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Old 01-31-2006, 02:01 AM   #1
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Default CAVITATING RUDDER

hi there

can anyone explain what exactly is the meaning of a "cavitating rudder" or when does the rudder tend to "cavitate"

is it because the boat is heeling too much? is it because the rudder is breaking clear of the water (as in while falling off a wave?)

What could be other reasons? why is this phenomenon dangerous? what is the best way to control the boat in such a situation?

cheers

chetan
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Old 01-31-2006, 04:26 PM   #2
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Hi Chetan,

I've never actually heard the term before, but as cavities in liquids are bubbles formed in a liquid when extreme pressure is applied, I can only assume it is a result of the rudder being stalled.

For those new to sailing, one can stall any foil by creating too extreme an angle twix the foil (rudder in this case) and the medium (water in this case) through which it is moving.

Most people appreciate if you stall a plane and the wings cease to work. If you stall a rudder by turning it too far too fast and it also ceases to work.

To correct this and re-establish flow over the rudder, one straightens the helm for a second or two until the flow is re-established and then turn the rudder again.

I had one fast JOG aluminium yacht that had its rudder designed with an aftward inclination to match the keel design. The yacht was also 15 foot wide and 30 foot long, with a very flat bottom.

That boat when reaching in very strong winds would would want to heel over despite 8 crew on the edge, and due to its width plus the aft inclined rudder, we'd find the hydrodynamic pressures continually further moving aft on our rudder.

The loading on the decreasing rudder area underwater used to get to a point where I'd need to continually 'pump' the helm to keep her in a straight line.

Dangerous - er yes.

I think it was our second race season saw the rudder rip off due to this repeated overloading.

Cheers

JOHN
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Old 01-31-2006, 07:23 PM   #3
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hi there John

from you description, this alluminium boat sounds like one of the more fun rides to be found on this forum!!!

thanks a lot for the info

could you enlighten me as to what part of the sail may be described as "roach"? is it the depth or belly of the sail?

tks again

chetan
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Old 01-31-2006, 08:58 PM   #4
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Hi Chetan,

Yo. 'Krakatini' as our Terry Ings designed aluminium flyer was named, was a great boat and we won many many races in her off the Western Australian coast.

But it was extreme racing her and we did have to change our underpants more frequently than I liked.

Re sails and terminology. I am sure another with more knowledge may correct my explanation, but I've always understood 'roach' (in the saling sense) means the amount of curve in the leach of the sail.

A bigger roach ends up providing more sail area to use without increasing foot or luff length, and 'no roach' usually means a straight leach.

The US Hunter range of cruisers which have no backstays do carry added 'roach' in their conventionally reefed mainsails. You can't build too much into a normally set up mainsail / rig, as the leach will then foul the backstay when tacking or gybing.

Er. If you wonder whay some performance yachts like IAC class, are able to carry roach in the top part of the mainsail, you'll see they have sprung devices at the masthead to help lift the backstay away when they tack, plus they have the bodies on board to co-ordinate the manouvre so's the rig does not fall over.

Cheers

JOHN
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Old 01-31-2006, 11:56 PM   #5
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thanks a lot John.... makes sense. I understand now that when you refer to leach curve it is as seen from abeam of the boat and not from behind as i initially thought and hence mhy query on "belly".

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Old 02-01-2006, 07:50 PM   #6
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Hi Chetan,

I'm hoping this is useful but please tell me to shut up if I'm trying to teach my granny to such eggs - or indeed if they are not even interested in eating them!

Your 'belly' is simply called the 'depth' in a sail - and the 'chord' is any line taken from the luff to the leach and parallel with the foot.

So when looking up a sail from below or down from above (hard unless you use a camera) one can view where maximum 'chord depth' is on any chord. It can be measured in both a fore and aft plane and actual chord depth expressed as a percentage.

Changing this sail shape is one of the gears one can use to make a boat go faster / slower and its' easy to use. When acceleration is required say out of a tack or off the start line to get it up to hull speed, one wants the sail set with a forward chord depth of possibly 20% - a bit like a deeply curved plane wing when it is going to take off or land. But once up to hull speed or if you wished to point real high, you'd want the chord depth moved further aft and for the depth to be reduced. Again - just like a plane wing when it is at cruising speed.

All this can be adjusted by backstay, any running backstays, main halyard, cunningham and foot, and usually once the settings are found and proved, they can be marked on halyards and lines to make repeating it easier.

Again - apologies if this info is not needed - but I often like to cruise fast and guess the race experiences help me do this...........

Cheers

JOHN
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Old 02-01-2006, 08:14 PM   #7
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ROACH refers to the sail's “fullness”, deriving from extra material between a straight vertical and the leech. A fuller roach provides extra drive (power) to the sail when running or reaching.

For a simple Sail Terminology & Measurement diagram, Goto the CruisersForum Photo Gallery at:

http://cruisersforum.com/photopost//...php?photo=1621

HTH,

Gord May
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