Are there any sailing performance reasons to make the rudder shaft vertical to the waterplane vs. the exit point below the hull.
(IOM under construction)
In power boats tipping the axis foward can raise the bows in a turn. In sail boats, it’s also vertical to prevent Lester introducing another variable into his spreadsheets.
This is only partially true (smile). The rest of the reason is that anything other than vertical gives an extra drag component which just slows the boat down more.
Hi Lester,
I had hoped that you would consider this question.
If I understand your ‘short’ version; the rudder post should be vertical to the water plane to minimise drag. If the rudder post is canted forward so that the post is vertical to the line of the hull, then drag is increased.
What gives me thought and caused me to ask the question in the first place is that if I look at the drawings for the TS2/3 and look at other skiff deck designs. They have the post vertical to the hull bottom(ie canted forward). These are respected designs. That seems to diverge from your comment.
I can see that an inclined post keeps the top of the rudder consistently close to the hull at various turn angles, and that a vertical post causes an increased gap at the top rear of the rudder blade at turn angles. This gap could be a cause of turbulance and drag.
Still confused.
I would also offer that with transom hung rudders on our full size cats, tilting the leading edge of the rudder forward so it is under the hull (slightly) reduces weather helm a great deal.
Not sure if this would also change lateral resistance if the entire blade is under water and under a monohull boat. By “raking” a rudder tip/blade foward or aft, the change in total lateral resistance is changed in a remarkable fashion. I know both Hobie Cat and NACRA/Inter catamarans include this as part of basic tuning of the boat - along with mast rake.
Yes, this is where ‘practice’ intrudes upon ‘theory’ (smile). If the axis is not normal to the local line of the hull rocker, the rudder will foul the hull when turned (unless there is an unacceptable gap between rudder and hull).
So, if I read you correctly, and putting it in my words; a post vertical to the waterline would generate less drag than an inclined post…while sailing in a straight line. . .
But a vertical post has an increasing gap between the rudder top and the hull at increasing turn angles and the gap would allow/create increased turbulance…
So in the real world, an inclined post that keeps the top of the rudder closer to the hull at turn angles would produce less turbulance and so could be faster???
That makes sense to me.
More here:
http://www.boatdesign.net/forums/showthread.php?t=20949
Sounds like a bad idea 
Cheers,
Earl
There will be a compromise/tradeoff in there somewhere, and only a spreadsheet (smile) could guide you because you’ll need to do the maths.
As you tilt the rudder axis, you get increased drag. As you increase the gap between rudder and hull bottom, you get increased drag.
Thing is, the rudder tilt drag really only kicks in when you turn the rudder, and then it hurts a lot. The loss of rudder lift with a tilted axis I think is slight and not a major issue.
The rudder gap drag hurts all the time (and when you turn the rudder even with a no gap setup, you’ll introduce a gap for the duration of the turn).
So if your sailing is mainly in a straight line, have zero gap. If your sailing is close-quarters match racing, I’d try and keep the axis as vertical as possible…
Thanks Lester and other responders. That reenforces what I thought.
For fleet racing my IOM, the absence of gap is the more important factor. It will be easier to minimise the rudder/hull gap across the steering range if I place the post at right angles to the bottom of the hull. The angle from vertical is not a lot and so any loss is not worth worrying about.
:zbeer:
I would think that if the shaft is vertical to water line, and you do a decent job of fitting the rudder to match the slope of the hull, then for most of the travel that rudder will go through where you would want to have the gap be minimized to reduce drag and keep speed up it will be fine. I would suspect that when you make considerable rudder movements, such as when tacking or mark rounding, that a lot of other drag is being induced by the hull trying to change from its more normal tendency to continue straight ahead. I would also think that this arrangement would continue to present the fin more or less in a plane where it is providing more of the desired turning effect than simply adding drag.
When I hold a rudder on an angle that would put it at right angle to the bottom of the hull it looks to me like it would induce a lot more drag than turning moment even during quite small rudder movements. It also seems to me that at that kind of angle it is going to tend to try to drive the stern up/bow down whenever it is at any angle off centre and that is going to introduce a bunch of other factors such as maybe more hull in the water (=drag through increased wetted surface) and will change the angle of the keel fin and bulb as presented to the water.
I think mine will remain vertical for now.
-Barry