Rudder Shaft

I’m figuring on using this shapeli for my rudder…[/li]

… but want some advice on shaft placement. Sometimes I center it, or slightly forward of center. Lately I have been putting it on the leading edge but wonder if that’s a mistake.

Any advice, theories, or previous thread I can refer to would be much appreciated.

thanks, yar

If you put it on the leading edge it’s harder for the sevo to move. The farther back it is the easier it is on the servo til you get close to halfway. Then the flow of water starts to overpower the servo. It should be somewhere in between, I use 25 or 30% roughly

when i desing my rudders. i usual put the shaft close to the center. BUT AHEAD OF THE MID WAY PIONT. the reason for this. is you dont want the rudder to spin around. you going to use a standard servo, you only got so much power. and water can push the rudder around. if you put the pivot piont ahead of the midsection. the water will try and keep the rudder straight. therfore the water would work with you, and all you need to work about is the servo keeping the rudder at a angle. to turn your boat:zbeer:
hope this helps

The thought on the shaft up front was less turbulance around the shaft (a totally baseless theory) and it just seemed innovative (another theory).

Thanks for the quick input. yar

My colleagues have spoken wisely Grasshopper, but fear not. Although we have recommended a different shaft placement, there are many full-sized boats with the pivot point at the leading edge, and with neither balance nor skeg. This is true especially in certain racing dinghies.

So your idea was not without merit.

Hi Yar, this is very interesting, I started racing RC sail boats about 5 years ago, before that I was running nitro RC boats and “ALL” the fast boats ran rudders that where pined from the front edge and sharpened vary sharp, they didn’t care how big of a servo you had to put on it they just wanted to go fast.

It is very rarely that I disagree with my learned freind Muzza but this is one of them - or at least in part.

Ok, if you put the pivot at the forward end you or the servo have to work harder. This may or may not be a good thing. An unbalanced and a balanced rudder will also have different optimum rates of change of helm angle when tacking, but model yachts tack so fst that even your pedantic ogre suspects that the effect is purely academic.

What can be a very serious problem is actually fitting the diameter of the stock (rudder shaft) into the rudder blade. You are buildin big and presumably very fast models. The bending load on the blade rises with the square of speed. And increasing the nose radius of an NACA foil has catastrophic effects - as I know to my very considerable cost: it was the 2500 lb ballast weight of a full sized keel that the builder sneaked through while I was on holiday. It was easier to make and he was running over budget. I didn’t have the guts to tell him to do it again and we had to relace the entire fin. We didn’t quite get the judges involved, but there were some very hard words.

Hope this helps


You are not disagreeing at all Angus. I was simply acknowledging that in many different types of boat there are rudders with the pivot point at the leading edge.

FWIW I position all of mine at about 30%.

It’s interesting to me that of the rudders that are transom mounted - with the pivot at the leading edge of the rudder - (as far as I can remember) most have the very long lever arm of a tiller. Even some open 60’s have had tillers and leading edge hung rudders…

Presumably, the tiller can overcome the forces on the unbalanced rudder blade with leverage.

Hmmmmm, I agree with you tbarjohn… VERY interesting. Thank you for that bit of information.


[FONT=Arial][FONT=Arial]Hi Yar

I think the fast (20mph plus) powered boats use a sharp leading edge (wedge shape in fact) rudder as a result of lower drag at their higher Reynolds numbers. (cutting through the water & clean water release at trailing edge at high speed?)

Sailboats generally go much much slower and at our lower Reynolds numbers a NACA foiled rudder has a lower drag and with it’s blunter leading edge it has better control at higher angles of attack (is less prone to stalling).

I hope someone of more knowledge and better writing skills pipes up here to explain more & better than I can…!! :smiley:



A rudder is simply a foil that generates lift at a particular point along it’s cross section…just like a wing, the center of lift can be measured. Typically, this lift centers at roughly the widest point of the foil…and in a really rough approximation is about 1/3 the distance back from the leading edge if you have a proper NACA foil section. If you were to place your rudder shaft in-line with your draft at this point, the rudder will be balanced and it will require very little force to turn it even at high flow and heavy load. If you place the shaft further back, the rudder will want to flop around and the servo will be trying to prevent the rudder from turning (not advisable). Ideally, you want the rudder to be balanced slightly ahead of the center of lift, so that it naturally tends to go straight. The servo then doesn’t have to work very hard to turn it and uses very little energy keeping it straight and narrow.

As for boats that have stern hung rudders, take a second look as many of them rake the rudder forward placing some of the center of effort in front of the hinge (pintle). I regularly race catamarans and we adjust the forward rake of our rudder foils so they give us the amount of helm we desire for the feel of the boat…mostly balanced in our case. My big’ol monohull has a stern mounted rudder with very little forward rake and it takes quite a bit of force to turn the boat at all.