GBE Rudder foil angle ?

[G Force 0]

Were in the middle of building a new set of foils and rudder boxes.

The current set up is mounted vertically off the transom ( not following the original transom ,as was done 25 yrs ago).This system loads up especially when flying masthead kites to the point of if i haul on it any more somthings gunna brake

Q Has anyone played around with the angle of attack ?

As i understand we want to rake the rudder box 5 deg foward so that the leading edge is just in front of the pivot point at its lowest point ?

The foils are similar to the originals in shape.

Any feed back apreciated ?

[SloopJohnB 323]

Thats the only problem with a cassette type rudder, trying to get the leading as close to the pintel

line.

On the PT's some have got be to the pivoting blades to achieve this.

[MartinRF 53]

Were in the middle of building a new set of foils and rudder boxes.

The current set up is mounted vertically off the transom ( not following the original transom ,as was done 25 yrs ago).This system loads up especially when flying masthead kites to the point of if i haul on it any more somthings gunna brake

Q Has anyone played around with the angle of attack ?

As i understand we want to rake the rudder box 5 deg foward so that the leading edge is just in front of the pivot point at its lowest point ?

The foils are similar to the originals in shape.

Any feed back apreciated ?

The centre of effort of the foil is 25% of the cord length back from the leading edge and ca. 45% of the depth down from the water surface. This point must be aft of the pivot axis or you will soon loose control. If the centre of effort is too far aft of the pivot axis you will have heavy steering.

 

A rule of thumb number I come across from time to time is CE at 15% behind axis.

One word of caution: This rudder will be at an angle to the pivot axis so lower parts of the blade might be over-balanced. During flat water sailing you may not notice but when sailing in a way that results in only partially submerged rudder blades you may have to cope with surpising tiller response and feedback.

 

/Martin

[G Force 0]

Many thanks for the info Martin very interesting ,i like your site ,obviously a lot of research has been done here .Have you raked your rudders at all ? If so how much angle from the vertical ?

[MartinRF 53]

Have you raked your rudders at all ? If so how much angle from the vertical ?

You might be able to make out the rake of the pivotal axis from the photos on my front page:

http://hem.bredband.net/b262106/

Here is a simple sketch:

As you see the rudder is vertical but the axis is raked.

I chopped the top off the rudder blade many, many years ago so these days it goes as deep as in the sketch. New, deeper blades with a slightly different plan form are on the to-do list since 1997...

 

/Martin

[MartinRF 53]

I just found another sketch showing what it is all about:

I have even included a dashed hull flying water line so you can imagine the possibility of over balance under such conditions.

/Martin

[G Force 0]

Thanks Martin

[Guest]

any thoughts on toe in -toe out martin RF?

[Guest]

Hey Tim,

 

I think toe in is the way to go I read a good article on it (I will try and find the link and post) anyway Chris and I changed the hooters setup to include toe in of approx 2-3deg and noticed a nice change in helm feeling especially when transitioning to flying a hull. I also think Chris has a way to change the toe angle depending on conditions? Another thing we played round with was the Ackerman setup so the rudders turn at the right rate compared to each hulls turning radius!!??? not sure we ever got it right!!

 

G-force: I really like Bobo and Greers kick up rudders on the Dirty deeds as it allows you to have a nice really nice balance on the rudders as well ad being able to incorporate a fuse system in case you hit something! I think the cases can probable be a little liter too? Anyway probable worth checking them out at http://www.bobfishermarine.com

cheers Paul

[MartinRF 53]

I have not studied toe-in as I (simplistically?) think the rudders should work in harmony. 2-3 degrees of toe-in smells like drag to me but then, again, I have not looked into this.

 

Ackermann, on the other hand, is something I have studied.

http://hem.bredband.net/b262106/Boat/acker.html

I am convinced it really pays off going for a proper Ackermann geometry.

 

/Martin

[SloopJohnB 323]

I am convinced it really pays off going for a proper Ackermann geometry.

/Martin

 

I have looked at this before, but always get lost in the formula.

 

To me the only and the biggest variable in the whole thing is "R".

What is the turning circle of a cat?

Or do you base it on a percentage of the WL?

[Guest]

CAn someone explain Ackerman for us beginners?

[TimW 1]

CAn someone explain Ackerman for us beginners?

 

Put the steering wheel on your car on hard lock.

Get out of your car and look at the angle of the front wheels.

 

basically when you turn your cars steering wheel the car wheel on the inside of the turn has to turn in a tighter radius than the wheel of the car on the outside.

 

So anytime you have a two wheeled boat the same thing happens.

 

the outside hull has to travel further to complete the turn.

 

Ackermann is a geometric principle that allows for a compromise solution.

the angle of the tillers to the rudders are the key to Ackermann.

The object is to have the two rudders pointing the boat in the right direction as you complete a turn.

If it is wrong (i.e. if your Ackermann set up is wrong it is just a giant brake )

[Guest]

Now I need to draw a picture, how on earth would you calculate that?

[MartinRF 53]

Now I need to draw a picture, how on earth would you calculate that?

Take a look at the PDFs down-loadable from http://hem.bredband.net/b262106/Boat/acker.html

 

/Martin

[robdavies 0]

Automotive text books give a simplistic diagram where a line is drawn from the centre of the rear axle through the steering axis of rotation for each side. The connection for the steering arms to the hubs intersects along this line. I guess that on a cat the steering wheels are the rudders and the rear wheels are a combination of hull resistance and centreboards. I guess this gives a starting point between mid hull length and where the boards are placed on the centreline.

It would be an interesting experiment to work it backwards. Disconnect the link arm and for a set angle of helm and measure the free tiller angle and work Martin's geometry backwards.

[MartinRF 53]

It would be an interesting experiment to work it backwards. Disconnect the link arm and for a set angle of helm and measure the free tiller angle and work Martin's geometry backwards.

I actually did that once (motoring at about 5 knots) and it confirmed to me that my geometry is about right. It was kind of fun going in circles just outside the marina observing the freed tiller and how the boat reacted when you re-attached that tiller to the cross-bar. No, no-one went out to 'rescue' me nor asked me what had gone wrong when arrived back at my mooring.

 

/Martin

[ScottiE 174]

yup - did that years ago with the GBE. Pretty sure Bill did it with Voom as well. Our ackerman is a little different - go figure.

[Clipper 343]

yup - did that years ago with the GBE. Pretty sure Bill did it with Voom as well. Our ackerman is a little different - go figure.

 

Maybe cause the rudders are effectively in different positions? - voom has the extended sterns

[Marshy 30]

With the extended sterns the rudder will be different relative to the Boards so the ackerman would be slightly different i would have thought?