Rudder servo strength

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Colin Helliwell
Posts: 24
Joined: 18 Jun 2014, 18:44
Location: UK

Rudder servo strength

Post by Colin Helliwell » 22 Aug 2014, 18:47

Is there any perceived/generally-accepted wisdom on the amount of torque an IOM's rudder servo should be able to impart?
(In case it's relevant: CSA of my present rudder is ~130cm2; 7cm leading-to-trailing edge ['chord length'?], with shaft @1.5cm from leading edge).
Ta.

Dave Pickett
Posts: 107
Joined: 18 Oct 2010, 19:33
Sail number: GBR3845
Club: Eastbourne
Design: TS2 and Shiraz

Re: Rudder servo strength

Post by Dave Pickett » 22 Aug 2014, 19:56

Colin in the Shiraz I have ordered they fit an HB325 which has 3kg.cm of torque - I can only assume the builders have experience to suggest that this servo is sufficient:

http://www.servoshop.co.uk/index.php?pi ... area=Servo

That said there is no reason why a more powerful servo shouldn't be used, and it may last longer as it is not under as much stress, I think the digital servos may also centre better, and hold rather more accurately.

There are a lot of experienced IOM sailors on here, guys what servos do you all use?

Colin Helliwell
Posts: 24
Joined: 18 Jun 2014, 18:44
Location: UK

Re: Rudder servo strength

Post by Colin Helliwell » 22 Aug 2014, 20:06

What I've got in there at the moment (i.e. it came with the boat) is a Futaba S148 - specs I've found online say this is 3kg-cm @6V).
[That reminds me - if it's a 6V max then I maybe need to consider voltage as well as torque, if I stick a7.4V LiPo in there!]

Colin Helliwell
Posts: 24
Joined: 18 Jun 2014, 18:44
Location: UK

Re: Rudder servo strength

Post by Colin Helliwell » 22 Aug 2014, 20:18

Dave Pickett wrote:Colin in the Shiraz I have ordered they fit an HB325 which has 3kg.cm of torque - I can only assume the builders have experience to suggest that this servo is sufficient:

http://www.servoshop.co.uk/index.php?pi ... area=Servo

....

There are a lot of experienced IOM sailors on here, guys what servos do you all use?
Just went to that site, as a starting point, and jeez there's a lot of choice - does the world, even boats cars and planes, really need that many servos?! :o Where to start.....? So yep, very interested in what others can recommend!

Dick Carver
Posts: 55
Joined: 23 Nov 2003, 22:06
Location: USA 22

Re: Rudder servo strength

Post by Dick Carver » 23 Aug 2014, 09:07

This is a good servo for the price.
Dual ball bearing, metal gears, enough torque.
http://www.horizonhobby.com/products/a5 ... MSA5040#t2
Dick Carver

David L Alston
Posts: 72
Joined: 24 Jul 2012, 17:38
Sail number: 3011
Club: Leicestershire RYC UK
Design: Fatboy

Re: Rudder servo strength

Post by David L Alston » 28 Aug 2014, 09:01

In reality there is very little torque exerted by the rudder on the servo because the rudder is to a degree balanced because the rudder stock is offset. Unlike on a dingy where the rudder pintails are located at the leading edge of the rudder a pivot point is at about 30% of the rudder chord.

This offset makes the rudder bite better as well reducing the loading.

Shock loading is not an issue on an IOM or even a RM so metal gears and fancy bearings do not really contribute much either in the long or short term performance.


To me the key is positional accuracy particularly as I run near 80% exponential making the middle of the stick travel very dead but very wild at 60% to 100% stick movement. This provides a good snap at the end of travel that will get you out of irons. A good pump of the rudder will send the bow around nicely in the tack in light and heavy air.

But to do this one needs a responsive servo … not a lot of torque.

Some skippers do not like this and prefer a dead boat setup, clearly it is a personal setup item. I like a smooth progressive downwind tiller response, lean of the stick and the boat moves left or right...but push on the stick and the boat jumps.

An analog servos will not provide that snap response and only digital servos provide the positional accuracy and / or repeatability. Another consideration is operation Voltage. I run at 7.2V . Most analog servos will not like this voltage level and will tend to die prematurely.

But let us be clear .... you can use a £10.. Analog Futaba servo or a £50 Hitec Digital servo … whatever you like. Most servos of this class are the same size … and you can upgrade at any time .

Currently I am using a semi SAVOX miniature servo directly mounted on the rudder stock of my IOM(2) and RM .. really nice .. very simple – quick change if needed … I prefer the Hitec to the SAVOX because the Hitec is programmable and is about the same price. But if you do not have a digital servo programmer there is no reason to choose a programmable servo.

But after all that it is you that will determine where you sail and not the servo… and where you sail will determine your success.

wim bakker
Posts: 12
Joined: 16 Sep 2006, 18:18
Sail number: NED 11
Location: NED

Re: Rudder servo strength

Post by wim bakker » 28 Aug 2014, 15:36

http://www.ibextrax.com/Update/

You may want to take a look at the link above.
Look for the March 2014 edition, page 51 near the bottom.
Here, Zvonko Jelacic, a top Croatian sailor gives his views on rudder servo's for the IOM.

The other newsletters are very interesting as well!

Colin Helliwell
Posts: 24
Joined: 18 Jun 2014, 18:44
Location: UK

Re: Rudder servo strength

Post by Colin Helliwell » 28 Aug 2014, 16:56

Yep, I'd been looking at the digital for the position repeatability. And I've got a winch and Rx that will run at 7.2v so it would make sense for the rudder servo to also, otherwise it means dropping to NiMH's or using a regulator - weight & space either way.
Came across a low-cost Hitec digital - HS5646WP - on the IOM Ireland site, seems respectable in all respects.

Colin Helliwell
Posts: 24
Joined: 18 Jun 2014, 18:44
Location: UK

Re: Rudder servo strength

Post by Colin Helliwell » 28 Aug 2014, 18:18

Have come over all impulsive and ordered a Hitec HS5495BH - digital, 7.5kg-cm@7.4V, 0.15sec, standard size. Seems a reasonable spec, and as David rightly points out, it's the Herbert on the tiller that will be a key factor.

Dave Pickett
Posts: 107
Joined: 18 Oct 2010, 19:33
Sail number: GBR3845
Club: Eastbourne
Design: TS2 and Shiraz

Re: Rudder servo strength

Post by Dave Pickett » 05 Sep 2014, 08:43

Looks like a good choice Colin, let me know how it performs please, do yo have to program it or can you just use the TX to set throw etc?

Dave Pickett
Posts: 107
Joined: 18 Oct 2010, 19:33
Sail number: GBR3845
Club: Eastbourne
Design: TS2 and Shiraz

Re: Rudder servo strength

Post by Dave Pickett » 12 Sep 2014, 08:31

Just one thing to watch out for on Digital servos is that current drain can be a lot more:

HB 425 - 180 mah operating no load
HS 5485 - 500mah operating no load

Not an issue for battery capacity but pre 2013 RMG winches only have a 1 amp BEC which may not be enough for a digital servo under load.

Dave Pickett
Posts: 107
Joined: 18 Oct 2010, 19:33
Sail number: GBR3845
Club: Eastbourne
Design: TS2 and Shiraz

Re: Rudder servo strength

Post by Dave Pickett » 30 Sep 2014, 14:39

Just an update I have purchased a Hitec HS-5495BH and tried it in my elderly TS2. Whilst it appeared to work fine, there was a small amount of interference with the RMG winch as a result of which I have gone back to my trusty HB325 for the time being.

RMG recommend only a standard non digital servo, as nothing better is required (as David A has pointed out) and the extra current drawn by a digital may cause radio black out, as the BEC cannot handle the peak current. RMG recommend if using a digital servo to bypass the BEC (also recommended by hitec).

I found a certain amount of black wire in the winch wires, as a result of this will do a rewire at some point, and will put the digital back in at the same time bypassing the BEC.

It's worth noting that this particular servo is designed for 7.4 volts, and running it on the 5 volt BEC wont get the best out of it anyway.

Colin Helliwell
Posts: 24
Joined: 18 Jun 2014, 18:44
Location: UK

Re: Rudder servo strength

Post by Colin Helliwell » 03 Oct 2014, 17:32

Dave Pickett wrote:Just an update I have purchased a Hitec HS-5495BH and tried it in my elderly TS2. Whilst it appeared to work fine, there was a small amount of interference with the RMG winch as a result of which I have gone back to my trusty HB325 for the time being.

RMG recommend only a standard non digital servo, as nothing better is required (as David A has pointed out) and the extra current drawn by a digital may cause radio black out, as the BEC cannot handle the peak current. RMG recommend if using a digital servo to bypass the BEC (also recommended by hitec).

I found a certain amount of black wire in the winch wires, as a result of this will do a rewire at some point, and will put the digital back in at the same time bypassing the BEC.

It's worth noting that this particular servo is designed for 7.4 volts, and running it on the 5 volt BEC wont get the best out of it anyway.
Current consumption is on the to-do list, to check how to wire everything up to avoid voltage drops off the winch....!

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