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Criteria for boat servos

What key features do boat servos need?

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Jeff Adams20/01/2021 12:25:40
5 forum posts

Hi Folks, apologies for the newbie question which may be an old chestnut. My experience is with standard sized servos in flying models, so I'm out of my depth [excuse the pun].

I'm looking at adding servos to Acer's 17" 'racing sloop' kit model. I've found some good folk who've done this before, and modifications have gone well. Time to choose the servos.

All I know for sure is that they have to be less than 1/2" wide, so I guess that means micro servos. One for the rudder, and one to pull the sheets.

How to choose though? Do I need metal gears, what torque, what speed? Should it be waterproof or not? (I'm not planning on getting them wet )

Could I ask for some general guidance on speccing sail winch and rudder servos please?

Specific part suggestions would be welcomed, but I'm also interested what the key features that I really need for both applications.

Ray Wood 220/01/2021 15:37:38
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2232 forum posts
777 photos

Hi Jeff,

If you could post a picture of the space available in the hull we could give better advice, If good folk have advised you so far what are their views ? the sail control only needs a standard servo with a longer arm fitted, are you sheeting the jib & main ? the best parallel I have is the Micro Magic yacht which roughly similar in size. I assume you have the keel weighted for stability ?

Regards Ray

Malcolm Frary21/01/2021 10:49:56
944 forum posts

Any guesses without some details of the actual model must be guesses, even if the principles are sound.

For a 17" sailboat - Look at the servo specs - "under 1/2" wide, no se looking at named sizes, "mini" and "micro" tend to be a bit flexible. There are analogue servos (digital not needed) around 0.45" wide. If working a balanced rudder, probably any of them, unbalanced, metal geared is preferable.

The sails are not going to offer a huge load, but there are no winches smaller than "standard" size plus a big lump for the drum and shroud. This leaves arm type servos, just look at metal geared and the highest torque figure available.

https://servodatabase.com/servos/all        has a fairly comprehensive list.

Edited By Malcolm Frary on 21/01/2021 10:51:40

Jeff Adams21/01/2021 13:13:01
5 forum posts

Thanks Ray, Malcolm, I wanted to avoid going straight into detail initially. My question was more or less along the lines of 'what is a sail winch anyway'.

So to go into a bit more detail about this particular project. I'm indebted to Alan Izzard primarily for translating a set of build log photos into some helpful hints.

Probably most efficient if I give a link to Alan's photo bucket site.

https://app.photobucket.com/u/montessa315/a/3cebd09a-843c-4bb6-b71e-2dc37a0faeba

I hope that works as expected.

Alan said he used whatever servos he had to hand, having built a servo carrier over the re-inforced keel, which provides the ~1/2" space for the servos. The photos don't show the part number clearly, but I've tried to avoid just copying what Alan did directly - at least try and get some understanding of his thought process.

You'll see the sail servo has main and jib sheets via a long arm, which is the 'model' I shall follow.

Thanks for your thoughts so far..

Edited By Jeff Adams on 21/01/2021 13:13:46

Edited By Jeff Adams on 21/01/2021 13:30:55

Malcolm Frary22/01/2021 09:55:09
944 forum posts

A normal servo has an arm that swings to mimic the position of the controlling stick on the transmitter. Usually a swing of about 90 degrees.

A "sail winch" is a servo that is geared to "swing" through rather more and drive a drum rather than an arm. A 6 turn winch will travel through 2160 degrees, a 2 turn through 720. There is extra gearing that lets the same position sensor "think" it is dealing with the standard 90 degrees.

Beware of "360 degree" servos. Manufacturers sales and marketing are a bit casual about the meaning. Some think that it means "will go round and round forever", some think it means "will act like a normal servo, but is arranged as a 1 turn winch". Usually, it is the former, which gives a box containing a motor plus a gearbox plus a small ESC. Works well with a spring centre control once you get used to it, minor drawback is that it doesn't have end stops - winding in it can try to compress the structure of the boat, over winding out can cause a reverse re-wind, which can lead to confusion.

Jeff Adams22/01/2021 20:21:13
5 forum posts

Thanks Malcolm - That makes it very clear.

Jeff

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