[C320-list] steering geometry question

[Graeme Clark]

I got down to my boat last weekend and made some measurements.

Given that it is a six spoked wheel. I noticed that I got 6 1/2 spokes, i.e. 1 and 1/12 full turns of the wheel from central to full lock.

Each spoke (1/6 turn) equates to approx. 1 inch of cable movement, which equates to approx. 9 degrees of rotation of the rudder, on the rudder quadrant (which is actually not a quadrant bit a full circle) which is approx. 12 inch diameter

So do the math and the max rudder angle would be just under 60 degrees either side of central - (that seems a lot to me!)

More math shows that the throw required to make the full 120 degrees of travel would be 10.2 inches, but only  if you could mount the end of the ram arm on the edge of the quadrant, which you cant unless you use a second separate crank arm clamped to the rudder post

If you did it at 5 inches radius point then only 8.6 inches of travel is required but of course you need more force!

I guess the bigger issue is that the ram cannot be easily mounted to permit full use of all 12 inches of travel?

How does this theory compare to real life experience?

Graeme
#366 Jaskar


On 22 Sep 2015, at 00:18, Jeff Hare <catalina at thehares.com> wrote:

> CORRECTION:
> 
> I believe I got this wrong folks.  When I said I get 1-1/2 turns of the
> wheel each way, that's incorrect.  I should have said I originally got
> exactly 1 full turn each direction.  Old age and memories don't coexist well
> in my head. Sorry!
> 
> With the below deck autopilot installed, I lose somewhere between 1/4 and
> 1/5 of a turn each direction and that's about the best I could get.  That's
> with mounting the ram at just about 7.5" out on the tiller arm.  So, that
> reduction is a bit more significant as a percentage.
> 
> I'll be able to assess it when I go out this weekend to see whether I can
> notice it or not.  I'm used to pivoting and making sharp turns while
> manoeuvring slowly, so we'll see how it reacts now.  
> 
> Since we dock in up to 1.5kts of current flowing straight into our slip
> (reversing tidal river), we point down river and crab our way over to our
> slip, then spin and hit reverse to keep from crashing the dock.  :)  We only
> get one shot or we have to call the fiberglass repair guys, so docking
> without the ability to spin sharply won't be fun (or we'll have to be sure
> we always come back in with an outgoing tide).
> 
> -Jeff Hare
> #809
> 
> -----Original Message-----
> From: C320-list [mailto:c320-list-bounces at lists.catalina320.com] On Behalf
> Of Jeff Hare
> Sent: Wednesday, September 16, 2015 1:53 PM
> To: C320-List at Catalina320.com
> Subject: Re: [C320-list] steering geometry question
> 
> Hi Graeme,
> 
> I just completed this project last weekend. Special thanks go to Allan Field
> and Scott Thompson who were great resources (and Jon Vez for sending
> pictures of the Rudder travel limiter configuration used on his C355)
> 
> Rudder post is 2.860" in diameter according to Edson.  The factory rudder
> travel seems to be around or slightly over 40 degrees each direction.
> 
> On our hull (#809) from the wheel amidships,  I used to get exactly 1 and
> 1/2 turns of the wheel each direction.  With the AP installed I now get
> exactly 1 and 1/4 turns each direction with my installation.  After studying
> it a bit and experimenting with mounting angles etc, there are several
> aspects of the geometry here that really does prevent a linear drive unit
> from doing the full lock to lock you get from the factory on the C320.  But
> with the mounting/drilling dimensions/angles I used, I give up very little
> turning radius and I suspect the final turning angle I get is a little over
> 75 degrees total.
> 
> Much of this is limited by the 12" maximum travel of the Linear drive's RAM.
> RayMarine wants the RAM connected out 10" from the center of the post which
> results in a max angle of 70 degrees.    
> 
> I attached the ram at about 8" out from center to increase the turning angle
> a little beyond the RayMarine max of 70 degrees.  If I went less than that,
> I might get more throw, but the RayMarine said the wear rate on the RAM
> would increase dramatically, not to mention that the average stresses on the
> aft cabin wall where the RAM attaches would be much higher and could cause
> the tabbing to weaken or break over time or in rough conditions.  The aft
> cabin wall is very thin and not great structurally.  It's weakened by access
> panel.  Hence the large backing plates to spread the load.
> 
> I've been documenting the process pretty carefully with lots of pictures.  I
> made a CAD drawing file for the Linear drive backing plate that I sent to a
> machine shop.  The backing Plates I used to sandwich the aft cabin wall
> where the RAM is mounted are larger than those used by Scott or Allan.  I
> didn't use a mirror polished plate like they did but would recommend it.
> Mine is brushed finish because it was cheaper and would be ready quicker.
> But I can have it polished later if I want.
> 
> The unit I installed is the RayMarine ACU-200 Sail Pack (p/n T70155) with
> Type 1 Linear drive and Rudder Position Sensor to allow software control
> over the Linear drive's turning limits.  
> 
> Edson sells a Tiller arm to attach to the rudder stock that is specially
> milled for the C320 rudder post.  It's a nice piece but needs to be
> shortened slightly to give enough clearance on the C320.  Also, you'll need
> to drill the rudder post.  I used a high quality Dewalt bit dipped in ALUMET
> 872 cutting oil before I started and with my cordless drill, I was done in 2
> minutes with a few 6" long coils of SS cuttings.  Rather than drilling all
> the way through the post and thru-bolting it, I drilled a 5/16 hole and
> tapped it for a 3/8 coarse thread high strength stainless bolt.  I believe
> this is sufficient. 
> 
> Edson also sells a Rudder Stop kit that (P/N: 862-WIRE) (mentioned earlier)
> that replaces the bump stop system used by the C320 in favor of a cable stop
> with rubber shock dampener used in newer installations and larger boats.
> 
> For mounting the Rudder Position Sensor, I made a simple wooden unit that
> clamps around the rudder support tube, therefore is already angled the same
> as the rudder post and is height adjustable for perfect alignment with the
> tiller arm.  I'll be working up an illustrated article for the website that
> others can also use for a reference but will be happy to discuss it with
> anyone starting the project before the write-up is done.
> 
> -Jeff Hare
> 
> -----Original Message-----
> From: C320-list [mailto:c320-list-bounces at lists.catalina320.com] On Behalf
> Of Graeme Clark
> Sent: Wednesday, September 16, 2015 11:58 AM
> To: C320-List at Catalina320.com
> Subject: [C320-list] steering geometry question
> 
> I'm 250 miles from my boat and planning a below deck autopilot installation
> so would like to know approximately what is the angle or travel for the
> rudder (degrees either side of central) and the approx diameter or the
> rudder quadrant, so I can calculate the actuator throw I will need and where
> to connect it to the quadrant (or possibly make up a new radius arm
> 
> Anybody have those figures to hand by any chance? Doesnt have to  be that
> accurate at this stage
> 
> Thanks
> 
> Graeme
> 
> #366, Mk 1, Jaskar.=
>