Twisted physics

J

jonjo

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Much as I enjoyed reading our Ken's humorous interpretations of yacht brochure speak in the latest YM, I think he has got his physics wrong in his quest to prove that the Twister is the ultimate sailing yacht :-)

Ken, what is wrong with a bit of weight in the pointy bits?

Think of it this way, imagine an ultra high-tech yacht with 90% of its displacement concentrated at a single point near the top of the keel. Such a yacht would have an awful motion as it would hobby horse over every minor wave.

By way of confirming this theory it is well know that yachts have an uncomfortable motion at sea after the rig has come down.

So there you have it, conclusive proof that high rotational inertia is good for you and that a modern anchor locker right up in the bow = comfort!

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The same is the case for in mast furling. Although the Cof G is raised, the moment of inertia is increased and this reduces pitching but much more important it reduces the tendancy to roll suddenly with a breaking wave in extreme weather.

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!!!! But in order to increase the longitudinal and transverse mass moments of inertia, most of us actually prefer to put the weight in the keel, not up the mast. Perhaps if what you say is useful then we have had it wrong all these years by not putting the ballast on top of the mast.

John

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I didn’t see the original article that started this thread, but...

As I understand it, adding weight high up raises your C of G and therefore reduces static stability. But it also increases the roll moment of inertia, which turns out to be helpful in terms of resistance to a knock-down.

Adding weight forward or aft increases the pitch moment of inertia, and actually tends to increase the amount of pitch (with respect to the surface of the water). In other words, the boat will tend to cut through the waves rather than sail over them. This is a much more comfortable motion, but takes energy out of the boat, and slows you down.

So there you have it – reduce weight for and aft and enjoy a fast, bumpy ride. Add weight and you go a little more slowly, but avoid spilling the soup...

Ruthie

PS Hello – I’m a complete newbie around here...


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Erm . my thoughts exactly ... and if you're hit on the beam by a big breaking wave a bit of extra weight anywhere makes no difference ... the only factor of concern is whether you come up again or remain inverted ... weight aloft here would certainly be counterproductive ..

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But suppose that the weight up aloft were something like a Blipper radar reflector, acting like a buoyancy chamber? Surely that should help recovery?

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Quote "By way of confirming this theory it is well know that yachts have an uncomfortable motion at sea after the rig has come down." /Unquote.

Well, true, but... the reason that the mast is there is not just to increase the roll period. It's mainly to hang things on.

Whilst increasing polar moment of inertia will increase roll and pitch period, it also ensures that once pitch or roll is initiated, it will carry on for longer.

Late and great Uffa Fox said that the only place to add weight was a steamroller.

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Re: a horse in his bedroom

> Late and great Uffa Fox said

Well he kept a horse in his bedroom.

Seriously though, traditional designs with elegant raked bows like a Twister have no option but to drop anchor chain into a locker positioned way back from the bow.

By contrast my Bavaria Match 35 has a near plumb straight bow with a deep and generous anchor locker that starts about 6 inches back from the stemhead. Performance aside, I think the weight of the anchor here will give the yacht a smoother motion in a chop.

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'Fraid not, Jimi - a bit of extra weight aloft reduces how quickly you will roll and may prevent capsize.
It is the knobkerry principle !
Ken

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Oh come on .. if your hit beam on by a breaking wave whether or not you've got a bit of extra weight aloft will make sod all difference as to whether you roll or not

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Oh my goodness, we actually do have people who think ballasting the mast of a yacht is a good idea /forums/images/icons/frown.gif.

John

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"ballasting the mast of a yacht is a good idea".
I am afraid that it is a bit early in the morning to think of a silly enough riposte !
Ken

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Ok then .. next time in heavy seas I'll haull the anchor chain to the mast head with a spare halyard to stabilise the boat .. erm .. and I'll attach a sandbag to the top of my laser mast to stop my frequent capsizes /forums/images/icons/smile.gif

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Given that the Fastnet 79 report is a largely forgotten historical document, and that it only really applied to racing yachts anyway.....

Go ahead and add a bit more mass up there. The average modern cruiser only seems to need a maximum (dry ship) 30% ballast ratio, so even if you've already added a vertical roller on the main, radar and a nice heavy masthead radar reflector, a litttle more won't do any harm. Enjoy your 'inertial knockdown preventer'.

Heaven help you when you find some real bad weather.

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Who me? /forums/images/icons/blush.gif

I am pretty certain you are referring to someone else boatless, I am the last person to want to add weight up my mast for no good reason.

"Fastnet, Force 10" by John Rousmaniere is certainly well worth a read and I see it still in the bookshops from time to time.

Regards

John

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John, sorry, clicked the reply in your reply, as it were. Was aimed at all poor souls who still think that Colin Archer was the greatest designer ever, and that Spray would still hold her own in a Volvo RTW.

ps. Shouldn't you be in bed?
<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by boatless on 15/08/2004 09:33 (server time).</FONT></P>
 
Whew, I was getting worried Boatless.

Just coming up to 9pm Sunday here so a while to go until bedtime. Was a very wet and windy weekend in the marina - the sun came out and the wind dropped just as we left around 5 pm!!!

John

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