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rudder alignment ackermann system #271920
04/29/14 04:02 AM
04/29/14 04:02 AM
Joined: Oct 2012
Posts: 104
gold coast
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fast energy Offline OP
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this is a question or an alignment issue I just carnt understand ?
should the rudder alignment follow the Ackermann system
on my boat the rudders are aligned with straight rudder arms
if the Ackermann system is the correct way how do you work out the correct toe in for the rudder arms
or do the alloy frame rudder holders bent at approx. 25 mm (25 mm just my gest )
if any had a rudder arm bend / toe in measurement would give me a start in trying to alignment

thanks for any reply
as I my pulling out my hair on this one and I don't have much hair left


Brett Kelly
Fast Energy
Gold Coast
-- Have You Seen This? --
Re: rudder alignment ackermann system [Re: fast energy] #271966
04/29/14 06:45 PM
04/29/14 06:45 PM

S
Scarecrow
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Scarecrow
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Brett, the whole point behind Ackermann steering is the rudders don't stay aligned when they're turned. The outer rudder actually turns less. On a car this reduces tire scrub, on your boat it means you generate similar turning forces on each rudder despite the outer rudder is travelling faster through the water as the boat turns.

So to check your alignment you need to have the rudders aligned fore and aft.

On a car there is a particular easily defined Ackermann angle which keeps all four tires tangent to curves centered on the same point when turning.

But your boat doesn't turn around a single point, it slips sideways and has a different centre point for any speed and rudder angle so there is no "perfect angle". Which is why people aren't rushing to answer your question. On a boat like a mossie, people will normally put about 75-100mm of inset on the tillers. Less will have less resistance if making small corrections at speed more will have less drag when turning.

Re: rudder alignment ackermann system [Re: fast energy] #271973
04/30/14 03:18 AM
04/30/14 03:18 AM
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Posts: 104
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fast energy Offline OP
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Hi Scarecrow
Thanks heaps for info will try your option and let you know how I go
Regards Brett


Brett Kelly
Fast Energy
Gold Coast
Re: rudder alignment ackermann system [Re: fast energy] #271976
04/30/14 03:54 AM
04/30/14 03:54 AM
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Victoria Australia
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Ackermann is basic stuff on cars.....

When you turn a car, the Ackermann setup makes the inside wheel "pull" the car into the corner, as the car body rolls the load is transferred to the outside wheel, it then takes over the turning of the car and "pushes' the front of the car around the turn.

To better simplify it.....
if you draw a circle and place the car's outside turning wheel on it, then the inside turning wheels path is on a much smaller radius'd circle.... Ackermann counters that by making the wheels turning rates differ to allow for the difference in the size of the turning circles of the 2 front wheels.

and yes there's a formula to work it out for the best angle desired in a car....... the formula works for all wheel based variations in the different sized cars we drive from a tiny "smart car" to a limo...

but for boats.......

As Scarecrow said we have other forces at work as we turn the boat, we also have the "steering" at the back of the boat where-as a cars is up front.... ever tried steering a car in reverse at speed ?????
The Ackermann principle for boats and cars is basically the same, the inside hull actually travels on a slightly smaller radius'd circle than the outside hull.


Depending on how sharp you "steer" the boat around the marks will have a big impact on the Ackermann amount you "need" to have......
If for example you slam the tiller across until it simply wont go any further and the boat speed isn't that great then you'd probably want a large amount of Ackermann
&
if for example you take big arc turns and rely on keeping the boat speed up as you turn then a small amount (if any) of Ackermann is desired


So if you slam the tiller across then you are doing small turning circles, and hence need the inner hull needs to turn tighter
&
big arc turns means you have a large turning circle so the need to turn the inner hull is less.



Hope you understand it all so far ..... wink




So.......
lets put 100mm of Ackermann into the boat, it will near turn on a dime but will be as touchy as hell to keep going in a straight line, the slightest movement of the tiller WILL result in the boat "turning"
as you decrease the amount of Ackermann the touchy steering dissipates as does the sharpness of the turning....

Its a trade off as to which way you want it to work but most importantly, it has to suit YOUR style of tacking wink


& just for the record.... Twice Shy has 55mm of Ackermann over a distance of 380mm from the pintles
It turns nicely until you reef on it at speed, then its like slamming on the brakes and boat speed is lost instantly. The up-side is it can round a mark in a blink of an eye and tacking is a breeze.
With Jurassic Karp I intend to lessen the measurement to ~30mm as its a faster boat and will need a larger turning circle to keep the boat speed up.

smile

The other side of it is...
if you've got dead straight rudder arms and the boat 'handles" the way you like it..... then why change it

wink

cheers grin


Yar, & this ere post be done without a sin'le drop o' rum passin' me lips

Kingy
started with Impara Cadet #3 / Mosquito #245
& now Mosquitos #1182 & #1740

Re: rudder alignment ackermann system [Re: fast energy] #271981
04/30/14 06:19 AM
04/30/14 06:19 AM
Joined: Oct 2012
Posts: 104
gold coast
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fast energy Offline OP
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Thanks pirate
Maybe my tacking style is a little abrupt

I tend to try to tack like a dingy
Maybe I need say the 55 mm and a bit more tacking practice
I think I may modify the rudders and instead of just raceing
But am becoming disappointed as I have used my boat 4 times with 3 times toed back from the rescue boat
I know that I will all the problem s storted

Once again thanks for your info
I hope to get this boat going for the nationals but don't want to embarrass the class for myself


Brett Kelly
Fast Energy
Gold Coast
Re: rudder alignment ackermann system [Re: fast energy] #272028
04/30/14 03:12 PM
04/30/14 03:12 PM
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Victoria Australia
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Originally Posted by fast energy
....Maybe my tacking style is a little abrupt

I tend to try to tack like a dingy .....


old trick but a good one wink
tie a small diameter rope around the centre tube where the centre tube meets the rear beam (Venetian blind cord is ideal) so that you have 2 ends hanging from the centre tube...... about a foot & a half of cord hanging from the tube is a good start

then tie one end around the cross-bar in a loop on one side of where the tiller extension is fitted....
tie the other bit of cord around the cross-bar on the other side of the tiller extension.....


by adjusting the length of the cords you can limit the amount of movement the cross-bar travels

By doing this your not slamming the rudders across that far that the boat looses all its momentum and your boat speed is maintained at a higher speed.
In all reality you don't need much more than 45degrees of swing on the rudders in each direction so that's a reasonable starting point, just shorten the cord up from there as you get used to the restricted movement.
Its a simple "fix" for those that tack like they are sailing a laser wink



cool


Yar, & this ere post be done without a sin'le drop o' rum passin' me lips

Kingy
started with Impara Cadet #3 / Mosquito #245
& now Mosquitos #1182 & #1740

Re: rudder alignment ackermann system [Re: fast energy] #272029
04/30/14 03:19 PM
04/30/14 03:19 PM
Joined: Apr 2013
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couple of things I forgot to add last night..... blush

Originally Posted by fast energy
..... the alloy frame rudder holders bent at approx. 25 mm (25 mm just my gest )
if any had a rudder arm bend / toe in measurement would give me a start in trying to alignment...

What your looking at is the amount of 'bend' there is in the rudder case's arm .....
what you need to look at is slightly different tho.

draw an imaginary line through the centre of the rudder / pintle pin / & down the rudder case's arm, then measure from that line to the hinge pin on the cross-bar, that's the measurement your looking for. Most boats have different fittings that connect the rudder arms to the cross-bar, depending on that fitting type as to where the hinge is actually located.

There may only be about~25mm of bend in the tube but you need to add the fitting that lives in the gap between the arm and the cross-bar, its the hinge pin at that point that determins the amount of angle you have

wink



The other one.....

Ackermann angle is usually measured in degrees and not in millimetres or inches....
but its easier to simply get the tape measure out and start adding up the numbers
Twice Shy has ~6.5 degrees of angle



smile


Yar, & this ere post be done without a sin'le drop o' rum passin' me lips

Kingy
started with Impara Cadet #3 / Mosquito #245
& now Mosquitos #1182 & #1740

Re: rudder alignment ackermann system [Re: fast energy] #272135
05/02/14 03:16 PM
05/02/14 03:16 PM
Joined: Oct 2012
Posts: 104
gold coast
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fast energy Offline OP
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thanks pirate
are those measurements for each rudder or over all
I am going to the sailing club to compare measurement's of different cats
we have two mossies down here
if I compare cats of equal width should get my a rough size
again thanks for the in depth info and reply
I think you must really know how to set up the mosquito cat pretty well with this knowledge you have
for us in queensland we don't have many other mossie to test and tune against so all your throughs are appreciated


Brett Kelly
Fast Energy
Gold Coast
Re: rudder alignment ackermann system [Re: fast energy] #272143
05/02/14 05:06 PM
05/02/14 05:06 PM
Joined: Apr 2013
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Victoria Australia
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Originally Posted by fast energy
are those measurements for each rudder or over all ?


just for a single rudder case....

[Linked Image]

as you can see in this pic the 'bend' is around that 25mm your seeing.... then when we add the crossbar fitting it certainly alters the amount considerably


Yar, & this ere post be done without a sin'le drop o' rum passin' me lips

Kingy
started with Impara Cadet #3 / Mosquito #245
& now Mosquitos #1182 & #1740

Re: rudder alignment ackermann system [Re: fast energy] #272165
05/04/14 04:08 AM
05/04/14 04:08 AM
Joined: Oct 2012
Posts: 104
gold coast
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fast energy Offline OP
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thanks pirate
today I modified my rudder a arms to 40 mm bend inward to achieve the Ackermann system
my thanks and gratitude goes out to pirate and scarecrow for your help
all looking good
can't wait to race next weekend


Brett Kelly
Fast Energy
Gold Coast

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