Dagger boards versus shared lift

[havliii]

So at stall speeds boards up? But it's not that easy. Bill has given some broad rules of thumb. Let's add in currents and tides. Where I sail you can have 3 knots of boat speed and 9 knots of current flow over the boards. So what rules apply here? With an ebb tide and river flow we can easily see six to eight knots of current.

[Bill Roberts]

havliii, you have it backwards. At stall, boards need more area. When your boat is sailing at 3 knots into a 9 knot current, you have 12 knots over the boards. They are happy. They are no where close to stall. The only speed that matters is water speed over the boards. The boards don't know when they are in a current. They were sized for 12mph boatspeed double trap righting moment and sailing to windward.

[Bruiser]

A little word about current. Your boat does not recognize current. Think of a boat sitting on your carpet that is moving. No matter which way you point your boat it will be going the same speed with respect to the carpet/current (body of water). The difference is apparent wind created by current, either positive or negative. Having your boards deep or shallow will make no difference as the entire body of water moves the same. The depth of your boards will be about your sailing not how much the current effects you. Differences can be found in shallow water versus deeper water, The body of water will be a little slower in shallow water versus deeper water. Now, was that 3 knots over the ground or the water? If it is 3 knots over the ground against a 9 knot current, you are sailing at 12 knots through the water, but only 3 over the ground. 3 knots though the water in a 9 knot current is still 3 knots through the water and minus 6 knots over the ground, (you are being swept away by current) board up or down will make no difference with respect to the current itself. However what Bill is saying is at 3 knots through the water your will need all the board lift you can get going to weather.

[havliii]

current = wind and has a very profound effect on boat speed and dagger board management.

Bill did what he always does and reframed my question.

If he boards are near or at stall speeds, up or down? That is the question.

Can I make it any more succinct? Let's leave the current out, okay.

If the boards are at or near stalling, up or down?

[Bill Roberts]

havliii,
When the boards are at or near stall, increasing the board area underwater will reduce the load per square foot of board area and this move the board away from stall.

Stall occurs when the load is too great for the amount of board that is down at the speed the boat is going. To move away from stall, increase board area and or increase boat speed or both.

[havliii]

Thanks Bill, as my father used to say, "you are a gentleman, a scholar and a judge of good bourbon." He was a Purdue man and they were "Boilermakers" of course.

I am aware that the boards don't know the difference between current and plain old motion. It relative right? Water moving past the boards.

Now, give me a discussion on 'angle of incidence' isn't this where the direction of the fluid flow (same as the air fluid) has a major impact on lift and drag. Since we haven't got a traveler or a mainsheet on the dagger board the angle of incidence is directly related to the direction of the current and of course the direction of the boat.

The Boilermaker part of this post is an obtuse reference to bourbon, for those of you who don't follow my twisted logic trails.

[Bill Roberts]

The driver of this train is sail side force. The boards automatically adjust and generate an equal and opposite force. The peak lift to drag ratio of the boards occur at a small angle of attack like 3 to 5 degrees max. Sailors can keep the boards operating at or very near peak lift to drag ratio by adjusting board area. Sail control devices like mainsheets and travelers applied to daggerboards are no benefit over adjusting board area to attain best lift to drag ratio on the daggerboards.

[havliii]

and the angle of attack is the leeway angle? If we are talking about 'motionless water' and the boat is moving forward the angle of attack can only be the leeway angle.

[Bill Roberts]

havliii,
Yes, the angle of attack of the water on the board is the leeway angle.
Many, many one design classes of sailboats have tried adjusting the angle of attack of the board in the trunk to the slip angle. No benefit has ever been found in any class. This tells us that there is no measurable change in hull drag for the small slip angle on our round bottom boats. Maybe if we had a hard keel angle and hard chines a measurable change in hull drag could be found associated with a 3 degree slip angle.

[havliii]

......... bringing this thread full circle ......... let's talk some more about dagger boards and shared lift.

As we all know the 15 and the 17 have no dagger boards, the 19 and the 20 both sail perfectly fine with dagger boards in the full up position. So what is going on here? We are talking about a balanced system where drag always equals lift. Where does the counter force come from that balances out the sail side force if the dagger boards are not down?

Well, when you sail any boat you are always "sailing with the designer!" He's along for the ride like it or not. (Hello, Bill) You (the helmsman and tender of the sheets) cannot change the hull shape or the characteristics that were designed into the boat. With the dagger boards full up the opposing side force has to come from somewhere, that somewhere is the hull shape and the rudders. There's no where else!

So now picture sailing merrily along daggers full up, the boat is perfectly balanced, maybe some weather helm, we are thinking about lowering the dagger boards a tad. Okay, we start lowering the dagger, what EXACTLY is changing? You are adding an element to the equation something has to change to stay in balance. You are adding both lift and drag, but where is the benefit? There must be one or we would leave the daggers on the beach! (some folks do exactly that on the SC19 and there is a version that was sold w/o the trunks installed)

more thoughts please..........
We'll get back to leeway and how current can effect the angle of attack later..........

[Bill Roberts]

havliii
You lost me when you said drag equals lift. Drag is the force resisting forward motion of the boat and occurs along the path of the boat through the water. The lift we are talking about is perpendicular to the path of the boat and they are NOT equal and are almost totally unrelated.

Here's a simple test. Go sailing with about 20ft of white string trailing out the back of the boat. Sail to windward boards down. The string will appear to hang straight out the back of the boat. Now raise the boards up and continue sailing to windward. The string will swing up to windward about 10 degrees or more indicating the increased slip angle and the boat will sail slower also. The daggerboard makes much less drag than the hull does when made to generate lift.
Induced drag, the drag due to making lift, varies inversely with the aspect ratio of the body generating the lift. A daggerboard might be 3ft deep underwater and 1ft wide. The aspect ratio or depth to chord ratio is 3.0. The hull waterline might be 19ft long and average 1ft deep. The depth to chord ratio of this hull is 1/19 or 0.05623. As you can see there is a major difference in induced drag and VMG made toward a windward mark.

[havliii]

I'm speaking of the entire system. All of the drag forces combined equals the propulsion forces. If it were not so the boat would continue accelerating until the drag force exactly equalled the propulsion force and we reach equilibrium.

[Bill Roberts]

It is true that propulsion forces equal drag forces at terminal velocity. It is true on sailboats and race cars and airplanes etc.

I'm lost.

[J Drew]

So what your saying is, less board is actually slower in some cases because the hulls create more drag, in a slide, than the boards do?

Now, getting back to asymmetric boards, given the "one up one down" necessity, could a board that creates better lift be shorter?

[Bill Roberts]

Drew,
If you had taken a course in school on basic aerodynamics, you would understand all of this.
The part of the centerboard drag that you are missing is the induced drag, the drag due to making lift part.
The total CB drag equals the parasitic drag plus the induced drag.
The parasitic drag, friction plus form drag, is a function of wetted area.
The induced drag is a function lift coefficient **2 times a constant.
The constant is related to 1/Pi X e x Aspect Ratio. You know what aspect ratio is because we have talked about that before
and e is that Span Effectiveness term we have talked about before.
Now the big deal is lift coefficient. Lift coefficient is Lift / Area.
Therefore induced drag varies with (Lift/Area)**2.

So, in this centerboard drag situation we have a parasitic drag term that varies directly with CB area.
We also have an induced drag term that varies with (LIft/Area)**2. Lift is the side force from the sail
so let's treat it as a constant. Now we can say that induced drag is a function of (1/Area)**2.
So, one term in our drag equation becomes larger or smaller directly with area.
But the other term in our drag equation does just the opposite. As CB area becomes larger, the induced drag becomes smaller
and as CB area becomes smaller, induced drag becomes larger. Not only does it become larger but it becomes larger as the square of the Lift/Area term. So, as CB area changes, the parasitic drag varies with it directly. As CB area changes, induced drag makes major changes inversely with CB area.

Now what we want out of our CB area system is for it to operate at the best or highest level of Lift to Drag Ratio. Therefore at a given side force or Lift, the drag is a minimum. This occurs when parasitic drag equals induced drag. This is the way the CBs have been sized on all SCs and ARC boats.