Has anyone ever considered, or developed a cam-cleat device to hold your mainsheets?
I have an RC Iceboat/landsailor model and I am looking into this option to control the mainsheet under loads.
Another reason to hold sail trim is that an iceboat can keep the sail sheeted in and manuver gusts by vessel direction rather than sheet control.
So my desire is to sheet the sail in and hold it there for long periods and under considerable loads.
Most servos I have used are not strong enough on thier own to hold the torque. I have stripped gears and melted electronics on several servos.
So, I have considered the cleat option as a possible solution.
My thoughts were to use the same type mechanism as a standard sailboat spring loaded cam-cleat.
I will make a minerature cam from metal gears and use another (3rd) small servo to “trip” the cleat when I want to sheet out.
you may want to try taking apart a small boat cam cleat (harken, ronstan etc). they’re spring loaded, so i am guessing you could connect it to a servo instead to have it open and close. or if you don’t have another channel, you connect it to the main sheet servo mechanically, so when the arm goes in a direction(ease), it will open the cam.
OK Iceratz, you’ll need to do some detective work, but there is a nice design out there. A locking/unlocking ball system that might work for you. I think Steve Andre is the inventor, if I’m wrong… my apologies… and no doubt I will be corrected here.
I ran a topic a year ago called “Got Moving Stay?” Do a serach for adjustable backstay here. The topic is the second one. Towards the last of the posts (s vernon), there’s a mention of the forum Windpower (http://rcsailtalk.com/). If you go there, and do the backstay cleat and/or backstay lock search as suggested, there’s a topic called Backstay Lock and mention of it but no pictures. I did the search. When there was pictures, it showed an ingenious device which you might try to get reposted (please post HERE).
Just a thought - have you tried digital servos? They have a much higher holding torque. They require a good power supply (re: larger gauge wires/switches/plugs but thier hoilding power is much higher than normal analog servos.
Glad to hear you’re getting ready for the upcoming ice sailing season. My thoughts are below
1)What size/brand of servos are you using? I used to strip out gears with small ones having plastic gears, but since I’ve switched to larger metal geared servos have not had a problem with the gears. I currently use the Futaba S3802.
2)Your rig doesn’t appear to have any sort of boomvang to resist uplift of the boom, as far as I can tell from the picture. So your servo is not only controlling lateral motion (angle of attack) but is pulling down. This requires considerable extra work that could be minimized with a vang.
The orientation of the servo arm is critical. If the arm approaches 180 degrees from the direction of pull as the sail is sheeted in, it will have a couple advantages. The effective length of the lever arm becomes progressively shorter & hence stronger (although length of travel is correspondingly shorter). Also, if the end position is “just past” 180, there will be essentially no force exerted on the servo, which is exactly what you’re trying to achieve. This is what I call “rotary-linear differential” & can be represented by a cosine curve similar to an engine pushrod & crankshaft.
One concern I would have about any sort of mechanical locking device (such as a cam cleat with a separate servo actuator) is the extra time it will take to release. As you well know, with the speed of these models it is sometimes essential to instantly sheet out to avoid capsize.
I will pursue the “backstay ball cleat” design to some extent.
The servos I have used:
HiTec HS755MG, in current use, TQ=200oz
Futaba S3801, melted elect., and stripped gears on 2 units
HiTec HS-5985 , digital, TQ=172 oz. Melted elect.
HiTec HS-5955TG, digital, TQ=333oz, NOISEY…returned before I melted it.
So, I feel that although the digitals state they have the torque spec, they do not have the “large scale” ability.(?)
My boat has dedicated 6v to the sail trim servo with “Y” harness.
The “next” servo was going to be the HiTec HS-815BB (333oz/in) but it is too tall to fit into my hull…at the moment.
I may add the boom vang, it seems the least thing I can do for improvement.
However, I have certian sail designs which have a “camber” or “boom-jaw” that forces shape against the mast. If I vang it, then the shape will be stuck on one side and not “rotate” through a tack.
I understand your arm/180 deg config, but I do not want to sacrafice so much main sheet travel.
I will post my developments for the cleat designs.
Any more info folks can add for this method, please do!
I look forward to seeing what you come up with. I’m sure it will be well thought out.
Have you considered mounting a pulley on the end of the servo arm & sheeted back to a fixed point? This will give twice as much linear movement for the same degree of arc. Also, as you well know from full sized iceboating, these things will be sheeted in pretty tight most of the time anyway because the apparent wind is forward all the time anyway (once you’re moving, you easily are going faster than the true wind). My rig only sheets out about 20 degrees off C/L & doesn’t need to be out more than that, except for the occasional times when I get stalled out facing dead downwind. I try not to do that.
My experience with these servos is that they don’t really “like” to have to work to maintain a loaded position, sometimes evidenced by a humming noise. Plus it’s terrible on battery drain. My guess is this is what yours have been doing, based on the problems you’ve had.
So I guess I would suggest that you not entirely reject the arm orientation concept. Especially if there is no way to vang the boom, it would be a simple way to achieve a solution to the problem.
ps perhaps the release for your cleat arrangement could be triggered by the arm itself, not a separate servo…just a thought if you decide to go that route
Take another look on Windpower after you log in. General Discussion. Last post date is Nov 16, 2006. Title is “Backstay lock - cleat takes tension off backstay servo”. The thread is on the first page of General Discussion today so it is not hard to find.
I put the images back on Steve’s thread on that Nov date. You cannot see images unless you log in. I had no trouble seeing the images today at bottom of page. Qty 2 images because they have to be small. I broke Steve’s one image into 2 and reposted. His got deleted when they changed the size limit.
More thoughts are under progress.
The spring-loaded cam cleats are going to be challenging to design and may not prove effective for the holding power desired. I tried a mockup and decided against it.
Now I am thinking of a locking ratchet arm type of device. It’s the kind of design found on an ancient Greek catapult to cock back the arm under loads.
The “backstay ball lock” is quite similar to my thinking here. Thanks for the photos! :zbeer:
The main sheet shall have a fixed ball (or knot tied) at a location which corresponds to a ratchet ladder lock arm with a series of several notches where the knot will stop and lock in. I picture notches like a saw blade profile, ramped up, then vertical stop down.
The ratchet arm engages into sheet/ball locking position with a servo and pushrod system.
The arm is made of a 3ply sandwich of aircraft grade 3/32” ply.
Outer layers (cheeks) to have the notches shaped in. The center layer is smaller to create a channel sized just for the mainsheet to travel through the outer cheeks.
As the sheet is pulled in by the sail sheet servo, the ball will lift up over the ratchet notches and then lock to prevent back-slip.
Next I need to design the placement of everything for mechanical advantage. :rolleyes: