[hpv-boats] Some thoughts on Outrigger Rudder Effectiveness

[Larry]

On Jun 19, 2011, at 12:56 AM, Ian Cassell wrote:

[[... For a tight turn the dipping rudders
could be used. Another way is to fit angled rudders at
a set angle on the aft end of the outriggers, they would
be set just above the waterline. By leaning into the side
you want to turn the rudder digs in and turns the boat.
Simple but effective, would need a bit of reserve of
volume in the outriggers. In rough water could be a problem
with waves hitting the rudders.]]

I'll leave it to each individual, how important "tight turns" are in  
his activity, remembering that we are talking pleasure/race  
watercraft, not fighter planes ;-)

*Note - I'm aware that my "dismissal" of mechanism design/ 
construction issues in the following discussion makes my observations  
"easy for me to say", while actualizing the concepts would be  
anything-but easy!

But let's at least have some fun with theory:

Trying to look at basic principles, it seems as though there is some  
potential for aft outrigger-mounted rudders to be the most effective  
of all, if satisfactory mounting/deployment can be devised.

If we assume that an asymmetrical (lifts better in one direction)  
rudder can be more efficient in it's preferred direction-of-lift than  
a symmetrical (can lift either way) rudder lifting the same way,
... and that any rudder drag offset in  the direction of turn at some  
distance from the hull adds to the turning effect, while rudder drag  
centered behind the hull impedes the boats forward motion, but does  
nothing to help turn the hull, ... and that, if the sterns of the  
outriggers are even with the stern of the hull, and the center of  
lateral resistance of the hull is some distance forward, then the  
straight line distance between the center-mounted rudder to the CLR,  
i.e., the effective lever arm with which the rudder effect is  
working, is shorter than the diagonal line from the outboard rudder  
to the CLR.

Then we see that the outboard mounted rudder has 3 advantages,  
compared with the center mounted rudder, i.e.:

Center-mounted aft symmetrical foil rudder = less efficient either- 
direction turning (lift) effect/ non-contributing drag/ turning force  
applied to a shorter lever-arm..

Outboard-mounted aft asymmetrical foil rudder = more efficient  
specialized one-way turning effect/ turn-enhancing drag/ both applied  
at a longer lever-arm.

Another consideration: Leaning into the turn, and thus immersing the  
outrigger hull to what ever depth, adds "drag" turning effect, but  
not all of it at the aft lever-arm point, and without any additional  
foil-effect turning force. Experiment would determine if the loss of  
hull speed from the additional drag is favorably offset  by reduced  
turning radius ( I imagine a rodeo "barrel race" type of comparison,  
down-to and around a mark, back to start, around and repeat, to timed  
finish at start/finish point).

It would seem more "ideal", ignoring difficulty of design/ 
construction, if the hull could be kept more upright during the turn,  
the outrigger hull NOT immersed, and the appropriate Reynolds-number  
asymmetrical foil deployed "dagger-board-style" at its best lift/drag  
ratio angle of attack, only deeply enough(variable) to give the  
desired turning rate.

Thus we would have the best lift/drag turning force applied at the  
best (long lever-arm) position, to only the degree desired, with both  
the foil lift and the inescapable accompanying drag contributing to  
the turn.

The twin (smaller)outboard rudders and linkages might end up weighing  
little if any more than the larger center-mount rudder. (better  
efficiency / better applied might make up for size disparity (lift/ 
drag ratios and points of application of the force being the  
determinants).

If the twin set-up weighs a bit more, anyone who is seemingly willing  
to carry 30-40 lbs. excess personhood constantly, such as myself,  
should probably not complain about the rudders ;-).

Re. "Reserve volume" in outriggers -   personal opinion only, but  
seems to me that an outrigger which will not prevent overturning,  
once  fully immersed, with any normally expected leaning by the  
operator, is inadequate in the first place.

Comment/criticism welcome, naturally. :-)