The ketch I have built has roughly 900 square inches (a US1M jib and two mains) of sail area. With three sheets, I have used one servo to control the lot. Much to ask of a single unit , an analog MS-747WB. No brand name on the label, I have used it and other 747s for years. The story went, from the guys I got it from (early 90s in Irvine CA), that it was for use on military drones.
It was very strong yet I stripped the gears and I’m looking to replace it with a newer (digital I suspect) and stronger servo. Please…
What is the strongest servo that rotates 180 degrees?
I got a HiTec 5995TG but found out I need to program (with a HFP-20) it for 180. And I’m doubting it will be as strong as my old workhorse.
I’ve been using the Hitech 815BB for the big multihulls as it has both the speed and power I needed. Size is bigger than most however, so it must be taken into consideration. It’s 1/4 scale size!
If you follow Maximo’s link to ServoCity, you will notice a lot of their analog servos can be purchased as stock, or with one of two additional upgrades. You can buy the servos already modified for either 180 degrees or 360 degrees or more travel. The cost is $10.00 more for each upgrade - and what’s neat is you can send in (most) current Hitec servos and they will modify for $10-$20 plus shipping and return to you.
There is a “Servo-Stretcher” for $20 - and it has the benefit to allow adjustmetnts to the mid-point and both end-points if you need to have servo swing more one way than another. I didn’t need that option for mine - I just needed 180 degree travel. The “stretcher” simply plugs into the circuit between receiver and servo… pretty simple.
Hope this helps.
Dick
EDIT - ADDED: Nope, they don’t modify the 815BB - but if 140 degrees travel works for you - cost is $48.99
Take a look at the RMG winches. They are great: strong, robust, have a stall cutoff, and have built-in programming capability. For your application the 280 may do, and the 380 will do it easily.
Another factor is to consider what kind of onboard voltage you are going to have. If you are going to run something like 5 cell AA Nimh packs (nominally 6V) then a 5995 doesn’t buy you anything over an 815 (except bulk and weight) as they have virtually similar torque and speed and the digital ones are twice the price. The 815 has 140 degrees of travel by default which is often enough with the right length of arm.
However, if you use something like a 6 cell AA pack (or LiPo’s) then a 5995 has a big advantage in that it has almost 100 oz-in more torque and so it doesn’t work as hard.
For travel adjustment, if you have a “computer” radio then you can adjust the end points independantly at each end and can even over drive the servo travel a little. The servo folks have a small kiniption when you talk to them about that but it is done all the time.
On the drum side of things it is hard to beat a Hitec 785 for value but it is maybe a little slower. An RMG is bulletproof but at a fair premium.
another frickin’ correction - Although the 815BB doesn’t show on conversion chart to continuous 360 degrees, if you look at the actual ServoCity servo page, left hand small specs it shows it as being able to be modifed.
If at all interested, an email or phone call to verify is your best bet. I am completely confused if it is - or isn’t - able to be modified !
A RobotZone Servo Power Gearbox from Servo City is the best bang for the buck. Couple that with a Servo Stretcher for easy end point adjustment & you’ve got a mighty powerful, adjustable set-up for a lot less money than any other option I can think of. RMG winches are sweet but of course they’re drum winches & they cost a small fortune. There are also the gigantic scale industrial servos but we won’t even discuss the insane prices. I’ve seen people run the RobotZone gearbox with a smaller servo as the drive & get good results in stuff as small as a Soling One Meter. With a fancy high torque servo as the drive, I’m not sure what the upper boat size limit would be. Pretty darn big would be my best guess. A fabricated guard of some sort to keep the sheets out of the gear teeth is always a good thing. With a fancy servo & the 5:1 gear ratio option, Servo City says 2000 oz-in is possible. Granted, the full transit time would be around 2 or 3 seconds but, that’s a whole lot of power for cheap. I’ve never had anything except warm fuzzy feelings about any dealings I’ve ever had with Servo City. They also sell the real nice 3M brand of heat shrink tubing. It works SO much better than the cheap crap!
The Servocity/Robotzone Servo Power Gearbox is great for larger model yachts. I have a Santa Barbara that I have configured for two power gearboxes for independent control of the main and jib. The gearboxes have the HSR-5995TG servo installed. The gearbox servos are fed 7.4v from a 4000mah sub-c pack, the rest of the electronics receive 6.0v via a regulator. I can sail all weekend on a single charge.
Seems that the HS-805BB is what I need. Its half the torque (343) at 6v of the HSR-5995 but the cost is more what I can do. And the HS-815BB is 140 degrees, I need that “extra 40”.
Besides, on the HiTec web site they say
Known as the “monster” servo the HS-805BB…
Monster!
Everywhere its NYLON GEARS, do any servos use metal gears anymore? Or a brass/nylon combination? Maybe plastic is just as strong nowadays.
You’ll may need a little more power than the HS805 has, given you have 900 square inches of sail.
I tend to design my configurations with excess servo power. I build for racing. That’s why I use the servo power gearboxes.
The 5995TG has 180 degree rotation out of the box. The 5995TG is discontinued, but you can still find it on eBay. The 5990TG has 120 degree rotation. It is the replacement for the 5995TG.
I have the 815 servo and use it on my IOM. It actually does 180 degrees when I use the fine adjustments on the side of my radio. So I set it up that I use the fine adjustment for that last few cm of adjustment. I do find if it is windy that I have to turn the boat as I sheet in to get thw sails in, I could not pull the sails in while going down wind. That gear box sure would be nice, you could tear the boat to shreads if somthing got cought?
Appendix A contains an evaluation of servos available to the researcher in 1997. The Condor MS-747WB servo is one of the servos.
The Torgue is in Newtons, but a quick calculation shows the MS-747WB has a torque rating of about 220in-oz and a rotation speed of around 0.20 sec/60deg.
This is precisely the reason I prefer to over power the sail arm servos, hence use the servo power gearbox. Up wind I can easily trim the sails regardless of the wind conditions. The extra power is a tactical advantage because you don’t need to change the course of the boat to adjust sail trim.
Switching tacks down wind is likewise effortless. This is very helpful when you are on a port tack down wind and need to tack to starboard to gain a tactical advantage.
I did have a binding issue once with the SPG800 gearbox and the HS-800BB servo. The job sheet got fouled on deck hardware. Luckily just the sail arm broke. I did learn from the experience to allow for a point of failure.
Keep in mind that you don’t need to buy the most powerful servo gearbox (the 5995TG with +2000in-oz torque). There are gearbox configurations that have less torque that would provide more torque than a servo alone (even the 5995TG servo). The SPG645 servo power gearbox provides 665in-oz of torque for $149.
All this worry over end point adjustment & total rotation is really not necessary. Just cough up the $20 for a Servo Stretcher from Servo City & all your worries will disappear. It’s a pretty slick little item. It makes it so much easier to dial in an arm winch set-up. Get the RobotZone Servo Gearbox with a drive servo & gear ratio appropriate to your torque loads & add the Servo Stretcher to make it all easily adjustable. I would personally lean towards “overkill” in the amount of available torque. I like David’s idea of a “weak link” built into the system to limit damage in case of a malfunction. The Servo Stretcher can also be adjusted “off-center” to enhance close-hauled sheet resolution. For $20 it’s a mighty sweet little circuit board. You won’t be disappointed.
Whether you choose to use the servo stretcher, program the servo, or use a programmable transmitter, you should arrive at the same result. Kip’s suggestion of the servo stretcher is probably the best one for you, Yar.
Overkill is underrated.
I’ve been at races where I see skippers swapping out batteries between races. When asked, the say they their battery pack can carry them through 1 or 2 races. As I said previously, I run with a 7.4v 4000mah sub-c six pack. I stay on the water through all the heats (2-3 hours). At at the end of the day, it takes less than 10 minutes to top off the charge. I’m told the big pack is overkill. But then, I’m not the one swapping batteries between races.
I’m also told the servo power gearbox is overkill. When skippers are rounding up to sheet in on a beat or doing crazy dances downwind to go wing on wing, my independent main/jib power gearboxes give me the power to do fine trim adjustment upwind and easily switch tacks or pop the jib over down wind without having to alter my course. And, after all, altering your course for any reason other than to achieve the best line is never a tactical advantage.
Yar, since you’ve built a ketch, my rant on racing probably doesn’t matter a great deal to you. All the same, the power gearbox will be your best choice, couple it with the servo stretcher that Kip suggests, you’ll have a more than capable configuration which I’m sure you will be happy with.
The design concept of the boat was total racing. The model is a daydream of a monohull design which, in full size, would beat the multihull (Dick thinks, “yeah right”) Earth Voyager in the PH-Mac race in Michigan.
Love the gearbox, but I’m personally still shy about the price.
