Pros and Cons of Transom Hung Rudders

Hi,

I'm working on a 30ft IRC cruiser racer design - my first design. I cant find any published discussion on the pros and cons of a transom hung rudder over a conventional rudder post type, from a performance viewpoint.

A transom hung type gives increased space in the cockpit but does it have a performance issue.

Any thoughts would be interesting,

Cheers

Andrew.

 

transom hung rudder

Probably the biggest disadvantage to a transom hung rudder is the tendency it can have to ventillate causing it to lose lift and not function very well. Any foil that pierces the surface will have a similar disadvantage.
In Pierre Gutelle's book he shows a potential solution: an endplate parallel to the bottom and flush with it that serves as a sort of extension of the bottom but that rotates with the rudder. Another solution I have seen is a "fence" just below the waters surface-similar to the endplate but covering just the forward half of the foil designed to prevent air from being sucked down from the surface. Used widely on surface piercing hydrofoils.

 

rudders

I've also read that having a rudder mounted under the hull is beneficial in the fact that the hull serves as an "endplate" which has an apparent doubling effect on the aspect ratio of the rudder. I haven't read anything about the plates at the bottom or just below the surface of a transom hung rudder. I've wondered if a device like that would work in that regard. Seems like it would. There would be additional wetted surface and maybe some interference drag associated with the additional appendages.

 

The above points are quite correct, however, it really comes down to three major points... firstly the efficiency (where you're not about to beat a post rudder in a hurry) the looks (where let's face it, yachts look nicer without a rudder hung off the stern) and whether or not you'd like a shoulder left after the race. The last point considers the helm force, and the actual rudder balance. Any rake on the transom (top for'd of stern) will induce pretty horrendous loads (I speak from experience). Conversely, whilst the post rudder may have a few degrees of rake to start with, it is easier to balance, thus the helm is lighter.

There is an advantage to the transom-hung rudder (there are a few in fact). Firstly, as has been mentioned before, the cockpit space is increased. Secondly, it is much easier to remove a transom-hung rudder at sea for repairs.

However... The use of a cleverly designed wheel helm can return the space that would have been lost... and why on earth would you need to remove the rudder? I presume this is an around-the-cans racer.

A 30' cruiser racer is an ambitious project, but it is achievable. Feel free to E-mail me if you need more help (or foil optimisation). Tim@MarineDesign.tk

Cheers,

Tim B.

 

Given the theoretical advantages of under-hung spades, it's sorta fun to see the world championships in International Canoes (which traditionally have beautifully efficient little spades hung under the boat) being lead by a guy who normally uses a transom-hung rudder. We know about the theoretical advantages of an underhung spade but are they practicallly all that easy to find? Some of the C Class cats with underhung spades were beaten by boats with notched transom-hung blades (ie Victoria 150). Same thing in sportsboats, the transom-hung rudders on Thompson designs are pretty quick. And many dinghies prove that a transom-hung rudder can be delightfully light in feel, if they are balanced (not swept like say the Laser).

If you had a rudder box, you can lift the foil out when moored and keep a very important part of the boat in top condition.

The problem with wheels is that they are either huge, or the driver gets stuck too close the centreline AFAIK.

 

you can balance transom mount rudders, by having sort of like a step in the rudder, so the leading edge of the foil is actually infront of the gudgeons (don't ask me by how much), this should balance it, just watch out if you are goin really fast, but its a cruiser racer, so it probably wont go that fast. my mate had a fireball (fast dinghy) & he did this to his rudder & it was perfectly balanced, but they had a bit of a problem when they broke of the part that was "stepped" by turning whilst traveling to fast.
on a 31fter that i have been sailin on, we have a under hung rudder with a tiller, doesn't take up much room, means the skipper can sit to weather & as far forwards as he can reach with the tiller extension & the rudder is balanced as well.

 

I didn't say you couldn't balance a transom-hung rudder, nor did I say that it will be less efficient regardless. The aparrent advatages in the dinghy world probably comes from the difficulty of sealing the rudder against the hull (ie. there should not be a gap) and the fact that the foils in question do not have the same sections. Of course, there are also issues about the CLR of the boat with two different rudders etc. etc. etc.

I have a LARK racing dinghy which has a beutifully set up transom rudder. it is very light, with just enough force to feel what's happening. It proves two things... firstly that a transom rudder vertically mounted can be balanced. Secondly that at planing speeds when you lose a lot of wetted surface forward, the considerable reduction in rudder force.

I would always advise a post-rudder on any boat, regardless of size, because if it is properly designed and properly made then it will be an efficient, fast solution.

Tim B.

 

Thanks for the input guys. Solution looks like underslung spade.

Tim - I may be interested in foil optimisation. Not quite ready for that yet but I will let you know if I need your services.

Cheers

Andrew

 

The angle of pivot of the rudder does not have to parrallel the transom. Also, The biggest advantage to me is the ease with which one can make a kick up rudder! no feer of loosing steerage etc. Also, if you are going to use a dagger, you can kick the rudder up and anchor in shalllower water....always a place to anchor a boat that draws only 1 foot! Cheers

 

You can contact me through e-mail, Andrew. my e-mail address is Tim@MarineDesign.tk

Give me a shout when you want an optimisation done.

Tim B.

 

one other thing...

Haven't seen it mentioned, but with a transom hung rudder its easy to set the max thickness at 12% (or whatever your heart desires).

On spade rudders, you are limited by the mechanical constraints of fitting in a strong enough post (usually set forward of the point of max thickness as well). This can be a problem on modern high aspect profiles of short root chord length... you might end up with a very fat section in order to get the post in.

Of course the way around this is to use a carbon post that is trapezoidal in section inside the rudder, then flares out to a suitably large diameter round section above the blade ($$$$ - and requires $$$$ large diameter rudder bearings).

 

Phil,

I was discussing this, only two days ago. We came to the conclusion that it is not hard to get a rudder-post into a normal size underslung rudder. Even a high aspect-ratio racing yacht rudder shouldn't pose too great a problem. 12% seems a lot too thick to me, I'd say the maximum was 10% (the Cd drops back quite noticably for given Cl).

Rudder bearings? well, that's the price you pay for efficiency.

Tim B

 

I thought the rudder is usually fatter (in % of chord) than the keel. A 10% foil will stall much sooner than say a 15% width, and high AoAs are a very important issue with rudders.

 

better to have the rudder hung on the transom than the bow (unless you like going around backwards, me handsome).

But seriously as long as the bloody thing is in the water 100% of the time there's pros and cons to where ever you have it - if it looks right it normally is; K.I.S.S. (keep it simple stupid) is the only way to go. If it can go wrong it probably will, normally in the worst possible place CAN YOU FIX IT?

 

Bigest advantage is the ability to attach a trim tab to the rudders trailing edge.

From there a light second tiller can be used for power steering ,
or even the smallest cheapest autopilot can steer.

FAST FRED