Skeg hung rudders

[sailaway21]

I think little eating of inedible fowl is required and would note that Baker wrote, way back in '43, a note explaining his use of the term semibalanced versus balanced, which I quoted above. But you hang out with the wrong crowd and pretty soon you're talking like them! (strictly tongue in cheek)

[Ilenart]

One negative that has not been raised with balanced spade rudders is when they "stall out" and cause the loss of all steering ability if the helm is turned quickly to moderate (12 to 15 degree) angles. Fitting a skeg hung rudder on a cruising boat lessens this risk by increaseing the stalling angle and reducing the chance of broaching.

Any Comments? The above is from a design book by Ted Brewer.

Ian

[Jeff_H]

That is somewhat eroneous these days. Brewer's book was written back in the 1970's and a lot has happened since. The reality is that post hung rudders are no more prone to stalling than skeg hung rudders. The issues are vitually the same and relate to the rudder's aspect ratio, foil shape and camber depth rather than whether the rudder is post hung or skeg hung.

Jeff

[Plumper]

Do you have a reference to support your "reality is" statement. It seems to me that any design where the leading edge turns out of the flow (post hung) would be easier to stall than one where the leading edge is always oriented towards the flow (skeg hung).

[Jeff_H]

Plumper, I will see if I can dig out some specific references for you when I am home and access to my library. I seem to recall a paper from the Chesapeke Sailing Yacht Symposium that might be a good source.

Just to comment on your thought that the leading edge of a skeg hung rudder faces the flow while a posthung rudder is not oriented with the flow, my thoughts are that boats actually move slightly sidewards as the pass through the water because of leeway so the skeg starts out slightly across the flow, and when a boat is actually turning, by its very nature of the stern moving sidewards relative to the course of the boat, the skeg also is moving across the flow of the water.

When you talk about stalling you are talking about high angles of attack, and at high angles of attack to the flow, when you turn a boat with a skeg, the fixed portion of the skeg is actually being driven sidewards across the flow as the stern swings where as a the whole foil on the post-hung blade is able to feather to the flow direction during the turn.

I'll try to look this up and get back with a source.

Respectfully,
Jeff

[Plumper]

It is an interesting question. I can see your point. I think that when you initially put the helm on with a spade rudder is when you may stall. You have to gradually put them helm on to avoid stalling it. It read somewhere that helming an America's Cup boat is a challenge because the long narrow rudders stall easily if the helmsman is too aggressive. I am not sure how much weight to put on that though.
I have owned boats with both kinds of rudders. On my Ranger 26 (Mull design) it was fairly easy to stall the spade rudder. On my Truant (Garden design) it is very difficult to stall the partial skeg. That may be because the Ranger had no stops and the rudder (tiller) turned 360 degrees and on the Truant ( wheel) there are stops at about 40 degrees. I could certainly turn the Ranger much sharper but it would stall if the boat was moving quickly.

[Jeff_H]

I think that you hit on one of the more difficult problems in resolving these discussions, which is our own experience with bad examples and being able to separate out the factors at play. We all have experienced bad handling spade rudders that stall out way too soon and it is only natural to assume that the fact that these are spade rudders was a contributing factor in their propensity to stall.

What is harder to sort out is the relationship between the fact that post hung rudders are generally a little more agressive in their design (higher aspect ratios, and thinner foils) than skeg hung rudders which as a broad generality tend to be used on more conservative designs and so would tend to be less stall prone because of their more conservative aspect ratios and camber depth.

Jeff

[Plumper]

So you are saying that "skeg hung rudders which as a broad generality tend to be used on more conservative designs and so would tend to be less stall prone because of their more conservative aspect ratios and camber depth.".

Then we agree.

[Jeff_H]

We agree if you let me add several words at the end for clarity...

"skeg hung rudders, which as a broad generality tend to be used on more conservative designs and so would tend to be less stall prone because of their more conservative aspect ratios and camber depth." rather than because they are skeg hung rudders.

Jeff

[Plumper]

Works for me.