[Rhodes22-list] GB tiller-motor link geometry problem

[ROGER PIHLAJA]

Hi Patrick,

Using your straight overhead photo, I estimate the ratio of the [Rudder Pivot R] / [Motor Pivot R] ~ 3.2 .  This is an example of a type of mechanical amplifier.  ie, The motor would turn about 3.2 X as many degrees as the rudder turns.  There is also about that much torque amplification.  However, mechanical amplifiers like this have a very limited range of linear operation.  It has to do with the linkage geometry and the nonlinear behavior of trig functions.  Outside of this range, the linkage behavior becomes very nonlinear.  If you moved the rudder pivot forward so that the above ratio is < ~ 2, you will find the linear range is increased and there will still be sufficient torque multiplication such that the tiller will not be too hard to move.  I’m afraid if you shorten the [Rudder Pivot R] too much and approach a ratio ~ 1.0; that, the tiller will take a lot of force to operate and you will lose “rudder feel” , especially under sail.

Roger Pihlaja
S/V Dynamic Equilibrium
1978. Sanford, MI

Sent from my iPhone

> On Jun 6, 2024, at 1:29 PM, PBR <pbryanriley at gmail.com> wrote:
>
> On this new to me 89 recycled 2012 I have issues with the tiller link.  On
> my first boat I made a link that worked pretty well.  I recall at higher
> speed it wanted to just go straight (which was fine).
>
> This one is really squirrelly.  If you start to turn it seems unstable and
> just wants to go full into a hard turn.  From these pics. at center, turned
> max port, and max starboard - you can see that the motor is turning much
> faster than the rudder and it actually stops the rudder from turning as
> much as I would like to cruise into dock.  It also kind of binds up as the
> max angle resists turning back from the stops.
>
> On my home built I tried to get the distance from the rudder rotation point
> to the connecting bar, and the distance from the motor rotation point to
> the attach point on it to be about the same radius.  In this GB setup, the
> rudder pin to connection pivot point  is about 16 inches whereas on the
> motor it is only about 5 inches.  I think this is the problem and I
> probably need to build an extension to move the attach point on the motor
> top out further behind it.
>
> Thoughts?
>
> I sort of need this linkage for maneuvering in shallow water where my
> rudder might get kicked up.  If I leave the motor straight I could have
> prop. impact.
>
> Patrick
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