Rudder height?

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mixedbag
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Rudder height?

Post by mixedbag »

What is the optimal height of the lower rudder bar? is there a maximum or minimum height? Just switched my rigging to a new hull everything raised by 3.5 ".
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JAMES
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Re: Rudder height?

Post by JAMES »

ladyblackwater wrote:A prop pushes air from center out 16 inches on each blade. If you want easy and fast turning make sure top and bottom of rudders catch all 16 inches. Basically getting the center of rudders close to center of prop is a good idea. I don't know what size prop and what motor you have but the distance between rudders make a huge difference also.
What part of SA did u read this ? Just wondering because the prop manufactures Have said other wise .
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Re: Rudder height?

Post by SWAMPHUNTER45 »

It was explained to me the propeller grabs air from the outer area of the blades and compresses it toward the center. The strongest portion of the thrust column is reported to be toward the center.

This may be where the old saying "diameter trumps pitch" comes from ?



If your rudders are centered and the geometry is taking the thrust column into account it should work adequately.

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kwanjangnihm
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Re: Rudder height?

Post by kwanjangnihm »

Smugglers Formula:
This formula seems to work pretty good as well.. Length of prop divided by 2 minus 10 = rudder spacing. Not saying it's the gospel but it works for me..

82” Prop
82 / 2 = 41
41 – 10 = 31” spacing

Here is a couple old rudder spacing threads..
http://www.southernairboat.com/phpBB3/v ... ng#p378279
http://www.southernairboat.com/phpBB3/v ... er+spacing

Rudder Height (cntry141)
the rule of thumb I try to use .. is 2/3 of the rudders below the centerline of the prop ..
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Re: Rudder height?

Post by Prototype »

IDK,
I've built a 3 rudder system that was outstanding on the shoreline after it turned too good. That's exactly where I was standing, on the shoreline after that sucker turned on a dime and went horizontal.
The old rule works really good as far as spacing apart for two rudder system, but height does play a major role! As low as possible rudder mounts will keep your rig more stable to the center of the prop but also nasty riding over weirs and logs that they could hit. Strictly speaking the normal shit if you hit your rudders on stuff up or down.

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Re: Rudder height?

Post by Slidin Gator »

If the prop did not compress and accelerate the air towards the center there would be no thrust. Consider that the prop pulls the air in from roughly a 180 degree segment (50%) of a sphere in front of the prop. If all that air were exhausted out into the other 50% (the other 180 degrees) then the net thrust would be Zilch.

The airflow/thrust profile does resemble a cone, but with a twist. The rotational component is the wasted power, normally called drag.

Start a garbage fire, move your boat in reasonably close, throw some leaves on and fire that mother up. That redneck air tunnel will show you what the air is doing, plus you can cook some sausage once it gets going good. :idea: The air flows in from the blade tips and beyond, but leaves concentrated towards the center in a swirling point out the rear.

As for the rudder, I want to catch all of the flow that I can (without being stupid). If I want the air to go straight I'll hold the rudder straight. Otherwise, I want access to all of that thrust in the direction I want/need it.

I run a 48" full airfoil, single rudder on a 72" prop. Rudder center is 4" below prop center, which puts 58% of the rudder below center (28/48). I think Kwan's formula is for Port/Stbd spacing of dual rudders?? :scratch:
I still think I-10 is the Mason Dixon line.
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Prototype
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Re: Rudder height?

Post by Prototype »

Wtf does radiant heat in a vertical leaf fire have anything to do with prop push horizontal?
Nice thought if your horizontal thinking about it but airboat air washes down from prop even on the rudders.

Simple flour will show you that!

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Re: Rudder height?

Post by mixedbag »

I wanted to know the height of the LOWER rudder bar from the transom, I got the other stuff down. Thanks for the info!
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Re: Rudder height?

Post by Slidin Gator »

mixedbag wrote:I wanted to know the height of the LOWER rudder bar from the transom, I got the other stuff down. Thanks for the info!
Mixed bag, the bottom of my rudder is 3" above the top of the transom, the rudder steering bar is 1" above the transom. Specifically, for an (my) open hull boat the steering arm just clears the transom and the rudder connection dictates where the bottom of the rudder itself is. You want to catch all of the air at the bottom of the prop that you can, and then most of the upper half. The very top of the prop pushes the nose down the hardest, so steering get's touchier when you grab more of the top half of the prop, as Prototype covered.
Prototype wrote:Wtf does radiant heat in a vertical leaf fire have anything to do with prop push horizontal?
Nice thought if your horizontal thinking about it but airboat air washes down from prop even on the rudders.

Simple flour will show you that!
Protype, I'm just talking about how the prop pulls the smoke off the fire. The prop pulls air from everywhere, the airboat hull (and ground) keeps it from flowing up from the ground so most of it comes in from above like you say. Yep, flour, leaves, tree bushes, all of that shows the airflow. I'm pretty sure we are on the same page here. :bom:
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.

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kwanjangnihm
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Re: Rudder height?

Post by kwanjangnihm »

ladyblackwater I sent you a PM! :thumbleft:
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mixedbag
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Re: Rudder height?

Post by mixedbag »

ladyblackwater wrote:Mixedbag it's hard to give you a correct answer with out knowing diameter of prop, height of motor stand, height from top of stringer to top of transom and size (lengt) of rudders.

My new hull has 7 inch stringers opposed to 3.5 inch in the old boat, everything has shifted upwards 3.5 inches. I do not expect a problem,

have put a few boats together. Going by some tried a true methods, just wanted to know if there was an exact. Thanks
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mixedbag
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Re: Rudder height?

Post by mixedbag »

BTW 74 " prop, approx 50 " to ctr of prop from deck, approx. 13" from top of stringer to transom, 48 " rudders.
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