High Speed Rudder issue....

Saturday we hit a new High on Terrapin. 18.5 knots, single reef, one dagger down, both rudders down, True wind 18 gusting to maybe 23.

https://www.youtube.com/watch?v=7vBKJrrUD8w&feature=youtu.be

The boat was planing nicely but the rudder feel got pretty shaky up above 18 knots. I was worried about it possibly careening out of control into a crash jibe. The tiller was very light, and seemed to be "hunting" a bit, never finding a steady groove.

Keeping in mind this is a home built boat, what should I be looking at to restore control at high speeds? I'm considering adding a bit of toe-in which worked when I had a similar problem on the Hobie 16. Are there other tricks here? How should I diagnose this?

I don't believe the rudders were ventilating. I've experienced that before on an F-28R stock rudder. Scary also, but that was a total loss of control until I cranked the tiller quickly to disperse the bubbles.

In the end, it may just be that I'm hitting a design and/or build limit. The boat is a Tennant Wildfire (GBE variant.) Or, I'm willing to accept that I may not be sailing it properly - School me!

And yes, there are plans for a new suit of sails....

Here is a video of the rudder rake: https://www.youtube.com/watch?v=8XZLZZpoOYU

 

I experienced that problem on boats that have a rudder profile that will cavitate above a certain speed.

 

I remember reading an article on Malcolm Tennant where he said that he and his builders did a fair bit of experimenting but always came back to 0009 sections (or was it 0012?) for the rudder sections. Sad thing is he is no longer around to check with.

As to the hunting - from what I remember about your boat the rudders don't point forward. If they were too far forward then the CP of the foil could be causing the hunting.

To get the correct toe in for speed try disconnecting the rudders and observing where the non loaded rudder wants to go. If it is markedly different that may be a clue.

cheers

Phil

 

Thanks Phil - That's another thing to try. Thanks for the idea. Experimenting at the high speed is tough - I am the only person I trust at the tiller!

The plans called for a NACA 0013 rudder, but I don't think what I actually have matches the radiused leading edge of the 0013. Mine are sharp, as you can see in the second video attached above.

 

Malcolm went to thicker sections later on, even 15% and experimented with above this figure too because he had a thing about stalling, also liked the increased strength. Even his earlier designs would have been 0012.
Remember earlier Tennants like GBEs and Bamboo Bombers, had aft sloping rudders which gave weather helm feel, then he went to vertical or sloped forward blades. Which can take over, meaning from what I read here that helm might feel like it is hunting. Try setting them vertical or sloping aft by a couple of degrees.

 

is there a dagger board in front of it?

 

There IS a dagger board in front of it - perhaps 12 feet forward and angled outward a bit (10 degrees maybe?) On that day I had one board down and one up. And it occurs to me that perhaps I should have had that board up a bit given we were headed deep downwind. That said, the apparent wind was well forward, which raises the question, "boards or no boards?!"

Gary - Thanks. This is one of Malcolm's later designs, a GBE variant called Wildfire. so, hopefully, it incorporates all his years of experience in that area. I think that changing the rake aft might exacerbate my weather helm issue, but I'll fiddle with it a bit.

Here's a couple of pics:

 

Drawing looks like rudder is meant to be forward so maybe do that but you might need to move the rig forward to get the helm load comfortable
Catch with cassette rudders is you cant make a balanced rudder unless you have offset pintle or a 4 link setup

 

Thanks Powerabout - Yes, it is raked forward )you can see it in the video https://www.youtube.com/watch?v=8XZLZZpoOYU)

You asked about the dagger, were you thinking that it is causing turbulence around the rudder at high speed?

 

could be depending on its angle of attack and NACA profile

 

In a brief segment of the sailing part of that video, it appears that the rudder is ventilating quite a bit. I'm not sure a change to the section shape would improve that, if ventilation is indeed what is occurring. A "fence" could help cure that but is probably not possible given the way your rudders are mounted and retract...

 

The video on the rudders alone answers some of my questions. I would do two things.

First get a good friend to hold the rudder as you disconnect the tiller bar. The second rudder should smoothly follow the first. If it doesn't then there is an over balance problem. I had a friend who got a really nice rudder designed by a proper foil designer. It was beautifully built by a great pro and then had an overbalance problem - you had to hold it in the centre of the boat otherwise it would go hard over when at speed. The designer, who made a living out of refitting foils, couldn't get it right a second time and in the end the builder just did it his way. There was something insigificant the designer and builder did that made this rudder special in a bad way. Your rudders could be similar.

Also I would try lifting the rudders up halfway. They are big anyway so this should be fine. As your rudders balance only when full down lifting them will reduce balance (It is the bottom bit that does the balancing) (On reflection there could be some interesting twisting phenomena that could give rise to your problem.

A forward raked rudder is balanced by its bottom third (the front part actually) At speed the rudder may be heavily loaded and twist. The bottom part may twist so that it increases its negative lift (opposite from the torque the rest of the foil gives - that is why you get lesser torque when balanced). This increase in load on the bottom reverse loaded foil will increase load and twist and you get a positive feedback loop and eventually too much balance. The effect will go away when the load is relieved at lower speeds. Solution - stiffen blade and reduce forward rake and balance.

So try lifting them to reduce rake and check balance. Look for bending and sail around on one rudder only to check for inconsistencies between rudders. If you have a rake problem then the epoxy may need to come out.

cheers

Phil

 

how much weather helm do you have in rudder angle?
maybe you exceed the speed versus angle for that naca profile?

 

I feel you need talk to Tom Speer on here, I believe he was involved with the A Cup boats in foils, drop him an IM he is an expert

 

Thank you all for the insights.

If we get a good breeze before the end of the season here, I intend to try in various combinations:

Raising dagger downwind to reduce possibility of turbulence disturbing the rudders.
Raise rudders to half depth.
Disconnecting the tiller bar and seeing if the rudders follow one another.