Probably alot...
But, if you grounded that board, at any significant speed, you might be testing your floatation.
I'm quite happy with the knowledge that my f/g dagger board will break before the hull...
Of course, another important question is how do you plan to raise that board?
Centerboard / Daggerboard Shape
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baldbaby2000
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The flow can still be asymetrical on a symetrical foil. If the flow is laminer, no turbulence, there can be accelerated streamlines on the windward side causing a lower pressure and hence lift. Marchaj tested several shapes; the best is parabolic, the worst is a sharp point. I don't know if the Macgreggors have a parabolic shape or if it's just rounded off.But while on the subject of lift created by a foil shape, isn't lift created only when the shape is not symetrical causing the flow to be faster on one side than the other.
Given a typical angle of attack of 10 degrees, and a 5 sq ft surface, Marchaj shows the foil to have a drag of 24 lbs and the flat plate a drag of 68 lbs. Seems pretty significant.
It's better to have the board at an angle so that hull doesn't crab. I don't know how the board sits in the trunk on Macgreggors relative to the hull.Is it better to have the board cocked a bit of an angle, if so in which direction, or is it better to have it pointed straight ahead?
BB
I recovered a little after "sailing puzzle" and I am now more in the mood to post something.
First let me point out major difference between water and air. Water practically do not compress. It practically do not change its volume under preassure in comparizon to air. Because of this we have aerodynamics and hydrodynamics as separate disciplines. Some of popular science approach to aerfoils do not quite translate to hydrofoils. Topics are complicated enough to leave it to few profesionals. On other hand I am not going to follow my own advice so bear with me.
There is no doubt that fancy hydrofoils are better in respect to drag, lift and lift to drag ratio then flat plates. But difference can be minimalized if someone will round up leading edge and taper trailing edge of plate to less then 10 - 15 percent. Those who are in doubt please note crossections of torpedoes. They can move trough the water very fast and usually there is no shortage in their developement budget. It is also worth to mention that badly design hydrofoil can be worse then flat, hardedged plate! In not easy forgiving environment of water the shape of hydrofoil must be kept very close to optimal design otherwise its efficiency will suffer significantly. One one the major errors is using foils designed for air (like free available NACA design)
underwater without critical approach.
Average home DIY builder should not even try to make one off centerboard with "arerodynamic" crossection as it probably will be worst then easier to make flat one. That is why I would stick to flat ones for home manufacturing. Of course if someone is willing to do research, make several models and test them go head and please share you findings.
As for shape the original one is not bad with the exception of the very bottom which I would make much less pronouced. I really did not put much tought into this so I reserve the right to change my opinion.
Hanging, pivot and raising must be redesigned totally. Mac26X center board design from factory is in my opinion as bad as it can be and still function. It was probably the very first and most important item on cost cutting list. I do not have at the moment any suggestions how to change as it depends on too many factors to which i did not form my opinion yet. I need to think about it a little more. Maybe some really high tech fibers can be used here?
As material for the board I already suggeted marine bronze for following reasons
1. significaly more weight gives better righting moment at higher angles (25 deg and up)where healing is critical. (At small angles hull is the major souce of stability and there is not much it can be done about its shape)
2. corrosion resistance
2. workability
Because of heavier board design entire trunk and its vinicity must be reinforced with stringers and additional laminate. I do not have enough data to even suggest particular solution.
Because new board will be thinner the original existing slot in the hull must be filled on the sides. I am thinking of some metal (bronze too) plating. Someone at the beginig of this thread had very good idea to make fillers and hanging support as one unit containig also raising mechanism. I think it is good idea and needs further analysis. Dimensional limitation would be critical and I do not have any data for suggestions.
Lastly what can be gained. Probably less then expected. There will be some stability gain. It may be possible to correct problems with board raising and correct 26X "rouding up" problem. This will not be easy nor cheap project if done to quality standards.
As for 26M I would not go with any more modification then maybe filling the lower part of dagger board with some weight (no more 100 - 150 lbs of lead pelltes in resin) - and that even with serious hesitation and concern of structural integrity of hull and dagger board trunk. I would keep second unmodified board as spare on the boat in any case.
First let me point out major difference between water and air. Water practically do not compress. It practically do not change its volume under preassure in comparizon to air. Because of this we have aerodynamics and hydrodynamics as separate disciplines. Some of popular science approach to aerfoils do not quite translate to hydrofoils. Topics are complicated enough to leave it to few profesionals. On other hand I am not going to follow my own advice so bear with me.
There is no doubt that fancy hydrofoils are better in respect to drag, lift and lift to drag ratio then flat plates. But difference can be minimalized if someone will round up leading edge and taper trailing edge of plate to less then 10 - 15 percent. Those who are in doubt please note crossections of torpedoes. They can move trough the water very fast and usually there is no shortage in their developement budget. It is also worth to mention that badly design hydrofoil can be worse then flat, hardedged plate! In not easy forgiving environment of water the shape of hydrofoil must be kept very close to optimal design otherwise its efficiency will suffer significantly. One one the major errors is using foils designed for air (like free available NACA design)
underwater without critical approach.
Average home DIY builder should not even try to make one off centerboard with "arerodynamic" crossection as it probably will be worst then easier to make flat one. That is why I would stick to flat ones for home manufacturing. Of course if someone is willing to do research, make several models and test them go head and please share you findings.
As for shape the original one is not bad with the exception of the very bottom which I would make much less pronouced. I really did not put much tought into this so I reserve the right to change my opinion.
Hanging, pivot and raising must be redesigned totally. Mac26X center board design from factory is in my opinion as bad as it can be and still function. It was probably the very first and most important item on cost cutting list. I do not have at the moment any suggestions how to change as it depends on too many factors to which i did not form my opinion yet. I need to think about it a little more. Maybe some really high tech fibers can be used here?
As material for the board I already suggeted marine bronze for following reasons
1. significaly more weight gives better righting moment at higher angles (25 deg and up)where healing is critical. (At small angles hull is the major souce of stability and there is not much it can be done about its shape)
2. corrosion resistance
2. workability
Because of heavier board design entire trunk and its vinicity must be reinforced with stringers and additional laminate. I do not have enough data to even suggest particular solution.
Because new board will be thinner the original existing slot in the hull must be filled on the sides. I am thinking of some metal (bronze too) plating. Someone at the beginig of this thread had very good idea to make fillers and hanging support as one unit containig also raising mechanism. I think it is good idea and needs further analysis. Dimensional limitation would be critical and I do not have any data for suggestions.
Lastly what can be gained. Probably less then expected. There will be some stability gain. It may be possible to correct problems with board raising and correct 26X "rouding up" problem. This will not be easy nor cheap project if done to quality standards.
As for 26M I would not go with any more modification then maybe filling the lower part of dagger board with some weight (no more 100 - 150 lbs of lead pelltes in resin) - and that even with serious hesitation and concern of structural integrity of hull and dagger board trunk. I would keep second unmodified board as spare on the boat in any case.
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baldbaby2000
- Admiral
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That's true that air is a compressible fluid. I'm no expert, but I believe that until air reaches velocities of 200 knots or so relative to a surface, veiwing it as a non-compressible fluid predicts it behaviour fairly close. A pitot tube on an airplane calibrated using compressible vs non-compressible equations will only be off a few knots between the two assumptions. The major differences between air at low speeds and water I would think would be their densities, not that one is compressible and one is not, and for air velocities below 200 knots the airfoil principles are the same. Maybe there's an aeronautical engineer on board that can set me straight.First let me point out major difference between water and air. Water practically do not compress. It practically do not change its volume under preassure in comparizon to air. Because of this we have aerodynamics and hydrodynamics as separate disciplines.
I do agree that adding weight to the board might be a good mod. I still think that it wouldn't stress the trunk anymore than putting an extra fat guy on the rail!
BB
Eric,
I realized that you have Mac 19 with damaged board and can not get new replacement from factory and you really need to make one. How bad is it damaged? Is anything left out of riginal?
Here suggestion for you if you have enough of it left so you can recreate original profile and shape. Use good grade marine plywood,epoxy glue two or three pieces for thickness leaving inside cavity for ballast, grind/shave it the best you can to the original crosssection. If not comfortable with this make it thinner like 1 inch. Just round up about 1 1/4 of leading edge to something more or less eliptical and taper 3 1/2 inches or so of traling edge to the point (but avoid knife edge -leave it slightly rounded). Just make sure that transitions are smooth. Keep the top of board around pivot as thick as possible to fit the slot and and make smooth 8 - 12 inch long trasition between part in the slot and the rest. weight it down at the bottom so it positivelydoes not float and laminate everything with 3 layers of s-glass cloth and resin. Do not kill youself over hydrodynamics. use some creativity and common sense to resolve manufacturing snag and just enjoy sailing with your new creation.
Keep us posted and share findings.
If really screwed up - grinding can erase some mistakes and you can always start over.
I realized that you have Mac 19 with damaged board and can not get new replacement from factory and you really need to make one. How bad is it damaged? Is anything left out of riginal?
Here suggestion for you if you have enough of it left so you can recreate original profile and shape. Use good grade marine plywood,epoxy glue two or three pieces for thickness leaving inside cavity for ballast, grind/shave it the best you can to the original crosssection. If not comfortable with this make it thinner like 1 inch. Just round up about 1 1/4 of leading edge to something more or less eliptical and taper 3 1/2 inches or so of traling edge to the point (but avoid knife edge -leave it slightly rounded). Just make sure that transitions are smooth. Keep the top of board around pivot as thick as possible to fit the slot and and make smooth 8 - 12 inch long trasition between part in the slot and the rest. weight it down at the bottom so it positivelydoes not float and laminate everything with 3 layers of s-glass cloth and resin. Do not kill youself over hydrodynamics. use some creativity and common sense to resolve manufacturing snag and just enjoy sailing with your new creation.
Keep us posted and share findings.
If really screwed up - grinding can erase some mistakes and you can always start over.
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baldbaby2000
- Admiral
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- Joined: Sun Mar 28, 2004 8:41 am
- Location: Rapid City, SD, 2005 26M, 40hp Tohatsu
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Yes you have a good point. Water and air are both fluids but have different densities. In both cases Bernoulli's equation (valid only for non-compressible fluids) can be used as long as you're not in a 200 knot wind where the non-compressible assumption falls apart; that would be the least of our worries in that case!I tried to point out that we do not need to be concerned with air properties and rather concentrate on water when discussing CB options.
Most readers of this forum avoid sailing or motoring with centerboard/daggerboard in the air and sails under water.
BB
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baldbaby2000
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The dealer told me a piece of plywood cut to the right shape makes a quick spare board. I'm thinking of making one just for an emergency. He didn't say anything about weighting it but that would seem to be a requirement. I think there are places that will build plastic boards to spec; I wonder if IdaSailor Marine could help?
BB
BB
I've considered repairing my existing board, having IdaSailor make one, or trying to make one out of wood. While all of these approachs would make the boat sailable I was hoping to come up with some ideas that would improve performace.
Of all of the power sailing models the MacGregor has built the CB arrangement on the M19 is the worst and the M19 is probably the poorest in terms of sailing performance.
One advantage of a CB over a DB is the ability to do some CB trimming for different sailing conditions. However the M19 has the line for CB adjustment going through a block under the V berth cushion. You have to go down into the cabin to raise or lower the CB. Could bring a line out to the cockpit through the hatchboard opening, but then would have a line running through the cabin while sailing.
Before I took on the CB repair I wanted to explore some options for improving performance and how the CB was raised an lowered. The idea of a metal plate seemed promising because while not an imporvement over the shape of the stock CB seemed it might not be any worse, but would also permit the closing in of the CB trunk opening on the bottom of the boat which produces drag under both sail and power. Also making a flat plate should be an easy construction project.
Of all of the power sailing models the MacGregor has built the CB arrangement on the M19 is the worst and the M19 is probably the poorest in terms of sailing performance.
One advantage of a CB over a DB is the ability to do some CB trimming for different sailing conditions. However the M19 has the line for CB adjustment going through a block under the V berth cushion. You have to go down into the cabin to raise or lower the CB. Could bring a line out to the cockpit through the hatchboard opening, but then would have a line running through the cabin while sailing.
Before I took on the CB repair I wanted to explore some options for improving performance and how the CB was raised an lowered. The idea of a metal plate seemed promising because while not an imporvement over the shape of the stock CB seemed it might not be any worse, but would also permit the closing in of the CB trunk opening on the bottom of the boat which produces drag under both sail and power. Also making a flat plate should be an easy construction project.
Here is a link I found elsewhere on the discussion board that talks about the kind of alternatives I was interested in hearing about.
I'd be very interested in any other reports of sucessful alternatives to the stock boards.
From this linkHere's a recent link on Zeno's Arrow from macgregorowners.com(including some pictures):
http://www.sailboatowners.com/forums/pv ... 1223508.10
Note that with a different CB he was able to go from about 14 tacks to about 8 covering the same passage, and it sounds like the board was made in part from SS plates. But haven't been able to find a really clear description of its design.Basically the Keel was modified for the following reasons;
(1) The stock keel can not take a heavy off-shore passage and will eventually fail. Although the good news is that even when this happens you just turn around and go to the closest place downwind. No worry about going bottom side up as the keel is not part of the ballast system. Trying to power upwind in any sea state, even for a short distance (meaning over 20 miles)is not practical and can not be done on autopilot. This happened to me in the Anagada Passage 14 miles West of St. Martin when I hit something(who knows what) at night and it took the keel off. I had to turn around and jib sail to Virgin Gorda.
(2) The stock keel does not let the boat point very well. I replaced the stock keel at Independent boatyard on St. Thomas and as a test tacked upwind through the narrows between St. John & Tortola (usually took the boat 14 tacks to get around to Coral Bay). With the new keel I was routinely able to cut this to 7-9 tacks.
(3) While on a sea-anchor (I use a 9' paratech unit to ride out storms at sea) with the stock keel the boat sailed heavely through the wind (up to 30 degrees), on the new keed the boat stays directly to windward with out any 'sailing'. This has been done n two serious storms, one of which had gusts of up to 70 knots for 4 hrs.
The new keel w/o the 'rotating wing' I use for ballast (even with two stainless steel shafts inclosed inside) actually has negative ballast. The wing on the keel is about 100 lbs.
I'd be very interested in any other reports of sucessful alternatives to the stock boards.
Eric O:
In its simplest terms:
The centerboard creates lift when it moves through the water at some small angle of attack. When sailing downwind with neutral rudder there is no angle of attack and therefore no lift is created by the centerboard, only drag. To eliminate this drag small racing boats often raise the the centerboard when sailing downwind. The rudders provide the lift necessary to change direction of the boat.
Upwind is a different story. Upwind, the forces of the wind on the sails and hull create sideslip i.e. the boat does not move on the same path (track) as the centerline of the boat (course). This small sideslip angle, which is called leeway, provides the angle of attack needed to create lift on a symmetrical airfoil like a centerboard. The lift created is a function of this sideslip angle, airfoil shape, size and speed through the water. The lift acts perpendicular to the centerboard and in a direction which will (1) help "pull" the boat to windward by opposing the wind forces on the hull and sails which want to push the boat off the wind and (2) add to the overall forces which make the boat heel.
You may have noticed that at high angles of heel the boat makes a lot of leeway. This is partially because a significant portion of the the centerboard force vector is pointing skyward. Reducing sail and sailing flatter is much faster, safer and more comfortable.
A flat plate will create lift but that shape is not nearly as efficient as an airfoil shape like a NACA 0012. The Mac centerboard and rudders have a decent shape. A flat plate would be a step backward in my opinion.
I have raced sailboats for years and have tried several improvements on my 26X. I have concluded that as a sailboat, the 26X is at the bottom of the heap performance wise (a phrf of 370 is close). There is nothing you can do to make it competitive with "normal" 25 footers so why bother. Enjoy the other things that you can do that keelboats can't and consider sailing a side benefit.
In its simplest terms:
The centerboard creates lift when it moves through the water at some small angle of attack. When sailing downwind with neutral rudder there is no angle of attack and therefore no lift is created by the centerboard, only drag. To eliminate this drag small racing boats often raise the the centerboard when sailing downwind. The rudders provide the lift necessary to change direction of the boat.
Upwind is a different story. Upwind, the forces of the wind on the sails and hull create sideslip i.e. the boat does not move on the same path (track) as the centerline of the boat (course). This small sideslip angle, which is called leeway, provides the angle of attack needed to create lift on a symmetrical airfoil like a centerboard. The lift created is a function of this sideslip angle, airfoil shape, size and speed through the water. The lift acts perpendicular to the centerboard and in a direction which will (1) help "pull" the boat to windward by opposing the wind forces on the hull and sails which want to push the boat off the wind and (2) add to the overall forces which make the boat heel.
You may have noticed that at high angles of heel the boat makes a lot of leeway. This is partially because a significant portion of the the centerboard force vector is pointing skyward. Reducing sail and sailing flatter is much faster, safer and more comfortable.
A flat plate will create lift but that shape is not nearly as efficient as an airfoil shape like a NACA 0012. The Mac centerboard and rudders have a decent shape. A flat plate would be a step backward in my opinion.
I have raced sailboats for years and have tried several improvements on my 26X. I have concluded that as a sailboat, the 26X is at the bottom of the heap performance wise (a phrf of 370 is close). There is nothing you can do to make it competitive with "normal" 25 footers so why bother. Enjoy the other things that you can do that keelboats can't and consider sailing a side benefit.
normo,
What improvements have you tried? Was your reference to a phrf of 370 a typo (maybe you meant 270)? The review in Practical Sailor mentions a phrf of between 240 and 250.
http://www.practical-sailor.com/sample/boatreview2.html
Even though we may never be able to get the Macs to sail as well as other boats of similar length nothing wrong with looking into ways to improve the performance. The board shape and construction would seem to be one possibility.
I'm much more interested in hearing about what you did try the results than hearing someone say don't bother its not worthwhile. Sounds like the folks at BWY have been pretty successful at making their new boat more competitive.
http://bwyachts.com/La%20Perla%20Noir/Pearl%20Home.htm
While we don't have the luxury of working with the factory on a new boat it doesn't mean there aren't things that can be done that will help. So how about sharing some of the you have tried and what you noticed in the way of results.
What improvements have you tried? Was your reference to a phrf of 370 a typo (maybe you meant 270)? The review in Practical Sailor mentions a phrf of between 240 and 250.
http://www.practical-sailor.com/sample/boatreview2.html
Even though we may never be able to get the Macs to sail as well as other boats of similar length nothing wrong with looking into ways to improve the performance. The board shape and construction would seem to be one possibility.
I'm much more interested in hearing about what you did try the results than hearing someone say don't bother its not worthwhile. Sounds like the folks at BWY have been pretty successful at making their new boat more competitive.
http://bwyachts.com/La%20Perla%20Noir/Pearl%20Home.htm
While we don't have the luxury of working with the factory on a new boat it doesn't mean there aren't things that can be done that will help. So how about sharing some of the you have tried and what you noticed in the way of results.
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Gerald Gordon
- First Officer
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- Location: O'ahu, Hawai'i
Replacing the X-CB with a look alike that has weight is IMHO a waste of time. What's needed is a small keel shape protruding from the hull. Maddmike, with his keel/CB design claims to have improved performance of his vessel. A course which needed 14 tacks can now be sailed in 7-8 tacks. That's about 50% improvement. That's worth considering!!
If I were to work on the CB I would have up to 12" inches or more of board protruding from the hull when the CB is retracted. I would even consider making the protruding portion of the blade extend up to 25 inches past the slot flush with the hull. I would also consider making the CB/keel heavier. I would be surprised if that mod would not improve sailing performance.
If I were to work on the CB I would have up to 12" inches or more of board protruding from the hull when the CB is retracted. I would even consider making the protruding portion of the blade extend up to 25 inches past the slot flush with the hull. I would also consider making the CB/keel heavier. I would be surprised if that mod would not improve sailing performance.