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  #31  
Old 07-16-2013
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Re: Backing up (or not)

Quote:
Originally Posted by bobperry View Post
The arm between the center of pressure of the rudder and the center of pressure on the hull in reverse is just too short to give the rudder the upper hand. The more you eliminate keel aft and move it forward, i.e. fin keel, the better the boat will back up.
Do you have a reference to that article.
I'm not sure I understand the arm thing you mentioned but is sounds like it is the answer I'm looking for.

IE what is the arm why is it a problem in reverse but not forward?
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  #32  
Old 07-17-2013
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Re: Backing up (or not)

Quote:
Originally Posted by smackdaddy View Post
Thanks for the explanation Force. After my last post, I looked it up and saw the explanation you just gave for how it works. Pretty cool.

When I'm at the boat this weekend, I'll do the prop-walk test in the slip to see how much push we're getting in reverse.
When you're looking for the prop walk it's best to "goose" the engine. Short bursts will result in the most prop walk. Sometimes you can use this to move the stern to your favor.
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  #33  
Old 07-17-2013
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Re: Backing up (or not)

Dave:
That article appears in the July? Aug issue 2010.

"IE what is the arm why is it a problem in reverse but not forward? "

Good question. I'll take a stab at answering it. I think the key is that the center of pressure on the hull moves. When in reverse the hull C of P moves well aft. In forward it moves well foward. This would be consistent with any foil and the keel foil shape is backwards in reverse, i.e. sharp leading edge, making it move even more aft. I think that's it.
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  #34  
Old 07-17-2013
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Re: Backing up (or not)

I've always looked at it like this:

When moving forward a keel with an attached rudder acts like an airplane wing with an aileron. The net effect on the lift of the wing is in the opposite direction of the movement of the aileron; the aileron goes up the wing goes down, and visa-versa. Now, turn the plane 90˚ so that the wing is pointing straight down and you essentially have a keel with an attached rudder. The rudder goes to the right the lift is increased to the left, and visa-versa.

BUT, reverse the flow and what do you have? The "aileron" is now acting more like a leading edge flap (and there's a good reason they put the ailerons on the trailing edge of the foil). It might reduce the stall speed a bit (that's why planes have leading edge flaps), and increase the drag of the foil, but it won't do much for the lift of the foil (wing or keel; OK, it may increase the lift a bit, but not as much as an aileron would). So, when backing a boat with a rudder attached to the keel the "control surface" is basically on the wrong edge of the foil.

A spade rudder works because moving it from side to side has the same effect as changing the "angle of attack" of an airplane wing. Increase the angle of attack and lift is increased, at least until the stall angle is reached. With a spade rudder such lift swings the stern right of left, and Voila! The distance from the center of pressure may also come into play, but the important dynamic is the effect of the control surface on lift (in my humble biomechanical opinion).
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  #35  
Old 07-17-2013
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Re: Backing up (or not)

Hmmmmm,,,,,,I'll buy that. I need to ponder it a bit but it sounds good to me.
I think you may have part of the answer but I still think that the center of pressure on the keel plays a big roll. If it were static you would still have the same turning arm in reverse. I think having the control surface on the leading edge also may be the reason the C of P moves aft.
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Last edited by bobperry; 07-17-2013 at 05:30 PM.
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Old 07-17-2013
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Re: Backing up (or not)

I like the explanations above; however in addition to the posts above, there's a very simple added explanation once you have momentum. In forward you can compensate for prop walk with the action of the prop wash over the steering rudder. In reverse there is no prop wash over the steering rudder.
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  #37  
Old 07-18-2013
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Re: Backing up (or not)

Quote:
Originally Posted by bobperry View Post
Hmmmmm,,,,,,I'll buy that. I need to ponder it a bit but it sounds good to me.
I think you may have part of the answer but I still think that the center of pressure on the keel plays a big roll. If it were static you would still have the same turning arm in reverse. I think having the control surface on the leading edge also may be the reason the C of P moves aft.
Another way to look at this is to think about what happens to the flow over the keel in the forward and reverse directions. Let's start with the boat moving forward; the flow is (or at least should be) laminar all along the keel and its attached rudder. Now, turn the rudder a bit to Port and the flow should stay pretty much laminar as long as it isn't cranked too far. There may be a few vortices being shed off of the trailing edge of the rudder, but these will only increase drag without having much of an impact on lift. So, turn the rudder to Port, lift increases on the Starboard side of the keel, the stern swings to Starboard and the bow turns to Port.

What happens when the flow is reversed (i.e., the boat is backing)? With the rudder centered the flow is again laminar. The maximum chord (the thickest part of the keel) is probably in the wrong place, but let's not worry about that for now. OK, turn the rudder to Port again and what happens to the flow? Now the edge of the rudder is the first thing the flow "sees" as it impacts the keel. Since this edge is relatively sharp (compared to the forward edge of the keel) the flow separates right at the leading edge (or what is the leading edge while backing). This causes vortices at slow speed and turbulence as the speed increases, in this case all along the Port side of the keel. This pretty much ruins any lift the keel might otherwise generate (i.e., the keel is "stalled"), and directional control is more or less nonexistent.
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Last edited by SlowButSteady; 07-18-2013 at 12:51 AM.
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  #38  
Old 07-18-2013
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Re: Backing up (or not)

Quote:
get the boat moving and then put it into neutral. ......... The idea is to go into and out of gear.
This is the only way to back a full keel boat, just enough thrust to make way then back to neutral
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  #39  
Old 07-18-2013
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Re: Backing up (or not)

Great thread; just to chime in, I have a 22' with a little 6hp outboard hung on the transom off-center with the transom-hung rudder. When I am backing out of my slip, I have found that if I keep the tiller straight and steer with the motor (with a little help turning the boat as we hand walk it out of the slip to the end of the finger pier, I get enough steerage to get her turned in the right direction. My motor won't do much to slow the boat when I am docking, so I rarely try to use it in that capacity, but I have found that unless I want to really bring her around fast, steering with both the tiller and the motor is not needed.


My only question is this: I have a swing keel that I can crank up, and I am wondering if it makes the boat easier or harder to steer in reverse if it is up vs. down. I suppose I can try it out but it ain't broke at the moment, so why try to fix it?


Any thoughts?
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  #40  
Old 07-18-2013
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Re: Backing up (or not)

I have tried to back my boat a few times and it was so slippery I gave up. I have a rh Folding prop. Not much help in rev or in Fwd for that matter. My shaft is a bit off-set as well.
Any Idea how to use the rudder against this prop configuration to get rev steerage?
Below is a shot of the offset.

Thanks for your help.



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