Greater Lift?
Printed From: Yachts and Yachting Online
Category: Multihulls
Forum Name: Dinghy multihulls
Forum Discription: For those who prefer two (or more) hulls to one!
URL: http://www.yachtsandyachting.com/forum/forum_posts.asp?TID=2225
Printed Date: 13 Aug 25 at 2:41pm
Software Version: Web Wiz Forums 9.665y - http://www.webwizforums.com
Topic: Greater Lift?
Posted By: lostatsea
Subject: Greater Lift?
Date Posted: 19 Sep 06 at 7:00pm
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Being a novice I keep reading that Catamarans with Daggerboards have "higher lift" than those with skegs and centerboards. Is this referring to the fact that they can point higher up wind, or something else?. |
Replies:
Posted By: 49erGBR735HSC
Date Posted: 19 Sep 06 at 10:31pm
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Greater lift will help pointing ability. Daggerboards also have less drag due to the fact that they don't have a centreboard slot on the underside of the boat creating more turbulence behind the board. Getting the bow down on the boat will also greatly help your pointing ability due to the hull generating a component of lift too, ie, the hull will be gripping water in a similar action to your foils. ------------- Dennis Watson 49er GBR735 http://www.helensburghsailingclub.co.uk/ - Helensburgh S.C http://www.noblemarine.co.uk/home.php3?affid=560 - Boat Insurance from Noble Marine |
Posted By: catmandoo
Date Posted: 20 Sep 06 at 3:04pm
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not a lot of grip from modern flat bottom cats bows compared to old style highly veed hulls , but getting stern out is fast ------------- |
Posted By: Port End Flyer
Date Posted: 22 Sep 06 at 9:17am
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I wasn’t sure that lift may be possible in fluids due to them being an almost incompressible medium, lift on a aerofoil is usually (non supersonic) created by having lower pressure above the wing than below as the air has to travel further over the curved upper surface and so speeds up and therefore reduces pressure. In fluids is this possible? An area of cavitation (low or no pressure) could be created but these are usually associated with increased drag, which is not a good thing, so any lift type effect experienced was caused simply by creating a deflection on an angled surface moved through the fluid. But I have since found this which appears to back up the lift principle;-
http://www.boat-links.com/foils.html - http://www.boat-links.com/foils.html  ------------- Close your eyes and "GO FOR IT!" What else do we pay insurance premiums for? |
Posted By: pdwarren
Date Posted: 22 Sep 06 at 9:47am
Just because it's (almost) incompressible doesn't mean that it doesn't change pressure. It just means that the volume doesn't change very much when it does so. Hydrofoils are pretty good proof that you can get lift from a foil in water  Paul |
Posted By: Port End Flyer
Date Posted: 22 Sep 06 at 12:54pm
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Paul Glad you mentioned Hydrofoils this is where I have a problem with the lift thing in water! With danger of hijacking the original thread? Lift, as I understood, was the LOW pressure caused by an aerofoil section as the air has to accelerate over the longer upper surface SUCKING the aircraft into the air (this can even happen with the roofs of buildings). Agreed that there may be some increase in pressure under the section due to the angle the section is presented to the airflow (Angle of Attack, AOA) assisting this, however this is not lift. Does a screw lift itself into a piece of wood as it is rotated? No the flutes of the screw cut at an AOA drawing the screw into the wood, does the propeller of a ship draw itself forward by lift, again no in a similar fashion to the wood screw it draws itself forward due to the AOA of its blades as they cut through the almost incompressible water. If you were to place a flat section horizontally in water with the leading edge angled towards the surface, then the water was driven past it there would be an upwards force, which could be used to lift something like a boat, a hydrofoil? But this upwards force was created in a similar fashion as the screw and propeller. I struggle to see this as lift as used in the aircraft model! ------------- Close your eyes and "GO FOR IT!" What else do we pay insurance premiums for? |
Posted By: JimC
Date Posted: 22 Sep 06 at 1:26pm
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yep. lift works just fine in water. There can and very easily is a pressure differential in water. Check out the phenomenum of cavitation, which is where the pressure drops so low the water actually boils at the ambient temperature.
As an example of how you get pressure in a water, in spite of it being incompressible, get out the garden hose. Turn on the tap, then block the end and feel the pressure. Bow turn the tap off. The pressure stays, you'll feel it, but if you unblock the end no great amount of water spurts out because the water hasn't compressed (what you get is because the hose walls have stretched!). |
Posted By: pdwarren
Date Posted: 22 Sep 06 at 3:13pm
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I don't think it's possible to distinguish between lift from an aerofoil shape and "lift" due to angle of attack. For example, many aerobatic aircraft have completely symmetrical wing sections. In order to get any lift it needs a positive angle attack but this doesn't change the fundamentals of what makes it go up compared to a non-symmetrical aerofoil. Similarly, if you push an angled plank of wood through water (or air for that matter) the thing that makes it go up is the same: more pressure underneath the plank than above it. It'll be high-drag and will stall easily, but I don't think there's a fundamental difference in what causes the lift. Have a look at http://www.av8n.com/how/htm/airfoils.html#sec-thin-wings - http://www.av8n.com/how/htm/airfoils.html#sec-thin-wings (scroll down to 3.10) for some information on using barn doors as wings. Paul |
Posted By: sailwave
Date Posted: 26 Sep 06 at 5:11pm
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There is a long but great thread on how sails work on the BoatDesign forums. http://boatdesign.net/forums/showthread.php?t=457 - http://boatdesign.net/forums/showthread.php?t=457
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