View Full Version : Capstan diamter

05-27-2003, 08:06 AM
I need some help in sizing the diamter of the capstan for the rudder cables of a 16 ft. tunnel boat. It will take about 9 inches of cable movement to turn the rudder to 60 degrees from center (max) Should the capstan diameter be such that the circumference is more or less than 9 inches?

Seems to me (a complete novice) that if the wheel makes less than one turn to move the rudder full stroke in either direction, the wheel would be too sensitive. On the other hand, if the one has to turn the wheel 1.5 turns to bring the rudder full stroke, one could spend all day trying to make a quick turn.

Your comments and suggestions are earnestly solicited.


Bob Price

05-29-2003, 01:08 AM
Bob I don't know if this will help or not but most of the cable steering systems I see are 3 1/2 turns. .......Phil

05-29-2003, 08:04 AM
Thanks for the response Phil but 3 1/2 turns on a 10 inch diameter vs a 3.00 inch diameter is the real question since the cable movement is a function of the capstan diameter.

And for that matter so is the torque required to turn the wheel. The smaller the capstan diameter vs. the wheel diameter, the easier it will be to turn the wheel. In my case that is not an issue since this boat will not be moving that fast. :^|


Bob Price

Del Lansing
05-29-2003, 08:18 AM
If I understand correctly you need 9 inches of cable motion from center to hard left (or right)so the total would be 18 inches hard left to hard right? if that is correct and you wanted 3 turns for full swing, the capstan would have a circumferance (sp?)of 6 inches. Divide that 6 inches by Pi (3.14) to get the diameter of the capstan. 1.9 inches would be the result for those conditions. Hope that helps.

05-29-2003, 08:24 PM
Del - are you suggesting that 3 turns of the wheel is a reasonable thing to go from full left to full right on the rudder?

That would mean 6 turns to complete a rudder reversal and in a panic situation, like approaching a dock too fast or something, that is a lot of cranking.

But basically, you are implying that I should have at least 1.5 turns for full left or full right rudder or a total of 3 turns of the wheel for a rudder reversal.

OK - that sounds like some good info.

Thanks very much. Bob

05-29-2003, 08:34 PM
And as long as I am asking so many questions what have you to say about using 0.094 in. diameter 7x19 stainless cable with a nylon or polyurethane coating for the rudder cables?

Thank wire has a breaking strength of 1,000 lbs. so with a Safety Factor of 3, a working load limit of 333 lbs.

The polymer coating would reduce chafing of the pulleys and keep the wire clean.

Again, my sincere thanks for the advice. Bob

05-30-2003, 09:10 AM
You can't have this discussion without considering the wheel size and boat type. It's reasonable to require multiple turns lock to lock if your wheel is small. Small wheels are easier to spin fast, since you hold them close to the center point. On the other hand, it would be unreasonable to have 3 turns lock to lock if you had a racing-style 6' wheel (which is pretty unlikely I know), unless you need the leverage on your 200' square rigger smile.gif

It also depends on how twitchy your boat is, and how much weather helm she has. If you want to reduce fatigue from heavy helm you need a higher ratino or a larger wheel. If your boat needs a lot of course correction then you want a smaller ratio or a smaller wheel--otherwise you'll be spinning madly trying to keep her going straight and fast.

That's why big heavy boats which steer straight but can have a lot of helm use high ratio gearing. And that's why fin keelers who use wheels basically rig them like 'round tillers'--a small motion on the wheel produces a lot of steering deviation.

For most course corrections, you don't actually steer much as you do it slowly, so your ratio can be slower than you might think. Even when tacking, it's best to spin the wheel slowly. And when you really DO need to have a fairly quick response--usually in dockage areas--you can compensate because you're usually 1) prepared, 2) going slower, 3) in relatively calm water so you can use both arms to spin like mad smile.gif

05-30-2003, 04:28 PM
Good Points Eric - the wheel is small, 12 in. diameter (and just for the fun of it, it only has five spokes). But the boat is small 16 OAL x 5 ft. 2 in. beam. It has a slight vee bottom and a skeg so it should move relatively straight and not very fast. I'm going to install a 8-10 HP Stuart-Turner P55M two cycle, two cylinder engine and I hope to get as much as 7 to 8 knots at full speed.

My instincts tell me that 3 full turns from full port to full starboard would be a reasonable compromise between sensitivity and too much cranking. With a little fussing I can change the capstan diameter if I have to but for now I'm going to design around those parameters.

Thanks for all the suggestions and commentary.

Bob Price

Del Lansing
06-01-2003, 07:45 AM
>>But basically, you are implying that I should have at least 1.5 turns for full left or full right rudder or a total of 3 turns of the wheel for a rudder reversal.<<
I only used someones suggestion of 3.5 turns lock-to-lock as a starting point. 3 turns was just an easier number to use since it divided easily into the 18 inches of cable movement you need. I see your scepticism about too many turns for a fast manuveur but conversly, it can get too twitchy also. Think about a choppy/sloppy day and you get knocked in the side by a rogue wave and almost take a tumble, the sudden move on the wheel could put you on the wet side. I bet that you would find that with a 12 in wheel you could give it a spin and the wheels inertia would keep it spinning most of the way. I thinking of outboards and I/Os I've run; never realy counted but they were seemed around 4 to 5 turns. Those needed that ratio to fight the engine torque but still 'came about'with 2 quick spins of the wheel.