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Old 11-05-2008, 13:30   #16
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I would also fair the trailing edge of the rudder to quite a sharp edge.
No. Absolutely not. The wing does not work that way. You need a trailing edge wider than you would think. There is a formulae for working that out. I imagine it is "spoiling" the water as it leaves the back of the foil and thus breaks drag.
Having things like wings on the bottom of the keel and rudder can make big differences. But for every action you take, produces some other reaction elsewhere. It is a law of Physics. The reactions may be negative in some aspects, but without them, the boat wouldn't move forward with wind coming form front to side.
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Old 11-05-2008, 13:35   #17
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I'm not recalling where I know this from, but I agree with Wheels on this one.

There is a purpose to having a rounded trailing edge on your rudder and it has to do with the "bite" it has in the water. A perfect razor edge would cause it to work improperly.

I don't know where I picked this up, but I have heard this as well.

(Please don't make me get my old physics books from college out of storage.... ha ha ha) My math is too rusty!

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Originally Posted by Alan Wheeler View Post
No. Absolutely not. The wing does not work that way. You need a trailing edge wider than you would think. There is a formulae for working that out. I imagine it is "spoiling" the water as it leaves the back of the foil and thus breaks drag.
Having things like wings on the bottom of the keel and rudder can make big differences. But for every action you take, produces some other reaction elsewhere. It is a law of Physics. The reactions may be negative in some aspects, but without them, the boat wouldn't move forward with wind coming form front to side.
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Old 11-05-2008, 15:32   #18
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Anyone spent time on reducing the under water drag? Care to share your experiences, what you did and the results?
If you really want to go for speed on liquid water (without an engine) you won't find it in cruising. Not even with a Gunboat. So why fight it?

You max speed will come from minimal surface area and low drag, not just low drag. So take up racing. I'm not joking.

International Moth class - over 27 knots in a sub 11-foot boat by hydroplaning. which can be done in winds of around 7 mph. Around 15k to get going.

Maybe you want a bit more room. Rave Hydrofoil holds two and gets over 30mph. From their website:
Nothing ticks off a jet skier like passing him in something without a motor. And it's possible in The WindRider Rave ... the world's first affordable production hydrofoil!
Nothing wrong with wanting to go for speed and cruise. But they don't really mix all that well. But remember kids:

Reduce parasitic drag by keeping your hull clean.
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Old 11-05-2008, 15:39   #19
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The latest trend on rudders is to narrow them down to a 1/4 inch thick perfectly square end. Belief being the flow of water nests back together as if the foil were tapered to infinity... which would be extremely long for a given foil aspect ratio, and very fragile... the trailing edge would have to be metal. Interesting point about all that... is that in some folks opinion you calculate the rudder force 10% or so larger than the surface area of the rudder. (Haven't found anyone applying this stuff to keel hung rudders... May have to give it a try.)

At the moment mine is an ear shaped slab sided barn door...
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Old 11-05-2008, 15:48   #20
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Back to the hull surface for a moment...

A friend of mine raced a J24. Before each season, he would paint the bottom with a hard antifouling paint, and then wet-sand it with successively finer grit sandpaper until he could see actually his face in it.

Did it work? Don't know, but it at least made him feel like he was faster.
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Old 11-05-2008, 19:31   #21
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In terms of cost effectiveness...Unless the Triton has a poor rudder and keel design you will not gain much by remaking them, and risk changing the balance of the boat. You can find NACA (NASA's origin) foil design software on the web, or buy templates to compare your keel section to an optimal shape if you really want to go through that exercise.

But...the most bang for the buck probably will be gained from a belt sander and a box of hole saws, so you can remove all excess material and weight from the boat. Got a door on the head? Throw it out, use a curtain, save weight save fuel. Those kind of alterations are strictly against most racing rules--but they are wonderfully effective. At a certain point, if you drill enough holes and remove enough structure, things break, but that's also why they say that if you don't break anything--you're not really racing. Get every extra ounce off the boat and out of the structure, and you'll accelerate faster and save fuel when motoring.

That also includes re-evaluating every line on the boat, looking for lighter weight lines that also don't absorb water (heavy stuff), and taking a second look at your anchor and tankage, too.
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Old 11-05-2008, 22:27   #22
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I've always wondered why racing crews have fat guys aboard.
Why not total muscled crew?
When they spend big bucks to shave weight what's up with crews with beer bellies?
Winning is all about the power to weight ratio.

An admittedly 20 lbs overweight, but not racing
Steve B.
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Old 12-05-2008, 02:08   #23
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The latest trend on rudders is to narrow them down to a 1/4 inch thick perfectly square end.
I am not so sure you could call it a "latest trend". It has been part of fluid dynamics for many many years. The guys that carried out the greatest research on this was in fact the Germans during the 30's with the major advancement in aeroplane wings.
The reason is actually more related to a Golf ball believe it or not. It is also the reason why some cars have spoilers on the boot. However, not may cars have a spoiler for the right reason. It's often just looks.
Anyways, I digress. It's about breaking laminar flow. In fluid dynamics, the reason why a wing, or in our case a rudder works, is due to the water wanting to "stick" to the surface and follow the curve of the surface. That "sticking" is what is then used by the rudder angle to give the rudder "lift". If I go back a step. Two actions are always taking place. When the rudder has no angle of attack, the flow of water creates pressure in the positive No's on the increasing diamension's of the foil and then goes neutral and back through the negative No's on the decreasing diamension's of the foil. The both sides cancel each other out. This is Bernallis principal [sic: Bernoulli's Principle]. As the angle of attack is increased in a direction, the flow over the high side side increases in velocity and decreases on the underside. Thus Bernallis principle tells us that the faster flow reduces in pressure, the slower increases in pressure and a force acts on the rudder to make it "suck" to the direction of attack.
However, a problem exists at the trailing edge. The water wants to "stick" to the wing surface as it leaves. It's called laminar flow and that flow has to be broken. By creatng a square edge, small vortices are created. These vortices or eddy's breaks the flow from the wing and thus reduces drag.
At least that is how I understand it.
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Old 12-05-2008, 22:03   #24
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Alan,

Thats how I understand it as well, except Bernoulli's principle doesn't quite describe why lift is made only looking at the foil. Instead of having one side of the foil longer than the other to develop lift... like an air plane wing or car spoiler foiled rudders are symmetrical. They rely on the angle of attack (steering angle) to develop lift.

Real interesting stuff, as various foil shapes are better for lift or drag reduction... but what the foils have in common is that they stall out! A slab sided rudder always works as a speed brake, but a stalled foil rudder may as well have nothing attached to helm. But... because foil rudders make lift, a smaller steering angle is required to see the same results! Less drag because of shape, or because of helm angle?

Makes one wonder about speed potential of a boat that requires a lot of steering angle to stay on course... anything off of straight, and its burning off power. (Though I must say it is fun to try and drop the rail under water, even if it means holding the tiller to my ribs....)

Thinner cross section foils do not necessarily mean less drag... where as a slab on a sided foil, thinner is better. The kink here? The 4 digit NACA foils don't mind being slapped on various thicknesses... 6 digit foils on the other hand, the leading edge radius is defined in the formula! In order to apply the latest formulas, the width is also key...

For me, I see this stuff more along the lines with Form drag. Most boats out there aren't going to play with adding foil shapes to keels and rudders... may as well pile the appendages on with the hull form part of the pie and stick to surface friction. (Some saying about sows ear and silk purses comes to mind...)
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Old 12-05-2008, 22:13   #25
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Hellosailor,

Carl Alberg updated his line drawings to a more modern design rudder. Squared off bottom end with a little more surface area. These boats have a fair bit of weather helm, and before I venture off shore to far I'll have rebuilt the rudder, replaced the shaft, and beefed up the heel fitting. Going back stock is always an option, but she's almost 50 years old... there have to be some tricks out there worth applying.

(The head door is gone, icebox is getting rebuilt... and a lot of foam cored panels are planned for use on the interior... creative interpretation of white painted "Plywood" with varnished trim. )


Hud,

I have a sneaky suspicion that most of this is just an academic exercise... and its all in my head anyway! If we really wanted to go fast, it'd be a lot easier to grab a cat/tri/ticket for a 747. (Thankfully airfare is expensive enough to warrant messing about in boats... at least thats my justification! )

Steve,
I've wondered the same thing when it comes to auto racing... Aluminum cylinder heads on a muscle car, with a beer gut to round it out. I guess so long as the center of gravity is low enough....
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