Dagger Boards --- I don't quite get it

[mikereed100]

Interesting stuff. The bit about the trailing edge being in the turbulent zone makes it easier to intuit. An interesting implication for daggerboard design would be that with a flatter trailing edge there would be less chance for damage to the case in event of collision.

[Tropic Cat]

Originally Posted by mikereed100

with a flatter trailing edge there would be less chance for damage to the case in event of collision.

Have you seen what damage barnacles can do to dagger board trunks? Is there a Gemini owner in the thread?

[Merlin Hawaii]

Originally Posted by mikereed100

This amazes me and I would love to learn more. Intuitively I would think that as fine a trailing edge as practical would be desireable for LAR foils as well. Do you have any online references for these studies?

Mike

High aspect ratio foils benefit the most from a truncated trailing edge

The flow separation on each side of the foil reunite and merge behind the foil with the least resistance if a small area of turbulence exists directly behind the trailing edge

Kind of like the golf ball effect,or the textured bottom surface on a race boat.

Low aspect ratio foils (designed for lower speeds) benefit less from a hydrodynamic standpoint, but gain a bit in terms of construction ease and durability

[BigCat]

Originally Posted by Merlin Hawaii

High aspect ratio foils benefit the most from a truncated trailing edge

The flow separation on each side of the foil reunite and merge behind the foil with the least resistance if a small area of turbulence exists directly behind the trailing edge

Kind of like the golf ball effect,or the textured bottom surface on a race boat.

Low aspect ratio foils (designed for lower speeds) benefit less from a hydrodynamic standpoint, but gain a bit in terms of construction ease and durability

In practice, what you actually see is usually that long keels are cut off foils, and daggerboards and rudders are not. This is probably in part because rudders and daggerboards are likely to be solid, and longer keels are likely to be hollow, and to have something inside of them, such as ballast or tanks.

[Merlin Hawaii]

Originally Posted by BigCat

In practice, what you actually see is usually that long keels are cut off foils, and daggerboards and rudders are not. This is probably in part because rudders and daggerboards are likely to be solid, and longer keels are likely to be hollow, and to have something inside of them, such as ballast or tanks.

I think you'll find that most serious race boats have truncated trailing edges on their rudder and keel/daggerboard foils

Prindle cats (just as an example) were doing that on their boats almost 40 years ago

So the design theory is nothing new

Cruising boats are continually becoming more sophisticated, and benefit from racing trickle down

[Limpet]

Forgot I started this thread long ago.

Aren't most dagger board shapes symmetrical...in which case there is no lift because the water flowing over the surfaces on both sides is moving at the same speed.
An airplane wing provides lift because the air travels faster over the longer top surface thus creating a low pressure on top surface of the wing relative to the pressure below the wing.

[ElGatoGordo]

Originally Posted by Limpet

Forgot I started this thread long ago.

Aren't most dagger board shapes symmetrical...in which case there is no lift because the water flowing over the surfaces on both sides is moving at the same speed.
An airplane wing provides lift because the air travels faster over the longer top surface thus creating a low pressure on top surface of the wing relative to the pressure below the wing.

That is so very basic a misconception that you must study more to understand the conversation. In relation to wings, please Google "angle of attach" and you will see that there is much more to it. You are thinking of a wing on something like a Cub, a flat bottom wing that generates lift just moving straight through the air. You might be surprised to learn this isn't the norm for wings, especially aerobatic wings. Most have symmetrical airfoils.

The only time a symmetrical Keel (are there any other types?) ISN'T making lift is dead downwind.

[Limpet]

Ah, I agree. So then to provide lift or a desired force to prevent leeward slippage the angle of attack is all important. So are the daggers aligned with the hulls or are they slightly angled?

[ElGatoGordo]

Aligned. It's the whole boat that has the angle of attack. The whole idea is the wing (keel or dagger board) is "flying"in relation to the forces of the wind trying to push the boat down wind. Think of the down wind force on a boat as gravity on a plane.

[Tomm0]

I have a swing keel- when it is up, and I'm trying to tack into a serious headwind, I reckon I can lose 30-50% of my distance upwind just from the slippage. Board down, it's maybe 20%...

[cal40john]

Originally Posted by Limpet

Ah, I agree. So then to provide lift or a desired force to prevent leeward slippage the angle of attack is all important. So are the daggers aligned with the hulls or are they slightly angled?

Most are aligned with the hull. Some race boats have some toe in. This requires you to lift the weather board on every tack or you'll have a lot of drag.

[Jimbo485]

Do some planes have wings that are symmetrical?

[ElGatoGordo]

Originally Posted by Jimbo485

Do some planes have wings that are symmetrical?

Almost all. Only very slow, low performance planes have flat bottom wings.

Edit: well actually semi-symmetrical is the most common

[Jimbo485]

Originally Posted by ElGatoGordo

Almost all. Only very slow, low performance planes have flat bottom wings.

Edit: well actually semi-symmetrical is the most common

So dagger boards, being so slow, should be asymmetrical?

[ElGatoGordo]

Originally Posted by Jimbo485

So dagger boards, being so slow, should be asymmetrical?

Not sure what you're asking. I haven't heard of asymmetrical boards. It would only generate lift on one tack...sounds like a pain to me.

I was just pointing out that planes designed to fly slowly with low power are often flat bottom winged. The comparison of sails and boards to airplane wings only goes so far. The physics is the same, yes, but the variables are very different. I was just trying to help the guy understand the way a board works.