Mono Rudder Placement - Why the Offset??

[Darin Jordan]

Every Mono I've built to date has had the rudder offset to the right, simply because that's how it seems to be done and I was following suit. Obviously, this position works well for Oval racing boats.

Been wondering lately, especially for Offshore boats, would it not make more sense to put the rudder inline in the center?? I figure that the rudder would have to be a little longer to get some of it out of the prop thrust cone and into "clean" water. Otherwise, does anyone have any experience with doing this that might be able to provide some insight?

Would love to hear your thoughts or insight.

Thanks,

[CornelP]

H&M obviously tried with the Fireblade, but even on their website, the only picture of the boat in the water has a traditional offset rudder...

[ED66677]

Darin,
I've done it on my very first FE boat, this was a cheap NQD Xcyclone that I upgraded from twin 540 to twin 600Race and finally to a single Feigao8XL, the rudder blade was very long (120mm-4.7") what I can tell is that the boat would turn better to the right due to prop walk but it would turn to the left better than my 29" DF copy that has the rudder offset of course.
The good and the bad... the good is that at low speed the boat turns much better, no comparison, for sport/fun running it has the advantage to be more controlable in both direction so more fun, the bad hum... probably some thrust loss due to rudder facing the prop but not sure!

[Fluid]

Offsetting to the right is due to racing, and the fact that most boats with a CC rotation of the prop will turn better to the right. With most monos the placement to the right works fine for turning either direction, I know my Aeromarine Titan 40 turns very well in both directions, but its rudder is long enough to stay in the water during a left turn.

Placing the rudder directly behind the prop does help low speed turning since the prop wash is directed by the rudder - rapid forward movement isn't needed to turn. Downsides are increased drag from the prop wash, and often a "hunting" of the boat when running straight ahead. This is due to the frothy, aerated prop wash bouncing randomly off the rudder.



.

[properchopper]

thoughts

Consider the forces acting on inline vs offset rudders : The offset rudder theoretically sees only "clean water" which singularly provides the lateral force to steer the boat. In contrast, the inline rudder derives its lateral energy both from the water rushing past it PLUS the energy that the prop creates by accelerating the water past the rudder. This secondary force may ( and usually isn't) constant, varied by throttle position, creating throttle steer. That's the rub, at least in my opinion. I remember reading Jay T.'s comments regarding inline rudders which point out straight line instability issues with inline rudders which I believe may be due to the above mentioned throttle steer hypothesis. Interesting topic - I await more learned ideas !

[Ctonez]

JMSCARD is selling a Triton with inline rudder in the swap shop...maybe he has some good input as well...?

[Rumdog]

the new venom p1 mono is also setup with inline rudder. another issue may be that once you move the rudder really far back to get away from propwash a bit, it will prob. Start causing lift in the stern making it run wetter than an offset setup. Personally, i dont want anything obstructing the thrust cone. Twin rudders is always another option.

[steveo]

also consider that any water forced up into the air by the prop hitting the rudder bracket will generate an amount of lift in the rear, i would put the rudder just behind the prop and offset enough to clear the roost which would be more to center than typical oval rudder layout

[properchopper]

also consider that any water forced up into the air by the prop hitting the rudder bracket will generate an amount of lift in the rear, i would put the rudder just behind the prop and offset enough to clear the roost which would be more to center than typical oval rudder layout

You need to place an offset rudder far enough away from the prop to have it clear the blades at full deflection plus some extra clearance to allow rudder flexing under hard lateral forces- I've run into that "problem" myself

[steveo]

of course you dont want to hit the prop and with any flex, set the prop just behind the prop so there is no conflict is what i am saying

[Basstronics]

My Micro Patriot is setup with the inline rudder. My frist attempt had the rudder entirely to close. So I moved it back another 1/2" she runs good now- sorta.

There is no straight line stability period. I jumps out the water. It bounces side to side violently. However it is a bit fun to run as its a challenge.

Vids in the vid section :)

[sjslhill]

I just remember Ed Hughey stating, less flipping over.

[DISAR]

I tried an inline rudder in my mhz racemaster boat, (deep V 98cm x 22cm). At low-medium throttle everything was OK but when hitting full throttle the boat was very unstable bouncing left to right and finally loosing control. Also because the rudder distance is very long from the transom, very strong linkage and servo are required. I started to move the rudder to the right and see what happens.

[Flying Scotsman]

Interesting thread.....Now to muddy the water.....inline with a cat hull ???

Douggie

[properchopper]

Interesting thread.....Now to muddy the water.....inline with a cat hull ???

Douggie

The same dynamics would apply to a cat hull as with a mono, although the twin hulls of a cat should provide more straight line stability and lessen the in-line rudder/prop interaction effect. The wider the cat hull, the better cancellation of the interaction in this regard I would think. No empirical data to back this up, just speculation on my part.