rudder shape/size

Mates and I own a 30' steel motor cruiser, displacement hull, 50h.p. diesel inboard, shaft drive. Partial keel. Expected weight 6000kg (ish, probably lighter). Expected hull speed 6.5-7kts. Full referbishment project.
We are wondering where to find some 'easy' to understand information on rudder size, shape and area requirements?
Does the rudder need to be shaped as an aerofoil?
How much balance is recommended?
Any direction to websites or books would be appreciated.
Cheers, Mat.

 

Need input !! need input !!

a little more info about the rudder would be a great help !!
will it be a spade rudder or have you got a a skeg or what ever ??
A picture or two of what you have and where and how its mounted would really be a big help !!
if its a refurbishing job it must have had one before ??
so what was there ??
Remember the more info you give the better the information you will receive to help and yes a foil shape is better than a flat plate ! do you know anything about making foils ??
what's your possible shaft length and diameter ?? this could be of help with the thickness of the blade !!

 

For a first approximation, very basic, I use the following formula: Total Area of ruder = (Lwl . D) / 30
Lwl = full load water line length
D = full load draft
Shape depends, among other things on stern profile. It would be good to know more about the boat.
I do not think that this boat needed a rudder with hydrodynamic horizontal section. In my opinión, would be enough with a flat plate.
Classification Societies have formulas to calculate all elements of the rudder.

The rudders compensation is used to reduce the torque on the stock. Therefore, to obtain more appropriate compensation, it is necessary to work with the value of this torque and the maximum diameter possible for the stock. Therefore depend, amongst other things, the physical and mechanical properties of the stock

 

Thanks, people. Will try to get some pics on this thread tommorrow (Tassie time).

 

If you are fitting a purchased(factory) steering system they may be able to help. Also consider a fishtail rudder ....it can make maneuvering a lot easier. Make sure the stock is the correct diameter.

 

rudder shapes are all over the place, from barn doors to elliptical foils and even lifting T foils. Almost any reasonable shape will work, it is only if you want to optimize control, lower drag, minimize tiller or wheel loads where these trade off become important. If you have a spade or transom hung type rudder I would use a NACA 00XX foil, it is predictable and a well proven shape. If it is on the trailing edge of the keel, than just a shape that slowly tapers to the trailing edge would be about the best you can do, and minimize the gap between the LE of the rudder and TE of the keel.

Only if you want to reduce drag, most rudders throughout history have just been large slabs. And they worked, but cause more drag than necessary. Also an optimized foil shaped rudder can be smaller for the same amount of steering force, meaning in can be lighter and have less drag, less wetted area, etc.

I think this is a matter of opinion, large ships should likely be fully balanced to reduce forces on the system, smaller boats can be under balanced so there is more tiller feed back to the helm.

"Elements of Yacht Design" by ellison and Larson is a good place to start, there are others.

 

There's two approaches to a rudder: a brick wall or an airfoil. The brick wall approach uses an over size blade, with enough area to effectively direct flow. It's not especially efficient in a true sense, but on slow speed craft like yours a viable solution. Efficiency on this type of boat isn't really a big deal, so why bother with NACA foil shapes.

The airfoil approach is for higher speed craft and/or hulls that are efficient at moving through the water. A steel motor cruiser, isn't (typically) an especially efficient set of shapes, so not much of a concern. If you want to squeeze the last 10th of a knot out of your boat, then using a 00 series NACA section will help, but again only in regard to a very slight improvement on the typical steel power cruiser.

As for area, just make a good guess, based on what was once there and similar boats like yours. Error on the too big side of things, as it's a lot easier to save a little off then to add more. I realize this sounds a bit hair brained, but frankly for your boat, just fine, mostly because it's not a set of shapes that need special refinement in this regard (like a racing sail or power boat might).

Post a picture of the hull profile and we can offer a very close approximation of the area and plan form you'll need.