Rudder stand off length and taper?

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Chris Kurtz

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Nov 15, 2020
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So I've been searching around and was wondering how you decide on the rudder standoff length or if you want a tapered or straight rudder.

I'm guessing that a straight rudder with more of a standoff extension would be more positive steering but is there more to it than that?

On my 60" MTI cat I have a shorter (around 2") stand off and on my F 41 Stryker cat they recommended a 5" stand off. What would happen if I switched them around?

I noticed on my F41 it will sort of hook and turn so quickly I can't hardly follow it if I get too aggressive on the wheel. It doesn't flip or roll over and seems to handle it but WOW? I have only ran the F41 a few times so far and I'm still playing around with different props and strut settings.

On my 60" MTI it will spin out more if I remember right rather than hooking and launching in a different direction.

Both boats are running Gizmo 30cc plus engines. Both boats I'm using the Zippkits large straight rudder with the cooling holes in the leading edge. On the F41 I cut it down about and inch or so because it was quite long I thought, for that hull.

I assume that a tapered rudder would be a little softer and not as aggressive?

Thanks for any insight.
 

Wasted wages

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Jan 2, 2006
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653
A longer rudder standoff makes the boat think it's longer and changes the fulcrum point .... (longer lever effect) You might try playing with the CG and see if that also helps with the hooking.

Make sure your rudder linkage is not too agressive at the servo arm pivot point and that the dual rate on the radio is not waiting and then bringing in all of the turn at once. You want full swing at full crank of the wheel, but it should be smooth and not "come in" all at once.

Of course each driver will have there own preference, but as you already know, driving a twitchy boat is not fun and can be very unpredictable... especially in heat races.
 

Mike Cathey

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Try slowing the servo speed down on your transmitter if it has that function. You can adjust in a few seconds rather than fiddling around with the linkage.
 

John Beardslee

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Apr 20, 2011
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A longer rudder standoff makes the boat think it's longer and changes the fulcrum point .... (longer lever effect) You might try playing with the CG and see if that also helps with the hooking.

Make sure your rudder linkage is not too agressive at the servo arm pivot point and that the dual rate on the radio is not waiting and then bringing in all of the turn at once. You want full swing at full crank of the wheel, but it should be smooth and not "come in" all at once.

Of course each driver will have there own preference, but as you already know, driving a twitchy boat is not fun and can be very unpredictable... especially in heat races.
I think you ment “expo” instead of “dual rate”.
 

Rod Fonda

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Apr 30, 2021
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After checking what Wasted wages suggested,,,,Are you getting enough bow lift?which props are you running?,,Both my 21 and 45 sprint cats with short rudder standoff lenght(around 2 inches), turn predictably at full speed in heat water with a more forward CG, using just stok s/balanced x4xx .low lift props,,When I detonged and slightly pitched up the leading egde on the 21's x440, I got too much prop lift and lost bow lift and speed, making it prone to hooking,,pivoting on the nose,,I think with cats,its a balance of CG and prop lift/ prop shaft angle to get the right bow lift,,too much and you're always blowing over with a stiff breeze,not enough and you pivot on the nose and lose speed,I somehow fluked mine lol,,extending the rudder might help a bit but I don't think it will solve a cat running with a wet nose ,,the extra rudder fulcrum might lift the transom easier with rudder drag applied in the turns and put the nose in ,,
 

Chris Kurtz

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Nov 15, 2020
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82
These are the props that I have.
Octura 470, V967/3
ABC 2716, 2719-19-38
Zip 6719/3, 7016/3
Prather 270, 275

So far with the 30cc plus Gizmo engine my boats really like the ABC 2716, 2719-19-38, and Prather 275, props.
The others are a bit much for the engine and I'm thinking of doing a Barr cut or reducing the diam of them to get them to spin up better.
I have only ran the F41 one day though so I'm still playing with it.
I've seen where people are running the strut between .5" to 5/8" above the ride pads and I'm was around 0 to 1/4" so maybe I need to raise the strut too.

Do I understand it correctly that a prop with more rake on the trailing edges has less stern lift? So if I add more rake to the prop by taking material off starting at the hub and tapering it to the tips it will take away lift and at the same time make it a little easier to spool up?
On my electric I de-tongued a prop and got it to work well.
 

Rod Fonda

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Apr 30, 2021
Messages
44
These are the props that I have.
Octura 470, V967/3
ABC 2716, 2719-19-38
Zip 6719/3, 7016/3
Prather 270, 275

So far with the 30cc plus Gizmo engine my boats really like the ABC 2716, 2719-19-38, and Prather 275, props.
The others are a bit much for the engine and I'm thinking of doing a Barr cut or reducing the diam of them to get them to spin up better.
I have only ran the F41 one day though so I'm still playing with it.
I've seen where people are running the strut between .5" to 5/8" above the ride pads and I'm was around 0 to 1/4" so maybe I need to raise the strut too.

Do I understand it correctly that a prop with more rake on the trailing edges has less stern lift? So if I add more rake to the prop by taking material off starting at the hub and tapering it to the tips it will take away lift and at the same time make it a little easier to spool up?
On my electric I de-tongued a prop and got it to work well.
Those ABC have plenty rake,19 deg,,I don't think just cutting the trailing edge adds rake,,it has to be cast in,as the whole blade raked,maybe the 2719 has a bit more pitch and less tongue/blade area and lifts a bit,,maybe try moving some weight back or adjust prop shaft angle pointing up a bit,,my prop was about 1/8 to 3/16 above riding surface,maybe more for your sized boat and props,,.5" probably good,does the bow get up out of the water?
 
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Wasted wages

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Jan 2, 2006
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653
I would try raising the strut first...
I love the Prather 275, it's a great prop on my .90 mono, surface drive...but it has to unload...
 

Rod Fonda

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Apr 30, 2021
Messages
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I would try raising the strut first...
I love the Prather 275, it's a great prop on my .90 mono, surface drive...but it has to unload...
Yes Frank raise the strut a bit and angle up the shaft a bit,,love the sirname,it suits this hobby lol,,,I understand Italian,,yeah,,the good ol Prathers
 
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Steve White

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Dec 11, 2020
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I use straight rudder blades. Dumped the tapered long ago. Tapered does nothing advantageous and can simply not be enough.

Straight blades length adjustment and throw is how I do it.
 

Chris Kurtz

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Nov 15, 2020
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82
I have a straight rudder blade on it now. It has been cut down to around 4 1/2".
 

Steve White

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Dec 11, 2020
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The overall function of the rudder varies with hull type and setup - beyond the obvious of steering the boat.

Get it right, boat tracks straight, turns good, runs fast and is predicable - easy to drive. Not right boat can hook and spin, wobble, arc down the straights never running in a straight line, porpoise along with transom hopping, etc. Proper rudder geometry is very important.

Really there is only speculation on what will happen if you change the standoff lengths - predictable results? Maybe.

Do it and see what happens - don't be afraid to experiment. Sometimes it takes 10 wrongs to get to a right - sometimes ya hit it the first try.

That's all part of the hobby for many, to learn. Have some fun with it.
 

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