be better, but even that would be way too heavy.

 

The mast..the knowledges..

Eric, been trying to get to your site..don't know what happened..it hang every time I had tried..Thank you for your opinion .The reason I joint this forum is to looks ouside of the box..to have views from different persfective..:!: It is a lot easier to just follow the proven way of doing things but from joining this thread I had came across lots of ways to overcome limitations and problems..in fact some of them are the accepted practices for the other part of the world.. The mast in discussion is for my new wooden boat,built 100 percent from timber, about LOD100 - 120 feet.Started thinking about how to built that mast..I will show the mast sample later..when it is done..might take about a month.

 

Manage to get to Eric Sponberg web site.Very interesting article and example.Thank you Eric. I do really want to have the mast unstayed.