Originally Posted by dpex
They seemed to have served Alex Rose well.
Its the gearing ratio which has me perplexed at the minute.
I have studied the Sarana site details and conclude the issues are all down to relatives. By that I mean the relative force applied to the trailing edge of the rudder required to to move it sufficiently to reset the desired vessel course.
That force is determined by the force applied by the vane to the tab and, the relative rate at which that force is applied. That seems to be what Rose means when he mentions the differential Hassler built into his system.
In turn, both the force and the relative rate of application (the differential bit) is inextricably linked to the force capable of application by the tab.
This second part is clearly determined by one of two factors.
The first being the distance of the tab's force from the gudgeons of the main rudder, and the size of the tab. Ergo, the further aft the tab, the less needs to be it's size.
In the Sarana model the tabe hangs out way back from the rudder and, in my opinion invites two problems. The first being fouling by underwater objects. The second being...Something hanging way out there is subject to multiplied forces when the vessel is in the sea from hell.
Also, I note the Sarana model requires the tab to be locked. One presumes this is for when the vessel is going in reverse.
My intention is to build first the tab and using racing dinghy
gudgeons, secure that to the main rudder.
That then results in the control shaft sticking up behind my rudder head
. Then I can build a simple brace for a bearing...a high impact nylon bearing.
Under the landing plate for that bearing will be a collet which will hold the tab in its gudgeons. If ever I wish or need to remove the tab I will simply undo the collet, lift
the tab from the gudgeons, then feed the shaft down and clear.
Next is the connection between tab-shaft and vane. I'me going to follow Hassler's approach for this but use one helical and one straight-cut gear
, made of high impact nylon...at a ratio of 3-to-1. Ergo, three degrees of turn of the vane will exert one degree on the tab.
I'm guessing at this ratio but I have to start somewhere.
However, that ratio is inextricably linked to the surface area of the tab.
My intention is to begin with my best guess, plus 50%, then try it all out.
As needed I will shave off the tab, vertically, till the best fit arrives.
I'm presuming my best guess plus 50% will create huge oversteering issues, so I'll have plenty of test-room as I shave the tab down. Sooner or later the ratios will arrive at the best fit. Never perfect, but good enough.
Furthermore, I will be able to modify the vane area by simply making it out of sail cloth then reefing it a bit, if required. Miniature roller reefing is easy enough to make.
You see, when ocean sailing, and not in a hurry, a few degrees of course, here and there is hardly likely to cause stress to the Captain
But I say agin, the thing I like about the Hassler concept
is it's robustness which, in counterpoise
to the thing I don't like about the Narvik is its delicateness. Lovely design, but what state will it be in after say, five days of being bashed around in a force ten gale?
In such a gale, either laying ahull, or on a drogue
, the tab I can lock. The vane I can simply make disappear with the roller reefing, and then slam the companionway
door shut, dog it and simply ride out the drama.
Heavy galv pipe is way stronger than solid anything. So heavy galv pipe will be used for the tab axle and the vane axle. The vane will be made of two bits of much lighter galv pipe, plus couter balance. The 'sail' will be made of 12 ounce cloth on a little roller reefer.
All bearings will be made of high impact nylon.
The tab will be of high impact nylon, bolted to the tab axle. The stuff is easy to work once you know how. It doesn't collect any foul, and breaking it is more or less impossible.
Most off all, given this system is about as complex as a mouse-trap, I can carry spares for Africa
Now all I have to do is make it work. :--((
I figure it worked for Alex. I have the design. It's just a matter of slowly modifying the modifiable bits till it works for my boat.