Upwind - Fast -- Rudder "slip"

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 <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

ermmm ........... all of the above? <img src="<>/wink.gif" alt="wink" title="wink" height="15" width="15" />

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.

Matt, am I reading that right? 75mm for the cord on a rudder?
2 15/16

? With your example of 10% that would make the rudder 5/16

at its thickest point...That seems insane...how do you make anything with those dimensions stiff/strong enough to function at all irregardless of keeping laminar flow?

what, you mean these things aren't just thrown together from parts that are sitting around? <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

Pete same problem. The f18 Shockwave uses extended pintles maybe cheaper than new rudders, you would only need the arms to be lengthened then

Looks like one way to solve a little of the

over steer

problem....especially on an existing boat...but how does that sit with the rules? And if what you have pictured is class legal...how much farther can you carry the pintles aft before the rules have been broken?

Curious to how much smoother the steering can be enhanced before it is considered an extension of the hull, and is included in the overall length.

My PERSONAL understanding of the rules is that rudder gantries are legal provided that they do not add bouyancy or waterline length. Therefore they must be solid, heavier than water or open vented tubing that cannot float.
The overall length of an F16 hull excludes rudder fittings.

Wayne, couple of questions...
1) Did you ever sail that boat with the rudder attached directly to the transom without the stand off pintels? If so what percentage improvement did you feel it achieved by moving the rudders back?

2) What's the motivation for mounting the rear beam so close to the transom?

Thanks for sharing your insight...

Regards,
Bob

It was to overcome a problem the F16's had started to get into design wise. We all have started moving the front beam back further in the boat like the A's basically to get the upwind Uni sail working better ( the uni sailor is not allowed to have a jib ) and bringing the Spinnaker COE back further into the boat to prevent pitch poling ( it does seem to do that ). Bitsa has got to the point in that despite quite fine bows, it will side slip rather than dig the nose in even when provoced to try and make it so. A part of that also is my belief that as an Uni sailor we are over square at 2.5m beam so reduced it to more the A's of 2.3m

The problem lies with the box rules in that we have only a 8.5 m mast height allowed which then makes the sail much lower aspect than the A's and creates a much longer foot. That means then that the rear travellor has to be much further back than an A, so the beam has to go back further in the boat which then immediately flags up the short fulcrum arms. The 150mm gantry is a waste of time as far as leverage goes, it was just a method of getting the arms longer.

There is an alternative to the arms we use and I wish I had used that method, Have a look at the C class boats, they use a much better, far less vulnerable to accident damage ( crew falling into the cross arm )and probably a lot lighter system. Knowing now the problems caused by the extra leverage of the gantry, I wouldn't do the same again particularly as we cannot fair the lower gantry in to make it more streamlined as per the box rules.

Bitsa as we speak is in the workshop getting this mod done ready for next season, it has a load of benefits, moving the tiller mount point 400mm inside the rear beam and giving central sheeting. The downside is I will have two short tiller sticks rather than the more traditional long stick.

I know I will regret asking this, but here goes anyhow: Why did you reduce the beam by 20cm?

The hull bouyancy is marginal for the 15m main, yes I can get my lardy 95 kilos out on the wire to compensate but all that will do is sink the hull further creating wetted area and drag which means slow. So much better to reduce sail area and let the hull ride in its optimum level.

Certainly proven with my current boat, having gone from 13.5m sail to 15m sail and then from 2.3m wide to 2.5m wide, I think the smaller sailed narrower boat was faster other than in light conditions.

But Mecca I'm sure you will tell me that I am so wrong <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

I won't tell you that you are wrong, but I'm sure plenty of people who design multihulls for a living will...

But hey, if it's working for you... Keep on trucking!