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cat1951
Admiral

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USA
636 Posts

Initially Posted - 10/01/2007 : 15:51:05

I have always been physics challenged. Yes it is physics... I have been thinking about this quite a bit and have held off asking till now. I have a question about the 'water' ballasted sailboats and how stable they are when close to upright. It seems to me that the physics of the ballast would be less affective than a lead, or steel, or other type of keel. For example, if you lower a bucket full of water into water, the only weight you have left when it is submerged is the weight of the bucket. (That is if it is not plastic or something else that floats.)

So am I crazy or just don't understand how water ballast works. It just seems like the example above, the ballast would not be very effective the more upright you are sailing. I guess I have too much time on my hands.

Mike
Grand Lake, OK
N.O. Catalina 25 #4849
In my opinion 75% of the earth is water for a reason. That's why I sail.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 10/01/2007 : 16:24:56

OO!! OO!! I know!!

You're not crazy, and this issue has come up before--just not recently. Along the same line, some people have thought that water ballast only tends to right a boat to the extent that the water in the tank rises above the level of the water outside of the hull. In two words: Not true.

Think of a boat with a bottom a couple of feet below the waterline, sitting perfectly upright. Now, imagine that you put a bucket of water on the bottom, close to the chine (outside edge) and completely below the waterline. The boat will tip because the water changes its center of gravity, regardless of whether it's water or where the outside waterline is. The same bucket full of wood would also tip the boat--just somewhat less. Even a seagull feather lying on the bottom has some effect--just an imperceptible one.

When a WB boat heels, the ballast tank (when completely full) is lifted toward the high side, changing the center of gravity in that direction. That creates a downward force, or "righting moment". The shape of the hull causes the low side to have more buoyancy, also contributing to the righting moment. (The high side is pushing down--the low side is pushing up.) You're right that a similar amount of mass on a lever (keel) below the boat will work better, because of the lever arm. That's why WB boats generally carry more ballast than wing or fin keel boats. But water ballast works, regardless of the waterline.

BTW, a partially-full tank can have the reverse effect. The water can migrate to the low side, causing the boat to tip even more. A partially swamped dinghy is a good illustration--try stepping into one and you'll be swimming suddenly.

Does this help?

Edited by - Dave Bristle on 10/01/2007 16:46:43

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 10/01/2007 : 16:32:00

Fill a coke bottle half full of water. Put on the lid. Toss it in the bathtub. Knock it around a little. It is quite stable.

No ballast helps hold the boat upright when it is vertical (it just pulls it down). All ballasts create a restoring force when the boat is leaning. The difference between the center of gravity of the boat and the center of bouyancy is the lever the restoring force acts against. Gravity provides the force. Thus the long, deep keel with a bulb at the end is favored. Water ballast just can NOT have a lever that long, so more weight is needed, and more beam.

Another really important factor is called form stability. This comes from beam and freeboard. Try the above experiment with a flat tupperware instead of a bottle. You will see a tremendous initial stability due to wider beam and high freeboard. Then it will flip and be just as stable upside down (not good).

The question for stability of a boat is, how far can the boat tilt and still come back upright? A fin keel can go to about 130 - 160 degrees. A wing about the same, actually a little less. Weight down deep is good. A catamaran only to about 80. Note a cat is more stable upside down (not a good thing). A waterballast is essentially a centerboarder. They are not as stable as a fin/wing but I have not seen the stability curves on the 250. I have sailed one and they are a little more tender initally and then seem to firm up nicely around 20 degrees of heel. The centerboard bumps around a lot. I would feel better offshore, in 30 knots of wind and 6 foot seas in a fin or wing. But I've seen one reefed, in over 20 knots and rough seas and she was doing OK on a day I turned back.

I've never sailed a swinger and if there is a way to 100% lock the keel down so that if you got a full knockdown it would not swing up, it is potentially even more stable.

Catalina has figured all this out which is why the wing weighs more than the fin, which weighs more than the swinger. 20% to 30% of the weight of the boat should be in the ballast. The 250 WB should be on the high end of that, since it is not deep.

For pointing a long, narrow, deep fin with a bulb is fastest and points higher, reference this photo of the keel of Wind Quest, Maxi Z86.

That bulb weighs 12000 lbs.

Edited by - on

Justin
Admiral

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502 Posts

Response Posted - 10/02/2007 : 07:47:42

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by JimB517
 
Catalina has figured all this out which is why the wing weighs more than the fin, which weighs more than the swinger. 20% to 30% of the weight of the boat should be in the ballast. The 250 WB should be on the high end of that, since it is not deep.
<hr height="1" noshade id="quote"></blockquote id="quote">

Actually, slight correction... My friend Dave has a wing keel C-250 and his keel is lighter than my swing. I just checked out Catalina's website and the wing is 1050 lbs and my swing is 1500 lbs. You can really tell the difference when the wind and waves increase; his C-250 is more tender.

Edited by - on

cat1951
Admiral

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USA
636 Posts

Response Posted - 10/02/2007 : 08:12:22

Thanks guy's, I had not considered the beam and freeboard aspect of the equation. It does make sense though that the keel would be more of a factor on a heel than when upright.

I do appreciate the feedback.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 10/02/2007 : 09:02:42

Edited by - on

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 10/02/2007 : 09:43:42

I stand corrected on the weight of the wing. It is shaped very different, the wing is much longer - more like a full keel and of course has the wings. Since it has the wings and is lead, there must be more weight down low (where it counts). Thus Catalina can get by with less weight overall.

I've noticed no change in stability between the wing and the fin, but the fin points better (slightly).

Less weight could be why the wing seems to do better downwind in light air.

This is the kind of stuff yacht designers worry about, and of course a sailor planning to go offshore.

As John Vigor says in nearly every chapter in his excellent book on this subject "The Seaworthy Offshore Sailboat" ---

Think Inverted!

Meaning plan for the big knockdown.

Indiscipline is prepared to go over past 45 degrees but the interior would be a wreck if we ever got knocked down to 90.

Edited by - on

Dave Bristle
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Djibouti
10005 Posts

Response Posted - 10/02/2007 : 10:26:04

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Justin4192
 Actually, slight correction... My friend Dave has a wing keel C-250 and his keel is lighter than my swing. I just checked out Catalina's website and the wing is 1050 lbs and my swing is 1500 lbs. You can really tell the difference when the wind and waves increase; his C-250 is more tender.
<hr height="1" noshade id="quote"></blockquote id="quote">
Justin: This is apples and oranges... The C-250 is a different hull (more beam), different rig, and a lighter boat overall than the C-25. It requires less ballast to maintain any degree of stability, and was designed to be lighter for trailerability. The overall design is indeed more tender. The keels are just a part of that.

Also, it's not unusual for wing keels to be lighter than fins and swingers, because the fin/swing, especially the fin, has much of its weight up close to the hull, and the wing concentrates much of its weight at the bottom of the keel. Look at a few in boatyards and you can see--there's a lot of lead down there.

Edited by - Dave Bristle on 10/02/2007 10:28:17

Justin
Admiral

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502 Posts

Response Posted - 10/02/2007 : 12:03:57

Oops, my mistake. I didn't realize the C-25 wing is quite a bit heavier than the C-250's. I was thinking the keels were similar even though I know the boats are completely different designs. Your explanation makes sense with the C-250 being lighter/wider and needing less ballast.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 10/02/2007 : 12:28:30

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 Also, it's not unusual for wing keels to be lighter than fins and swingers, because the fin/swing, especially the fin, has much of its weight up close to the hull,...<hr height="1" noshade id="quote"></blockquote id="quote">

Are you sure Dave? If memory serves me correctly, my cast iron fin keel is relatively thin on the upper 1/3, just below the stub, then gets wider in the lower 2/3 of the keel making it a little more bulbous towards the bottom which would seemingly put more weight down low.

Edited by - dlucier on 10/02/2007 12:37:35

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 10/02/2007 : 15:00:49

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 ...If memory serves me correctly, my cast iron fin keel is relatively thin on the upper 1/3, just below the stub, then gets wider in the lower 2/3 of the keel making it a little more bulbous towards the bottom which would seemingly put more weight down low.
<hr height="1" noshade id="quote"></blockquote id="quote">Your O'Day or your Catalina? I don't remember my C-25 lead keel looking fatter at the bottom than the top. I believe it was both longer and a little fatter at the top. The fact that Catalina makes the fin heavier than the wing would suggest the wing has a lower CG even with a shallower draft.

Edited by - Dave Bristle on 10/02/2007 15:06:15

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 10/02/2007 : 18:01:06

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 [quote]Originally posted by dlucier
 Your O'Day or your Catalina?<hr height="1" noshade id="quote"></blockquote id="quote">

O'Day? I've never owned an O'day...I thought we were discussing Catalinas.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 10/02/2007 : 18:04:04

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 [quote]Originally posted by dlucier
 Your O'Day or your Catalina?<hr height="1" noshade id="quote"></blockquote id="quote">O'Day? I've never owned an O'day...I thought we were discussing Catalinas.<hr height="1" noshade id="quote"></blockquote id="quote">Oh... and I thought I was talking to Duane. Oops.

Edited by - on

existentialsailor
Master Marine Consultant

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USA
1180 Posts

Response Posted - 10/02/2007 : 19:15:37

The concept of the wing keel having less pointing ability than a fin is overstated. True in a lake with no current, but easily compensated for in the ocean with current and trim.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 10/02/2007 : 20:33:41

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by existentialsailor
 The concept of the wing keel having less pointing ability than a fin is overstated. True in a lake with no current, but easily compensated for in the ocean with current and trim.<hr height="1" noshade id="quote"></blockquote id="quote">I agree it's overstated for general cruising purposes, but for a racer, pointing is pointing, and is independent of current. On C-25s with equivalent sails and trim, a fin will track higher through the water than a shallower wing, and the wing will run downwind a little faster than the fin. The current will change the net track and speed over the bottom--equally for the two.

Edited by - on

GaryB
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USA
4300 Posts

Response Posted - 10/02/2007 : 22:17:39

Would the difference in sailing techniques between average weekend warriors and the difference in sails between any two boats make up most of the difference in pointing ability?

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 10/03/2007 : 07:13:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by GaryB
 Would the difference in sailing techniques between average weekend warriors and the difference in sails between any two boats make up most of the difference in pointing ability?<hr height="1" noshade id="quote"></blockquote id="quote">Of course. Sails can significantly affect pointing as well as upwind speed. Sailing ability and local knowledge of wind shifts, eddies, and currents could allow a C-25 to beat a J-105 to the finish line! I crewed in a light-air match race where the other boat slipped behind some rocks, caught a current, and did a horizon-job on us. No luck involved (except my luck of being with the wrong skipper).

Edited by - Dave Bristle on 10/03/2007 07:17:30

DaveR
Master Marine Consultant

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USA
2015 Posts

Response Posted - 10/03/2007 : 07:32:32

My wing keel hasn't pointed all that well but then in July I bought a 150 Jenny and it increased my pointability a fair amount. I bet if I replaced my original old stretched out main I'd really be grinning!

Edited by - on

OLarryR
Master Marine Consultant

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USA
3436 Posts

Response Posted - 10/03/2007 : 13:09:17

Dave,

I am having same experience with my sails. I had a blown out 110 and replaced it with same age 150 genoa but that sail was in much better condition - looked like it was used much less frequently. My boat also pointed up to thw wind better.

Right now I am going to get a quote on a new main and a 110 or 120 (if they make it) genoa. (My boat currently has the original sails.)

Edited by - on

Nautiduck
Master Marine Consultant

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USA
3704 Posts

Response Posted - 10/03/2007 : 15:53:44

I have found that if I crank up the Honda I can point directly into the wind.

Edited by - on

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