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Re: Carnac 2012 - results [Re: Gilo] #247849
05/06/12 07:53 AM
05/06/12 07:53 AM
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Australia
macca Offline
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It is Sunday... I do take some time out every now and then smile


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Re: Carnac 2012 - results [Re: macca] #247851
05/06/12 08:05 AM
05/06/12 08:05 AM
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waynemarlow Offline
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Now I'm actually interested in Macca's views on this as he does come up with some good sound engineering knowledge at times. Yes Macca can be a pain, but we all have opinions and as far as I'm concerned the more varied opinions aired,the more the development our class will have.

My problem has been as a solo I have no jib and therefore no slot effect over the main. With the very flattish spinny's we are now running, downwind the spiny is now so far foward and out that there will be negliable slot effect on the main, drop the clew back a bit and the sail combination now starts to look more like a screacher and main with a better slot.

Views and sail set up knowledge would be appreciated.

Re: Carnac 2012 - results [Re: Gilo] #247852
05/06/12 08:17 AM
05/06/12 08:17 AM
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Australia
macca Offline
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On the m20 we made a 1m longer pole than std. Was always better. Yachts have the same issue, longer pole faster and deeper is the result.

To deal with lee helm you have several options. Most effective is to move boards forward, a 25cm longer pole is usually offset by 5-7cm forward with the board. not so easy to do but most F16 designs I have seen are on the aft side of fast from my observations. Particularly the taipan which is what Wouter is basing his observations on.

You could fit higher aspect boards in the original cases, this will have a similar effect.


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Re: Carnac 2012 - results [Re: macca] #247854
05/06/12 08:26 AM
05/06/12 08:26 AM
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Macca, Are you saying that on a boat with lots of apparant wind, that there is no slot effect from the Spinny over the main and one would be better utilising the lift from the spinny to get the bows out more ( less drag )

There are other ways of getting lee helm or weather helm out of a boat as well such as simply moving the rig more upright or back ( or even moving the base from its usual position on the crossbeam, or simply balancing the rudders themselves better by altering their angle of incidence slightly.

Forget the M20 Macca and lets talk F18 A Class type of knowledge which are much more compariable to the F16's

Re: Carnac 2012 - results [Re: Gilo] #247855
05/06/12 08:47 AM
05/06/12 08:47 AM
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Australia
macca Offline
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If the boat is balanced and fast upwind but has lee helm downhill then it's not so good to mess with mast rake or rudder rake, it is much better to move the board position forward. It will have none or super small effect upwind (In a reasonable range) but will fix the lee helm.

The slot effect is minimal compared to the increase in air to the front of the kite. There are two things that make you faster downwind:-

1. Longer luff length
2. Longer pole

If I could have a 4.5m long pole on the F18 I would. But right now I will be happy that I can paint my boats smile


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Re: Carnac 2012 - results [Re: macca] #247863
05/06/12 10:50 AM
05/06/12 10:50 AM
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waynemarlow Offline
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Originally Posted by macca
If the boat is balanced and fast upwind but has lee helm downhill then it's not so good to mess with mast rake or rudder rake, it is much better to move the board position forward. It will have none or super small effect upwind (In a reasonable range) but will fix the lee helm.

The slot effect is minimal compared to the increase in air to the front of the kite. There are two things that make you faster downwind:-

1. Longer luff length
2. Longer pole

If I could have a 4.5m long pole on the F18 I would. But right now I will be happy that I can paint my boats smile

Moving the board forward will I guess move more effort onto the board and away from the rudder, the A's seem to have such small boards that the A has to take some of the loadings onto the rudders to compensate and the rigs position becomes far more critical to balance things up a bit.

The F18's seem to be going the other way with almost all of the lateral effort placed on huuuuuuge loooooooong high aspect boards with diddy small rudders just touching the water at the back. I guess thats all to do with the change of balance of the boat when that big spinnaker goes up.

Still not convinced about the spinnaker not forming some form of slot, the Shadow used to have a mere hankerchief as Spinny run straight from the bow, it was a very fast boat considering how little sail area it had. The new X version seems to be more normal in its spinny arrangement and seems to go just as well with a larger area.

I guess one will have to give it a try some time, I guess just like being able to paint your boats ( thought that had been resolved in your favour )

Re: Carnac 2012 - results [Re: waynemarlow] #247864
05/06/12 12:14 PM
05/06/12 12:14 PM
Joined: Sep 2007
Posts: 571
Hamburg
Smiths_Cat Offline
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Quote
Still not convinced about the spinnaker not forming some form of slot

Make you free of thinking of "slot effect".

The first sail is always in the upwash of the second one, which is always in the downwash of the first one. The more seperated the sails are the lower the drag by the way. But aerodynamic drag is not very important on downwind legs.
To increase lift the slot must be very narrow (about 40 mm to give you a number), stable and form a convergent nozzle with the mast. All of this is not present on todays soft sail designs, hence no slot effect.
Hence, as macca said, a longer pole is better (for exact the reasons he mentioned above) as long as you can steer the boat.

Another question is, is leehelm a bad thing (a bit of lee-helm, not massive)? I have the feeling that with leehelm the rudder pushes the stern down and bows up. Hence you can have abit more power in the rig before you start to submerge.

Cheers,

Klaus

Re: Carnac 2012 - results [Re: Smiths_Cat] #247865
05/06/12 12:48 PM
05/06/12 12:48 PM
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waynemarlow Offline
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I wish you could tell that to both texel and SCHRS as to regard the slot effect. Try inputting a 14 m main and 3 m jib and then a 17 m main only, into there calculators and there is considered to be a huge difference.

I am sort of getting very confused about this whole scenario of slot and non slots and jibs and thingies cry

Re: Carnac 2012 - results [Re: waynemarlow] #247872
05/06/12 05:32 PM
05/06/12 05:32 PM
Joined: Jun 2001
Posts: 9,582
North-West Europe
Wouter Offline
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North-West Europe

boards on the Taipan arre far back as the V-ed bows runs deep and narrow like a plate, but the aft sections have rounded keel. There is always a play between boards, rudders AND the hull. Rounded keel line on the bow required different board placements. Can 't easily compare board positions on boats with significantly different keel lines.

I shied aways from making complete new daggerboard wells over gaining 0.25 mtr pole length. Sue me !

BTW lots of leehelm is bad under spinnaker as rudders act like a brake then. I know because I tried. My current setup is the fastest without completely rebuilding the hulls or the daggerboard wells.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
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