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 Post subject: Lost My Mirage Drive
PostPosted: Sat Sep 06, 2008 4:59 am 
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Location: Punta Gorda, FL
File this one in your "don't do this" file.

I decided to sail yesterday evening without the Mirage Drive, but wanted to bring it along in case the wind died. I just put it behind me, kind of looped under the bungees.

When I got away from the boat ramp and pulled out my sail, the mainsheet must have snagged it. I reach up and grab the clew of my sail and just rip the whole thing out, then take up the slack in the mainsheet.

There was pretty good wind, and when I felt the sheet coming tight and hauled it in, I heard this "splooosh!" from behind me. Looked back to the empty well. :( I know pretty well where it is, but my experience with getting things back out of the muck has not been good. :(

We sailed for a while, then the wind died and I got to paddle home. My barber hauler setup is really in the way of paddling, and I'm very spoiled on the power of the Mirage Drive.

I have to go open the office in an hour, and we have Mirage Drives on inventory boats that I can take and replace later. I'm going to go sailing again this afternoon, this time with a leash on my Mirage Drive. ;)


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 Post subject:
PostPosted: Sat Sep 06, 2008 4:02 pm 
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Location: Central Coast NSW Australia
Tragic! :(
I too enjoy sailing without the drive if the conditions are right. I have found that the drive fits nicely resting on the front crossbar with one fin under the hatch bungee.
Image
The axle fits into the hatch groove which helps to stop it moving around.

Image

A bungee attached to the crossbar is all that is needed to hold the drive down.
Image
I always tether the drive before pulling it though! :wink:
With the drive in the front it makes it easy to put back in when needed. :)


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 Post subject:
PostPosted: Sat Sep 06, 2008 6:36 pm 
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Great idea stringy, and a nice looking small sail adapter! 8)


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 Post subject:
PostPosted: Sat Sep 06, 2008 9:15 pm 
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Location: Dallas, TX
Had a customer in today who has a couple of Mirage yaks... Apparently Chris knew the story, but I hadn't heard it till today.

Guy stores his yaks on White Rock Lake in the middle of Dallas. The lake is prone to flooding. The guy stored his drives in a dock box.

Flood comes. Box floats off. Guy buys new drives to replace those lost when box floats off.

Two years pass.

Different guy walks into store with two drives in a box he found on the shore of the lake, partially buried in mud. Didn't know what they were but asks around. Someone tells him to find us. He does.

You can guess the rest.

Karma.


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 Post subject:
PostPosted: Sun Sep 07, 2008 5:05 am 
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Location: Punta Gorda, FL
Cool, Dog.

Stringy, I like your solution, but have two problems with it. My legs are long, and my feet are against the bulkhead, so that pedal would be in the way. Also, the harbor chop is steep around here, and my boat frequently dives through waves. I'm not so sure it could hang on in that position.

I grabbed another Mirage Drive yesterday and went sailing, but winds were light so I was actually using it at times, and never removed it from the well. I'm thinking that if I bungee it down a bit better in the well behind me and put a leash on it, that will work OK.


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 Post subject: On Daggerboards
PostPosted: Sun Sep 07, 2008 5:20 am 
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Location: Punta Gorda, FL
By the way, in the course of sailing without my Mirage Drive, I tried to sail across the wind briefly without the daggerboard. I almost reached the corner to turn downwind, and pulled the daggerboard a bit early.

Wow. Don't do that either! Boat goes sideways! I quickly grabbed my paddle to help myself around the corner. With no Mirage Drive and no daggerboard in very little wind, my boat was only slightly faster than my wife's boat running downwind. Then she started pedaling.

I have noticed several posts by Matt Miller regarding the angle of the daggerboard, and have experimented with mine. It makes no sense, but it seems as if the boat performs better in almost all situations with the daggerboard swept back. The exception is upwind work in strong winds when hard on the wind with good boat speed.

To learn about efficient fluid dynamics, all you have to do is examine gliders. They don't have swept back wings. It would be easy to make swept back wings, but only high speed aircraft have them. Or look at racing sailboats. They don't have swept back daggerboards, but again it would be easy to make them.

Angling the daggerboard back on the AI should cause more drag, but all I can say is, it seems to make my boat go faster and straighter. Anyone else?


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 Post subject: Re: On Daggerboards
PostPosted: Sun Sep 07, 2008 9:10 am 
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Tom Ray wrote:
I have noticed several posts by Matt Miller regarding the angle of the daggerboard, and have experimented with mine. It makes no sense, but it seems as if the boat performs better in almost all situations with the daggerboard swept back. The exception is upwind work in strong winds when hard on the wind with good boat speed.


That's no surprise to those of us who've sailed centerboard boats. And the principles are sound.

Ever seen an F-111 in flight? It has wings that can be swept. Wings fully forward for maximum lift at take-off. Swept back for maximum speed. The fastest aircraft have swept wings.


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 Post subject:
PostPosted: Mon Sep 08, 2008 5:11 am 
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High speed aircraft have swept wings to reduce the drag caused by flying near the speed of sound. Adventure Islands are fast, but not that fast! ;) Wiki has a pretty good explanation:

http://en.wikipedia.org/wiki/Swept_wing

If swept back daggerboards were faster, we would see them on race boats.

I'm a former flight instructor, and sail lots of centerboard boats (we're also Precision and Com-Pac dealers). I think I know what's going on with the observed increase in speed when my daggerboard is swept back.

In the Com-Pacs and Precisions, if the sail is properly set and there is excessive weather helm, the correct response is to pull up the centerboard a bit. This moves its center of effort aft, and makes it a less efficient swept back wing, with more drag. The boat goes faster because you have to use less rudder to go straight, and the rudder is always a brake.

With the balanced rudder on the AI, I'm probably dragging my rudder along sideways in order to go straight when the daggerboard is straight down. I angle it back and it allows me to use less rudder, so I go faster. I'm unaware of the effect because the balanced rudder has little to no "feel" to it, and the little lever is not great for judging the precise angle of the rudder, so I never notice the change.

That's my current guess, anyway...


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 Post subject:
PostPosted: Mon Sep 08, 2008 5:51 am 
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Exactly.

Quote:
It makes no sense, but it seems as if the boat performs better in almost all situations with the daggerboard swept back.


So define your version of "better." My definition is "faster with better helm control."

Yes, my explanation was over simplified. But Hydrodynamics and Aerodynamics are not that different. One fluid is just a lot denser than the other. What gives such excellent lift also creates all the drag. The AI is pretty dang fast for a 16 foot "boat". I've had one into double digit speeds on several occasions. That's pretty significant for something with a theoretical hull speed of less than 6 knots (yeah, yeah... I know. 1.34 x sq rt of LWL doesn't really apply to multihulls).

And I can't remember seeing a truly vertical leading edge on a keel in a long, long time. It can't always be just about shedding debris and weeds.


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 Post subject:
PostPosted: Mon Sep 08, 2008 8:32 pm 
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Tom, it makes sense that you're moving the CE back (closer to where it probably should be) and putting less load on the rudder. Tacks a little quicker? Reefing the sail may achieve a similar benefit.

How do you control the board's position? 8)


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 Post subject:
PostPosted: Tue Sep 09, 2008 4:15 pm 
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RR,

I just rotate it, as it is designed to do if you hit something.

Dog, here are some vertical boards. The reason they are not swept back is because they're on race boats.

Image

Image

Image

It's true that on the latest Oracle boat the board in the center hull appears to have some sweepback to it, but the boards in the amas (though curved) are not swept back. Race boats with any sweepback angle are hard to find. Similarly, you can't find a performance glider with swept wings.

Image


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 Post subject:
PostPosted: Tue Sep 09, 2008 6:03 pm 
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I notice that 3 out of 4 boats are multihulls. And all are race boats. Since the AI isn't a race boat, how about a comparison to other non-race boats? :wink:

p.s. Let's not use the knuckleheads over at Oracle as a good example of anything. Their motivation is a whole lot different than yours or mine. If you're an Anarchist, you know what I mean.


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 Post subject:
PostPosted: Wed Sep 10, 2008 5:36 am 
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Dog,

I think you're missing my point. Angling back the board on the AI will generate less lift and more drag than leaving it straight down. If you want to see a vertical board on a non-race boat, check the MacGregor 26M, but my point in using race boats (and gliders) as examples is that when efficient lift really matters, people who know fluid dynamics use shapes that look like glider wings. Swept wings are not used because they're less efficient, at least until we're talking about dealing with shock waves in transonic flight.

They are also not used on aircraft because they tend to stall first at the tip, not the root, which is very unsafe. On boats, we don't care about that effect, and there are some good reasons why many boats have angled keels. They're less efficient, but not all that much less efficient.

An angled wing still creates lift, just less lift per unit of drag created than a straight wing. On the AI, it is creating that lift (and a bit of extra drag) a little further aft. Because a vertical board is creating too much lift too far forward, the effect must be counteracted with the rudder, which is a HUGE source of drag when compared to the difference in drag between a swept and a vertical board.

That's why I think that what is happening is this: I'm angling my board back, making it a bit less efficient, but I'm going faster because the small amount of extra drag created, and small amount of lift lost, is more than made up by moving the whole center of effort aft, resulting in the need for less rudder.


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 Post subject:
PostPosted: Wed Sep 10, 2008 5:44 am 
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Getting back to the thread topic, I spent a couple of hours dragging the creek bottom for my lost Mirage Drive without success. I figure eventually one of my neighbors will catch it while fishing, or it will stay down there forever.


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 Post subject:
PostPosted: Wed Sep 10, 2008 6:38 am 
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I saw the point you were trying to make, but don't agree with the conclusions. I do agree with less lift. I disagree with more drag. Though we don't really include another important variable, point of sail. But I think neither of us is going to convince the other, so I'll let it go. Our experiences are just different.

Cheers.


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