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Daggerboards -- Aspect Ratio Explained #88503
11/07/06 05:42 PM
11/07/06 05:42 PM
Joined: Mar 2006
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Orlando, FL
tback Offline OP
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I thought I'd extract the discussion on daggerboards and put it here

The link to the original is here.


Specifically I wanted to inquire about some points that Wouter made and gain further knowledge:

Quote
Roughtly speaking the higher the aspect ratio is the better the ratio between lift and induced foil drag is. So you can get more sideways resistance for a smaller penalty in drag. But in real life things are never as easy as this.

You can overdo it in aspect ratio and end up with a boat that just won't lay still on the start-line, it will drift sideways quite fast when its hull speed is below a certain minimal value.


Why is this true? Would not the longer boards (higher AR) cause the boat to have more underwater resistance to sideways motion?

Quote
This is a big disadvantage as a another boat with normal boards can just park underneath you and wait till you hit him and then protest you out of the race. Only option is then to sail over the line early and restart. It needs no explanation that this is can be a serious drawback.

Low aspect boards have the advantage of allowing the boat to quickly get up to speed. And they do allow very harsh turns. In heavy winds the low aspect boards will allow you to depower later and thus put more power into the rig.


Is this true both upwind and downwind under spi'? This past week we sailed the first couple of races with the daggerboards raised (my crew was conditioned to do this from racing F18 and N20's). The result was we got tripped up on puffs with no weather helm to drive down ultimately causing us to pitchpole. Matt and Gina sailed by between races and encouraged us to "keep the boards down" ... no further capsizes and helm was maintained.

Later, when asked the "science" behind this, Matt stated that with boards down we can sail faster (no side slipping) maintaining apparent wind ... thus, when the puffs hit, there isn't as much difference between apparent wind speed and the puff.


Quote
In ligher winds the high aspect boards put you on the trap sooner and thus lift the luff hull clear of the water earlier allow some drag reduction that way as well.

So you see it all dependents where you put the most weight.

Currently the newest F18's have all aspect ratios between 5.0 and 5.5. This is unimaginable high with respect to only 5 years ago. Allowing carbon cloth in the F18 daggerboards excellerated this development.

The stealth F16 featured higher aspect boards since the beginning; aspect ratio = 3. The Spitfire has a daggerboard ratio of 3 as well. The Viper F16 will probably end up around 4 to 4.5. The Taipan and Blade have ratio's of 1.9. This is growing into a significant difference. It was time for the Taipan and Blade F16 to go with the times. Well the Taipan is now being replaced by the Viper and the Blade is going for a new daggerboard design.

Funny enough in really light winds and strong winds the low aspect boards are very comparable to the high aspect ones in the way of pointing and speed. It is the mid section of windconditions were the low aspect boards are lacking a bit.

There is also another drawback of low aspect boards, one that is linked to one of its best advantages. You can keep the low aspect boards fully extended to very high windranges. This means less work on board and nothing in the way to hit your elbow on. However when you do raise them a bit their lift to drag ratio decreases rapidly. Also the F16's are more and more sailed both as a 1-up and 2-up boat, raising your daggerboards is becoming more and more a standard procedure to depower a boat. The high aspect boards do allow a singlehanded sailor a better fine-tuning of the exposed board area without decreasing the lift/drag ratio too much. It should also make the boats calmer when doing 1-up in a blow while retaining pointing.

In order to win back some midrange performance and to not get the difference with the main competition get too big it was decided to develop the Blade F16 daggerboard further.

But it is not going as high in aspect ratio as the newest F18's. The new boards have an aspect ratio of about 4. This should give them enough boost in the midrange without trading-off too much of the advantages the old design did have.

For all you low aspect board sailors out there (and I'm one of them myself). There is a trick to get some additional performance out of your boards. Especially when the the boat is flightly you can try to almost fully lift your luff board and keep the leeward board fully down. You'll load up the leeward board alot more but that is the intention and it can easily withstand the forces. The intended effect can be explained like this :

A high aspect board cat can raise both boards (aspect ratio 4) halveway and end up with halve the exposed area operating at a new aspect ratio of 2. If the low aspect ratio boarded boat (=2) would do the same then he would end up with halve the area too but operating at an aspect ratio of only 1. However, if this boat would fully extend its leeward board and fully raise the windward one then he would also end up with halve the area but now operating at an aspect ratio of 2 as well.

The only drawback is that you'll have to move the boards up and down during each tack. But this may be worth it. Having higher aspect boards makes this easier.


Good luck,

Wouter


Can I assume that most comments made were beating to weather? How would high AR foils be placed under spi? half way up like the bigger boats?


USA 777
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Re: Daggerboards -- Aspect Ratio Explained [Re: tback] #88504
11/07/06 06:09 PM
11/07/06 06:09 PM
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St Petersburg FL
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The only reply I think I can provide is, I have learned that sailing with spin up and boards down is easier to control the boat. What Matt told you is true. When the puff come in, with boards up the boat feals extremely twitchy. With boards down the boat just gains speed and lift is NOT as violent.

It is also much easier to trap out while running spin with boards down compared to trapping out with boards up.

Re: Daggerboards -- Aspect Ratio Explained [Re: tback] #88505
11/07/06 06:12 PM
11/07/06 06:12 PM
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Thanks T.

Re: Daggerboards -- Aspect Ratio Explained [Re: tback] #88506
11/08/06 03:19 AM
11/08/06 03:19 AM

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Hi all,

not realy big on theory, but my experience downwind with low aspect boards is. I have always been a boards down, under spinnaker sailor, but recently in survival conditions have been pulling them up half way, I think this allows boat to pull away easier and it doesn't follow waves as much and slips sideways depowering some?

I would imagine the longer the boards the sooner you should pull them up some. <img src="http://www.catsailor.com/forums/images/graemlins/tongue.gif" alt="" />

Regards Gary. <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Re: Daggerboards -- Aspect Ratio Explained [Re: ] #88507
11/08/06 03:36 AM
11/08/06 03:36 AM
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Essex, UK
Jalani Offline
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The Stealth, as you know, has high(er) aspect boards. I've found that solo in a blow the boat's much happier on threequarters or even half boards.

Downwind in all conditions I'll lift the boards, less in lighter conditions than when it's blowing, but never more than half up. You need to retain some grip on the water, as Matt says. If you get a puff downwind without boards the boat just doesn't accelerate as well and if you then try pointing up to accelerate the boat just overpowers and trips in.

I've tried sailing downwind several times with full boards but it's just too much like hard work. The boat is too 'grippy' on the water and flies at the slightest change in apparent.

Just my experiences as I see them......


John Alani
___________
Stealth F16s GBR527 and GBR538
Re: Daggerboards -- Aspect Ratio Explained [Re: Jalani] #88508
11/08/06 04:06 AM
11/08/06 04:06 AM
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Netherlands
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The point is that the boat needs a bit of point to rotate around. When you pull the boards up the force direction is more side ways when the puff is coming in and then you don't have enough grip ( rotate point) to stear the boat down and you are always to slow in accelaration, but when the boat have some grip you are able to stear quick enough, the boat will "rotate" around the board. When you leaf a bit of grip available the boat will not go sideways only it will go forward also and this will give you the possiblity to stear in a puff lower. In A-class we leave ,when doing wildthing, the lower board half in to have grip and the higher board full in the hull, when we put both boards up the wildthing is not going that fast because the sideway motion is to big. When you have more sail (spnaker) to generate more sideway motion the problem is even getting worse.


Best regards,

Hans Klok

Web : http://www.catamaranparts.nl
Blog : http://catamaranparts.blogspot.nl
Mail : info@catamaranparts.nl

Raptor F16 and A-class builder
Re: Daggerboards -- Aspect Ratio Explained [Re: Hans_Ned_111] #88509
11/08/06 06:44 AM
11/08/06 06:44 AM
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Posts: 461
Victoria, Oztralia
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Hi All

Just while were on the subject of daggerboards, which would be the best profile to use, I'm thinking in terms of a board at 290mm wide x 23.2mm thick, i.e 8%, do you stick with the 0008 section or go for something like a 63008 section?

Regards


Matt Harper Homebuilt Taipan 4.9 AUS 329 'GOT WOOD' SEEDY PIRATES RACING TEAM
Explaining my own statements [Re: Jalani] #88510
11/08/06 08:50 AM
11/08/06 08:50 AM
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North-West Europe
Wouter Offline
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Explaining my own statements.

Low aspect daggerboards do not take well to being raised. So our current Blade and Taipan boards are best left fully down in all conditions except survival ones. Here raising the boards just a little can have a settling effect, but again only in really heavy conditions where you can otherwise not hold proper sail shape.

Additionally, with low aspect boards it is also always better to raise only 1 board a little while leaving the other fully down. That is if you really have to raise some board. This way you reduce area and sideways resistance a little (because you have far too much for the conditions) while not impacting too much on efficiency (aspect ratio)

High ratio aspect boards are different in this. Because they maintain sufficient effciency even while raise significantly. However their advantage is also their drawback. They are nice and firm in sideways resistance (good pointing), moreso then the low aspect boards, BUT in strong or unstable conditions they are TOO firm and lead to tripping and thus need to be raised. Luckily that can be done efficiently because they have a high aspect ratio. If this was not the case then no builder would ever use high aspect boards.

I hope this helps.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Explaining my own statements [Re: Wouter] #88511
11/09/06 07:12 AM
11/09/06 07:12 AM
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Thailand
Buccaneer Offline
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Quote


The Viper F16 will probably end up around 4 to 4.5.


Yikes! Not sure about actual speed but it's sure to produce a significant inconvenience increase over the Taipan… <img src="http://www.catsailor.com/forums/images/graemlins/cool.gif" alt="" />


"House prices have risen by nearly 25 percent over the past two years. Although speculative activity has increased in some areas, at a national level these price increases largely reflect strong economic fundamentals." – Ben Bernanke – 2005
Re: Daggerboards -- Aspect Ratio Explained [Re: mattaipan] #88512
11/09/06 12:55 PM
11/09/06 12:55 PM
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CA
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Quote
Hi All

Just while were on the subject of daggerboards, which would be the best profile to use, I'm thinking in terms of a board at 290mm wide x 23.2mm thick, i.e 8%, do you stick with the 0008 section or go for something like a 63008 section?

Regards


The 63008 is a laminar flow section, and should be avoided for daggerboards. Yes, laminar sections are lower drag, but any vertical scratches (inevitable on daggers) will destroy the laminar flow property, and they will behave worse than the normal 00XX series. Laminar flow sections must have a flawless surface to realize their benefits, and are therefore better suited to rudders and fixed keels on racing boats stored on land.

As for thickness, I've heard that 0009 and 0010 are really hard to improve on for marine applications. Going thinner makes the boards weaker, and you will not be able to build as high aspect ratio board. Assuming you want the highest aspect ratio boards possible to minimize drag, reducing thickness from 0009 to 0008 will force you to reduce aspect ratio by more than 20%, for the same area, load, and construction. With all the stories of boards breaking,
I don't think I'd drop below the conventional wisdom of 0009.

But I'm not expert.

Re: Daggerboards -- Aspect Ratio Explained [Re: Glenn_Brown] #88513
11/10/06 01:53 PM
11/10/06 01:53 PM
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So Ok we select a high aspect dagger board, so now where in relationship to the mast ( beam ) and rudder do we site it in the hull to get optimum upwind and downwind. Some A's seem to be going further back toward the rudder than the F16's, any sages on this.

Re: Daggerboards -- Aspect Ratio Explained [Re: waynemarlow] #88514
11/11/06 05:12 AM
11/11/06 05:12 AM
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Wouter Offline
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Quote

Some A's seem to be going further back toward the rudder than the F16's, any sages on this.



I don't see any correllation to us.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Daggerboards -- Aspect Ratio Explained [Re: Wouter] #88515
11/11/06 06:11 AM
11/11/06 06:11 AM
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Quote

Quote

Some A's seem to be going further back toward the rudder than the F16's, any sages on this.



I don't see any correllation to us.

Wouter


Me neither.

Reasons for them moving the rig back in the boat? Well we'll all agree the A is optimazed for going upwind. Maybe now they are looking at downwind performance, and in a blow, moving the rig back in the boat will make it less pitch-pole-prone.


F16 - GBR 553 - SOLD

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Re: Daggerboards -- Aspect Ratio Explained [Re: scooby_simon] #88516
11/11/06 04:44 PM
11/11/06 04:44 PM
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Eastern NC, USA
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tshan Offline
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Are there any limitations to the ratio/size of F16 daggerboards? Did not see them in the rules, but spent a total of 15 seconds looking.

What would be the results of adding 8 or so inches to the Taipan/Blade daggerboard length?

Last edited by tshan; 11/11/06 04:45 PM.

Tom
Re: Daggerboards -- Aspect Ratio Explained [Re: tshan] #88517
11/11/06 09:03 PM
11/11/06 09:03 PM
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Tom,
If you did that you would reduce one form of drag at the expense of another. Basically the drag due to lift would drop while the drag due to wetted surface would go up.
There can be some benefits by increasing the length of the board provided you don't increase area. But as mentioned earlier these benefits can come at a price.

BTW:- There is no limit to ratio/size of dagger boards in the rules.

Regards,
Phill


I know that the voices in my head aint real,
but they have some pretty good ideas.
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Re: Daggerboards -- Aspect Ratio Explained [Re: tshan] #88518
11/12/06 06:40 AM
11/12/06 06:40 AM
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Wouter Offline
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Tom,

For singlehanding you would most likely end up with too much board area and only make the boat more flighty and more easily to trip. The only solution would then be to raise the boards a little which would directly negate the lengthening of the boards in the first place.

For the remainder I side with Phills comments.

In effect, simply lengthening a board is not interesting.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Daggerboards -- Aspect Ratio Explained [Re: Wouter] #88519
11/12/06 12:42 PM
11/12/06 12:42 PM
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waynemarlow Offline
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Guys you didn't answer my question, where abouts should one position their daggerboards in relation to the mast, ie is there a magic spot where the sail forces should meet the daggerboard and I'm talking about position front to rear here.

Re: Daggerboards -- Aspect Ratio Explained [Re: waynemarlow] #88520
11/12/06 12:52 PM
11/12/06 12:52 PM
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Wouter Offline
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Quote

is there a magic spot where the sail forces should meet the daggerboard and I'm talking about position front to rear here


Not that I'm aware of. This is because the underwater shape of the hull is a factor in this as well as mast rake and distribution of sailarea.

Without these things you can only guess and I don't do guessing

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: Daggerboards -- Aspect Ratio Explained [Re: waynemarlow] #88521
11/12/06 12:53 PM
11/12/06 12:53 PM
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West coast of Norway
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You want the daggerboard CoE (Center of Effort) to balance the sailplan CoE. With the A-cat, which has only one sail, they can put the daggerboard further aft. F-16s need to consider the spi and jib sailarea, so the daggers are a bit further forward.
Then some designers start to spread the load generated by the sails between the daggers and rudders, either by mast rake or by daggerboard placement. There are written whole books on this subject, Marchaj has written what I think is the definitive work: http://www.amazon.co.uk/Aero-Hydrodynami...TF8&s=books

Re: Daggerboards -- Aspect Ratio Explained [Re: Rolf_Nilsen] #88522
11/13/06 01:38 AM
11/13/06 01:38 AM
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Sydney Australia
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Most sailboat design principal relates to mono's not cats and if adopted can put you out by a significant amount.

It's been my experience that for good balance, the CoE in the sail configuration should be directly over the CoLR, which in cats is most likely at the dagger boards. Variations in mast rake will effect this position.

I think a good test would be to put an existing boat in the water without a rig or rudders, with boards down and apply a lateral force at the c/l of the boards. I plan to do this next time the boat goes in the water.

When I designed the 430, I opted for a mid position between the hull CoLR without foils and the CoE in the rig. The boards could be further back.

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