sail servo torque

I have an old EC12 hull and am wondering how much torque I need in my sail servo to control 1200 sq in of sail. The servo that was in it did not do the job. I am thinking of a Hitec 815BB with an SPG 800 gearbox but don’t know what ratio to go with.


Hi Rick,
By comparaison with an AC120 with 7800cm they uses often an Hitec 785 and some time an Eurgle .
1200sq inches = 7740cm²
Certanily the 815 is very powerfull and probably too much !!!
The expensive solution is the RMG servos as :


I am trying to get away from the RMG winch as the set up would be very expensive. RMG is what all the new hi tech boats are using and work very well.



I knowe I’m blowing smoke here. but the RMG is by far IMO the best winch going. with multiple drum options, including a self tension drum which eliminates the need for the bungie cord/springs to keep tension on the line.

I run the 280el on my ec12… with a spiral self tension drum. it has 268 oz/in of torque… the 815 has 231. it is a popular unit on the soling 1m with arm style winch I’ve never seen the gearbox setup you are talking about. but another option would be a digital servo… HT 7955 has torque of 333 oz in

you might want to post on the ec 12 forum…

yes the rmg is pricey. but its a bullet proof setup.

are you having issues with getting a turning block retrofitted in an older hull?

I’ve looked at going down the cheap road a couple of times but RMG is just the best there can be. Also if you have issues their repair service is excellent. Very nice people to deal with too. I’m talking about the manufacturers in Australia.

If you use Hitec leverarm how do you program travel?

Here is a link to ServoCity. They sell a range of gear boxes that fit several servos. They mod the servo for 360 rotations and install the gear box, or supply the components for you to do it yourself.

and Hitec servos with power ratings

The HS815 is a strong, fast, robust servo for sail trim using an arm. It is a very large servo and there may not be a servocity gearbox that fits it. The usual choice is a standard sized analog or digital servo with 3, 4 or 5 to 1 gearbox for increased torque, at the expense of slower speed.

Have you considered the HS785 sail winch. It has the power you need but is a bit slow for top competition. Same price as the 815 but the work is already done for rotation.


My brother has a Dudinsky hull and has all the hi tech go fast stuff that he is setting up in his hull. My hull is a Treasure Tooling hull that was built in the mid 70’s. I have redone the deck and refinished the hull ready for paint but would like to keep the electronics fairly simple and less expensive till I see if it is going to be too tender in a stiff breeze. I have new aluminum masts coming and new sails as the old ones were pretty shabby. The hull is in great shape. I am thinking I might just as well order the RMG for it and be done with it but I would like to be able to set it up so I can depower the main while keeping full power to the headsail in the heavy puffs using one channel. Don’t think I can do that with the RMG.


with an arm setup using lester gilberts website, you can come up with a ratio fo servo arm length, and turning points. to get what you desire.

keep in mind that most competitive ec 12 are running three channels rudder, winch, jib trim…

so if you find an arm winch that works for you you sill could add a jib trim servo later

another option… you could run two sail servos. one for the jib, one for the main. run on separate channels. or hook them together so they both run on the same channel. this would allow you have a smaller servo but you’d need two of them… but that makes it more complicated…

Have decided to go with a Hitec 815BB in a 3:1 Robotzone gearbox which will give me 1029 oz-in of torque at 6 volts. Just a bit slower but plenty quick enough for that size of hull. If it does not work out as I hope I can use it in one of my smaller (45"0 boats)


take pics and keep us informed…

one thing i would do. engineer in some sort of a failure point. with that much power you could easily break something…

on my RMG I use 65# spectra as my “shuttle line” Loop…but my sheets are only 20# spectra. this gives me a planned failure point. so I don’t break booms, rip out bulkheads, tear sails, ect… if something binds up or get fouled.

Thanks for the tip Marc. I will definitely build that in.


Much easier to replace a single sheet that breaks during a 5 minute hold than it is to re-sheet both main, jib, fix bent boom, and replace a turning block, that you can’t see

don’t ask me how I know… : )

With the Robotzone gearboxes I have the choice of standard rotation ( 180 degrees I assume) 360 rotation , multi rotation ( up to 1400 degrees) or continuous rotation. I am leaning towards the standard rotation. What do you guys think? Even the 2:1 would give me 686 oz-in of torque and would be a tad quicker. Decisions , decisions.


if you are going with an arm servo i would not go more than 180… if you go drum the skys the limit. juts depends on how big of a drum you want… bigger drum means fast sheet in but you lose resolution, and torque

smaller drums are slower sheet in, but giver you better resolution on sheeting in and better torque…

I would clarify with the supplier if this 180(or whatever) rotation is at the servo or the output of the gearbox. If you get 180 at the sevo and use the 2:1 reduction you’re only going to get 90 at the output of the gearbox. That’s the problem with gearboxes, you get lots of grunt but you lose travel. If they can make the servo go 1400 degrees I would be tempted to use it as a drum winch. Hmmmm, can you post a link to the page that has this info?

I do have a question !
In order to rationalise the choices and based upon the fact that the servos are specified with a torque in kgcm,

how it is calculated the real torque needed for a sail servo in order to cope with the sail area and wind speed ?

Thank you

Hi Rick
I found your post with the link in it. It looks good. They have modified the servo by putting an external pot on the gearbox so the rotation they are giving is on the output of the gearbox. Sorry for butting in. When I looked at these gearboxes before they had limited travel. I’ll shut up now.


Why not buy an RMG winch, fit it, and sit back smiling while you sail!!.
I did !.


The main reason I don’t want to go with an RMG (besides cost) is I want to be able to depower my main in the puffs but still keep full power to the headsail using only one servo and one channel on the radio. I don’t see how I can do that with an RMG winch. I still might end up going with an RMG but it is fun to try new options and see how things go.


This is, indeed an interesting question. In reality it has two parts, and you need to think of it in terms of how much work does the servo need to do?

First, What is the force on the sheet? This is not just the sail area, but equally, how stiff is the boat? 800sq" on a Marblehead with an 8lb bulb and a cg to CoB of 10" is much stiffer than 800sq" on a soling with an 8lb keel and a cg to CoB of 5". Though both are 800sq" the Marblehead will require a stronger winch.

Second, comparing an arm to a drum is not a straightforward torque comparison. An arm has to do the same work in ~120 degrees of rotation that a drum does in ~1400 degrees. Therefore, the arm has to be 1400/120 or more than 10 times as strong to handle the same sail. The sacrifice with the drum winch is of course speed. Also, this assumes 0 friction in the sheet system.

So, having said all that, what is the answer? I like going with a drum, personally, because being underpowered will kill you and your winch. Also, a drum has a higher work to weight ration. The time difference is usually about 1/2 second versus 2 seconds. Smooth sailing takes the bite out of that. I look at my experience and the experience of others first. A 100 oz-inch drum winch turning 3 revolutions will easily handle a Marblehead. The GWS drum winch turning 1 turn will easily handle an RG65.