I don't know exactly what to call this--or explain it, but here goes:

Sunday, beating to weather in 12 knots (+gusts -- my speedPuck registered 15.3 mph max speed), I noticed on several occasions that as the F16 accelerated into the puffs the rudders would lose attachment to the water flowing over them; thereby causing the steerage to be "squirrely" as the rudders attempted to "dig in" again.

Anyone experience this? Any explanation on why this occurs?

T - Cavitation of the rudders can be caused by a couple of different things but it usually has to do with boat/sail trim. It was a "problem" on the 20 with the 1st gen rudders. If you had your weight in the wrong place or sails trimmed improperly, the rudders would cavitate. Get everything right and in the right place and there was no problem. Rudder cavitation was a great indicator that you were doing something wrong and needed to adjust some things on the boat.

Yep. Usually under spin. Saturday, I had lee helm for the first time (under spin).

It really didn't feel like "cavitation". Where I'd normally have a slight weather helm ... it changed to being VERY NEUTRAL...thus the feeling of the rudders "slipping".

Maybe "cavitate" wasn't the right word, but what you described is exactly what would happen on the 20 when things weren't trimmed right or weight wasn't in the right place.

Or maybe you got it right and you just don't know what that is supposed to feel like.
Sorry T, Cound not resist.

Hi Karl!


T, is that on the Viper? I haven't experienced that with mine yet.

I'm sure that was it! It was a fast ride up the river. Seth/Nancy and I were never separated by more than 10 boat lengths for 2 hours ... only until the wind laid down about 2 miles from the finish did we separate (we went off on different tacks looking for wind. Seth finished 32 seconds ahead of us (after 2.5 hours of sailing).

No, I'm sailing the VWM Blade prototype.

Hi Karl

Terry, did the rudders feel heavy when this happened? I've had mine stall out when trying to bear off quickly in a puff (going downwind) but no problems going upwind. A quick wiggle of the tiller (unload the rudders) will re-attach the flow.

I have also had them cavitate but only on a blast reach, going very fast on one hull. It felt like I had hooked a trash bag with the low rudder but when I looked down there was just a huge air bubble where the rudder was, it was hard to move the tiller, I had to ease the main, slow down a little to get the flow back on the blade. This would also happen on the I 20 first generation rudders quite a bit.

I seem to remember the rudders became a tad longer after the first few blades.

Could be something specific to the first series of 5 boats.

I know exactly what you are talking about. Seems to happen on my boat more with the starboard rudder. Letting out the main a tad and taking a step back towards the transom usually helps. However, it is slow. I have the rudders from #707.
IMO, I think the rudders on the Blade need to be redesigned. A friend has several extra 18ht rudders and I have really thought about trying those out on the Blade.

Done. There were 2 generations of Blade Rudders produced and a 3rd developed for the Falcon.

The original was a slightly enlarged version of the style used on the Oz A class at the time. The second generation foil increased the section and provided a slight increase in the aspect ratio, all to improve lift and make the rudder less susceptible to stalling out. There were also some changes to the head to make it work better with the pull down system and the castings.

The very thin sections in theory have less drag. They are in practice a bit more sensitive to stalling (or cavitations) Pinching being the best way to stall them out, but also they are much less forgiving to radical control movements and junk in the water.

I'm a "pincher" -- I know! Another reason to not pinch ... but I was trying to stay "high" so that I wouldn't have to tack to make each causeway!!

so the falcon rudders are thicker and longer than my rudders? are they a one-to-one replacement? would I need to modify my castings?
My boat has always felt a little squirelly (sp) when driving downwind. never been sure whether that was a function of light boat/overly sensitive rudders/piss poor driving

ermmm ........... all of the above?

Matt so are you saying the Falcon rudder blades are thicker in section but narrower from front to back? And if so, does that mean you made them slightly longer to make up for the lost area?

Unfortunately they are not interchangeable. In the generational changes we modified more than just the foil section.

Squirrely down wind and having cavitations or your rudders stall are 2 different things. 1 of the design compromises on the F16 is transom length behind the rear beam. There needs to be some minimum foot length on the main sail, so unless you are willing to place a stiff beam for the boom section, most sheet systems opt to directly attach to the clew of the main. There is also a desire to keep the maximum tramp size to allow for easy handling of 2 crew. The tiller length then on the Blade is then pretty short. The same arm travel in your steering can result in almost twice the angel change in the rudders relative to some of the 18 and 20 foot boats. Of course a lighter weight powered up platform will be significantly more sensitive to the conditions than the heavy weight, or even the longer light HTs.

Foil thickness is expressed as a percentage of the chord length. i.e in a 10% foil, a 190mm fore and aft measurement will result in a maximum thickness of 19mm. Where that thickness occurs is dependent on the foil section chosen. There are a variety of foil section theories so I will not even get into that.

I have tried to keep fairly close to a certain surface area on the foil sections. There is definitely a higher aspect on the new Falcon rudders, but the driver in the shape change was to provide some other features than just a higher aspect foil. The latest trends in the A class and you can see this on the C as well, is to go to an extreme aspect ratio. Some of the A class rudders I have seen are down to 75mm in chord. This is for that whole reduced drag kick. The problem with this is that you have to keep water moving over them to keep flow attached. Sporadic control movements, pinching and parking on the line really become an issue. This kind of follows my view of the curved foils. There is a potential for a small performance edge in certain conditions, but the skill level required to drive the boat to that point becomes an order of magnitude higher. The top guys are going to make any platform work and will win races. To me a great design is one that the average guy can get on and be able to drive to its numbers over the whole range of conditions.