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PostPosted: Tue Jul 24, 2012 8:29 pm 
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So did you try it?

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PostPosted: Tue Jul 24, 2012 9:04 pm 
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Chris, waving a Miragedrive sideways in the water would only have relevance if Islands could sail through the water at 90 degrees to the direction the hull is pointing.

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PostPosted: Tue Jul 24, 2012 9:42 pm 
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tonystott wrote:
Chris, waving a Miragedrive sideways in the water would only have relevance if Islands could sail through the water at 90 degrees to the direction the hull is pointing.

That's precisely what they are trying to do when they are sailing close hauled. The whole point of the daggerboard is to stop them doing that. If the daggerboard was highly effective, there would be no side slippage and the Islands would be able to sail at maybe 30° to the wind instead of 45°-50°. The point is there is substantial side slippage, despite the best efforts of the daggerboard, so there is plenty of lateral force on the drive fins. The point about dragging them sideways through the water is to demonstrate how effective they are at resisting that force.
In practice, a number of experienced sailors on this forum have reported they can make the boat point a little higher by using the drive with the fins down.

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PostPosted: Tue Jul 24, 2012 10:18 pm 
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Chris, are you really suggesting that Islands suffer from 15 to 20 degrees leeway? I think you will find that measured leeway is more in the order of 5 to 10 degrees tops. With such small >real< leeway angles, combined with the looseness at the "clew" of the fins plus their ease of flex (required for them to form variable pitch "propeller blades" when pedaled), I simply do not believe they will contribute any meaningful resistance to leeway.

Also, the only instances "a number of experienced sailors on this forum have reported they can make the boat point a little higher by using the drive with the fins down" relate to them PEDALING while going to windward in light airs, where the additional forward motion from the Miragedrive generates higher apparent wind which in turn increases drive and pointing ability from the sail. That is an unrelated issue.

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PostPosted: Tue Jul 24, 2012 10:26 pm 
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I normally use only the Mirage Drive down as a daggerboard below about 6mph winds and I use both the Mirage Drive down and the Daggerboard when sailing tight to the wind and need to point high. That's what works best for me.

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PostPosted: Wed Jul 25, 2012 12:21 am 
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I have no doubt that the extended mirage drives influence pointing somewhat when beating upwind, but from a practical standpoint, pedaling slowly with them allows me to go faster and point even higher.

So that is the newbie sailing tip I would suggest.


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PostPosted: Thu Jul 26, 2012 10:44 pm 
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chrisj wrote:
tonystott wrote:
Chris, waving a Miragedrive sideways in the water would only have relevance if Islands could sail through the water at 90 degrees to the direction the hull is pointing.

That's precisely what they are trying to do when they are sailing close hauled. The whole point of the daggerboard is to stop them doing that. If the daggerboard was highly effective, there would be no side slippage and the Islands would be able to sail at maybe 30° to the wind instead of 45°-50°. The point is there is substantial side slippage, despite the best efforts of the daggerboard, so there is plenty of lateral force on the drive fins. The point about dragging them sideways through the water is to demonstrate how effective they are at resisting that force.
In practice, a number of experienced sailors on this forum have reported they can make the boat point a little higher by using the drive with the fins down.


These boats do suffer quite a large amount of slippage, I compare this to my Hobie Wave and it can point considerably higher than my AI. It can be seen easily when you sail close hauled with the fins and daggerboard, the pivot point moves forward and the boat will change heading. I use this when I get lazy on a starboard tack to steer the boat without having to stretch for the rudder. (the rudder needs to be in the right spot to start with).I still enjoy my AI more way more than the wave tho!

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PostPosted: Thu Jul 26, 2012 11:30 pm 
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Perhaps the larger sized centreboard on the TI makes a difference. Yesterday I was out sailing for nine hours, in winds from 5 to 15 knots, and I repeatedly tried to find any measurable difference in pointing ability with the fins down or flat against the hull. Nope. Maybe there is a placebo effect in place...

I know I am flying in the face of many people's opinions, but I have read lots regarding the hydrodynamics of keels and juggling the lift/drag ratio (there is just as much science in choosing keel foil profiles as there is for aeroplane wing airfoils, especially as water is incompressible so gains or losses can be greater. For instance, on my quarter ton level rating yacht, I shaved the rear of the keel into a really sharp 1/4 inch wide edge, as per the Kamm effect used in racing cars). It is for this reason that the daggerboard and centreboard have somewhat of an aerofoil cross section. Compare this ti a miragedrive fin. Not only is it flat, but being flexible means that the more sideways force is exerted on it, the more the back edge bends out of the way. Obviously there must be some force generated to cause this bending, but I submit that it would be almost too small to measure.

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PostPosted: Thu Jul 26, 2012 11:50 pm 
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tonystott wrote:
... Yesterday I was out sailing for nine hours, in winds from 5 to 15 knots, and I repeatedly tried to find any measurable difference in pointing ability with the fins down or flat against the hull. Nope. Maybe there is a placebo effect in place...

Were you in the back seat? I think it works, because in the AI and the front seat in the TI, the Mirage Drive is forward of the daggerboard and forward of the pivot point.

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PostPosted: Fri Jul 27, 2012 12:05 am 
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I was in the front seat

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PostPosted: Fri Jul 27, 2012 2:50 am 
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chrisj wrote:
tonystott wrote:
Chris, waving a Miragedrive sideways in the water would only have relevance if Islands could sail through the water at 90 degrees to the direction the hull is pointing.

That's precisely what they are trying to do when they are sailing close hauled. The whole point of the daggerboard is to stop them doing that. If the daggerboard was highly effective, there would be no side slippage and the Islands would be able to sail at maybe 30° to the wind instead of 45°-50°. The point is there is substantial side slippage, despite the best efforts of the daggerboard, so there is plenty of lateral force on the drive fins. The point about dragging them sideways through the water is to demonstrate how effective they are at resisting that force.
In practice, a number of experienced sailors on this forum have reported they can make the boat point a little higher by using the drive with the fins down.


As I understand it, most of our pointing ability is determined by the type of sail rig, and has much less to do with the centerboard or daggerboard than one would believe. The simplicity of our boats is their weakness, cat-rigged, no boom, no traveler on the sheet to adjust it's position, and no jib. Bermuda rigs point 30, cat-rigs point 40~50, look it up!

I sailed into a regatta by accident once, and I sailed with them for a while. On a broad reach and running before the wind, the TI's performance was equal or better than anyone else except a big cat with bermuda rig. However, once they rounded a buoy and sailed close hauled, the only sailboat I could keep up with was a cat-rig (it actually was a cat-ketch rig). Wind was about 15-20 and gusty. I could make pace with the fleet if I a sailed off the wind a little more, but couldn't do both without cheating.

j

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PostPosted: Fri Jul 27, 2012 4:10 am 
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bassetm1 wrote:
chrisj wrote:
tonystott wrote:
Chris, waving a Miragedrive sideways in the water would only have relevance if Islands could sail through the water at 90 degrees to the direction the hull is pointing.

That's precisely what they are trying to do when they are sailing close hauled. The whole point of the daggerboard is to stop them doing that. If the daggerboard was highly effective, there would be no side slippage and the Islands would be able to sail at maybe 30° to the wind instead of 45°-50°. The point is there is substantial side slippage, despite the best efforts of the daggerboard, so there is plenty of lateral force on the drive fins. The point about dragging them sideways through the water is to demonstrate how effective they are at resisting that force.
In practice, a number of experienced sailors on this forum have reported they can make the boat point a little higher by using the drive with the fins down.


These boats do suffer quite a large amount of slippage, I compare this to my Hobie Wave and it can point considerably higher than my AI. It can be seen easily when you sail close hauled with the fins and daggerboard, the pivot point moves forward and the boat will change heading. I use this when I get lazy on a starboard tack to steer the boat without having to stretch for the rudder. (the rudder needs to be in the right spot to start with).I still enjoy my AI more way more than the wave tho!


Three things make comparing those two boats irrelevant: first, the Wave is a MUCH faster boat which means it can point closer because it's own speed changes it's apparent wind (that's why peddling the mirage drive lets us point higher) and second, you have a planning cat compared to a full displacement tri and third, you are comparing a cat-rig to a bermuda rig.

Put a bermuda rig on a AI or TI and they point much higher (see Fusioneng's posts, also note the way he turns the mirage drive into a hydrofoil. The angle is maintained by a spacer, if left loose they are just drag.) or put a cat-rig on your wave...

j

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PostPosted: Fri Jul 27, 2012 5:54 am 
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kayakman, I'm not claiming personal expertise in this area, but looking around a few sailing forums, the relative importance of rig vs hull in pointing ability seems to be hotly debated, with the most common consensus being that they are about equally important.

I found a few comments that Bermuda rigged boats have a slight edge upwind, but couldn't find any statement that the difference was as much as 30° vs 45°-50°. Do you have a link to that?

The superior upwind performance of Bermuda rigs seems to be attributed to the triangular shape of the mainsail being less prone to twist than the four sided cat rigged sail. In that respect, fusioneng's setup isn't really a Bermuda rig, since it lacks a triangular mainsail (and likewise the Hobie Wave).

Couldn't your inability to keep up with the other boats in the regatta going upwind be due to you being the only trimaran, so more prone to leeway than the other boats?

Incidentally, do you think a TI being sailed solo from the rear should be thought of as a full displacement or a semi-displacement hull? I'm sure you would have seen Eduardo Cavendish's clip of him sailing close hauled from the rear seat:



I don't know if you'd call it planing, but his bow is mostly up out of the water.

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PostPosted: Fri Jul 27, 2012 2:27 pm 
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That video is always fun to watch. Ed does a great job as both skipper and daredevil cameraman! But much of the extreme lift/heeling seen there are the result of hopping the waves that day. In flat water, same wind, I think his bow would be wetter and his lee Ama always buried. This would illustrate the natural displacement mode of the hull.

When Ed really guns it on a starboard tack, you can see the dagger getting air. Long before it does, it's at a poor angle and losing grip with the water. The long, buried TI Amas are providing lateral resistance.

In our current TI3 tests, we are trying to keep the dagger/rudder fully submerged while lifting the bow. We're making progress moving the bow wave back, but are a long way from planing while riding a "balanced" hull. :cry:

We are still trying to enter a true plane mode, but I don't think it's possible with the standard TI config and sail. Some high wind testing with the Amas in the rear position will tell us if it's going to happen.

Hey - Didn't this thread start life as "Tips for Newbies?"


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PostPosted: Fri Jul 27, 2012 3:06 pm 
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Chrisj

chrisj wrote:
kayakman, I'm not claiming personal expertise in this area, but looking around a few sailing forums, the relative importance of rig vs hull in pointing ability seems to be hotly debated, with the most common consensus being that they are about equally important.

I found a few comments that Bermuda rigged boats have a slight edge upwind, but couldn't find any statement that the difference was as much as 30° vs 45°-50°. Do you have a link to that?


I used your numbers from a from a previous post, but they seem fairly close in my experience. In optimal conditions, I think 35 is reasonable, 40 in less optimal conditions, and 45-50 over 25-30 knots of wind.

chrisj wrote:
The superior upwind performance of Bermuda rigs seems to be attributed to the triangular shape of the mainsail being less prone to twist than the four sided cat rigged sail. In that respect, fusioneng's setup isn't really a Bermuda rig, since it lacks a triangular mainsail (and likewise the Hobie Wave).


From my research, it's the jib that is the big upwind engine. The mainsail is relatively inefficient upwind. So any boat with a jib should point further upwind and at greater speed than one without. I don't have a jib or a bermuda rig so I don't know these things by personal experience.

chrisj wrote:
Couldn't your inability to keep up with the other boats in the regatta going upwind be due to you being the only trimaran, so more prone to leeway than the other boats?


Two trimarans flew past me, one came from way back in the pack. Both had bermuda rigs. So no, I don't think the number of hulls has much to do with leeway. Hull design certainly can affect leeway but I think Hobie has done a very good job with the TI hull. I have forgotten to lower the centerboard on occasion and found it does okay without.

chrisj wrote:
Incidentally, do you think a TI being sailed solo from the rear should be thought of as a full displacement or a semi-displacement hull? I'm sure you would have seen Eduardo Cavendish's clip of him sailing close hauled from the rear seat:

I don't know if you'd call it planing, but his bow is mostly up out of the water.


I would call it planning, I've topped out at 13.3mph. With a minimal load and enough wind the TI will plane, however that doesn't mean it's planning hull or even semi-displacement. It is very much a full displacement hull. It does have a large flat bottom that can act as a planning surface, but with a full load the hull design limits the speed to five or six knots.

j

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