Old 25th March 2007, 08:07 PM   #1
tonymatta
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Default centreboard size

Hello Roger

I have a Starboard Z-class board which I have discussed with you before. I am considering enlarging the centreboard to improve upwind angle in light wind in displacement mode. I would like your thoughts on how much improvement if any I would get from say 20 to 30% increased area.

The thickness would need to stay the same because of the gap in the board. I can go 25mm longer and maybe 50mm wider.

Appart from getting in the way above deck when retracted, will it do more harm than good with increasing drag?

At the lower displacement speeds, how important is having the perfect machined foil shape compared to hand shaping?

Any other problems to consider?
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Old 25th March 2007, 08:44 PM   #2
Roger
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Default RE: centreboard size

Hi Tony,
I've never personally "shaped" a centerboard (nor a fin for that matter)
so I can only answer your questions from a "what I've seen others do" perspective.
I've seen very few other sailors make oversize centerboards, and mostly they were pretty "oversized" guys.
They seemed to think the larger centerboard was an improvement, but I never noticed them moving up in any racing fleet, so I don't know whether there's really an advantage or not.
If they normally finished in about 15th position in a fleet of 30 sailors, they seemed to finish at about the same position in the fleet with the oversized CB.
Longer and wider might give you some increase, but what makes you think your board has less than an optimum centerboard from the factory?
Part of the problem here is that if you make a larger more efficient CB with more "bite" to go upwind in really light winds, you probably can't control the thing when the wind and boardspeed increase.
So, what you gain in super light wind displacement mode you tend to give up when you have to retract or partially retract the CB in higher winds to keep your board from rolling over.
I think balancing light wind upwind performance with higher wind performance is the real criteria for CB's on sailboards.
At one time the Fanatic Cat was an all carbon skinned board with a 3 different sizes of CB's you could buy. I sailed the new board with what the rep said was the "lightwind" CB (larger with a foil designed for about 10-12 knots boardspeed).
I took the board out and it immediately turned over. I had to sail that CB half retracted just to get back to the beach. The board didn't turn well or go upwind. Guess I was just going too fast for the big CB.
Changed to the mid size CB and things were much better.
You could make up a couple of new CB's in marine grade plywood (glassed over of course) and see what effect a larger CB will have on the Z-Class.
Yes, the drag will increase, so you might get a little boost upwind only to find that your overall board speed is less.
Having a good foil shape is always important, but probably less so at displacement speeds.
I know that Starboard put countless hours of development into the Z Class board, and a ton of on the water testing, so I'm not sure you are going to be able to improve on what you have.
Are you "railing" your Z class to get the optimum upwind angle?
Hope this helps,
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Old 27th March 2007, 08:26 PM   #3
tonymatta
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Default RE: centreboard size

Hi Roger
I'm not sure that the CB I have is less than optimum.
I have been racing this summer against the dinghies at my sailing club and I have no problem keeping up in stronger winds but I am much slower in light winds. The comparison to other sailboards is less important to me than to the dinghies I am sailing against because there is not much sailboard racing here.

I baught an RSX to see how they compare because no one else around my area had one. I found that it was significantly faster in the light wind and railed more easily. It has several differences to the Z-class but one of them is the longer and wider CB.

Both boards don't rail easily because they are so wide and do point much better once the wind is strong enough to rail. the RSX has the nonplaning footstraps so it is easier to force the rail but I still need a bit of wind in the sail to do that. On the other hand I love the stability that the width provides because it is almost impossible to fall off going upwind or downwind.

My purpose in improving the light wind performance is also to be able to use a smaller sail to reduce the chance of being overpowered when the wind gets strong and sailing the smaller rig tends to be more pleasent.

The dinghies sail with one sail size so I prefer to also sail the same rig in all conditions if possible. I have been sailing my Z-class with a 10.6m V8 2004 model and I would like to get to the same performance with a 9.5m if I can.

I suspect the Z-class is optimised for planing performance and the CB needs to come up long before the 1m wide board will roleover.

I keep looking enviously at the comparatively giant centreboards on the dinghies that are pointing higher and going faster than me with smaller sails. I could leave it all as is but curiosity is getting the better of me.



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Old 27th March 2007, 09:36 PM   #4
Roger
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Default RE: centreboard size

Hi Tony,
I see!
I'd check with Tiesda You (the design guru at Starboard and see if they still have the mast box that slides laterally as well as for and aft.
Then you coould apply your mast foot pressure off the centerline (to leeward) and this (plus the fact that you can move your feet more to leeward) might help you rail the Z-Class.

also, look around for the top half of the next longer size mast for your big V8. Usre a stiffer top section of your mast to put more tension in the upper part of your sail.
This should serve to improve your light wind sailing.
Sails that twist off are good for fully planing speeds, but compare the top of your V8 with the dinghy sails you are racing with. Bet the dinghy sails do not twist off at all.
They have adjustable cunninghams and boom vangs to control the tension. You have twist and full battens.
As far as the larger CB, if you are already going lower and slower (due to hull drag and CB drag), it would seem to me that a larger CB would just induce more drag unless you can get one of the foil guys to make you something really special in a high aspect low drag foil with lots of "bite" at lower speeds. You'd need to figure out what boatspeed it needs to work best at, and then have something with a tall (high aspect ratio), but shorter chord and thicker foil I'd guess.
Or, get some marine plywood, and experiment.
Hope this helps,
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Old 28th March 2007, 06:42 AM   #5
tonymatta
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Default RE: centreboard size

Thanks Roger,

I will try increasing the tension on the top of the sail. that is easy enough to do.

I get the message that the centreboard option is not too likely to help. I can try an experiment as you sugested, just for fun perhaps.

Are the race sails in the same size like the RS:6 and new RS:racing likely to be better for nonplaning than my 10.6 and 9.5 two cam sails?

Does a side sliding mast box fit over the the other box or is it built into the board? How do i get in touch with Tiesda? can you pass on my enquiry?

You always take the time to answer my question in detail and with consideration. Thank you for this help and for this great Forum.

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Old 28th March 2007, 10:00 AM   #6
Roger
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Default RE: centreboard size

Hi Tony,
Ummm.... increasing the tension in the top of the sail may not be as easy as you seem to think it will be.
First, you have to find a stiffer mast, and then you need to make sure the ferrules are the same size. You would have to build some sort of a half mast test bench to really establish that the longer top piece is truly stiffer.
Race sails larger than 9.0 m2 are basically formula race sails and formula boards are "apparent wind machines". So, I think you are better off with your existing sails or perhaps sails a little smaller in the Free Race category.
More tuning range, and not so "high apparent wind" oriented.
I'll send a note to Tiesda and see if any of the prototype mast boxes are available. You would have to remove your existing mast track and dig a large cavity in your board, so maybe it's just too much modification.
Hope this helps,
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Old 28th March 2007, 07:50 PM   #7
tonymatta
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Default RE: centreboard size

Hi Roger
I wouldn't bother to replace the mast track if I needed to dig up the board. It sounds like I should just accept that I will come last on the very light days and keep working on my skills rather than the equipment. Spend the time on the water instead of the workshop.

Again Thanks for sharing your experience.
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Old 28th March 2007, 08:05 PM   #8
Roger
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Default RE: centreboard size

Hi Tony,
I got a note back from Tiesda this morning, and they are still working with the "lateral mast track system" but the hardware isn't fully developed yet and they have none that they could sell.
In the meantime, try to step as far across to leeward as you can, and
rail your board as much as possible.
Also work on your tactics.
Hope this helps,
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Old 30th March 2007, 02:53 PM   #9
infinity
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Default RE: centreboard size

Hello Tony,

I've been looking into building a centre plate, I've already built one to maximum dimensions (low wind) but am curious to find out how different profiles perform (eg a reduced thickness profile) for higher wind. I've noticed exocet's longboard comes with two plates.

Here's a photo of my first attempt
http://silmaril.math.sci.qut.edu.au/~jonesmb/wind/ahd_08.jpg
http://silmaril.math.sci.qut.edu.au/~jonesmb/wind/ahd_16.jpg

Anyway you might found some useful info at
http://www.moth.asn.au/building_registration.html
look for the file building_moth_hardware.pdf

Also do you compete against other windsurfers? I also live in Brisbane but did not know anyone was actively racing long/hybrid boards. While I've never raced, longboard racing is far more appealing to me than short or formula board racing. I might be tempted to give it a go even though the board I use is from the 80's!

Malcolm
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Old 30th March 2007, 06:43 PM   #10
tonymatta
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Default RE: centreboard size

Hello Malcolm
I have been racing at lake cootharaba. There is one other person who also races a sailboard regularly at that club. He is sailing a mistral superlight long board he uses 9.3m to 11.m sails depending on coditions. He usualy finshes four laps for each of my three in the races. The 80's race boards are very competive. The latest phatoms seem to be closer to the long board designs than the modern hybrids.

I am sailing tomorow Saturday afternoon at 2.00pm at the Brisbane Sailing Squardron at Bulimba against the Laser fleet if you want to try your hand. This is the last weekend of the season for most clubs but there are odd events on through the winter.

I'm sure ther are many long board sailors who would like to get back into it. I have found sailing with the dinghies to be lots of fun. Since there are some that are faster and some that are slower so there is always a bench mark to work towards. The dinghies have always been happy to sail mixed classes and most clubs if not all have handicap systems so you can be in the race no matter what you are sailing or how good you are.
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