Rudder Research?

[PAR]

If you leave the rudder hung the way it is (keel hung) then you can't use balance (lead). If it's a spade you can use balance.

If this was my boat, I'd just redo the rudder, but with a more effective profile and enough "meat" to get the job done. No, it will not have balance, but a keel hung rudder doesn't need, nor is it desirable to have any balance.

The first thing to do is unship the rudder, then fill in the huge gap between the aft end of the deadwood assembly and the gudgeon, below the LWL of course. A bit of timber, faired in nicely will do the deed. Next would be a better rudder with considerably more area, with of this area below the prop aperture. The rudder that's there is clearly a cobbled together piece of crap, so toss in in the can and start anew with a strip planked, laminated or "drifted" timber replacement.

You'll have to account for the prop aperture, but get the leading edge of the rudder blade up close to the filler piece you installed on the deadwood. How much more area? I'd try for 30%, even more if you can, as it's easy to remove area, but difficult to add it.

In the end, if the gap is closed up and more area employed, your steering issues will improve dramatically.

[Simes]

Hi PAR,

I hear every thing you have to say about the existing rudder, it is a piece of crap that is one of the many reasons why I want to change it.
You say;
"No, it will not have balance, but a keel hung rudder doesn't need, nor is it desirable to have any balance"
Why does a transom hung rudder not need balance? the rudder will still have to move the same amount of water to be effective won't it? Why is the absence of balance seen as being desirable? The tiller is already nearly 6 feet long.

However with the centre of pivot being so far from the transom if I can produce an aerofoil shape with the leading edge forward of the centre of pivot then the whole of this area will be the balance area, yes / no?
This would also give the rudder more area.

I agree with you about the bearings, they are foul things and will be replaced with nice bronze bushes and new pintals and gudgeons. I will also machine a nice bronze pressure plate for the foot of the rudder to sit on.

Ad Hoc,

as I under stand it an aerofoil shape works best when the flow of liquid is across the length of the chord? As the rudder will be hung on a transom that is declined by 20 deg' off the vertical the flow of water will also be at 20 deg' off the chord. So I guess that I align the chord with the flow and not with the leading edge of the rudder?

Thanks every one, this all good stuff.

Simes

[PAR]

If you put balance on a keel hung rudder, the balance portion of the rudder will "uncover" itself from behind the deadwood assembly during helm deflections and promptly present itself to the flow as a brake. I'd strongly recommend you not employ balance in a keel hung rudder.

Your rudder lacks efficiency because of insufficient area and it's location well aft of the deadwood assembly. If the gap is closed and the current rudder left as is, a substantial improvement would be noticed. If the gap was closed and the profile improved, more improvement would be noticed. If the gap was closed and the profile improved, foil shaping the rudder blade would hardly be noticeable, though it would stay "engaged" longer.

Employing all three things will get the best you can get from this particular arrangement. If you skip the foil shaping part, your steering will still be improved substantially. If you leave the gap and or leave the profile as it is, you will not notice much improvement.

[Simes]

Hi PAR,

Many thanks for the explanation. Simple and clear. I will do as you suggest.

I will also post details of the continuing saga as the rudder is improved.

Many thanks to you all,

Simes

[Ad Hoc]

Originally Posted by PAR

If you put balance on a keel hung rudder, the balance portion of the rudder will "uncover" itself from behind the deadwood assembly during helm deflections and promptly present itself to the flow as a brake. I'd strongly recommend you not employ balance in a keel hung rudder.

Excellent point....go with this advice.

[Petros]

That existing rudder is really a bad idea. I see similar rudders at the local wood boat shows and always marvel at the stupidity of building any replica with such a poor rudder.

There are two things you can do. One is as PAR suggests, close the gap way down, enlarge the rudder area (increase the chord), and taper the trailing edge to a fair shape. You might consider a simple NACA 0008 or 0012 shape.

The other solution I would suggest is to remove the rudder from the TE of the keel, and taper the trailing edge of the keel to a nice fair shape, and than mount a new rudder as far back as possible, perhaps with a though deck pivot with the shaft sweeping aft. With the rudder separated from the keel it can be a true foil shape, and be away from the messed up flow off the keel. It can be much smaller and more powerful, less drag, more effective and responsive. Also use the old NACA 0008 or similar sections, they will work fine. IF you do this option I suggest you contact a marine engineer who can design you the proper strength rudder and fittings, or an NA who is knowledgeable about using foil rudders.

Good luck.