RHM pendulum for double enders and outboard rudders

Design, Construction and Use of Self-Steering Gear

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RHM pendulum for double enders and outboard rudders

Postby jan alkema » Sat Oct 24, 2009 10:47 am

Hi all,
I have had quite some enquiries in the recent years regarding the use of a RHM pendulum for the outboard rudder of a double ender or Colin Archers type of boat. After a short investigation it became clear that the RHM pendulum system cannot easily be fitted on that type of boat.

On a boat with a transom it is quite simple. Axis a-a is the horizontal pendulum axis and b-b the vertical axis. For a normal mounting there is always an arm X available and with a chosen position of the pivot point P there is a suitable ratio of X/Y possible. Furthermore there is no problem for finding a position for the connection points D on the transom. The angle between the restraint lines and the heart line of the boat can be 50 degrees or more. (The nearer to 90 degrees, the better)
This system is very suitable for the outboard rudder versions of f.i. Westerly's, Moody's, Sadlers etc.
I am already sailing with a RHM pendulum for more then 25 years.
For a short impression of RHM (in combination with a USD vane) see : http://www.youtube.com/watch?v=I-2GxsrSmo0

On the double ender as shown on the sketch, the intersection point of the axis a-a en b-b is very near or just on the rudder axis c-c. There is no arm X and so the RHM principle will not work. Because of the narrow stern sections the angle between line P-D and the heart line of the boat is smaller compared with the boat with a transom and hence the forces will be much higher.

Solutions may still be possible, when the axis a-a is lowered and/or the pendulum axis b-b is set further aft. Also the restraint lines can be guided by a mounting so that its angle to the heart is more favourable. It can be an interesting challenge to find a solution.

Jan[attachment=0]rhm1_0001.jpg[/attachment]
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Re: RHM pendulum for double enders and outboard rudders

Postby wa4chq » Sun Oct 25, 2009 1:22 pm

Hi Jan
I looked at the pdf you have for the trim-tab arrangement. Very nice. I own a Nic. 32 and wondered if it would be suitable with the 'rhm trim-tab'. Could I use the existing inboard rudder or would you recommend adding a transom hung rudder? If it is the latter, would I tie off the tiller?

Thanks...

Neil
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Re: RHM pendulum for double enders and outboard rudders

Postby jan alkema » Sun Oct 25, 2009 3:12 pm

Hi Neil,

I am a bit puzzled about your questions, as the Nic.32 has no transom hung rudder. Can you present a sketch of what you mean ?

Jan
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Re: RHM pendulum for double enders and outboard rudders

Postby wa4chq » Sun Oct 25, 2009 3:23 pm

HI Jan--
Yes you are correct about the Nic32. I guess my question is could I use the system you describe with my existing rudder (sort of like the Sayes design) but looking back at your drawings I see that it wouldn't work. So if I installed a transom hung rudder, I assume that the main rudders tiller should be tied off?

I hope that clears things up a bit...

regards

Neil
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Re: RHM pendulum for double enders and outboard rudders

Postby jan alkema » Sun Oct 25, 2009 5:07 pm

Hi Neil,

If you are using an auxiliary rudder at the transom, then the main rudder should fixed. The book of Peter Foerthmann "Self-Steering under Sail" gives a good overview of all kind of systems. It can be downloaded from his site : http://www.windpilot.de

Jan
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Re: RHM pendulum for double enders and outboard rudders

Postby wa4chq » Sun Oct 25, 2009 6:27 pm

Jan, thanks for the latest link. It looks like it has some great information.
All the best-
Neil
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Re: RHM pendulum for double enders and outboard rudders

Postby jan alkema » Thu Oct 29, 2009 3:40 pm

In the posting "Atkin's Thistle" a sketch is presented showing a possible solution for using RHM on a double ender.

Jan.
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Re: RHM pendulum for double enders and outboard rudders

Postby jan alkema » Fri Aug 13, 2010 10:45 am

Hi all,

I started this posting by explaining that the RHM is sometimes difficult to mount on some types of boats with outboard rudders, specially the double enders.
I already mentioned a possible solution in this thread.
Now I designed an alternative solution which may be interesting for boats with small rudder heads or double enders. The principle is shown in the sketches. Left is the original RHM. The vane and linkage are not shown. Point P is a "fixed" pivot point, when the restraint lines are tight. X is the "tiller" used by the system. The ratio between rudder angle and pendulum swing is governed by the values of X and Y. Right is the alternative. Now point P is not a fixed point. Arm Xr is in fact the tiller. The ratio between rudder angle and pendulum swing is now governed by the values of X, Xr and Y.
For the type of rudder and transom shown here there is no need for an alternative system. My RHM system is already working for more then 20 years.
In a next post I will show on which type of rudders and boats the alternative system can be used.

Jan


[attachment=0]RHM variations_NEW.jpg[/attachment]
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Re: RHM pendulum for double enders and outboard rudders

Postby jan alkema » Tue Aug 17, 2010 11:25 am

Hi All,

In the first posting of this thread I mentioned that the RHM principle can give problems on double enders. In the thread "Atkin's Thistle needs wind steering" I already gave a solution for the problem. Next I show a sketch of a possible solution for double enders using the alternative way of the RHM principle.
For those who are not familiar with the RHM, you can read my article about it on Joern Heinrich's site http://www.windautopilot.de and then click on "Artikel"
In the sketch the arm X is zero, but the rudder will be operated via the arm Xr. Again a strut is used for getting more favourable angles of the restraint or steering lines.
I tested this idea on a model system, but I am quite sure that it will work on a real boat.
In a next posting I will show the alternative RHM for a rudder of a Twister type sailing boat.

Jan

[attachment=0]RHM alt.for double ender_NEW.jpg[/attachment]
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Re: RHM pendulum for double enders and outboard rudders

Postby jan alkema » Fri Aug 27, 2010 1:42 pm

Hi all,

In this posting I will show the normal solution of the RHM pendulum for a raked transom, like on the Twister. As the rudderhead on that type of boat is very small in length it has to be extended to 0,3 or 0,4 m, see the value X , as this is in fact the "tiller" for the system. The arm X is depending on the rudder balance. For an unbalanced rudder and a aperture in the rudder blade for the propeller X should be approx. 0,4 m. to keep the forces in the restaint wires on an acceptable level.

The alternative system accepts the small rudder head. In the sketch a point R on the rudderblade is shown and Xr is now the "tiller" . Note that it may be necessary to make an extension on the rudderblade for the point R, to have an arm length Xr of approx. 0,3 or 0,4 m.
This is just an idea, only tested on a model.

Jan


[attachment=1]Alt.RHM for Twister type_NEW.jpg[/attachment][attachment=0]RHM for Twister type_NEW.jpg[/attachment]
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RHM for Twister type_NEW.jpg
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Alt.RHM for Twister type_NEW.jpg
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