Is an inline rudder faster than an offset rudder?

[sundog]

This is something that has piqued my interest lately - which rudder setup is faster. It has been my understanding that an inline rudder (strudder) has the potential to be faster than an offset rudder since it is in aerated water - which has less drag than undisturbed water. But I have recently encountered differences of opinion on this subject. So what I'm hoping for here is input from those that have done it both ways (on the same boat with the same setup) and tested (GPS) the results.

Just to clarify, we're not talking about handling improvements, as it is pretty much unanimous that an offset rudder gives superior handling, as surfing the miriads of Youtube videos will eventually convince you. In another post, member Martin said a friend swapped rudders on a Genesis from the stock inline to an offset which resulted in a faster speed (as well as the aforementioned improvement in handling). Anyone else get those results?

[sundog]

Assuming that the inline rudder does have the 'potential' for higher speeds, it might well be the faster choice, but depending on the hull design may be too difficult to control at those speeds to reach its' potential. Ya gotta control the boat. Bada-bing.

[LarrysDrifter]

Wish I could help. Been watching your posts on this thought. Im surprised a few SAW racers havent commented on it. Best of luck in your findings.

[jevmax]

If you are interested in the ultimate speed, keep the rudder out of the prop wash. The prop blast pushing back on the cross sectional area of the rudder creates a force in the opposite direction of the boats direction.

[sundog]

If you are interested in the ultimate speed, keep the rudder out of the prop wash. The prop blast pushing back on the cross sectional area of the rudder creates a force in the opposite direction of the boats direction.

No, I don't believe that holds up. That is what Rumdog was alluding to in a different post. If you are standing on the dock and push on a sailboats mast, the boat will move. But if you are standing on the sailboat and push on the mast, nothing will happen. Propwash pushing on itself (rudder is connected to the prop via a strudder) would cancel out any such forces.

[keithbradley]

No, I don't believe that holds up. That is what Rumdog was alluding to in a different post. If you are standing on the dock and push on a sailboats mast, the boat will move. But if you are standing on the sailboat and push on the mast, nothing will happen. Propwash pushing on itself (rudder is connected to the prop via a strudder) would cancel out any such forces.

Not really true. given that the hull does cause some drag, and it requires energy to push it, the water behind the prop will be moving AWAY from the boat at an accelerated speed in comparison to the water that is not being "pushed" by the prop.
The man standing on the sailboat is not a fair example. Everything in that scenario is attached to the boat. The water that is being pushed into the rudder, however, is a seperate media and a variable that you're not considering.
This is the same reason an inline rudder can produce better turns at low speeds, becaue the water in front of the rudder is traveling at an accelerated speed.

Also, anytime you put something directly in front of or behind a prop it usually results in a decrease in load and power output.

[sundog]

Not really true. given that the hull does cause some drag, and it requires energy to push it, the water behind the prop will be moving AWAY from the boat at an accelerated speed in comparison to the water that is not being "pushed" by the prop.

Yes, I understand what you are saying. Do you agree that aerated water has lower mass or drag than un-aerated water (at any speed)?

[Jeff Wohlt]

I run the strutter on a HOR cat and she runs great....Also, most have to realize the right length of the rudder on a set up like that IS below the aerated water line so it is in clean water.

[Make-a-Wake]

I have often wondered this..................no such thing as a stupid question right?

Does the boat move due to the prop pushing water out at a greatly accelerated speed behind the boat....................or...............gimme a second here..............does the prop "bite in" to undisturbed water just in front of it to "pull" the boat forward? If the pulling scenario was true, then the roost would basically be the "exhaust"......................hmmmmmmmm. The roost itself obviously cant propel the boat, but there could be some kind of a "cone" of super accelerated water propelling it in the pushing scenario.....................

I know one thing............they move so they're fun, i understand that part.

[sundog]

I run the strutter on a HOR cat and she runs great....Also, most have to realize the right length of the rudder on a set up like that IS below the aerated water line so it is in clean water.

Wow, I wasn't considering that. That changes everything. So an inline is only partially in frothy water, but also in clear where the drag is higher. This complicates things. I see now the advantage is smaller than I had previously thought. Thanks Jeff, for mucking up a good theory.

I have often wondered this........Does the boat move due to the prop pushing water out at a greatly accelerated speed behind the boat....................or...............gimme a second here..............does the prop "bite in" to undisturbed water just in front of it to "pull" the boat forward?

I've seen a water 'jet pack' that you strap on your back to 'fly' from the force of water escaping out nozzles. It was supplied by a long hose from a pump in a small boat. And jet drives work on a similar principle. So the answer must be: A. moves due to the prop pushing water out at a greatly accelerated speed behind the boat.

[TheShaughnessy]

tell ya what send me your gps unit and i'll record numbers. Will use the same prop and strut adjustments. For one of the test i'll leave the rudder how i run it on my geico, which is offset and the other run i'll put it back in the factory position, behind the prop. Then i'll tell you those numbers. We could then go further and see if using a larger diameter prop increases or decreases the effects of speed gained or lost. Perhaps a larger diameter creates more frothy water so more low drag water for the rudder to pass thru or maybe it pushes against the rudder with more force.


Now i wonder if it would vary from a catamaran to a v hull.

[Make-a-Wake]

Hey Sun..................I'm just down the road in the "Rock"!

[Fluid]

This question was put to bed years ago, but like everything else comes up from time to time as new folks join the hobby. The inline setup suffers from two fatal flaws. The propwash does indeed push back on the rudder, increasing drag. Since the water coming off the prop is moving faster than the water around it, it increases drag on whatever it hits. Same thing with a prop wash water pickup. These aren't sailboats! Put your hand in the propwash of a boat and then tell me it does not push back.

The worst offender is the aerated water. This causes random air pockets to form on the sides of the rudder, inducing a slight occilation or 'hunting' down the straights. This causes drag and may require steering corrections to keep the boat running straight - again, more drag.

For SAW running where top end is most important, or for hydros and boats without a transom dragging in the water, don't use an inline setup. The fact that no hydros and few racing cats or monos (nitro, gas or FE) run them should be a clue.....


.

[sundog]

Hey Sun..................I'm just down the road in the "Rock"!

C'mon up to Gods country some time! Lots of fresh water to lay in. Used to stay in Mempho, but the crime got so bad. I'm sure you know all about that. Begood, Sdg.

[Make-a-Wake]

C'mon up to Gods country some time! Lots of fresh water to lay in. Used to stay in Mempho, but the crime got so bad. I'm sure you know all about that. Begood, Sdg.

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