Can a Mono Hulled Sailboat Right Itself

[barnakiel]

Most cruising monos have companionway doors of MM design that will allow loads of water when she gets rolled. Electrics will be gone and some electronics too.

For a boat planning to go where she has some chances of getting rolled a complete rebuild of the companionway/washboards is a good idea.

Hatches and windows/portholes tend to do fine unless they are too large and not designed for their job.

b.

[sparau]

Originally Posted by barnakiel

No. Not in any case. Many come back with they rig still up.

As you said, when the rig is gone the boat is more likely to get rolled over again.

b.

Do you mean because of inability to control the situation, to control the angle of wave to beam?

General knowledge would lead toward thinking that a boat under say a parachute anchor off the bow would be more stable without a rig than with, simply due to weight aloft.

i dont understand the pendulum analogy also if you would care to shed light

I have only experience in little skiffs and it can be quite difficult to get one up with the sails as drogues under the water ! It makes me think if you flipped a keel boat in calmish water you could sink before it righted.

cheers
Shane

[barnakiel]

Originally Posted by sparau

Do you mean because of inability to control the situation, to control the angle of wave to beam?

General knowledge would lead toward thinking that a boat under say a parachute anchor off the bow would be more stable without a rig than with, simply due to weight aloft.

i dont understand the pendulum analogy also if you would care to shed light

I have only experience in little skiffs and it can be quite difficult to get one up with the sails as drogues under the water ! It makes me think if you flipped a keel boat in calmish water you could sink before it righted.

cheers
Shane

There are many factors. Possibly the main one is the fact that a boat without rig is much easier to get rolled over.

Physics. Take a stick, grab it mid-length, add some weights to its end. Now try to turn it as if it were a plane prop. Now move the weights towards the center of the stick, now try to turn it again. When do use more energy to turn it?

What mitigates the roll is a) the mass (the more mass, the more energy required to roll it), b)the distribution of the mass (the further off-center, the more energy required).

Then one can think about the drogue effect of any rig and sails - they will slow and limit the roll acceleration also once submerged.

Both theory and practice dictate that a boat that happens to lose her rig is more likely to get rolled over again than a boat which rolls over without shaking off her rig. (Contrary to what you can read in one of posts above, getting rolled over does not imply the rig is lost, much as it may be lost due to such simple facts as not being designed for extreme weather sailing).

So, avoid losing yours. And if you have the rig that will snap at the spreaders rather than go overboard, DO NOT chop it ALL off and toss it overboard "because it may sink the boat". Secure the broken off part, stick to the remaining part. They may as well save your boat from getting rolled over again (and you may be able to set up a jury and sail to safety once the weather eases).

b.

[sparau]

Originally Posted by barnakiel

There are many factors. Possibly the main one is the fact that a boat without rig is much easier to get rolled over.

Physics. Take a stick, grab it mid-length, add some weights to its end. Now try to turn it as if it were a plane prop. Now move the weights towards the center of the stick, now try to turn it again. When do use more energy to turn it?

That experiment describes losing the mast and the keel, a closer experiment would be to just remove the weight from one end, now how hard is it to turn the the stick like a prop with the single weight hanging downward?

The more weight you add to the other end (mast) the less force required to turn it.

I can see that if you have sail up then that will stabilise rocking motion, but with bare poles sure the extra mass will slow down initial movement, then work to continue the motion.

Shane

[barnakiel]

Originally Posted by sparau

That experiment describes losing the mast and the keel, a closer experiment would be to just remove the weight from one end, now how hard is it to turn the the stick like a prop with the single weight hanging downward?

The more weight you add to the other end (mast) the less force required to turn it.

I can see that if you have sail up then that will stabilise rocking motion, but with bare poles sure the extra mass will slow down initial movement, then work to continue the motion.

Shane

No, Shane. The more mass you add to the other end (the mast) the MORE force required to roll the system (roll the boat). It is about 1) the mass, and the 2) distance of the mass from the center.

Read here : Moment of inertia - Wikipedia, the free encyclopedia

I think this better explains the property involved. But you can take it for granted (or not) that the MORE mass the more moment of inertia hence the more energy required to roll the system. Hence a boat minus her mast = less mass AND shorter radius = hence LESS energy requitred to roll her over.

Also, the matter get actually so much worse because losing the mast lessens both the mass AND the radius of gyration, which is squared in the formula (the basic formula being I= m(ass) x k^2 (radius squared)).

I am not a physicist so this is more difficult for me to explain. But doing a simple experiment takes the load of the grey cells. Just remember when the mast disappears, the center moves too - you get a new center and you calculate your forces from this new center.

Look up Marchaj's book, chapter 'Moment of Inertia' explains this phenomenon in its sailing aspect.

Cheers,
b.

[valhalla360]

Originally Posted by sparau

That experiment describes losing the mast and the keel, a closer experiment would be to just remove the weight from one end, now how hard is it to turn the the stick like a prop with the single weight hanging downward?

The more weight you add to the other end (mast) the less force required to turn it.

I can see that if you have sail up then that will stabilise rocking motion, but with bare poles sure the extra mass will slow down initial movement, then work to continue the motion.

Shane

The difference with eliminating one weight is you have to adjust the center of gravity. If you eliminate the weight on one end, you must hold the broomstick close to the remaining weight in order to hold it at the center of gravity. Do that and you find it very easy to spin relative to the two weights on each end.

On the sail boat the center of gravity doesn't move a great deal but you eliminated the weight that is very far away from the center of gravity and does a great deal to increase the roll moment of inertia.

[boatman61]

Check out Tony Bullimore's account of losing the keel and surviving 4 days upside down in the S Ocean...
Also this account of a Bendi being rolled... again in the S.Ocean...
http://bwsailing.com/cc/2013/01/17/d...outhern-ocean/

[sparau]

Thx gentlemen

It seemed counter intuitive, 150kg (even with a ~5 to 1 average leverage) of mast affecting the 10,000kg boat and keel so much ! It still seems kinda counter intuitive but i found this which explains the simple answer.

(I clearly need to update what i assumed was intuitive physics knowledge )

Cheers Shane

Beam vs. Ballast

A dismasted sailboat is more likely to capsize due to the greatly reduced "Roll Moment of Inertia" and the consequent relative ease with which a heavy roll can be suddenly induced. This cannot be demonstrated by any kind of static analysis as one would normally expect. Said differently, while the dismasted boat obviously has 'more' static stability without its mast, in the ocean where dynamic forces are at work, the effect is the opposite...!

More explicitly, Roll or Mass Moment of Inertia is a way of expressing resistance to being put into motion by a force. This is quite different from static Torque or "Righting Moment" which is calculated as a quantity of mass times its distance from an axis. Instead, Roll Moment of Inertia is calculated as a quantity of mass times the fourth power of its distance from an axis.

It is helpful to perform a few basic calculations... For example, assume we have 2,000 pounds of ballast located 5 feet from the roll axis. This will provide a Righting Moment (Torque) of 10,000 lb-ft and will yield a Roll Moment of Inertia of 5^4 * 10,000 = 3,125,000 lb-ft^4. By contrast, a 200 pound mast located 25 feet from the roll axis will introduce a Heeling Moment of 5,000 lb-ft, but will have a Roll Moment of Inertia of 25^4 * 200 = 78,125,000 lb-ft^4.

In this example, the mast exerts exactly half the static moment (torque) as compared to the ballast, but the mast is some 25 times more effective at resisting being put into motion...!! It is by this simple example that we can observe the extraordinary benefits conferred by well-distributed masses.

[sparau]

btw - I had read how old square riggers and such put weight aloft in storms, until now i had thought it was to help it lean with a bit of sail up which would increase the righting moment of ballast and steady the ship.

Now i see it is minimally that and really roll inertia, its actually making me think of what is a good boat design differently !

Cheers
Shane

[sparau]

Am i misunderstanding or is that example i have linked to above is incorrect?

Isn't it a squared relationship distance to axis rather than 4th as quoted?

from barnies wiki link

The quantity I = mr2 is the moment of inertia of this single mass around the pivot point.

[barnakiel]

Basic formula has r(adius)^2.

Depending on factors, the formulas may look slightly different, as sampled here:

List of moments of inertia - Wikipedia, the free encyclopedia

Yes. It is amazing how often facts of physics act in their mysterious ways. I was there too, many, many times!

Perhaps a good subject for another thread: how what there actually is differs from what we think there is.

b.

[Capt Rottnest]

Originally Posted by sparau

Am i misunderstanding or is that example i have linked to above is incorrect?

Isn't it a squared relationship distance to axis rather than 4th as quoted?

from barnies wiki link

I think you've found the error in the previously posted Kastenmarine link. His maths looks wrong & a bit muddled at that

But the point remains, weight aloft makes a big difference to roll.