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Author Topic: Why is a yacht stern that shape?  (Read 3377 times)

andrewa

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Why is a yacht stern that shape?
« on: February 19, 2010, 10:38:45 AM »

Hi All,

As a newby here this is probably a very obvious question but Im trying to understand the fundamentals of yatching as well as the model building aspects as well.

All yachts have a nice pointy, knife edge bow. I understand this, it makes sense.

Full size yachts have wide stern with a cockpit, wheel and controls so the hull tapers up almost parallel to the waterline to accomodate this space for people. I understand this too.

Now, if you look at most model yacht racing designs they still have the same basic shape with an almost flat bottom at the stern rising gently to the waterline or just above. Why would this be? Surely a knife edge stern would be at least as hydrodynamic? Is there a good reason for this shaping or is it just "How boats are"?

Cheers,
Andy
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malcolmfrary

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Re: Why is a yacht stern that shape?
« Reply #1 on: February 19, 2010, 12:02:35 PM »

As I understand it, its to give the smoothest possible exit.  At the sharp end, you dont want to waste energy piling water up suddenly, equally you also want to avoid creating extra drag at the stern by generating eddys.
Another consideration with models is that mostly they heel over.  A hull roughly triangular in plan view, when heeled, is a longer hull, and this is considered beneficial. 
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andrewa

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Re: Why is a yacht stern that shape?
« Reply #2 on: February 19, 2010, 03:47:59 PM »

Thnaks for the additional info Malcom.

Once I started including "eddies" in my googles I managed to find Ivor Bittle's excellent website which contains masses of technical information about model yacht design.

From reading that site it looks more to do with decreasing the "wetted area" of the hull as the minimum wetted area occurs when the hull is a semi-circle shape.

The ideal appears to be a knife edge at the bow to reduce the generation of bow waves to a minimum and then a smooth transition to a semi-circular hull asap which then flows right through to the stern and out of the water to keep the wetted area to a minimum.

If anybody else is interested in this stuff then take a look at http://www.ivorbittle.co.uk/

Cheers,
Andy
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tobyker

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Re: Why is a yacht stern that shape?
« Reply #3 on: February 19, 2010, 08:22:01 PM »

Cod head and mackerel tail. Most of the water displaced at the bow goes underneath the boat - not as you might think to the side. In a racing kayak-possibly one of the most "streamlined" hulls available-you can tell by the feel of the boat when the water is less than four feet deep.
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Jimmy James

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Re: Why is a yacht stern that shape?
« Reply #4 on: March 03, 2010, 08:54:16 PM »

Large sailing vessels usually have a fairly broad stern above the waterline but are very fine below the waterline... As most vessels controls (Wheel) are aft so the tiller lines were kept reasonably short and, to give the helmsman a good view of the sails, the stern was broadened out to provide more room and also very importantly the broader quarters gave the vessel lift in a following sea and helped to prevent the vessel getting pooped... But it is all a trade off because if the vessel is too fine forward she will dig her bows in and lift her stern and rudder part way out of the water and become very hard to steer (called Gripping) or even slue around beam on to the sea (Broaching)
  The ideal situation (for most vessels) is a clean entry at the waterline with the bow flaring out at the shoulders to give lift when needed, and a clean exit at the waterline aft with fairly broad quarters for lift and to provide room for accommodation  :-))     Most Yachts also follow this basic rule though some Raceing boats can be a bit extream...
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malcolmfrary

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Re: Why is a yacht stern that shape?
« Reply #5 on: March 03, 2010, 10:02:47 PM »

Quote
Most of the water displaced at the bow goes underneath the boat - not as you might think to the side
Not too sure about that - I was always told that water was incompressible, to go under a boat implies compression, the bow wave suggests water being displaced sideways, and trying to climb over that which is already there.
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dreadnought72

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Re: Why is a yacht stern that shape?
« Reply #6 on: March 03, 2010, 10:38:26 PM »

That said, Malcolm, you also get dinghies which try to climb their bow waves until they (if fast enough) plane - i.e. become non-displacement craft - which suggest they are riding on their bow waves. I can see a well-paddled kayak doing that "uphill" thing, if not actually planing.

Andy
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malcolmfrary

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Re: Why is a yacht stern that shape?
« Reply #7 on: March 04, 2010, 05:14:31 PM »

So, the hull has to push the water away sideways, but while moving forward, tries to climb the wave thus formed, and if it succeeds, becomes a planing hull and doesn't have to shove as much aside?  Obviously much depends on the shape of the hull and the way the power to move it is applied, whether it be motor or sail.
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tobyker

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Re: Why is a yacht stern that shape?
« Reply #8 on: March 04, 2010, 11:11:32 PM »

Indeed it does. I find the design of displacement sailing hulls with rudders no deeper than the keel interesting, as they need hollows in the buttocks to accelerate the water past the rudder and give some steering effect at quite low speeds. A pity that the designers of men o' war never cottoned on to tip  fences on rudders.
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Jimmy James

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Re: Why is a yacht stern that shape?
« Reply #9 on: March 07, 2010, 08:15:38 PM »

Tobyker
Re tip fences--- it's been tried and it works... the down side is its venerable to damage at high speeds and its noisy :-))
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