mill creek dager board?

Has anyone tried putting a dager board trunk into Mill Creek 16.5? It looks to me like there's plenty of room for one and seem pretty straight foward.




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RE: mill creek dager board?

A leeboard would work just as well, plus you would not have to cut a hole in a perfectly good boat.  Also, think of this.  


You are cruising along at a good clip with the daggerboard down.  Suddenly you ram a rock just under the surface going full speed.  The dagger board breaks loose and since it is mounted between your legs it goes straight into your.....  Well you can guess where.  Just something to think about.

RE: mill creek dager board?

 You could add a daggerboard or a set of leeboards. The problem with the daggerboard grunk is once it's glued in it's there to stay. If the dagger is not positioned in relation to the center of effort of whatever sail you use the boat will not sail properly.

A proper sailboat can always bring you back to where you started because it can tack against the wind. If it doesn't tack you will have to paddle home every time.

Leeboards  can be mounted on a crossbar and clamped in place. Then you can juggle the position till you get it right. I don't much care for leeboards but they are the only set-up that has that adjustability factor. 

RE: mill creek dager board?

The Oxford Shell includes a skeg that is fastened to a single slotted piece of wood which is inserted into the shell via a cut in the hull, and the standard epoxy fillet to secure it. The skeg itself is attached using marine caulk.  When you hit the rock, the skeg breaks free of the boat, and the hull of the boat remains intact.

It is  unobtrusive, and probably would serve.

RE: mill creek dager board?

Actually, for a leeboard to work correctly, it must be at the point of max beam. That pretty much sets where the sail's CE has to be just as strongly as if you had a daggerboard in a well.

As far as being damaged by a broken daggerboard - the trunk should be strong enough to contain it. I wouldn't worry. Besides, even with a sail, how fast is a Mill Creek going to go, anyway?


RE: mill creek dager board?

I read about this 'point of max beam' thing recently (perhaps on this forum?). I'm very happy to accept this as useful knowledge but I wonder why it must be so? can anyone enlighten this hydrodynamically challenged builder?

I'm in the process of making a sail rig and associated bits (leeboard, rudder, thwarts...) for an existing 'canoeyak'. The picture becoming clear is: leeboard at max beam, sail COE at leeboard COE (+ a bit for weather helm), net result is there aint much choice about position for mast step (given sail rig).

Any other gems like this one? seating position re COE? rules of thumb for leeboard and rudder sizes - relative to sail area? I need Yacht design for Dummies! I'm afraid after so many hours of reading / doodling / and standing around holding offcuts and sticks at strange angles to my hull that I'm still about to break one of the golden rules!

   Dave P

RE: mill creek dager board?


Understanding Boat Design by Ted Brewer is what that book is actually called. Great inroductory book, though focused more on larger boats than kayaks and canoes. Designing Power and Sail by Arthur Edmunds is the next step up. It brings in the math (simple formulas) needed to actually design boats. Again, mostly a non-kayak focus, but many of the principles are the same.

For lateral planes (skegs, dagger, lee and center boards) to be effective, they have to be aligned parallel with the boat's centerline. Otherwise they create a torque which causes the boat to yaw. The perfect example of this is the rudder. It is a lateral plane that occasionally gets deliberately misaligned to cause a yaw which changes the boat's course.

Unless you're building a Bolger Brick, your hull is going to have some curvature when viewed from above (plan view).  The only way that a leeboard which is tangent to the hull can be positioned to be parallel to the centerline is at the point of max beam. In front of that point it will be pointing in, behind it will be pointing out. In either case it will be trying to make the boat yaw toward the side away from the leeboard.

How about making a bracket which holds it parallel, even when it's not at the max beam? someone calls out. 

First, mechanically that can get complicated. Among other things, the point of max beam is usually stronger than in front or back of it. Think of an arch turned sideways. Then there's the extra complexity of the support structure. But even if that's all solved, you'd end up with some odd hydrodymanics as the board interacted with the hull.

If the board was in front, it would act as a scoop and create a high pressure point at the back of the board. This pressure would exert a force which would cause the boat to yaw.

If in back, it would create a low pressure point which suck the boat into a yaw and slow the boat down.

This is not a problem at the max beam point because the 2 effects pretty much cancel out fore & aft. They end up forming a low pressure point in the center which is directed toward the center of the boat, sucking the boat in the direction away from the board. If the board is on the downwind (lee) side of the boat, the boat gets sucked upwind, thereby cancelling the leeward motion of the boat.

That's why the name leeboard. To work best it needs to be on the lee side of the boat. That's also why serious leeboard sailboats have a board on each side. The one on the weather (upwind) side is normally retracted while the one on the lee side is in the water.

You can get by with one leeboard, but then it has to be bigger and deeper because it will be relying on brute force sideways resistance instead of the elegance of the venturi effect. On small low performance boats this is fine, but as you scale up the off-center weight of the board and the extra drag become unacceptable.

Finally, you're absolutely right Dave. Once you've set 2 of those parameters the 3rd one is pretty much defined.

Welcome to the world of boat design,


RE: mill creek dager board?

L. wrote:

>If the board was in front, it would act as a scoop and create a

>high pressure point at the back of the board. 

I think you meant "low pressure point", yes? 

Also, why would this low pressure area cause a net yawing force?  This was not clear to me.

RE: mill creek dager board?

Hey Camper,

High pressure aimed backwards, low pressure aimed toward the centerline.

The off-center high pressure on a single side of the boat causes the yaw. Picture a model boat and push on the back edge of the board with your finger. The boat will turn since there's nothing balancing out the push.



RE: mill creek dager board?

Laszlo, thanks for the very good info. I never expected the venturi bit, I presumed the leeboard was just lateral resistance, but now that you've explained it I can see I should have wondered (more) why people would bother to have two and why it was called a leeboard!

So a daggerboard IS just lateral resistance? I respect the honesty. The idea of a single leeboard pushing me the wrong way half of the time using complicated  suction vectors feels devious. Chris, go get your jigsaw and cut a slot for that daggerboard, I vote for hydrodynamic honesty over cockpit space and fertility! (so easy to say when its not my hull ;-)

    Dave P

 (the blind man gropes fowards less tentatively.....stumble..mutter......ooof )

RE: mill creek dager board?

Laszlo, I did understand that the pressure on the leeboard creates an outward force on the leeboard, which would be a yaw force if located forward or aft of the CLR.

What I did not understand is why there would be a net force, since the pressure pushing outward on the leeboard at a given point is also pushing inward on the hull.  The two forces would seem to cancel each other.

RE: mill creek dager board?

Thanks for all the input guys. Dave, I was actually going to use a router and the fertility issue is a moot point. Physical pain is a completly different story though.



RE: mill creek dager board?

Hi Chris, I too have contemplated a daggerboard, now I wince when I think about it...

Some thoughts I had: frame the slot with thick ply and build a seperate daggerboard case that bolts down to that frame (SS nuts embedded in epoxy in the frame on the floor?), with cross brace at top that clamps to 'carlins?'. That way daggerboard case is removable. Bolt in a shaped plug to fill the hole when removed. (Heck make a wiiiide slot and make that plug in lexan so you can watch the sharks...) Make daggerboard smaller in fore aft direction for some adjustment relative to sail COE, (and perhaps slide over that rock!).

Finally: disguise daggerboard case as: beverage holder, GPS / depth sounder mount, rod holder, live-bait tank, umbrella stand, iPod dock... now everyone will want one... let's see a leeboard do that!

Seriously, I'd like to see some pics of your MC sometime, with or without daggerboard. 

cheers, Dave P 


RE: mill creek dager board?


Sorry, it's not that simple :-) Daggerboards aren't just lateral resistance, either. They're actually a foil shape (fatter at the front, skinnier at the back & smoothly streamlined), just like a symmetrical  cross-sectioned airplane wing. And just like an airplane wing, they generate lift, just sidways. Depending on the boat's speed and other factors, this sideways lift can actually be more effective than just the lateral resistance alone.

Just as with symmetric aircraft wings, the direction of the lift is controlled by the angle of attack (the daggerboard's angle relative to the waterflow), which in a boat is controlled by the rudder. With a positive angle of attack (the bow pointing to the weather side by some amount) the daggerboard will lift the boat into the wind, cancelling the leeway.

Since even a flat wing can generate some lift as long as it has a positive angle of attack, a flat board can be used as a daggerboard. However, as the boat reaches some combination of speed and heading, the daggerboard will start vibrating in its well and leeway will increase. This is because the board is stalling out. The usual precription for this is to streamline the board, which lowers its stall speed.

Long narrow planes (high aspect ratio) have less resistance than short fat ones. That's why daggerboards are dagger-shaped.

So it's all forces, vectors, thrusts, pressures, etc. What I think is really neat is the way our boatbuilding ancestors worked out all this stuff empirically before the science was developed.


If I get time, I'll post a diagram, but the quick answer is that the force is centered on the back of the board and is aimed parallel to the centerline, not in toward it.



RE: mill creek dager board?

Thanks, Laszlo.  Yes a diagram would make it clearer.  Truth be told, it is only  to satisfy my constant curiousity about "how boats work", as the Sharpie already has her daggerboard trunk installed, and Chris's and Dave's questions have already been answered.

I spent much of our summer vacation last week at the Jersey Shore reading Ross Garrett's "The Symmetry of Sailing", if that is any indication of how enthusiastic to learn more about this subject I've become!

There's one other reason for my interest in your explanations.

I also spent a lot of time last week gunkholing in the back bay behind Avalon and Stone Harbor, in my beloved scratched up Old Town kayak (yes, it is possible for a wood boat person to love a Tuppayak, as long as it's not shaped like a giant Nike sneaker and colored Day-Glo lime green.)

I can't paddle for very long without my mind drifting to the idea of strapping a couple leeboards on the thing and somehow hoisting a patch of Dacron, to permit an occasional switch from TastyKakes-crabcakes-and-ale (a non-renewable energy source) to my favorite alternative fuel--a 10 knot breeze 5 or 6 points off the bow.


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