|Nov 16, 2012, 11:06 AM|
The dynamic CLR of a rectangular hull (CLR when a ship is moving) is located at approximately the 1/4 chord point: at 1/4 of the hull waterline length aft of the bow. I call it the chord because the hull forms a symmetrical airfoil in the water, so using wing terms is appropriate. The center of lift of a wing (nautical terms CLR and CE) can be at different spots for different airfoils. For Wawona, the dynamic CLR is about at the forward edge of your forward hatch if I use the 1/4 chord point.
Here's Wickipedia on center of lift:
"For symmetric airfoils in subsonic flight the aerodynamic center is located approximately 25% of the chord from the leading edge of the airfoil. This point is described as the quarter-chord point. This result also holds true for 'thin-airfoils'. For non-symmetric (cambered) airfoils the quarter-chord is only an approximation for the aerodynamic center."
The summation CE of her sails is located at about the mainmast.
Because the CLR is forward of the CE, the ship will try to turn into the wind (weathervane). The CLR is the fulcrum for the sails' tettertodder. You'll need to hold the rudder over to fight this tendency to weathervane; this is called "weather helm" since in the days of tillers, you had to hold the tiller upwind, or "to weather." If you have a large rudder, you may be able to control the weathervaning effect, albeit at the loss of ship speed due to the drag of the rudder. I personally have not had much luck with this technique with my models; the power of the wind on the sails pretty much rules what's going to happen on Pamir and Aldebaran. If my ships are not balanced, they are going to do what the wind says and pretty much ignore my joysticks :-/
In the old Sterling model kit of 2-masted schooner America (which had no fin keel), the instructions were to let the mainsail sheet slacken to keep the boat from turning into the wind. You could do this on Wawona by spilling wind from the mizzensail. By spilling wind, making the lift off the mizzensail less than it's capable of, you should be able to balance the ship, according to my worksheet. I used to use this slacken-sails-aft trick with my freesailing schooner before I installed a finkeel. There is nothing wrong with using sails to steer. In fact it's why I always urge builders to put the aft sails on a separate servo from the forward sails. If you happened to read my Pamir thread, you'll know that I initially sailed her by steering with sails alone (no rc rudder). The only real negative from slackening the mizzensail sheet is that the sail with slat like crazy if the wind shifts forward during a gust.
The static CLR, the hull's fulcrum when the ship is not moving, is located at the geometric center of the hull. For your ship, that's about 1/2 way between the aft hatch and the mainmast. For the static setup, therefore, your ship would fall off and need leeward helm. But when she starts moving, the CLR will shift forward. I can see this happening with my simpler models - When the wind hits, they initially drift to leeward (static CLR aft of the CE), but as they pick up speed, they curve to windward (dynamic CLR shifts forward, moving CLR foreward of the CE). If they are balanced, the curving path stops with the boat on the proper course for a beat.
You may see the same: your ship starts off controllable, but as she picks up speed, she weathervanes more than you can overcome with rudder. If you see that, it's a clue that she is not balanced.
When ships were designed prior to the understanding of flight forces, the rule of thumb was: "put the CE a ways ahead of the CLR". This allowed for the movement of the CLR forward once the ship started to move. No one knew why this happened because no one understood the lift/drag forces. Nautical designers depended on "secret formulas" and rules of thumb to design sailboats that worked. However, once flight forces were understood, and once nautical designers realized that lift in the air was the same as lift from sails and lift from hulls, then the mystery went out of a lot of the design process.
I don't know anyway you can modify your hull to move the CLR and CE into alignment w/o a fin keel. Since you have "too much" sail aft to balance the ship, you could just put a reef or two into the mizzen sail. Removing (ie furling) the sail would shift the CE too much, according to the worksheet.
But like I said, you can always just sail her and see what happens. Math at the level I am doing, won't provide all the answers. Perhaps the inefficiency of the aft sails will be enough to allow Winona to balance w/o fancy dodads added underwater.
|Nov 16, 2012, 11:25 AM|
I'm kind of a fanatic on controllability. There are plenty of modelers on rcgroups and other forums, though, who get plenty of fun from sailing unbalanced models. As long as you can get to all sides of the pond, there is no danger of watching your model sail out of sight (which has happened to some really sweet models, as reported on rcgroups and other model sailing forums, alas). It's your hobby, whatever makes you happy is what you should do.
I played what-if wrt mizzen reefing and no fin keel.
If the mizzen sail area is reduced to 1/2 (via reefing) and if the dynamic CLR is 1/3 chord (within the realm of possibility, if I understand airfoils correctly), the the ship should be close to balancing, with just a little weather helm. No fin keel needed :-). Once Wawona heels, her hull no longer presents a symmetric airfoil, so it's possible the CLR will move aft. I have no idea how far aft, or even it will move at all, but it's possible.
This is one of those things that will have to be confirmed by experiment on the water. I could throw a bucket of water over the computer, but I am not sure if this would really increase our understanding of Wawona...though it would confirm suspicions held by some here on the forum, I suspect *grin*.
|Nov 29, 2012, 03:51 PM|
I think you are right about the mezzen mast, so just in case I have problems I built a mezzen sail 1/2 the size as you suggested, should be a quick change at the pond, If the sails are not balanced for the air on that day.
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