length or rudder in relation to length of boat

[jpay]

I have a 19ft mirror offshore,

The rudder isnt orginial, the existing drops lower than the keel and bilges and casues me to get stuck in mud when it's low tide at my river berth.

Obviously not good enough...
Do i make a new one... or do i cut off the bottom.

Could i have a lifting rudder?
Pro's cons...

and if i was to make a new one which would be the optium length and width for the new one to be?
Im no good at physics but im sure somone knows some clever little mathmatics.

 

[Tranona]

No clever maths required. Rule of thumb is 15% of area below the waterline. However, in practice varies widely. Clearly a deeper, higher aspect ratio rudder is a more efficient foil, but when you have limited draft is of little benefit.

You probably need to experiment a bit. Cut the bottom off and see how it goes. You might add the lost area back to the blade, and adding some area forward of the pivot point might help.

Pretty sure the original was not deeper than the keel and suspect that somebody tried to "improve" it by making it hgher aspect ratio. You could do worse than finding a standard boat and replicating the rudder as it is probably the best compromise - given that a shallow rudder is never going to be super efficient!

 

[Lakesailor]

You could try making the rudder like the Seahawk 17. I'll post a link to the page as the site owner doesn't like hot-linking to pictures. Scroll down to the bottom. Seahawk rudder

I have found it very effective. It doesn't seem to stall when heeled although weather helm does increase with angle of heel. I think that's more to do with the shape of the keel than the rudder though.
Oddly enough, as I am on a non-tidal lake I am considering trying a deeper, straighter rudder. Just to see if it makes any difference.

 

[VicS]

Thses outlines might help. They come from a review of the Mirror offshore in PBO some years ago. Mk 1 boat on the left Mk 2 on the right.

Also dimensions of a modified Seawych rudder as its the same sort of size boat. (Original design had a swept back front edge and even more swept back trailing edge but has been modified to give a small balance area but retains the original total area exactly)

 

[Lakesailor]

I tried that Vic to make a new blade for a Prelude. I found a diagramatic side plan and scaled it up. Made the blade and it looked too short. I joined the Prelude Yahoo group and one star came up with a blueprint. My scaled up version was 7" too short!
No one guaranteed the diagram was to scale.

 

[VicS]

They look as though they are just above the level of the bottom of the keel. That should be fine. Below can be a real pain!

Meant to add the comment that most small boat yards have a Mirror or two knocking about . It would be a good idea to get a look at one, bearing in mind the apparent difference between the M1 and Mk 2 boats