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Old Dec 14, 2004, 04:34 AM
speedchaser
Brighton, Great Britain (UK)
Joined Sep 2004
167 Posts
Question
Offset rudders or not offset rudders...

Do rc racers have offset rudders on their boats purely for reasons of oval or 'none-offshore' racing (because boats are turned more one way than the other way in the course of a race), or is there also another reason? I.e. nearly all the pictures of riggers that I've seen have offset rudders, so it would be interesting to know if rigger types used for straight-line (offshore) speed events also have offset rudders, or if they have or can have the rudder behind the prop...
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Old Dec 14, 2004, 09:40 AM
Useful Idiot
Asturias, Spain
Joined Mar 2001
3,542 Posts
I think you'll find that the American off-shore class is limited to semi-scale mono's and cats. I think even the European Naviga classes for hydros also use a triangular course.
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Old Dec 14, 2004, 04:17 PM
speedchaser
Brighton, Great Britain (UK)
Joined Sep 2004
167 Posts
Cheers, could a rigger type have the rudder behind the prop anyway just like for any other surface drive setup...(for speed/strait line events) - many thanks
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Last edited by speedchaser; Dec 14, 2004 at 04:20 PM.
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Old Dec 14, 2004, 11:09 PM
Useful Idiot
Asturias, Spain
Joined Mar 2001
3,542 Posts
I've never seen anything but offset rudders on hydros, be they saw or oval racing. For saws, everything that produces drag, like rudders and turn-fins is kept to a minimum and the 120+mph boat used cut down stainless steel kitchen knife blades
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Old Dec 15, 2004, 06:18 AM
Registered User
Panama City, Panama
Joined Nov 2004
88 Posts
Could it also be for ease of mounting and/or weight savings? It is easier to mount the rudder pivot directly to the transom, and stronger, than putting it on a strut assembly to get it behind the prop. also mounting it to the transom saves the extra weight of the strut assembly, when every little bit counts, thats needless grams hanging of the back. Mind you I don't know from experience, as I am building my first boat, and it is a scale boat, not a fast electric outrigger.
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Old Dec 16, 2004, 12:20 PM
speedchaser
Brighton, Great Britain (UK)
Joined Sep 2004
167 Posts
To 'm.r' - right many thanks on that, so an offset rudder will generally produce less drag than one behind the prop for rigger types made for strait-line (offshore) events even.
I say 'offshaw' even though I will try to run the boat (when finished) in 100% calm conditions. Initially I was erring on behind-the prop as my main tub will only be 4 inches wide - and owing to how most servo's are arranged it will be very tight to get an offset mounting, but I think it's just possible. the other reason was because I thought that the wake/thrust created by the prop would assist in turning the rudder more quickly... As to kitchen-knife blades for rudders, well you learn something new every day, and thanks also to 'knotaddicted', I agree it would be better/easier to mount it to the side of the transom. I'm also building my first totally scratch buit-boat as well - why I couldn't have started with a mono I don't know...
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Old Dec 16, 2004, 02:41 PM
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P_J_Glor's Avatar
Valencia, CA
Joined Oct 2002
3,538 Posts
The offset rudder also makes it more convenient to remove the prop shaft. [You don't need to remove the rudder first.] This was done on the DE-1052 Knox-class destroyer escorts, even though they were hardly race boats.

Pete G.
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Old Dec 17, 2004, 03:31 AM
speedchaser
Brighton, Great Britain (UK)
Joined Sep 2004
167 Posts
Thanks - that settles it for me, as I'll probably need to reposition the motor a few times over the first few test runs, so beng able to remove the prop shaft easily will be essential...
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Old Dec 28, 2004, 12:54 AM
Registered User
Northern California
Joined Feb 2004
20 Posts
a rudder mounted behind the prop would be far more likely to cavitate. The swirling water coming from the prop would cause areas of high compression on one side of the rudder and a vacume on the other side.
Cavitation occurs when the water is comressed to the point that it breaks down the chemical composition releasing gas bubbles. Another form of cavitation is when air travels down an object from the surface due to a vacume created by the motion of that object. Cavitation can quickly eat away metal as well as cause vibration and stresses that could cause parts to fail.
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Old Dec 28, 2004, 12:59 AM
Registered User
Northern California
Joined Feb 2004
20 Posts
forgot to mention all of the drag that would be induced as well.
I watched a full size water speed record on tv and removing a small piece or the rudder 1/4 inch made a difference of 25 mph average speed which was all that was neccesary to break the record. They then removed another 1/4 inch and gained 33 mph avg speed breaking the record by an even larger margin. water creates lots of drag and the faster you go the more drag is created
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Old Dec 29, 2004, 04:12 AM
Boaters are nice people.
Sneek, Netherlands.
Joined May 2004
4,754 Posts
The drag issue is indeed what counts.

I run a Hopf Baracuda in the races, standard set-up (as the rules are) and gained just that bit of top speed, allowing me to clinch the Dutch mono 1 championship by changing the rudderblade from the wedges aluminum one to a steel (wedged) blade that was aprox. three times as thin.

Regards, Jan.
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