toe in???

can someone please breifly explain the reason for toe-in with twin bilge keels. i have an idea of why it's there and i want to know if what i think is correct...i.e.... as the boat heels, it rolls over on it's wetted surface which, towards the bow, is closer to the center line of the boat, so the boat'c C/L rotates to leeward as it heels... is this basically correct?. is there any on-line literature concerning keel toe-in?. thanks in advance.

 

Search Tool

Hi Capt'n Ron,
I did notice that you just recently joined the forum. Just in case you are unfamiliar with some of the features, I might make the suggestion that you try the "Search" tool. If you are on the general forum page it will search all of the discussions for any wording such as "twin keel". Here is just one such thread, http://www.boatdesign.net/forums/showthread.php?t=5315&highlight=twin keel Pretty handy tool

 

You may be right about the change in angle with heel. But I suspect the main reason designers use toe-in is to transfer the load to the leeward keel when making leeway. This has the deeper keel generating the side force, which it can do with less drag than the shallower keel.

 

...that under heeling is giving the most toe-in for richtning i asume?...

 

I'ts simpler than that. Look at a modern twin keel (or bilge board) boat. It is shallow with hard bilges and a wide transom. Sort of like a triangle with rounded sides. The idea is the windward bilge will lift out of the water to provide the righting moment (as opposed to a swing keel boat that cants the keel to windward). Because the boat is light the immersed body shifts greatly to leeward. Basicaly along a line drawn between the stem and the transom bilge. Now it is like a narrow hull with a great big hiking platform. The keel, while it appears to toe in to the normal CL, is actualy straight along the "sailing" CL.

"Old" twin keelers that were made for grounding had much less pronounced or no toe in.

 

thanks for the explination fellas.
i guess i had the right idea, almost, just put it in different terms.... as the boat heels, the sailing c/l and the true hull's c/l differ as the product of the hull's conical wetted shape. the leeward keel's alignment is approximately on the hull's sailing c/l when it is more or less verticle..... so the windward keel's alignment to the actual sailing line is essentially doubled in toe-in under way and is really doing allot of work trying to right the boat!.... that part i never thought about!! so a bildge keel's shape should be flat outside, foil inside(?).

 

Ron,

One recent issue of Seahorse magazine had an article on the subject, and a couple of good pictures describing the effect.

Yoke.

 

Good read ...

Hello...

See http://johnsboatstuff.com/Boat%20Designs/Grenada%2024/twinkeel.htm ...

SH.

 

I think you've got it backwards. As an extreme example, think of a boat with a triangular hull like something from Group Finot. As the boat rolls about the leeward bilge, it points the bow to windward and increases the leeway angle and angle of attack on both keels relative to a boat that had round sections and rolled about her centerline.

The effect of heel on the windward keel is to make the flow run more along the length of the keel from root to tip. The rotation of the boat in yaw as it rotates about the leeward bilge has the effect of swinging the windward keel back some more relative to the course through the water.

Imagine this exaggerated case: the angle at the bow is 30 degrees, with each bilge angled 15 degrees from the centerline. The boat rotates to 90 degrees heel about the leeward bilge. In this attitude, the centerline of the boat is pointed 15 degrees to windward of the original course. A keel that was originally vertical would be horizontal and raked back 15 degrees from the the direction of travel. Of course, it would also be out of the water. But the direction of the trends holds true for lesser heel angles and less extreme hull geometries.

Putting the flat side of the keel to the inside is effectively toeing out the keel because the angle of zero lift points out the curved side of the section. Putting the flat side to the outside effectively toes the keel in, even if the flat is aligned with the centerline.

When the boat is tracking straight upright with no leeway, both keels are lifting toward the centerline if the flats are to the outside or symmetrical keels are toed in. The windward keel is pushing the boat to leeward and is opposed by the leeward keel pushing the boat to windward. As the boat makes leeway, the lift on the leeward keel is increased and the (negative) lift on the windward keel is decreased. At some leeway angle, the change in lift will be equal to what each keel was making when the leeway was zero. The lift on the leeward keel will be doubled and the lift on the windward keel will be zero. This would probably be the design condition for going upwind with bilge keels, and you'd either camber or toe-in the keels to just zero out the windward keel's lift and the design leeway angle.

When you cant the keels outboard and heel the boat, the same situation applies, but the precise details are different. You'd still be looking to zero out the windward keel's lift at the design conditions for going to windward. You could do some vector analysis to figure out what the orientation of the zero lift line should be from the windward keel's axis. Knowing that, you'd use that as the toe-in angle for a symmetrical section or use it to align the zero lift line of a cambered section.

Mathematically you want the velocity vector through the water, the spanwise axis of the keel and the zero lift line of the section to all lie in the same plane with the boat at the design angle of heel and design leeway angle.

 

tom ,thanks.... i have to look( visualize) at this more closely, i thought i had it figured out!?????!!!! . i'm building a boat with twin keels and i want to better understand how the work.... if the keels are toed in, i assume they are aligned on the boats conical(so-to-speak) roll center, so that whichever keel is leeward, or "down in" would be in line with the boat's leeway, which would point the bow to windward.... ok... i see how this cancels the toe to the lee side fin now. so the foil should be on the outside, where the lee fin's lift would be a righting moment trying to pull the hull back onto it's true center. i have to go back and read a little more about keel hydrodynamics. in my non engineer mind, i was confusing the term "zero lift", thinking the foil shaped side of the fin is the "lifting" side, when actually as in an airfoil, it is the negative pressure( zero lift) side.....thanks again.

 

Every section shape has an angle of attack at which it will produce zero lift. As a first order approximation, you can take the slope of the camber line at the three-quarter chord location as being the direction of the zero lift line.

So if you draw a line tangent to the camber line at three-quarter chord, this is a good reference to use for measuring your toe angle since it's a hydrodynamic reference instead of a geometric reference like the chord line. This will lie along the chord if the section is symmetric. But if you use a cambered section, it will account for the lift at zero angle of attack (measured to the chord line).

Another approximate guide is to allow a degree of angle of attack for every 0.1 change in Cl. So a cambered section that produces a Cl of 0.3 at zero angle of attack will have a zero lift angle of attack near -3 degrees. This would be typical of a flat-bottomed 12% thick section like the Clark Y.