Leeway - Ignore It?

[AndrewB]

Older bilge keelers and negative leeway.

Oddly, there was once a theory that bilge keelers should in theory make less leeway than fin keelers, and even create negative leeway, i.e. drive the yacht to windward relative to the sea. This would be achieved by an airfoil shape to each keel. Once the yacht was heeling, the deeper leeward keel would provide lift to windward, while the upper windward keel would provide downward 'lift', stopping the yacht from heeling further.

I believe that racing yachts were tried .... presumably the theory didn't pan out. However, as is the way with radical racing experiments, the idea did find its way through to some production cruising yachts.

<hr width=100% size=1>

 

[boatless]

Re: Older bilge keelers and negative leeway.

Andrew, I think Qsiv might hope to disagree, although I think he's a little bit busy at the minute. His new boat, Wraith, is a Schock 40, which, along with a canting keel, has fore and aft rudders. Using what they are calling a 'collective' linkage, it's possible to give both foils positive incidence and climb to windward (obviously at a cost in speed/drag, but tactically very handy).

I think he's down at HYS screwing it all together as we speak. An exclusive for you?

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[racingron]

There are several points here but basically leeway has to exist for a boat to go forward when the wind is forward of the beam.

Pull up a sail on the dock in neutral tide (i.e. the boat is not moving through the water) and see which way the boat is pushed - it goes to leeward.

When the keel generates lift it works as a sail under the water pushing the boat to windward and reducing the sideways action. For this to happen there has to be greater pressure on the leeward side of the keel than the windward side. This can only be achieved by the boat going (however little) to leeward.

The faster you go the less leeway you have as the appendages generate more and more lift at higher speeds. This is why lightweight race boats can get away with shorter chord lengths (width of the fin) than slower cruisers because they go faster. i.e. at, say 25knots windspeed, you'd have smaller sail area above the water line but the same keel/rudder area below the water.

With no leeway you would generate no lift on the appendages. Try it in a dinghy with the centreboard up, the only part of the boat generating lift would be the hull itself.


<hr width=100% size=1>

 

[bedouin]

There are ways you can increase the "lift" generated by the underwater sections. The section that generates the lift is a combination of the keel and the rudder (and any canard etc). So for example weather helm increases the lift of generated by the keel, and so reduces leeway (it may also reduce drag, but I'm not sure).

I gather that some modern racing designs can actually make "negative" leeway

<hr width=100% size=1>

 

[racingron]

I would have thought that weather helm only increases drag, though I can see your point about lift, however with the rudder at the back of the bus not too sure of the full implications.

Negative leeway? Be keen to know how that works? Perhaps self tacking canards? I've seen these on minis etc.

<hr width=100% size=1>

 

[MainlySteam]

The rudder produces lift with its angle of attack being the leeway plus the weather helm (assuming it is in clear water).

John

<hr width=100% size=1>

 

[bedouin]

On boats like mine, with a long fin and skeg, the rudder acts rather like the rudder on a aeroplane, so the angle of attack should be considered to be the angle between the flow of water and the line joining the front of the keel and the rear of the rudder (assuming the rudder is not stalled). I really don't know how well this would carry over to a modern design with a high aspect fin and spade rudder - still less a twin ruddered boat.

The usual advice is that a few degrees of weather helm is beneficial because of the increased lift.

As to the drag - if you would normally be making 5 degrees leeway than using 5 degrees of weather helm to counter act that could well reduce drag (by improving the flow across the fin and probably reducing turbulence on the leeward side) but we need someone with a much greater knowledge of fluid dynamics to give us an authoritative opinion

A google search for "canard" gives some interesting pages but I don't have time to browse them now

<hr width=100% size=1>

 

[Robin]

Canard

A l'orange peut etre? Orange source recipe from our local butcher: Add marmalade to Bisto or Oxo gravy mix, very good too!

Apologies /forums/images/icons/blush.gif

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[racingron]

I'm probably being dense here, but surely if you have weather helm you're increasing drag more so with the effect of leeway, example:

on starboard tack I have 5 degrees of weather helm - i.e. I'm turning the rudder to port to keep the boat in a straight line, does this not put it more across the water flow - particularly as the boat is going left (as well as forward) here with the effect of leeway?

If there was no weather helm the rudder would not be required (to move from centre) so there would be less drag as it would be in line with the water flow under the hull.

I could be being really stupid here...

<hr width=100% size=1>

 

[jimi]

Think that what Bedouin is saying is that the turned rudder is creating a basic wing shape in combination with the keel and the water travelling faster over the outer bit than across the chord is creating lift?

<hr width=100% size=1>

 

[bedouin]

As you said earlier, you need an angle of attack between the keel and the water flow to generate the lift that will stop the boat from going to leeward. For the sake of argument let us say that is 5 deg.

Now with the rudder straight that means you have to be making 5 degrees of leeway to achieve that angle of attack, so you are effectively pushing the keel slightly sidewards through the water which I suspect will cause turbulence on the leeward side of the keel - rather like sailing with a luffing sail.

If you apply a bit of weather helm, then you achieve the 5 degrees by the aerofoil effect of the combined fin, skeg and rudder, like an aerilon on an aircraft wing, in that way you can eliminate the sidewards movement of the keel and probably keep smooth water flow on both sides.

Apparently some Centurions were built with a trim tab on the back of the keel to do this - but there is no sign of one on Bedouin.

<hr width=100% size=1>

 

[DogStar]

"Now that GPS is the universal means of navigation, as cruisers we can ignore leeway"

Again, this should surely be dependent on the boat type. I have a very old bilge keeler, with a high straight stem and I make a fair amount of leeway if the wind is on the beam. I have GPS, but always work up a course corrected for leeway and then TRY to keep to it, rather than constantly correcting the leeway almost as it effects you. I have been told, quite forcibly, that on a long run you can use a lot more fuel, and time, by correcting even a small amount of leeway on the fly. Apparently even with GPS you should be on a heading a few degrees upwind of the bearing to your destination, so you get blown onto the correct heading.

<hr width=100% size=1>I'd rather have a bottle in front of me than a frontal lobotomy!