Sailboat with a heavy helm-rudder analysis

Part of the problem on this boat is her aft buttocks, which make her "cranky" upwind. Though Eric's point about an appropriate plan form approach, for this style of rudder is valid, it's difficult to do on this setup, because of the aperture. There wouldn't be much purchase at the bottom of the stock and the blade would be pretty weak. I've made this modification previously with good results, if Crazer would like to contact me privately (click on my icon).

 

Yes, the advice I gave about changing the rudder planform is the simple solution, and any other radical solution would be more difficult and expensive. I did do a radical change on the Spencer 42 Copernicus, the story of which is on my website: http://www.sponbergyachtdesign.com/CopernicusRud.htm

The original rudder was attached to the back of he keel and inclined. The new rudder is a separated spade design set further back. The back of the keel and apperture had to be filled and faired in where the original rudder was. The total cost for the rudder design and the new rudder itself was nearly $16,000, and the yard bills and other modifications to the keel and hull were on top of that. So it is not a job for the faint of heart or the small of pocketbook.

The story on my website contains a report from the owner about how well the performance improved, all in all a very successful job.

Eric

 

Ok, so let's first rule out a spade rudder. It's just too much expense for this boat and this sailor, especially with the constraints of having a shoal draft. Moving on from there, we have the need for a more hydrodynamic section and planform. Mr. Sponberg, the following comment with regards to the half-heart shape is based on something I read a year or two ago and is in no way based on any knowledge or experience, so please correct me. I had read that the rudders effectiveness (I don't know the correct terminology) was mostly determined by the shape at the deepest point. In other words, in my mind, the shape you would want would be in an inverted half-heart, narrow at the top and wider at the bottom. From your description, the rudder you had in mind would look something like the attachment below (please excuse the crudeness, you get the idea.)

One benefit of all this discussion is that it has laid to rest any doubt in my mind that removing the inboard is the correct course of action. In addition the improved access and ease of maintenance of having an outboard, I look forward to the improved sailing and handling characteristics afforded by the loss of the prop aperture. In addition to all the problems already mentioned, I can pile on another: the boat squats at the stern. She does not sit on her lines, and indeed I have never seen a Wanderer that really does. Losing the 350 lb. gas inboard will help with that tremendously. Would that also improve the rudder's performance?

PAR, a PM will be inbound shortly.

 

Generally, yes, deeper rudders are more effective, particularly when the boat is heeling a lot. But you have to qualify that statement to what kind of rudder it is. Most rudders these days are spade rudders, and so what applies to a spade rudder may not apply to an attached rudder such as what you have. All these older boats with the inclined attached rudders such as yours experience a heavy load on the tiller mostly because of the incline of the rudder stock. You mitigate the bad effect by reducing the area at the lower end, which reduces area away from the rudder stock pivot line, down low, and where the water flow is really good and there isn't much wake. You don't want the area of the rudder too small, however, so the trend was to make the top of the rudder a little fatter in profile with the heart shape. The flow near the top of the rudder at the waterline is more disturbed with the wake of the boat, so the water speed past the rudder up there is less--so more rudder area up high is OK. In your sketch, I would reduce your red lined area a bit more--the suggested shape looks too big to me. But generally, you understand my suggestion correctly. In every instance that I can think of in my experience, this solution of changing the rudder shape on these boats has worked well.

I don't think getting rid of the engine and propeller is really going to affect your rudder performance, although getting rid of the engine will certainly affect the trim of the boat as you suggest. If you do get rid of the engine and the prop, then you should close in the aperture, certainly. An alternative might be to keep the prop, but switch to a much lighter electric motor system--put the batteries forward to change the trim, but you still would have the power and maneuverability when motoring. Of course, an outboard is another solution which might be OK--an electric outboard makes sense too.

Eric

 

One option that hasn't been mentioned is switching to worm gear steering which eliminates all feedback to the helm. You don't have to fight the wheel when holding course. You can let go of it and it will stay put.

 

This is a rough idea of what would be typical for a CCA era rudder, before they started to separate the appendages. To me it still looks a bit fat, but I haven't run any numbers, just an eyeball thing.

The second sketch would requires a little surgery, but you'll retain shaft security and gain hugely improved steering response. The shaft comes up about in the same place, so steering gear and/or tiller arrangements don't need to change.

 

Crazer,
assuming there are no maintenance, bearing, or setup problems, the most cost effective solution would be a rudder replacement. Your rudder is a POS if it is anything like the picture. My interpretation of your need is to get more turning force on the boat with less force on the wheel and what you need is the same or GREATER turning moment per angle of attack. The force on the wheel is most directly proportional to the length of the rudder away from the hinge. My approach would be a planform that parallels the pivot (a parallelogram). I would be reluctant to move too much area from the bottom (where it is most effective) to the top where it is out of the water in desperate times. An added benefit of the parallelogram is that it makes shaping as simple as possible.

You are limited in length by the shallow draft. I would understand if you didn't want to extend below the keel but there are solutions -a pivoting or telescopic retracting extension is possible and worth consideration. There is one other old Yankee shallow draft rudder solution that I promise is not as dumb as it looks ...bolt a horizontal plate on the bottom of the rudder. If you don't like the result you can just unbolt it and fill the holes -but I have never known anyone that did.

 

An end plate on a raked shaft is a really foolish idea. Picture the end plate with some rudder deflection on a raked shaft. It quickly becomes a very effective brake.

 

As I said, I know a plate looks foolish, but it should be considered because it's cheap, easy, reversible, and I think you will find, it works. The raked axis is good for a telescoping extension, and bad for a horizontal plate.

I really don't want to defend the plate because it's a 'fix' -like well placed duct tape or a coat hanger, but I will say that at the 15 or 20 shaft rake the plate does not become a brake 'quickly' ie. at the 7 or so degrees it should be operating at. If the plate were angled up at 2 degrees or so it should be good through the operating range. What does happen 'quickly' at (7AOA) is that water flows around the bottom of the low aspect rudder reducing turning force and generating induced drag. I think the trouble Crazer is having is that his barn door rudder stalls at relatively small angles of attack leaving him to crank the rudder over to get the turning force he needs from the pressure side which is diminished by the vortex around the bottom of the rudder. The plate simply reduces this vortex and restores the effective area of the rudder, likely allowing it to operate at a lower angle of attack with less drag.

If Crazer is ready to spend a little, or just doesn't like the look of the plate, a telescoping rudder would be the best fix.

 

No, an end plate shouldn't be considered for obvious reasons, on a raked shaft. The suggestion indicates a considerable lack of understanding in the dynamics involved. This boat sails at a considerable angle of heel, compared to a modern vessel, as most CCA's era yachts do. Get a grip. The problems with this particular yacht and many like her, are fairly well understood.