natural period(Tn) and yacht motion

Hi Urisvan,

I'm looking forward to Guillermo's response as it will no doubt clear up some confusion, he usually does! But meanwhile, the quote you gave above from “Principles of Yacht Design” has really intrigued me. I have the book, so I looked it up and found that it continued: “In fact, some fatal accidents occurred when people were thrown towards the bulkheads in heavy seas.” That's some accelerations, for sure.

But a problem I'm having is this: accelerations from what source? Exactly how does an increase in "weight" stability increase accelerations all other factors being equal?

I can understand how in the case of an ore ship it would be desirable to shift from too much “weight” stability (by raising the CG) and thus allow more form stability in order to cause the ship to follow the wave more and thereby keep the freeboard above the surface. But I don't think this reasoning applies to a sailboat at all! Marchaj (in Seaworthiness) presents a very interesting figure (Fig. 80, p 148) showing the effects of wave motion on the two types of stability, form and “weight”, comparing the effect on a catamaran and a traditional deep-keel boat of a large wave presented from abeam. The cat, being almost totally dependent on form stability, keeps its pole perpendicular to the wave surface. The traditional boat tends to turn it's pole toward the wave peak because of water circulating below the surface. If the wave is steep enough the cat will flip over and point it's pole toward what is probably the nearest land (down), but the more traditional design will remain upright, and this effect is stronger the deeper the keel and the lower the CG. Where is the downside here? Forgetting “performance” issues for the moment.

I would also think that (and Marchaj confirms) the accelerations experienced by the more traditional design would be much gentler than on the cat, contrary to the implication of the “Principles of Yacht Design” quote above. Marchaj also quotes W. Froude (p 144): “The effort of stability is the lever by which a wave forces a ship into motion --- if a ship were destitute of this stability, no wave that the ocean produces would serve to put her in motion.” Marchaj continues: “When talking about stability, W. Froude had in mind the metacentric height (GM) or inertial hydrostatic stability and this, according to him, is frequently mistakenly given in excess for safety's sake.” But Marchaj seems to be in favor of as much “weight” stability as you can fit into the design. The lower the CG, the better! Both for seaworthiness and for comfort.

So, what have I missed here?

BillyDoc

 

Billy, read Marchaj's a little further downwards:.... "I believe in a low metacentre and a low centre of gravity for a wholesome ship"

Principles of Yacht Design is right: A low CoG jointly with wide beams brought extremely strong and dangereous accelerations to those ships. Accelerations depend on Tn which on its side depends on beam and CoG. The lower the Tn, due to high values of GM (and this can be due to a low CoG), the bigger the accelerations.

urisvan,
I think there is not contradiction, although apparently it is. Both statements may be valid, as the effect of weight depends on the position of CoG but also on the distribution of masses. Combined effect can be an increased or decreased Tn, it depends.

I just want to bring again to your attention the fact that the variation of righting arms for up to 20-25º, let's say, will usually be more dependant on forms than on weights, so accelerations and comfort/discomfort of movements for sailing boats in this range (the more usual) will also be.

Cheers.

 

I and Guillermo seems to agree on most things, also this time, I don’t see a contradiction either.

Almost everything when it comes to yacht design is a compromise, go to far and you will negate the positive effect, or even reverse it. Look at the Seaworthiness thread, 682 posts of discussion and arguments when most of the time it simply comes down to where the best compromise is

I am at work now and don’t have “Seaworthiness…” here, wasn’t the quote “I believe in a low BM and a low centre of gravity for a wholesome ship”?

Mikey

 

hi guilermo, i got you.
there is no possibbility to increase gyradius by adding weight(ballast, or inside ballast) without increasing GM. Depending on the shape of the hull, the effects of changes will be different.
In some boat the increase of the inside ballast may increase the Tn.
But in some boats, increase of ballast, even locating it lower can decrease Tn.
It sounds logical. Because gyradius and GM are also interrelated.
BM=I/disp and GM=BM-BG (B is the center of buoyancy)
Now let’s try to make it more certain. The numbers don’t lie.
How can you decide about a boat, if its Tn value will increase or decrease by adding ballast?
For exampe you are designing a boat. You want to maximize the Tn of it by modifying the shape.Would you make it by trial and error? Or how can you optimize your solution by deciding your parametres defining boat shape and weigth distribution and the intervals of them that they can vary, and the aim is maximization of Tn.
See you.

 

hi
maybe i exagerated the problem!
in a yacht, if you want a big Tn value, make it deeper, add weight as much as possible and add it as bottom as you can and make the vessel as narrow as you can.
right? is it valid for all kind of sail boats?
cheers

 

Try using a single displacement model and modifying your two elements singly.

Reducing beam/increasing draught of hull will lower your center of bouyancy(COB) towards the center of gravity(COG).

Raising COG towards COB is the other way.

To lengthen the roll period for a given displacement, reduce the distance between COB and COG.

Does that help?

 

Hi rayk
This could be an interesting and valuable test. Of course, if it works it will be an experimental proof .
But are you sure that distance between COG and COB is the only parameter that effect the Tn value for a given displacement? And Why?

As you read, i asked a question and quillermo answered 3 days before. I asked that:
“please consider this: there is a boat, which has a bulb keel, that you can lower and rise up. in which case the Tn of the boat will be higher? when you lower the bulb keel or when the bulb is up?”

so you think that the bulb in a place, maybe somewhere in the middle that makes the distance between COB and COG will be the best.
cheers