High Speed Rudder issue....

A ventilation + " load inversion" problem occured on an A-Cat

The origin of the problem seemed to be ventilation, which affects mostly the upper part of the blade.

But as for beach cat, the axe of rotation is in front of the section's CoE in the upper part, and behind it on the lower part of the blade

When ventilation of the blade occured at high speed, only the lower part provides lift, and this lift is in front of the axe of rotation.

So I had to hold very firmly the tiller, if I tried to push a millimeter , the load on the blade increased the movement, and conversly, if I pull a little bit, the tiller extension tends to "kebab" me.

Box rule allow a max 76 mm at the rudder, so I planned to put a little end-plate near the waterline of the rudder blades.

It has not been done yet as now, the point is more to get rudder with T foils

Hope it can help

EK

 

Very interesting....I still don't believe I have a ventilation issue, but your symptoms sound familiar.....

 

Pax,

In the video it looks like an air bubble is streaming down the rudder a good ways. This is definitional ventilation.

A simple gate or fin would stop it, but it would require a different rudder attachment system.

 

That's what I saw too. (see my earlier post).

 

Of course a balanced rudder ventilating in it' s upper part will be overbalanced , giving a " strange" feeling at the helm.

As we all know, a negative angled blade ( about 4 degrees) is suffering later on ventilation.
Most sailers think that a negative angle only can be obtained by balancing the blade.
No.
One have to lenghten the upper , transom mounted part of the rudder hinge only.
Raking the axis negative ( forward).
I think Tennant has foreseen this in his drawings .
Building a rudder this way , you are not only suffering ventilation later ( means at higher speeds) , your rudder also will never be overbalanced , which sometimes can become dangerous.


So, rake your rudder forward without balancing it.

 

Angle too far forward (by lengthening the upper bracket) and the rudder will take on a life of its own, meaning sudden lee and/or weather helm resulting from its imbalance.
Agreed a positive angle/raked forward, will slow ventilation but a better design setup would be to have an underhung rudder with hull acting like a top plate, not a dopey thing hanging off the transom - which is just asking for air suck-down trouble when at speed. We found that out with a transom hung rudder on a B24 doing close to 20 knot speeds, steering disappeared, you had to hold the tiller steady and wait for it to come back. Which is no fun. Extended stern and balanced underhung rudder fixed all that.
The other alternative is to fit a top plate to act like it is underhung design. But then you can't lift rudder blade. Life is hell.

 

I agree with Gary. I had a transom hung rudder fitted on my B24. It was a thin section, and since it was surface piercing, it sucked, ---literately.
I cured this by fitting a small skeg forward of the rudder, about 200ml deep and fences on the rudder, formed by thin 1/2" aluminium angle, screwed on with countersunk SS screws for a smooth surface. Worked well as vortex generators, spinning entrained air off the rudder.

I notice the rudder on the above tri is very high aspect ratio. Great for daggerboards ----but not for rudders.
High Aspect ratios on foils, whether in water or air have very good L/D ratios. But only at low angles of attack. Fatter sections with more rounded noses are necessary for rudders, as they are required to operate at fluctuating, higher angles of attack without stalling.
Of course no rudder evaluation can be of any use if the sails are not trimmed to keep the boat sailing practically "hands off".

 

if your doing 20 kts how do you get a rudder to a high angle of attack?
cure the problem not the symptom, move the mast maybe?

 

Move the mast maybe?
Good answer, But maybe just trimming the sails would be enough.
Harden down on the jib clew.? Ease out the Main traveller? Things like that.
But my bet would be, get a new rudder.
Thicker. More rounded nose. Lower aspect ratio. Just my 2c worth.

 

are we cruising or racing?
need to determine where full power is and configure to that so you have those adjustments as you mention for over powered/reaching angles days etc.
J24 at 20kts anyone?

 

Errr----Did you mean B24.? I've never seen a J24 do 20kts

 

lol
I was joking but it has the rudder you describe and works perfectly on that boat.
If the photos in this thread had an even smaller chord rudder it wouldn't surprise me and no reason they wouldnt work.
I sailed a passage on a modern 55' carbon cat got up to about 17ish knots offshore in light air (fast cruiser racer) when I looked at the rudders they looked smaller chord than an A Class, I really couldn't believe what I was looking at and that was after I sailed it without a problem.

 

Doesn't matter if you're cruising or racing, get a sudden wind power increase and you have to steer/trim/spill accordingly.
It's not like, "Oh, we're only cruising, tell those strong gusts to go away."
By the way, when we lost steering on the Crowther Buccaneer 24 (B24 for short, not a plodding J24) we would have been doing over 20 knots in heavy gusting and cold (in New Zealand) southerly winds - because we averaged over 15 knots for over an hour period ... so when rudder was over ventilated, working in completely aerated water (because the B24 rudder was transom hung and the main hull had exaggerated rocker profile) there was no solid water for the blade to work in. Even the Tennant cats have considerable rocker in the after section profile of the hulls, a steep upwards turn aft of amidships - and that plus the air sucking transom rudder design - is maybe doing the same thing that we experienced on the stock B24?
Here's the new and underhung rudder shape and size.

 

The Bucc 24 plans come with an alternate underhung rudder.
The Bucc28 comes as standard with an underhung rudder and believe me, we experienced wild high speed broaches when a big gust hit.
As Gary said we had to rapidly push the tiller away to re-establish the lamina flow over the rudder. The AYRS has an excellent book, which is the bible on rudder design. Still available for a modest sum http://www.ayrs.org./ayrslist.htm. List # seventy nine.

 

Guys - I couldn't see it at first, but now, on slowing down the video, I see that recurring bubble sliding down the face of the rudder. next time the wind is high, I will have a gopro watching it.

Fascinating discussion all. I am grateful to see all this knowledge coming together on this thread on an issue that apparently many have encountered over the years.

After distilling your comments, I'm left with the thought that this will persist without a rudder re-design, and I am OK with that. I do intend to try some different things to reduce the impact, but I think at this point, I'm bumping up against the limits of this home built boat, and am left to consider the cost-benefit of upgrades.

All of your responses will help in that process. This boat is primarily a daysailer 30X a year, and will race maybe two 50 to 100 mile races per year.

I did have a rudder casing blow up this year. I was executing a jibe at about 10 knots of boat speed and the rudder blade ripped out of the back of the casing. My initial thought was that I had hit something. But there was no evidence of damage on the leading edge of the rudder.

In retrospect, after reading your comments, I believe the cassette blew up because the rudder was overbalanced. You see, the builder had a piece of line that limited the travel of the tiller to roughly 35 degrees. When I jibed, that line broke. I did not think anything of it at the time (still thinking I had hit something.) But now I believe the line broke, the tiller over-traveled at 10 knots and the lower forward portion of the rudder suddenly levered it's way through the cassette with a loud BANG! I think I am very lucky BOTH cassettes did not blow up, and there is a crack in the gudgeon of the one that survived that I had not noticed before....

I have since replaced said lines and rebuilt the cassette to good effect. Now I understand the importance of that line in my rudder configuration. Thanks again to all for your thoughtful comments.