Rudder design

[Pugwash1952]

Hi all,
I am building a 24ft GRP sail boat (still not managed to find design). I want to make/buy a transom hung rudder but would like some advice on size, shape, thickness etc. I calculate she will weigh circa 1.5 tons. She is fin keeled with an 18 hp Kubota engine.
If more info is required, let me know.
Any advice would be great

 

[reeac]

[ QUOTE ]
Hi all,
I am building a 24ft GRP sail boat (still not managed to find design). I want to make/buy a transom hung rudder but would like some advice on size, shape, thickness etc. I calculate she will weigh circa 1.5 tons. She is fin keeled with an 18 hp Kubota engine.
If more info is required, let me know.
Any advice would be great

[/ QUOTE ]

Sounds like a brave effort but at the risk of stating the obvious, can't you find an existing similar sized boat and copy the rudder dimensions and shape? Such a "prototype" would probably come in useful in answering many other design and construction questions.

 

[William_H]

There are a range of compromises involved in rudder design. My rudder on a 21fter is about 1 metre deep from water line and about 300mm chord (front to back) I have in fact extended it a little because I need a lot of rudder power when running shy with a spin in strong winds. To stop the boat rounding up. It has a cut out adjacent to the hull so that the rudder centre of effort is as far forward as possible with about 15% balance of area in front of the pintle line. All this to reduce helm loads.

I imagine if you have a fin keel then you don't need a lifting rudder so a fixed plank with pintles will be OK. Mine pivots in an aluminium box although stainless steel tubing box would be better.

It is important to get the lower pintle as low as possible to minimise strain of leverage sideways on the pintle. You need to beef up the structure of the huill where the pintles attach.

Now from my extreme performance rudder you can have a shallower rudder if you are not going to be skidding along on the gunwhales.
I think the balance however is highly desirable.
Deeper rudder is more strain so must be stronger. Shallow /shorter rudder is less strain and less susceptible to damage from ropes etc.
good luck olewill

apologies I have posted these pics before but they may give you some ideas.

 

[ningcompoop]

I would imagine that copying/using a an existing design would be easier, but if you want to design your own, try posting on (or searching) the boat design forum?

 

[kds]

The AYRS have a small publication on rudder design - but you can summarise it by saying that it is a part of the boat experiencing so many forces which change in size and direction that predictions are very hard to make.
I put twin rudders on a 26 foot hull based on observing many other boats and they worked incredibly well - but I know it was more by good luck than good judgement !
Ken

 

[Tranona]

Size of rudder is a function of the underwater area of the boat - usually between 10 and 15%. Aspect ratio depends on draft - deeper draft allows deeper,narrower rudder. You also need to consider balance, that is area forward of the pivot point. Thickness would be typically 50 to 60mm tapering to next to nothing - there are standard aerofoil sections you can use.

Suggest you consult a book on yacht design such as Skenes, plus looking at similar boats to see how they have solved the problem - although suspect you will find a wide range!

 

[William_H]

Just a few more comments.
I mentioned power of the rudder. When a sail boat heels over it tends to turn into the wind. (rounds up) This is partly because the drive of the sails goes out to leeward. This tendency to turn is a force which needs to be overcome. It is generally not a factor in power boats. (look at how tiny by comparison the rudder is on a super tanker)
So if you intend to race your boat hard especially with spinnacker you will need a more powerful rudder than for gentle sailing.

On a mobo as you turn the rudder the boat tends to follow so the angle of attack is never very great. But if as on a sail boat the boat has a mind of its own you can end up with a significant angle of attack for a long time. Hence you need an efficient foil. An efficient foil gives maximum lift (force turning the hull) with minimum drag. (slowing the boat down)

A foil loses efficiency when water flows from high pressure to low pressure side across the bottom and top of the foil. The higher the aspect ratio (Length to breadth or chord) the smaller the losses are compared to the size. So a long thin rudder is more efficient. Also a semicircular bottom shape gives less drag from this loss.

Obviously the longer the foil the more susceptible to damage and stronger it must be if it is canterlevered down from the hull. The strongest rudder has a pintle at the bottom to support it.

If the rudder is too small you may find it will stall. This is a bit like if you turned a rudder to 90 degrees ie square to the water flow, the water tends to flow around both front and back causing a lot of drag and very little lift or turning force. I have observed this effect on mine where you get a stiring sound from the rudder lots of foam and a tiller hard over with not enough effect.
A foil with a greater thickness especially toward the front of the foil up to semicircular profile at leading edge will have greater resistance to stall but at the price of greater drag. A fine angle at the leading edge has less drag but more tendency to stall. So a flat plate is ok with low drag but not so much turning power (resistance to stall as proper foil shape. The thickness especially at the top gives more strength too.

One of the problems with a normal rudder is that as the boat heel angle increases the rudder is trying to lift the stern as weell as turn the boat. At 45 degrees of heel ( a lot) there is equal force trying to lift the stern as is available to turn the bow down wind.

So when sailing hard there comes a point where the rudder drag starts to detract from speed more than the power in the sails that heel the boat will increase speed. However one small advantage is that in struggling to push the stern up to windward you are aiding the keel in counteracting leeway.

An article a while back ion PBO was sobering relating to how when running the rudder is most likely to be damaged as loads increase with speed. The worst scenario was running down the face of a large wave which may be breaking. The hull accelerates in breaking water only to hit water moving back wards so water speed is very high just at a time when you might need rudder power to avert a broach. So rudder strength is important.

So a powerful deep high aspect ratio rudder will give more steering power when heeled in strong winds but will need to be very strong if not well supported and will give some added drag when plain sailing.
However I like my deep rudder even if I can see it bending under load. Obviously you should get a rudder design with the boat design but this may give you some ideas. olewill