I have noticed that my sail servo is much less powerful than advertised. It is a HS-81MG, rated at 36 oz-in with 4.8V. It should be more than sufficient to handle a McCormack rig, which is partially balanced. However, it is losing control downwind in gusts, making it impossible to jibe.
In troubleshooting the problem, I made some torque measurements on this servo, as well as another one of the same type. They both have the same problem, as they will not provide more than 20 oz in with 6V. This of course gets cut in half again after going through all the changes of direction before the sheet gets to the boom. I am guessing that maybe the motor is good fort 36 oz-in, but the friction in the internal gears is losing most of it. I could be wrong.
I was originally thinking of going to a slightly bigger servo, like a HS-85MG, but these measurements indicate that maybe more drastic measures are needed. Any comments?
1 - somewere on the forum this argument has been already discussed and it seems that Hitec HS81 is not very popular
2 - unfortunately I can confirm that during liverpool GP my Hs81 refused to work ( to be honest wind gust have been guessed at 30 kts …), leaving my boat with rudder control only
3 - a more reliable servo seems to be the bigger Hitec HS 225 ( declared torque is 66 oz @ 6v )
4 - to replace my sail servo I have found other two intersting alternatives :
GWS IQ-220BB and the Blue bird BMS-380MG
both of them have good torque, and are almost half the weight of the HS 225,and are also cheap enough
asking myself "how can they be so much better ? " it seems to me that manifacturers have selected different gear ratios, because their angular speed are lower ( this is a point of concern on airplane model but not on our small boats )
Most probably I will buy soon one GWS, because it is ready available at my local model shop.
I will give notice about its performances on my folgore
They don’t mention unreliability of the HS81 but do say thet it has been replaced by the HS82 MG - metal gears but still plain bearings. These are interchangable size wise with the HS81 but are rated at 39 oz. ins. @ 4.8V and 47.2 oz. ins. at 6.0V. They weigh in at 19 grams - only 2 grams more than the HS81 but with the metal gears.
The HS85MG is both ball raced and metal geared, weighs 22 grams and is rated at 41.7 oz. ins @4.8V and 48.6 oz. ins at 6.0V
I originally had the same trouble as you Walt, plus the odd instance of gear slipping, so I replaced my HS81 with a SuperTec Micro 2BB MG with 75 oz. ins @ 4.8V but weighs 28 grams. No problems so far on the performance front.
Purely in the search for low weight I am thinking of trying the HS82MG in my next hull. My wind pressure calcs suggest it ought to have sufficient pull - particularly at 6.0 V