Posted By: tback
Daggerboards -- Aspect Ratio Explained - 11/07/06 10:42 PM
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:
Why is this true? Would not the longer boards (higher AR) cause the boat to have more underwater resistance to sideways motion?
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.
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?
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.
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.
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
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?
