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Catalina 25 Specific Forum

swing, wing sming?

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collinz88
Deckhand

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USA
2 Posts

Initially Posted - 11/02/2003 : 14:24:22

until reading here today on-line i did not know there was a difference. can you explain the difference between a swing & a wing keel please. my 25 catalina has a wench handle & a new cable to what i call a swing keel. what is better? why>

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 11/02/2003 : 14:45:08

The "wing keel" is a shallow-draft fixed (bolted to the hull) keel. Adding a horizontal 'wing' (essentailly a heavy plate) to the bottom of the keel puts extra mass there... and allows the keel to be shorter than would otherwise be needed to provide the same stability.

Advantage is less maintenance and more 'solid' construction than the swinger. Better for saltwater. Disadvantage, draws about 8" more water than the swing when the swinger is in the up position and is more expensive. Wing keel boats command a higher price than swingers and 'fin' keels.

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/02/2003 : 20:14:35

I think the difference on the C-25 is much less than 8"--more like 2", so the difference for trailering or gunkholing is virtually nil. Also, the wing at the bottom contributes to "lift" when the boat is heeled, which is when a straight keel, especially a shallow one, loses some lift.

Just be careful how you handle that wench... You could end up on Arlyn's site.

Edited by - Dave Bristle on 11/02/2003 20:18:18

Randall
Navigator

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123 Posts

Response Posted - 11/03/2003 : 08:33:10

Can somebody explain "lift" to me, in language that even I can understand?

Randall
79 TR/SK dinette #1459

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 11/03/2003 : 10:03:44

Simplified... 'lift' is a force produced by the movement of the keel and rudder through the water that opposes the efforts of the wind to blow the boat sideways.

This link may help.

http://www.sailnet.com/collections/learningtosail/theory/index.cfm

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 11/03/2003 : 10:17:19

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Randall
 Can somebody explain "lift" to me, in language that even I can understand?
Randall
79 TR/SK dinette #1459
<hr height="1" noshade id="quote"></blockquote id="quote">
Sailboats fly through two fluid mediums, air and water. Foils are the shapes we call lifting surfaces. A lifting surface (an airplane wing) creates a low pressure area on the fat side of the foil. That low pressure area "sucks" the airplane wing up. Rather than call these surfaces suckers we call them lifters. A keel should provide some lift as well, it will suck the boat to windward helping you "point" into the wind with as little leeway (down wind slip) as possible. Really nice racing boats have very nice keel forms (shapes) that lift well. A wing keel has the lifting effect to weather as well as an oportunity to lift verticaly. The vertical lift will come from the cross member or "wing" that is connected to the bottom of the short keel. It attemps to make up for the poor performance of the short keel section that a wing keel will have. These wings and the lift they generate help more to stabalize a boat rather than help it point to weather. Of the three keels on Catalina 25s the wing is the poorest keel for pointing into the wind. If it were not for lift we would not be able to sail into the wind.The biggest lifting foils on our boats are the sails. You are actually sucked into the wind when you are sailing above a beam reach. That is why a full sail shape is powerfull and why you flatten the sail (kill some of the lift) when you are over powered. Sailing is the art of flying through air and water at the same time.

Edited by - Frank Hopper on 11/03/2003 10:20:19

timpky
1st Mate

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USA
41 Posts

Response Posted - 11/03/2003 : 10:39:06

Don't overlook one other advantage of the swing keel. If you sail in waters where you are likely to run aground, being able to crank up the keel and get free can be quite an advantage ... and I speak from first hand experience.

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JoergK
Navigator

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USA
140 Posts

Response Posted - 11/03/2003 : 14:54:19

I can second Tim. The swing keel is great for shallow water cruising. I touched ground several times this season in Barnegat Bay (all sand/mud bottom), a few cranks and I am free. However, the knowledge of having this "assist" may lead to complacency. I suppose that's the case with me, and that is definitely no good.

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Raskal
Navigator

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USA
162 Posts

Response Posted - 11/03/2003 : 17:08:41

Interestingly, the latest Catalina Direct catalog offers a conversion kit to turn your swing into a wing--which may mean that over time swings become less common and wings proliferate.

Rich Kokoska

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Charlie Vick
Captain

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USA
423 Posts

Response Posted - 11/03/2003 : 17:54:11

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by JoergK
 I can second Tim. The swing keel is great for shallow water cruising. I touched ground several times this season in Barnegat Bay (all sand/mud bottom), a few cranks and I am free. However, the knowledge of having this "assist" may lead to complacency. I suppose that's the case with me, and that is definitely no good.

<hr height="1" noshade id="quote"></blockquote id="quote">

I am in touch with that emotion!
I have no doubt that if I had a wing I would have had to replace it by now, if not the whole boat!

CVick
PanaceaII '81 C25 #2439 SRSK
Fort Smith, AR

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 11/03/2003 : 21:12:56

I suppose you can think of the swing as a nice 6 foot long 'curb feeler'...

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 11/03/2003 : 22:01:08

Hey guys, throw me a bone here. Someone tell me I did a good job explaining lift!

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/03/2003 : 22:27:46

Frank: Sorry, but I have to go with Clam's explanation. The keel is a symetric foil that doesn't act like an aircraft wing (which uses an asymetric shape to create a pressure differential). It's function is to translate a diagonal force from the rig into forward motion by making forward motion much easier than sideways (or diagonal) motion. That force translation, in the parlance of sailboat designers, is "lift". Some racing keels are a little freakier than that--using trim tabs similar to the elevators on an aircraft wing, but not ours. The wing at the bottom of a keel basically (1) adds some mass down low without adding draft, and (2) contributes to "lift" as the heel angle increases.

But you've done a great job explaining some other things!

(I wonder if Arlyn is about to jump in on this...

)

Edited by - Dave Bristle on 11/03/2003 22:32:48

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 11/03/2003 : 23:21:40

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 Frank: Sorry, but I have to go with Clam's explanation. The keel is a symetric foil that doesn't act like an aircraft wing (which uses an asymetric shape to create a pressure differential). It's function is to translate a diagonal force from the rig into forward motion by making forward motion much easier than sideways (or diagonal) motion. That force translation, in the parlance of sailboat designers, is "lift". Some racing keels are a little freakier than that--using trim tabs similar to the elevators on an aircraft wing, but not ours. The wing at the bottom of a keel basically (1) adds some mass down low without adding draft, and (2) contributes to "lift" as the heel angle increases.

But you've done a great job explaining some other things!

(I wonder if Arlyn is about to jump in on this...

)
<hr height="1" noshade id="quote"></blockquote id="quote">
Ow Dave that hurt.
The keel is not symetrical in motion.

Lateral resistance is the primary function of a keel/centerboard. Designers were smart enough to recognize a foil shape could add a secondary function to the keel. Lift.

Edited by - on

Randall
Navigator

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123 Posts

Response Posted - 11/04/2003 : 08:16:41

Hey, now I get it! Thanks guys. The part I was missing was "water having to move farther, faster on the upwind side, thus creating lift. The drawing helped. I didn't know about the Sailnet tutorials, either-- I shall peruse them further.

I was visualizing it incorrectly. Try and follow me on this weird line of reasoning:

I remember reading about "prop walk" on a long ago thread on this site. As I remember, the upshot was that water gets denser the deeper you go, so the half of the propeller below the shaft rotates in denser water than the top half of the prop. This causes sterns to "walk" sideways as the prop turns.

From that, I incorrectly deduced that "lift" must mean that when a boat is heeled, the downward facing side of the keel is moving through denser water (deeper water that is compressed further by force of side-slipping). The denser water then creates a righting force (lift) to help reduce heel.

Pretty silly, huh?

Thanks for all your insights, and Frank---
I love you, man, don't you pay that mean old Dave no nevermind.

Randall

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 11/04/2003 : 08:29:58

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Randall
 
Thanks for all your insights, and Frank---
I love you, man, don't you pay that mean old Dave no nevermind.
Randall
<hr height="1" noshade id="quote"></blockquote id="quote">
Now that's a bone!
But... Dave is right the vast majority of the time and a huge resource to this forum.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/04/2003 : 09:57:51

Sorry, Frank... I really am... My head is a little screwed up these days--I'll defer on explaining that for the time being. A good debate tends to take my mind off things, but I certainly don't intend to be "that mean old Dave." And I'll defer to Steve Colgate (although I don't agree with him on several things). Thanks for the diagram.

Randall: I seriously doubt that water density (which is virtually constant as pressure varies) has anything to do with prop walk. What's happening has to do with the effect of the anticavitation plate or, if close enough to the prop, the bottom of the boat. As the prop spins, much of the flow is rearward (driving the boat) but some of it is outward (in a spiral), as from a paddle wheel (except that in this case that is reduced by the pitch of the blades). The anticavitation plate interrupts that outward spiral above the prop and creates turbulence in that area, but there is no counteracting interruption below the prop. Thus, the lower part of the spiral provides more force than the upper part, which is actually pushing on the motor as well as the water around it. So why not put a cylindrical "tunnel" around the prop instead of a flat plate above it? I suspect that it's been tried, and that the risks with obstructions, dents, and such are worse than a little prop walk.

Edited by - Dave Bristle on 11/04/2003 10:05:35

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 11/04/2003 : 10:09:04

In school, we are all taught about the aerodynamic shape of wings generating lift... but angle of attack is pretty important too. (most aircraft won't fly without some angle of attack).

High speed jets have almost no airfoil shape and fly almost exclusively on the angle of attack effect.

My own suspicion is that most of our keel lift is generated by a slight angle of attack. (The slight angle of the keel relative to the motion of the boat through the water). Boats with pure 'slab' keels (like the Potters) still sail fairly well.

More romantically put, "Sailing is a balancing act... one hand in the water and one hand in the sky." (don't know who to attribute the quote to...)

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 11/04/2003 : 10:24:31

WOTAM? Whazzat?

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Charlie Vick
Captain

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USA
423 Posts

Response Posted - 11/04/2003 : 11:38:55

Now I know why there are so many pilots that are sailors!

CVick
PanaceaII '81 C25 #2439 SRSK
Fort Smith, AR

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 11/04/2003 : 12:14:13

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 WOTAM? Whazzat?

<hr height="1" noshade id="quote"></blockquote id="quote">
http://www.wotam.com

http://netforbeginners.about.com/library/101/acronyms/bl-acronyms-w.htm?iam=sherlock_abc

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Steve Milby
Past Commodore

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USA
5902 Posts

Response Posted - 11/04/2003 : 12:47:32

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Randall: I seriously doubt that water density (which is virtually constant as pressure varies) has anything to do with prop walk.<hr height="1" noshade id="quote"></blockquote id="quote">
I've heard the "water density" explanation, but, like Dave, I don't buy it. However, I also don't accept Dave's explanation that it has to do with the effect of the cavitation plate on the flow of water.

The best explanation I have heard is that prop walk is caused by the fact that, on an inboard, the propellor shaft is angled downward. On a properly adjusted outboard, the prop thrust is directed somewhat downward. Because of this angle, and its relationship to the direction of the flow of water, the ascending blade of the propellor has an effectively greater pitch than the decending blade. As a result, the ascending side of the propellor produces more thrust than the descending side.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">So why not put a cylindrical "tunnel" around the prop instead of a flat plate above it? I suspect that it's been tried, and that the risks with obstructions, dents, and such are worse than a little prop walk.<hr height="1" noshade id="quote"></blockquote id="quote">

Actually, prop walk is not a bad thing. It's a very useful principal. You don't want to eliminate it. If you have an inboard boat, with a single, fixed prop, prop walk enables you to maneuver the boat, even when you are moving so slowly that the rudder doesn't have steerageway. Because of prop walk, an inboard sailboat with a single, fixed prop can be rotated almost within in its own length.

Edited by - on

Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 11/04/2003 : 13:37:09

I thought prop walk was a result of water flow from the prop being driven unevenly across the rudder. A boat with a perfectly aligned prop (in relation to the center line of the rudder) would, theoretically, have no prop walk with the helm held at dead center.

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Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 11/04/2003 : 13:40:20

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 WOTAM? Whazzat?<hr height="1" noshade id="quote"></blockquote id="quote">

Bruce - Is that what you've named your C-25? Also, didn't your signature previously say, "currently maintaining two holes in the water"? If so, what happened to the other boat?

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 11/04/2003 : 13:48:47

Although I think the meaning of WOTAM is funny I have to disagree with it! There's nothing better to spend my money on and my time with ..... both are precious but time even more so!

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 11/04/2003 : 14:07:44

I thought prop walk had to do with the direction of rotational torque, similar to what happens to my spinning golf ball when it leaves the tee...takes a ninety degree turn to the right!

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