Rudder Size

[Capt_Marlinspike]

I have a 27' 1979 motor / sailer. She sails quite well but the tracking is really bad. Under both motor and sail she wanders about all over the place. Even is a sea state of slight every wavelet knocks her of course. The semi-balanced rudder is quite small, less than 2 square feet. WOuld a bigger rudder help her track better?

 

[Csail]

Sounds like it could be too big if anything!

 

[Salty John]

It seems undersized. The submerged portion of the rudder is typically 1% to as much as 2% of the sail area. Power boats have smaller rudders but if you consider yours a sail boat then as a minimum I'd expect 1% of sail area. Your sail area must be at least 300 square feet, I would guess, so a rudder area of 3 square feet would be about right. The submerged rudder area on my 20' boat with 196 square feet of sail area is 2 square feet.

 

[Capt_Marlinspike]

Thanks John. My sail area is 300 square feet so by your rule it would seem it should be bigger.

 

[boatmike]

You dont say what make/type it is but its probably more to do with the keel and hull configuration than the rudder size. The test for rudder area is really about how much rudder angle you need to put on to correct deviation from course than anything else. A boat with too small a rudder will be difficult to get back on course without stuffing the tiller or wheel hard over and holding it. If you can keep her on course manually with fairly minor corrections (not winding the rudder way over and waiting for a reaction) then it's OK. Perhaps you are asking too much. Many 27 ft boats can be a handful in a chop. A long deep keeled sailing vessel will track well with little attention. a motor sailor can often be a compromise having relatively shallow draught and less keel. Horses for courses......

 

[boatmike]

From a hydrodynamic viewpoint the most meaningful ratio is the ratio of rudder size to keel area. Even this is totally arbitory and is speed dependent. There is no design rule that I ever heard that gave a relationship between rudder size and sail area. Where did you get this from?

 

[Capt_Marlinspike]

She has a relatively long keel, cut away at the fore foot. She is shallow draft at just 3'1". The boat is a Kent 27.

 

[boatmike]

Just looked again at your bio. Is it a sailing version of the Kent 27 you have? If so you only have a nominal draught of 2ft 11 ins which means the rudder is quite big enough for the keel area you have and you are asking rather too much of her! She was never really designed to be a sailboat in the first place, rather a shoal draft motor launch...

oops! we crossed posts there. Thought so. My answer still stands if it is the boat I remember..

 

[Salty John]

Just one of those useless pieces of information you pick up when you spend most of your life with boats. I've always been interested in the various ratios that determine the sailing qualities or otherwise of different boat types.

 

[Poignard]

Just a thought - is there excess play in the linkage between the wheel [I assume she has wheel steering] and the rudder?

 

[macnorton]

Has the tiller got a leading edge (in front of the pivot) solved this problem on different boats by extending this.
you could also fit angle plates at the top on each side of the tiller, this stop water "spilling" over the top and gives more precise steerage, also improves reverse control.

 

[jimbaerselman]

Good 'Tracking' is usually a result of high directional stability. Just in case the term is a bit un-familiar, directional stability is something an arrow/dart has a hell of a lot of - it's why they fly point first.

So first, a question to determine whether we're actually talking anbout the same thing - when disturbed off track, does she then continue wandering further and further, winding up into a turn in the same direction?

If so, she's directionally unstable. This arises when the centre of lateral resistance is in front of the centre of gravity of the vessel. Cured by adding a fixed fin aft of the keel, or increasing fin area.

Problems with improving directional stability - she becomes more difficult to steer in reverse!

A larger rudder will only cure directional instability if it is held fixed in position, rather than allowed to wander, free of constraint.

So a balanced rudder (with area in front of the hinge axis) will tend to wander more than an un-balanced one, and sometimes this reduces directional stability.

A different factor which makes for 'wandering' steering is a lack of rotational damping. OK - something which is characteristic of a deep, narrow keel. Makes the boat easy to spin on a sixpence. As apart from a long keel, which resists all types of turning moment. But this doesn't seem to be your case!

A final factor which makes for wandering is relative lack of a keel! Think of a canal boat - long, shallow keels (if any); thames sailing barges with their centre boards up; sailing dinghies ditto. Always easily diagnosed, because the vessel will not sail efficiently to windward.