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Old 03-04-2010, 09:46   #46
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Interesting you lump daggerboards and centerboards together. They are really not the same thing at all, are they. Its also interesting to see monohull owners talking about catamarans. Kind of like getting your Chevy review written by a Ford fanatic....

.
That I own a keel monohull now does not mean the experiences or lessons of having owned and adjustable board multi-hull are lost to me. If anything having owned both provides a better comparison. Why do you assume I'm a monohull fanatic? I'd buy a Leopard 38 cat in a second if I had the income to make that practical.

Also, many of the issues of putting a board in a trunk below the water line have little to do with how many hulls are attached to each other.
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Old 03-04-2010, 09:56   #47
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I disagree. I owned a 69 Chevy. Doesn't make me qualified to talk about today's Chevies. If I was driving a 2008 or 2009 Chevy, then that would be a different story.

But whatever.
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Old 03-04-2010, 10:07   #48
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I disagree. I owned a 69 Chevy. Doesn't make me qualified to talk about today's Chevies. If I was driving a 2008 or 2009 Chevy, then that would be a different story.

But whatever.
I made no reference to cars or current model anythings. I was talking about the pros and cons of a certain system that I had experienced. I am very qualified to talk about my views regarding a system I have owned and used which is all I did. That system and my experience with it was a large factor in both my decision to purchase and sell that boat. The OP is free to judge for himself whether or not the context of my experience is useful to him or not.
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Old 03-04-2010, 10:22   #49
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I don't get the concept that an underwater appendage generates "lift" to windward. Water is not air. I believe lift produced by laminar flow over a sail is a product of pressure differential between the "thinned" fast moving air to leeward (low pressure) and the "compressed" slower moving air to windward (high pressure). Water is not compressible and therefore resists any disturbance.
Where's Bernouilli when you need him. I'm no expert but as I understand it Bernoulli's principle is valid for incompresible liquid flows and also for compressive gasious flows moving under Mach, or thereabouts. For sailboats, air acts like it is incompressible (unless it is contained). So "lift" applies as much in water as it does in air.

Think of it this way...If the air compressed, then your boat wouldn't move.

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Old 03-04-2010, 10:43   #50
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Just as additional info. The MC 41 does NOT have any minikeels, just daggerboards. The only disadvantage is that the rudders and saildrives are the lowest hanging appendages and beaching is not really practical. That's okay by me but I suppose it could be a limitation 'out there' someplace. On a real 'worst case' scenario, I'd pre-position some sandbags so that my stern was somewhat elevated when drying out should I need to do so for some emergency repair or something.

One or two boards? I usually use only one board at a time going to weather. It seems to work best. In shoal waters, I use both boards partially extended. This also provides some semblance of protection for the aforementioned saildrives and rudders.

With regard to simplicity. Since I can sail with only one board, I essentially have a spare. Also, it only takes 5 minutes to pull the board entirely out of the slot for inspection/repair etc if needed.

Ultimately, I'll repeat my assertion that unless one is race oriented, buy the boat you like for other reasons and you'll find advantages and disadvantages to whichever method of leeway control your boat has.
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Old 03-04-2010, 12:20   #51
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I owned an aluminum canoe one time. It was a monohull. Hated it. ALuminum monohulls all exhibit these characteristics.

Don't you dare try to tell me I don't know monohulls! I had one. Grumman. Aluminum.
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Old 03-04-2010, 14:26   #52
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Boards stall, leaving a daggerboard equipped boat free to slip sideways.

Minikeels continue to offer "barn-door" lift even when the boat has little forward motion.

This distinction is quite obvious. A minikeel sailor at the helm of a daggerboard boat must learn to keep the boards flying, or he will slide down wind to an embarrasing degree.
This hasn't been my experience. The daggerboard has offered resistance to sideways movement at extremely low speeds. The difference in docking the boat in a crosswind, board up vs board down is amazing.

Even at less than 1 knot boatspeed. Either it didn't stall, or it continued to offer plenty of "barn door" resistance. (Which would make sense, given that the area of the board is at least as big as a mini keel.)
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Old 04-04-2010, 04:13   #53
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Hey, Dave, Thanks for the thoughtful response. I'm no expert myself, certainly not in physics or hydrodynamics. I'm still puzzled by the issue of lift generated by immersed foils, especially asymmetric ones. I'm not sure this is an issue which even Bernoulli could have resolved in my aging mind.
I am pretty well convinced that the lift vector produced by a well shaped sail is a result of a pressure differential between the windward and leeward sides, mostly ala Bernoulli.
The fully immersed sailboat fin operates in an incompressible liquid in a totally open channel (even my cat hulls - separated by 28 feet - don't really close the channel between them). Subsurface wave making, rooted in form and skin resistance, and significantly disturbing the water, must be a major factor, maybe even more than Bernoulli.
When we were designing the boards for our cat (two hulls would allow one of two asymmetrical fins to be deployed in the proper orientation for either tack), I played with this, hand holding either of two high aspect ratio test fins, one symmetrical and one asymmetrical, along the side of a motorboat (ugh!) at about 20 knots. What I felt subjectively, but did not measure objectively, was about 90% resistance to forward motion, much greater for the asymmetric fin, and less than 10% (if any) lift from either fin (maybe a bit from the asymmetrical fin - hard to assess in face of the much greater resistance to forward motion). Any deviation from a strictly fore and aft orientation resulted in irresistable resistance to forward motion wrenching the fin right out of my old man's grip.
On the way home at 55mph in my pickup truck, the same experiment repeated horizontally out the window showed zero lift and not too much resistance to forward motion for the symmetrical fin, but beaucoup lift (albeit with increased resistance to forward motion) for the asymmetric fin. Changing the angle of attack to even +/- 15-20 degrees did not prevent me from maintaining my grip on either one.
What does this all mean? I am sure I don't fully know.
We settled on symmetrical fins. I tend to feel vindicated as I watch the (mostly French) ultrafast racing cats do over 40 knots to windward, invariably with symmetrical fins.
I am sure I am not adding any clarity, likely only clouding the issue, an issue probably only of interest to another speed freak like me, cruising along (just not sedately). Maybe this discussion beongs in the unconventional cruisers subforum, where another interesting aspect of performance sailing could be added, the whole issue of wingtip vortex (which surely is replicated by immersed fins, and was extensively addressed by naval architect, Henry Scheel, some 50 years ago - whatever happened to the Scheel keel?).
Anyway, thanks, Dave, for sending me back to the Physics I last studied almost 50 years ago. Your comments were well taken.
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Old 04-04-2010, 12:06   #54
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A lot here could be commented, but mostly Iíd then just immerse into too theoretical and detailed issues, making it maybe off topic. For the specially interested, there are several good books. My favorite is the old classic ďbibleĒ: C. A. Marchaj ďAero- Hydrodynamics of sailingĒ. A big book with huge amounts of theory, formulae, graphs, research and a good amount of straight forward good explanations to complex phenomenons. But this book is only for nerds. No point in reading it if you just want to know sailing. For high level racing though, itís useful. Itís easier to trim all perfect when you know WHY a change works. Also at that level, the boat will be developed continuously to stay at the edge. That needs more than guesswork. But I have to admit, I got that book because it triggered my curiosity. It is actually quite interesting.

@ Dwightsusan.
I just have to comment a little bit on your thoughts, but will not go into explaining too much, from the above reasonsÖ Itís obvious that you know sailing, and that you have given the theoretical side of it a good thought. Still some of your impressions are a bit wrong: You are 100% right about the lift of the sails being caused by a pressure difference between the sides. But pressure in this case is not related to compression of the medium. For the pressure levels relevant to sailing, both water and air can be considered incompressible, even though they actually are compressible. Water too.

The pressure in this case is related to inertia of mass: The unwillingness of weight to be accelerated or decelerated. (Please excuse my probably incorrect language.) The principles of dynamics in water are practically the same as in air. But of course water is so much denser (heavier) than air so a much smaller area and smaller angles are required to generate the same lift. Keels do create lift by altering the direction of a flow. Just like sails. The most effective way to do this (with some not too useful exceptions) is a laminar or smooth flow. The less turbulence/swirls, the better.

When a foil (sail, keel or wing) sits in a flow and is rotated at an angle, the pressure will be different on each side, with the low pressure side creating the most lift. An 80% - 20% relationship has been said to be normal. Thus, on a plane the top of the wings is the important side, as it does not mainly float on the pressure under its wings but rather hangs by the suction on their top sides. Equally the leeward side of the sails and the windward side of the keels are the important ones. The appropriate angle to the flow around sails is of course trimmed by the sheets etc. The angle of a keel to the water flow normally comes from leeway created by the sideways forces from the sails. Some keels can also be trimmed, and some are set at a fixed angle (on racing tris).

An asymmetrical foil (keel) will give quite much more lift at a lower drag penalty than a symmetrical one, and it will tolerate larger angles of attack without stalling, and it will regain laminar flow much more easily after a stall out. This is why all planes have asymmetrical wing profiles. Most ultimate racing multis do too. For most cruisers they are not suitable though, since they are much more expensive to make (exact shape is more critical) and the windward one should always be pulled up since it will create a lift even at zero degrees angle, which is detrimental when the lift side faces to leeward. It will also never just slice through the water like a zero angle symmetrical foil can, since it will ALWAYS generate a lift, and thus get the drag. But of course a symmetrical foil at zero degrees angle is no point having. It makes zero lift too. A fine profile symmetrical keel will work nicely at good speed, but will stall out more easily than a similar fatter one. This is mostly noticeable just after a tack or in low wind conditions when angles of attack tend to be larger.

Tip vortex is indeed an important point, but very hard to address properly. Mostly one tries to minimize it by means of the plan form. Elliptical pressure distribution being the theoretical ideal. Square head main sails are related to this. One tries to not make the max pressure area extend to the end of the foils. This is also one of the reasons for tilting the rig to windward. On keels, several designs use various fencing methods. A bulb on a mono also serves this function. This way the shorter keel will have a lift comparable to a longer unfenced keel. The latest of the now dormant Formula 60 trimaran class used asymmetric banana shaped keels with winglets at the end in the amas. Very efficient. Larry Ellisons BMW Oracle 120 foot monster tri that won the recent Americas Cup utilizing among other things a semi rigid wing more than twice the length of a Boeing 747 Jumbo Jet wing , and Groupama 3, that recently set the new record around the world, 48 days averaging almost 25 knots, both have copied that.

@ 44cruisingcat.
Sandy is right. Fixed keel boats do have a better grip on the water due to larger total area and barn door effect when speed is very low. But your daggers will definitely help you massively when maneuvering by engine at close to zero speed. The daggers will stall out a bit, but since side forces are negligible compared to when the sails are taut, the stalls quickly disappear and sufficient lift is generated at quite low speeds.

This got to be a lot of nerdyness too, even though Iíve actually deleted quite a bit of what I wrote. I hope itís understandable and somewhat interesting.
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Old 04-04-2010, 15:01   #55
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@ 44cruisingcat.
Sandy is right. Fixed keel boats do have a better grip on the water due to larger total area.....
Really?

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Old 05-04-2010, 02:58   #56
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I'm with 44'cruisingcat. His board looks very much like my two (8' below bottom of hull / 30" cord). Fully extended, that's a lot of lateral plane. I doubt if many 45' cats with mini keels deploy much more than my potential 40 sq. ft.
I am, however, wrong about some of the latest racing multi's (and some innovative beachcats and windsurfers), which do, as pointed out by Stein, have asymmetrical fins.
The more I Google, the more confused I get.
For sail / air considerations, Boyle's Law tells me that any pressure differential across a sail will result in relative decrease in volume ("compression?") of the air flowing on the higher pressure side. But in my simple mind, there isn't too much confusion about laminar airflow around a properly trimmed sail.
Immersed vertical fins are another matter. I can't see that Bernoulli is the only consideration. Wave making, Froude number, form and skin resistance, Hagen–Poiseuille, turbulence, Reynold's Number, Coanda, ooh, how my head spins.
I can see little reason to doubt the performance advantages of a deep, retractable, high aspect ratio fin (symmetrical or asymmetrical). Laying up a scientifically designed asymmetrical fin might exceed my abilities in fiberglass over foam - symmetrical taxed me plenty enough.
I have been considering tip vortex, and maybe adding some very small variation of the Scheel keel to the tip of my boards, but so far my estimate of the benefits does not exceed my approximation of the trouble.
For all of us out there sailing our multi's (and mono's, and any other wind propelled vessel operating in the sea - air interface), science be damned, I know for a fact that the worst day sailing offshore is far better than the best day in a chair on the beach.
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Old 05-04-2010, 05:39   #57
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My thoughts on the keels versus boards.

Really depends on what sort of sailing you like to do.

mini keels are the easiest solution in that you dont have to worry about how much board is down, and you dont lose space inside for the board slots. You can dry out without concern especially if you have a sacrificial strip on the bottom of the keels. It also provides a nice stable platform. The minikeels provide great directional stability. They can also provide space for tanks or other stowage.

Boards provide a better performance solution, with slightly better windward performance, and better downwind (less wetted surface). Drying out means that you have worries about stones getting in the slot and jamming. However, they do ensure a smaller draft.

Personally for cruising it is a no brainer - minikeels, but if you are performance orientated it is a harder decision.
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Old 07-04-2010, 02:12   #58
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A parting shot on this issue, and another reason why I love my retractable daggers.
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Old 23-04-2010, 06:11   #59
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I think we're all pretty agreed on this topic, and I don't think there's much important you're confused about Dwightsusan. :-) Even though the different laws and theories do apply, in this context many of them can be disregarded since the effect they have on actual performance is negligible. So the remaining is actually understandable without really knowing much of the theory explaining why. I actually see the very basic laws of physics as the most useful here. The ones about mass related to static and dynamic energy.

To express it relatively non scientific: The sails of course forces a flow of air to alter its direction of movement. To alter a that volume of air to that angle and distance, takes an exactly opposite force. As you say, it's quite easy to grasp in air, as the wind is the flow being altered. The water, for all useful purposes in this context, is a non moving fluid. How can its direction be altered when it's not moving?

Well, there is still a flow. The wind makes the boat move on the water surface. Measured from the boat, the water is flowing past. If it's still water measured from a moving vessel, or a river measured from an anchored vessel, is impossible to know from that measurement. So the wind makes the boat move, which "creates a water flow", the direction of which can be altered to create a lift of equal strength as the one from the sails, but the opposite direction. Voila: The equilibrium that makes the magic of sailing possible.

@44cruisingcat. That's one big dagger. Quite a bit wider than normal for racing daggers, which is useful for several cruising related topics without loosing much of the advantages of daggers. It will of course give you a considerable "grip" on the water even when stalled out. But much of that area isn't relevant as it will be inside the hull. I'd guess at least one third of it. Also, as mentioned, mini keel boats tend to have a different hull shape all over, presenting more lateral area than a similar dagger equipped boat. In total, a "minikeel" boat normally has way too little lateral area and efficiency in light and medium winds but may work almost as well as daggers in stronger winds. When stalled out the combination of normally more area total, and a deeper hull shape, will give it a better "grip" on the water. Still this difference is not significant when it comes to maneuvering in tight quarters etc. Daggers all down normally gives plenty of "grip".

Also as mentioned: Mini keel boats are good choices for many cruisers, but I will never consider anything in that direction. I want a boat that performs better in many ways than a mini keel boat can do. And I don't mind that it means I have to be more attentive. After all, I sail because I love to do just that. I want to be attentive. :-)
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Old 23-04-2010, 06:36   #60
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I own and sell mini keel cats. so I have a reason to push them. They have many advantages and positive aspects.

BUT A good dagger board boat will kill me to windward every time (everything else being equal). If my boat was made with daggers as an option, I would have gone that way.
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