Overbalanced rudder with no balance area

Hi,

I've got a problem with my 19ftMicro Gem 1970s micro tonner, that the rudder feels overbalanced, pulls out your hand and takes more effort to return amidships than steer. It's not got weather helm or lee helm for that matter! I can't let the helm go even momentarily before it veers off one way other the other.

However, there is no blade area in front of the pintle and the rudder is not balanced in the conventional sense at all. The rudder blade is in a cassette.

Is there anything I can do to resolve this?

I've got two ideas i'm thinking about:

1. Shorten the rudder chord where it's in the cassette so that I can set it further aft in the cassette, increasing the distance from CoE to pintle / lever arm.
2. Rake the blade aft so that the CoE moves aft

Both seem like they'll increase weather helm.
Any advice?

Cheers
Andrew

 

Sounds strange. Could you shim out the lower pintle?

 

Have you also considered adding a small tail fin, about halfway up the rudder ? Seems easy to test (at different distance aft), and to remove if this does not work, before reducing the chord length option.

 

Does this happen on both tacks? A trailing edge beveled in one direction will have a moment that turns the rudder.
Do you have a side profile photo of rudder and cassette including the pintles and gudgeons?
It's possible for thick airfoil sections at low Re numbers to behave like this. I *guess* anything less than 15% thickness/chord ratio would not have this problem.

 

Hi Andrew, you could try a steering dampener, like for a motorbike, to slow the wrenching of tiller out of your hand. Does it behave the same with different sails up; eg try with storm mainsail or max reefed main and full genoa; then try with storm jib and full mainsail, to see if it's a sail / keel balance issue; the mast may need a small angle adjustment. If the rudder needs more effort to end a turn than to start one, it probably doesn't need any more foil the the rear; has someone messed with it previously?; what does the standard rudder blade look like? . Does the boat float on her lines, or is it down at the bow, up at the stern, causing runaway steering. So many questions and possibilities. Pictures please.

 

Weather helm has to do with the balance between the sail set and the lateral resistance of the hull. Moving the rudder aft to balance the rudder won't produce weather helm unless the sails aren't balanced properly. Note on small boats like this, they tend to heel a lot, and weather helm will show up at high heel angles. That's normal.

 

There's no weather helm, just wants keep turning rather than centering itself either way you turn.

 

The rudder is a NACA 12 which i made myself. It's probably not 100% accurate but reasonable. However, all the boats in the class have the same issue, including the standard rudder.

My rudder is shorter chord and deeper than the standard. The shorter chord means I have about 50mm wiggle room in the headstock where I can either rake the blade or move it to the front or back of the stock very easily. I'll get some pics.

 

It's been heavily messed with in that it's not the class rudder and I built it. Its a NACA 0012 section shorter chord and deeper than the original.. But but, but, the standard rudder has the same issue so i don't really think it's my blade.

To say snatching out my hand is an exaggeration really the worst of it is when tacking or gybing, expecting the tiller to help itself back to centreline, it always wants to pull the other way.

 

Yeah easy to do, that would move the whole CoE back a bit, but wouldn't move it back relative to the pintle / steering axis so I'm not sure if that would help?

 

Maybe a strange take, but could it be that the keel placement makes the whole boat "overbalanced"? If you steer the boat in one direction the area in front of the keel makes the boat "pivot" around it?

 

Or may be the hull releases a vortex in front of the fore edge of the rudder ?

 

I never helmed a dingy type where you could let go of the tiller without a crash gybe or massive stuffup, ,maybe use heavy bungie upon the tiller, see what happens eh, this way you maybe able to open your next flagon of beer

 

You'd have to push the lower pintle forward on the cassette. I assume this is some sort of cheek plate that lets you slide the rudder up like a daggerboard while still hanging on pintles and gudgeons?? I normally think of a cassette rudder as a barrel that holds the rudder blade and turns on large diameter bearings. You can see how this type is prone to being overbalanced.

 

In the UK, the commonly accepted description of a cassette rudder installation is something that fits through a daggerboard type slot and is then pinned in place. So that the slot only needs to be a small amount thicker than the rudder itself. I must admit to being mystified at the description of the rudder's behaviour in this instance. I wouldn't expect to be able to release the tiller on any form of racing boat and have it maintain a course,but to have to force it back to a central position is really odd. I am inclined to think Dolfiman's notion of a vortex from the keel may be correct. I would be interested in seeing a picture of the rudder assembly,out of the boat.