• If you log in, the ads disappear in the forum and gallery. If you need help logging in, send request for help to: webmaster@southernairboat.com

Steering Thrust

Prototype

Well-known member
I have been testing the heck out of some 4 blade warps and 3 blade sabers in all kinds of configurations with a 6 blade warp hub from 2 to 3 to 4 blade. I've noticed a serious low speed steering advantage to some combo's vs others? I can't find much on props and steering on here but thought it may be worth a question to anyone that has noticed a difference in steering with a prop or blade change?
 

Waterthunder

Well-known member
It pertains to the thrust cone and how much thrust a blade profile makes at certain RPM,s the lower the RPM the blade makes thrust the more turning capability it will have. Also many boat riggers are still setting up rudder dimensions and locations based on the props they were running 10, 20,30 even 40 years ago. I have a customers boat here with a 4 blade 84'' R on the 3rd mark that cant turn under a mile radius and it's all because of the rudder location.
 

Prototype

Well-known member
Thanks Dave,
I thought there may be a missing x factor somewhere with all the new blades out. I was testing for low speed handling wet and dry often the same day with a few configurations but I'm running four foot rudders on a mini so that also may be an influence to my own results vs a normal setup.
 

bman

Well-known member
There are many variables with the different props out there.
The thrust cone will change depending on prop length and style of blade, so your rudder spacing and set back would have to dialed in based on that. I would imagine rotation of prop is also a factor as to which way turns the best, high pressure side vs low pressure.
I would like to see manufactures testing their props with some type of smoke screen that would really show the thrust cone and help guys dial these boats a whole lot better.
Maybe some one that has a thrust tester would be willing to play around with this?
Btw, I run a single rudder with a 10" setback, boat turns within its own length.
 

One Eyed Gator

Well-known member
Wouldn't also matter as to the steering setup you had PTO cable 6" travel, teleflex cable 8" travel and dog leg (what ever you want to make it).?

Same thrust with more rudder travel should turn quicker.

Just thoughts.
 

Prototype

Well-known member
Bman,
"There are many variables with the different props out there.
The thrust cone will change depending on prop length and style of blade, so your rudder spacing and set back would have to dialed in based on that."

That's what I was trying to figure out after some test.There are some good rudder spacing numbers out there for a normal setup but looking at this path, it's about impossible to test to the extent to say and r blade likes these number or a warp likes those numbers and so on for the rigging. It really shouldn't be hard to define with unlimited resources :) but for now it's just whats on hand and why one prop does better than another for a particular situation on the same setup. I think Dave kind of summed it up that even new rigs still don't have a standard for new props.
 

glades cat

Well-known member
If you want accurate measurement, you might want to invest in a rotary vane anemometer to measure air velocities at various points behind the prop and rudders. It would be a good tool to avoid the guessing game of their placement. Spiral slipstream, or prop wash, is rotational as well. The air behind the prop twists. A wide prop with more pitch will have a greater effect than a thin blade with less pitch. That's why you can't go with a ball-park rule of thumb calculation. It may not be right for your boat.

Here's a reasonably priced unit at Grainger.
http://www.grainger.com/product/28AF77?gclid=CNrUsvP6i74CFWuhOgodlw4APA&cm_mmc=PPC:GooglePLA-_-Test%20Instruments-_-Air%20Movement-_-28AF77&ci_src=17588969&ci_sku=28AF77&ef_id=33VPG1xxWR8AAAez:20140502001539:s

Through trial and error over many years and numerous builds, older manufacturers have fine-tuned the placement of the various components on their boats. Well placed rudder height, separation, set-back and size can mean the difference between a responsive boat and one that feels like mush.

Lee & Judy did an awesome job on my Scorpion. The steering is light and neutral, is very responsive underway and pivots on its' own length at rest.
 

bman

Well-known member
I always have liked a longer rudder over a taller rudder for lower hp smaller propped boats, takes more leverage but will perform much better at slower speeds. Having a long pivot point with a long travel 8" cable makes that possible.
Hell, my rudder is a 1/4 of my boat length. :lol:
 

Prototype

Well-known member
glades cat said:
If you want accurate measurement, you might want to invest in a rotary vane anemometer to measure air velocities at various points behind the prop and rudders. It would be a good tool to avoid the guessing game of their placement. Spiral slipstream, or prop wash, is rotational as well. The air behind the prop twists. A wide prop with more pitch will have a greater effect than a thin blade with less pitch. That's why you can't go with a ball-park rule of thumb calculation. It may not be right for your boat.

Here's a reasonably priced unit at Grainger.
http://www.grainger.com/product/28AF77?gclid=CNrUsvP6i74CFWuhOgodlw4APA&cm_mmc=PPC:GooglePLA-_-Test%20Instruments-_-Air%20Movement-_-28AF77&ci_src=17588969&ci_sku=28AF77&ef_id=33VPG1xxWR8AAAez:20140502001539:s

Through trial and error over many years and numerous builds, older manufacturers have fine-tuned the placement of the various components on their boats. Well placed rudder height, separation, set-back and size can mean the difference between a responsive boat and one that feels like mush.

Lee & Judy did an awesome job on my Scorpion. The steering is light and neutral, is very responsive underway and pivots on its' own length at rest.

I tried my old two stage one! Can't even begin to describe the numbers but I'm not sure it applies with rudders on?
 

glades cat

Well-known member
Yea, the airflow is crazy behind a prop.
When I raced hovercrafts back in the late 80's we used ducted fans and multiple (4) rudders. The idea was to direct the entire airstream without losing any peripherally. The prop was only 32" in diameter on a 10' boat so we had to capture all of it. The rudders were flat to reduce turbulence and create more laminar flow.
There is a fine line with having "too much rudder", especially airfoil type. The more surface area, the more control, but it also increases "skin friction".
 

Steel Armadillo

Active member
So once we have a general idea of where our thrust cone is at a certain rpm, is there some guideline on where to place the rudder(s) in relation to it?

Jesse :old_glory:
 

Prototype

Well-known member
Steel Armadillo said:
So once we have a general idea of where our thrust cone is at a certain rpm, is there some guideline on where to place the rudder(s) in relation to it?

Jesse :old_glory:


That's another unknown! There are some good formulas on here but I started this with just a why on the same boat using them formulas the props make handling better or worse in blade pitch and configuration, Both in bounce and torque roll. It may never be answered but asking doesn't cost a dime and so far it's just move things in your setup to fix it verses an incorrect prop configuration or rudder configuration.
 
Top