Pros and Cons of Transom Hung Rudders

[Gone Ballistic]

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.

[Doug Lord]

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.

[LP]

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.

[Tim B]

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.

[CT 249]

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.

[yachtie2k4]

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.

[Tim B]

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.

[Gone Ballistic]

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

[terabika]

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

[Tim B]

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.

[Phil Locker]

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).

[Tim B]

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

[Skippy]

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.

[safewalrus]

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?

[FAST FRED]

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

[tspeer]

... 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)....

Here's a different way of looking at the drag of foils. Induced drag depends on the span and the lift, but doesn't depend on the area. The lift from a big rudder and the lift from a small rudder is the same, because that's what's needed to balance or maneuver the boat. The pilot controls the lift on the rudder as required, and will use as much, or as little, deflection as needed to get it. So the intentional loads are nearly independent of rudder size.

If you know the maximum load you want to apply to the rudder, and you set the depth to minimize the induced drag that results from it, that pretty much sizes the rudder stock. What's left is to size the chord so you can achieve that maximum load.

If you look at the profile drag normalized by the thickness instead of the chord, you get a completely different picture. You can easily do this with published section data just by dividing the drag by the thickness ratio. What you'll find is thin sections and thick sections are closer together when compared on the basis of the same physical thickness.

For example, at a Reynolds number of 1,000,000, XFOIL predicts the following minimum drag coefficients, with the last column using the NACA 0012 as a reference for comparison:

Foil Cd_min
NACA 0006: 0.00334 62%
NACA 0009: 0.00428 79%
NACA 0012: 0.00539 100%
NACA 0015: 0.00634 118%
NACA 0018: 0.00712 132%
NACA 0021: 0.00791 147%

it looks like there's a big difference in drag between the thick and thin sections. But when you divide through by the thickness ratio, you get:

Foil Cd_min/t
NACA 0006: 0.0557 124%
NACA 0009: 0.0475 106%
NACA 0012: 0.0449 100%
NACA 0015: 0.0409 91%
NACA 0018: 0.0396 88%
NACA 0021: 0.0377 84%

This is equivalent to basing the minimum drag on the frontal area of the faired rudder. Now the difference is not all that dramatic, and the thin sections fare worse than the thick sections.

If you keep the physical thickness of the rudder stock the same and the depth of the rudder the same, a narrow high aspect rudder will have less drag than a rudder with larger chord. As you might expect because of the reduced wetted area. But the difference is not all that great. A rudder with a NACA 0009 will be twice the chord of a rudder using a NACA 0018, but the large rudder will only have 20% more profile drag when they share the same stock. A NACA 0009 rudder will have 50% more wetted area than a NACA 0012 rudder, but only have 6% more profile drag.

The wide rudder will have a greater maximum lift by virtue of its larger size, even though it has a somewhat smaller maximum lift coefficient because of its smaller thickness ratio. Here's the maximum lift coefficient from XFOIL at a Reynolds number of 1,000,000 (which probably over-predicts the max lift somewhat):

Foil Cl_max
NACA 0006: 0.765 56%
NACA 0009: 1.218 89%
NACA 0012: 1.373 100%
NACA 0015: 1.425 104%
NACA 0018: 1.421 103%
NACA 0021: 1.387 101%

Scaling by thickness again to get the maximum lift for their physical thickness:

Foil Cl_max/t
NACA 0006: 12.75 111%
NACA 0009: 13.53 118%
NACA 0012: 11.44 100%
NACA 0015: 9.50 83%
NACA 0018: 7.89 69%
NACA 0021: 6.60 58%

So the 9% thick foil will stress its rudder stock the most. If the rudder is wider, the larger area does not make up for stalling at a lower angle of attack. And the converse is true for the thicker foils - their larger stall angle of attack doesn't make up for the reduction in area when the foils are compared on the basis of equal physical thickness.

Probably the best metric is to divide the maximum lift by the minimum drag. This means the rudder is sized by the maximum lift and you want to have the minimum drag rudder that will do the job. Again, the span and planform shape are kept fixed, and just the chord is varied.

Foil Cl_max/Cd_min
NACA 0006: 229.0 90%
NACA 0009: 284.6 112%
NACA 0012: 254.7 100%
NACA 0015: 224.8 88%
NACA 0018: 199.6 78%
NACA 0021: 175.3 69%

9% to 10% thick is again the optimum size when optimized on a purely hydrodynamic basis. But there's not a dramatic difference between 6% thickness and 15% thickness when the foils are designed to the same requirements. The minimum drag for the same maximum steering power is within +- 12% of the 12% thick section I've used as a baseline.

Finally, the maximum stress in the rudder stock scales inversely as thickness cubed for the same load. The required chord is inversely proportional to the maximum lift: Cl_design*chord_baseline = Cl_max*chord; chord = Cl_design/Cl_max * chord_baseline. The load acting on the foil is also proportional Cl_max*chord. The minimum drag is proportional to Cd_min*chord. When you multiply the stress by the minimum drag, you get a metric that looks like the minimum drag coefficient times the maximum lift coefficient-squared divided by the thickness ratio-cubed. Here's what the same three foils look like using this metric (lower is better):

Foil Cd_min*Cl_max^2/t^3
NACA 0006: 9.05 154%
NACA 0009: 8.71 148%
NACA 0012: 5.88 100%
NACA 0015: 3.81 65%
NACA 0018: 2.47 42%
NACA 0021: 1.64 28%

So what does all this mean? For the cantilevered spade rudder, thin sections pose a difficult challenge. They produce almost as much lift as thicker sections and there's no room to put in strong stock.

There's not much reason to go any thinner than 9%, even with a transom hung rudder that uses the whole section for strength instead of just a post in the middle. The drag you save is offset by the loss in maximum lift, forcing you to use a slightly bigger rudder.

For spade rudders, it is worth it to go with a thick section to reduce the maximum stress in the stock. The drag due to the extra thickness is not prohibitive, and it can be reduced further by designing sections specifically for the purpose instead of using off-the-shelf NACA sections. Hence the typical practice in spade rudders of using a very thick section at the root and tapering the stock to get down to around 9% -10% thickness to get the hydrodynamic optimum.

What you save in induced drag in a spade rudder by going deeper is well worth the added profile drag due to using thicker sections for strength.

[tspeer]

...If it can go wrong it probably will, normally in the worst possible place CAN YOU FIX IT?

I think this is a big advantage of transom hung rudders - you can see and remove it if necessary. But it's also easier to damage a transom hung rudder because it's exposed. It's also harder to make pintles and gougeons as strong as the rudder shaft bearings supported all around by the bottom skin and cockpit sole.

I often wonder about the performance advantages of spades. As others have pointed out, a small gap between the hull and rudder can spoil much of the end-plate effect.

[D'ARTOIS]

That is fully correct. The gap between rudder and hull should be as minimal as possible.

[Skippy]

tspeer: a small gap between the hull and rudder can spoil much of the end-plate effect.
D'ARTOIS: The gap between rudder and hull should be as minimal as possible.

That's an interesting argument for the keel-hung rudder, at least for cruising. In order for a spade rudder to get a full end-cap effect, the hull around the rudder post must be "conically symmetric" within the rudder's range of motion. First of all, that means the upper edge of the rudder has to curve upward from the post, which is structurally not great but maybe not a huge problem. More importantly, the conical shape is not ideal for that part of the hull, which needs to be angled upward toward the transom but horizontal athwartship. The hull is more horizontal just aft of the keel, so fitting the rudder to the hull will be much easier there. It just won't work very well in tight racing or similar situations.

[yokebutt]

Personally, I'm not sure that the transom-hung rudder is any stronger either, the bending load is still concentrated in the small section that is thickest anyway. When you use one on a boat much larger than a dinghy you usually have to notch out the leading edge so there isn't too much weather helm, and that's an obviously unfortunate place to put a substantial stress-riser.

Further problems can ensue when plywood is used for a core, if the rudder deflects too much under load, the lignin holding the chord-wise wood-fibers together can break down and allow them to turn into "rollers".

My preference is for rectangular or slightly trapezoidal section carbon shafts, that way the load is shared between the fibers a lot less unevenly, and the primary load-bearing fibers are just below the skin of the blade.

Yoke.