Rudder configuration

Discussion in 'Boat Design' started by AdrienJousset, Jul 10, 2010.

  1. AdrienJousset
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    AdrienJousset Junior Member

    Hi everyone,

    I would like to have a discussion regarding two different rudder configurations on a 7.9m sailing sport boat. The aim is to develop a one design class that can also race in GP26 class. We are pretty far in the design process and so far are really happy about the design.

    Anyway here is my question...

    How much a rudder standing behind the transom would affect the performances compare to a fin rudder located under the hull? I am not sure about the name for those two configuration so I hope you will understand me.

    I strongly believe located under the hull the rudder will have better performances. However we would like to have a lifting rudder for multiple reasons. Here is an example of solution for this configuration.


    How heavy such a mechanism could be? how expensive?

    A removable rudder located at the transom is for sure, simpler, cheaper... less efficient???:?:

    I thank you for your advices, talk and ideas...;)
  2. daiquiri
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    daiquiri Engineering and Design

    The rudder in the pic looks like the one Eric Sponberg ( has used in one of his projects. He will surely be able to tell you all the details regarding costs, weight etc.

    Regarding the efficiency of the outboard (transom-hung) vs. inboard (or spade) rudder, the second one will nearly always be more efficient. The reason lies in the fact that the outboard rudder pierces the water surface.

    When the tiller is turned to some angle, a pressure distributon is set around the rudder blade (foil), which results in a lift force. Without entering too much into details of this pressure field, it can be simply summed up as: low pressure over the dorsal face of the foil, high pressure over the ventral face. The difference between these two pressures (times the rudder surface) gives you the lift force.

    Now, consider the inboard rudder. Down at the tip, the pressures on the dorsal and ventral side of the foil must equalize, because the tip is one single point for the waterflow and only one pressure value can exist in that point.
    So, looking at the rudder in a spanwise direction, what you have is that the lift force distributon will go from some finite positive value at some mid-span point, to zero at the tip.
    At the root, if there is no gap between the rudder and the hull a finite pressure difference can exist between the dorsal and ventral side of the foil, so the lift can be bigger than zero at the root.

    But, if there is a gap, you have the same situation seen at the tip - the lift distribution will have to go to zero at the root because pressures will have to equalize between the two sides. Technically speaking, the aspect-ratio of a rudder with a root gap becomes half the aspect ratio of the rudder with no root gap, with the consequent increase in induced drag.

    You can read more about root-gap influence here:

    In the case of an outboard rudder (transom-hung), you have surface-piercing rudder. Down at the tip side, all the considerations done before remain valid.
    Along the line where the blade pierces the water surface you have again the lift going to zero, plus another phenomena hapening: the ventilation. The low pressure zone on the ventral area below the water surface sucks down the air and creates a hollow in the water surface shape - which creates an additional wave drag component.
    The lift distribution in the spanwise direction becomes similar to the lift distribution of the inboard rudder with a root gap.

    I have made the attached drawing which will hopefully explain the things better than words.


    Attached Files:

  3. messabout
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    messabout Senior Member

    An inboard rudder has some advantage if compared to a transom hung rudder of identical size and shape. Transom hung types are almost always surface piercing units which may be somewhat less efficient than totally submerged units. BUT....Submerged units will be placed forward of the transom and nearer the CLA so there is a moment arm difference that favors transom hung types. From a practical viewpoint, the transom hung rudder is the hands down winner. Get some sea weed, trash bag, or pot warp on the TH rudder and you can clear it in seconds. With the underbody type you will get out the scuba gear. If I interpret your picture correctly, you could clear this rudder pretty quickly too. Although if clearing it underway you would be rudderless for some interval that is larger than the TH interval. TH wins the decision when grounding too. It's not a question of whether you will ever ground the rudder but when you will ground it. The TH kickup type is not immune to damage but is far less so than the alternative type.

    Given the complication and expense involved, your clever inboard layout is not my cup o tea. I would have reservations about buying a boat so equipped.
  4. tspeer
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    tspeer Senior Member

    The rudder on my Chris White designed trimaran has a cassette that gives it some of the characteristics of both the inboard and transom-hung rudders.


    It is inboard, but can still kick up. When kicked up part way, it is still steerable, because the pulleys on the steering cables are aligned with the kick-up axis, and there is a fixed skeg at the top of the rudder that is the distance between the kick-up axis and the end of the sugar-scoop.

    The fixed skeg contributes to stability, but not maneuvering. There will be some maneuvering force contributed by the skeg due to favorable interference with the rudder (it acts like a winglet), but nowhere near as much as if the skeg itself were part of the rudder.

    This particular rudder has all of the taper in the leading edge, with the trailing edge parallel to the axis of the rudder. The axis is located so there is enough area ahead of the axis to balance the hydrodynamic moments, making for a very nicely balanced helm when sailing. However, when under high power, the portion of the rudder on which the prop blows has proportionately more area ahead of the axis than does the rudder as a whole, making it grossly over-balanced under power. It can be a real hand-full until you get used to it. Much better just to put it on autopilot when motoring!
  5. Paul B

    Paul B Previous Member

    Both the Henderson 30 and Melges 32 sportboats have used this type of inboard cassette. I believe thay Henderson/SOCA had a patent on it. I don't know if it has lapsed.

    In order for this to work you have to have a FLAT in the hull shape(longitudinal and athwartships) large enough for the drum. If you have any curvature in this area the drum will not align with the hull as it rotates.
  6. AdrienJousset
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    AdrienJousset Junior Member

  7. brian eiland
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    brian eiland Senior Member

    Bridgedeck Centerboards

    Hi Daiquiri,
    I was wondering if you care to add insight as to how the asymmetric centerboards I am suggesting here might be best configured to experience the least ventilation.

    I do realize it is not the most optimum configuration to have a surface piercing foil with a free surface at its top, but then there are many other reasons to employ such an arrangement. So how would you suggest lessening the ventilation??
  8. daiquiri
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    daiquiri Engineering and Design

    Hi Brian,

    Without entering into complicated talks about the boundary layer, pressure gradients etc., for practical purposes it is IMO sufficient to know following few facts about the ventilation:

    1) The onset of surface-piercing foil ventilation can be triggered by any water-surface perturbation (waves, aerated water, wake of objects in front of the foil etc.), when the foil is working under a sufficiently high hydrodynamic load (in this case, lift). And, for a given speed, hydrodynamic load is proportional to the AoA.

    2) The onset of ventilation is particularily sensitive to:
    a) loading of the foil area closest to the water surface;
    b) pressure peak immediately behind the leading edge of the foil.
    3) Once the water surface is breached by the air, the air stream propagates downwards along the path connecting areas of minimum pressure, approximately until the point where the atmospheric air pressure equals the local static pressure on the foil (which can be quite deep).

    Knowing these basics, one can draw some conclusions about ways to minimize the probability of ventilation.

    First, minimize flow perturbations. In practice it means - avoid placing water-piercing objects in front of the centerboard. That's all you can do about it. The other forms of perturbations are out of your control (waves, air bubbles etc.).

    Next, you can decrease the hydro load on the centerboard in the proximity of the water surface. In practice it means decreasing the camber (variable-section centerboard) and/or decreasing AoA (variable twist). From the practical (constructive) point of view, both are a demanding and hardly doable solutions, and I am mentioning them for the sake of completeness.

    Third, you can decrease the pressure peak by adopting foil sections with the point of max. camber and thickness placed more towards the trailing edge. NACA laminar sections (6-series or 7-series) could be a good place to look, for example.

    Fourth, increase the depth (span) of the centerboard. This increases the probability that deeper parts of the foil will remain washed by a clean flow even when the more superficial ones are ventilated. The drawback is an increased root-bending moment, which requires heavier, bulkier and more costly centerboard scantling.

    Finally, impede the propagation of the air stream downwards along the span. It is done by placing one or more fences (plates) along the span of the foil.
    A single fence is used when the water surface is at a known fixed height. This is not your case, since wave motion can be significant in between the two demi-hulls of the cat, so you actually don't know at what point along the span will the foil pierce the water surface at a given moment.
    For that reason, a series of fences is IMO a better option. The first one should be placed some 10 cm below the still water line, and a couple of additional ones should be added down to the lowest expected water level along the centerboard span. Since the ventilation propagates along the suction side of the foil, it is sufficient to place them on the outer side of each of yor centerboards. In this way, there will be no mechanical interferences between centerboards.
    As an example of multiple anti-ventilation fences, see the hydrofoil of Hydroptere:
    This is IMO probably the most effective and simplest thing you can do to minimize the ventilation on your centerboard. In this CFD work you can visually appreciate the difference in ventilation levels between a clean foil and the one one with a series of fences:

    Hope it helps.

    Last edited: Feb 11, 2017
  9. brian eiland
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    brian eiland Senior Member

    WOW, thank you Daiquiri very much. :cool:

    I think I will wait for a good timing to inject this information into the conversation. In the meantime I'm going to reread your reply several times over.

    Ah ha, you already did so....GREAT.
  10. Skyak
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    Skyak Senior Member

    Well stated! A page for the text books.

    One trick that did not make the list is forward sweep. My unsupported view is forward sweep should be good for some ventilation resistance/recovery but I am at a complete loss to quantify it. Superstition?

    While we are on the topic, I am wondering why fences are the length they are -do they only need to cover the suction peak or do they just not care so much about ventilation in the recovery? I ask because if it is a trade-off then slower boats might want longer fences.
  11. daiquiri
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    daiquiri Engineering and Design

    IMO, you are correct that raking the board should have been added to the list. The reason why I didn't do it is that I don't have experimentally or field-proven data to support that claim.
    But it is a known fact from the general aerodynamics that back-swept wings have a reduced aerodynamic loading in the root area and increased loading near the wing tips. Conversely, the fwd-swept wings have increased loading at the root but reduced loading towards the tips. See these examples - same wing area, airfoils and spanwise chord distribution, different rakes:

    1) Straight wing lift distribution:

    2) Back-swept wing lift distribution:

    3) Fwd-swept wing lift distribution:

    4) Comparison of lift distributions for the above three cases:

    Both back-raking and fwd-raking chould be useful in preventing the ventilation.

    In other words, considering that centreboard root is the part which pierces the water surface, the above considerations mean that:
    1) back-raking gives a lower hydrodynamic loading near the water surface, which might delay the onset of the ventilation
    2) fwd-raking increases the loading near the water surface but decreases it towards the tips, which might help preventing the propagation of the ventilation.

    Once the ventilation has started, the air stream will move downwards pulled by the low-pressure areas it progressively encounters along the path. But the flow field, the boundary layer and the pressure gets modified too in the process. Mathematically speaking, it is a multi-variable chaotic system in which at any given moment each variable influences and modifies all the others. So it is difficult to quantify precisely the true path and extension of air propagation. The only certain thing is that it will be the path which gives the maximum enthropy increase, because the mother nature likes it that way.

    Why did hydroptere team place the fences only at the forward half of the foil? My guess is it comes from both CFD analysis and sea trials.
    Each fence is a wetted surface which increases the drag, so the number and extension of fences had to be kept to the minimum. Evidently, they have set their goals and have determined that the forward part of the foil is the most important one for the onset of ventilation, and for fulfillment of their design goals. :)
    Last edited: Feb 13, 2017
  12. Stumble
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    Stumble Senior Member


    With the rapid deployment of winglets to rudders for attitude control what would your thoughts be about using a set of winglets to also act as fences? For non-foilingboats the recommendation has been to instal them about 2/3 of the way down the rudder with the max camber of the winglets and the rudders intercecting (I am not sure why). But instead if you moved the winglets up, they could do both, with no additional drag.
  13. daiquiri
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    daiquiri Engineering and Design

    Hi Stumble,
    this sounds either like a case of a typical misnomer, or a case of my mis-interpretation of your idea.
    A winglet serves to reduce the induced drag of a hydrofoil (or rudder, or centreboard) and it is placed at or near the foil tip. Once you move it away from the tips and place it in a random position along the hydrofoil span, it is no more a winglet but a T-foil designed for some other function.

    So, if it is placed down at the rudder's (or centreboard) tip, then you have a winglet. If it is placed more upwards towards the water surface, then you might have an anti-ventilation fence. A single device evidently cannot perform both functions, because the two phenomena - the foil-tip induced drag and the ventilation - occur at different sides of a foil.

  14. daiquiri
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    daiquiri Engineering and Design

    Notice of an error correction: when replying to Skyak about the forward raking the board, I was having in mind the more common backward-swept wing, though the reply was implying the opposite. Too much contemporary tasks, sorry guys.
    Corrected that one, and expanded the previous post by including the effects of both forward and backward-raking of the CB.
    In substance, nothing changes, both types of board raking have their potential beneficial effects for delaying the ventilation.

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    I do agree. Excellent post resuming the facts. I'll just add some examples.
    I made dozens of rudders, as I was involved in racing. The cheapest and simplest are simply in laminated wood, covered with glass and a bit of carbon fiber. The good all around profile is the good ole symmetric NACA somewhere around 10%.
    For very hard racing on sport cats hyperlaminar profiles with max thickness around 35%, and sometimes turbulators but they are delicate to use, and ask for a total respect of the profile while making them.
    Plenty of small boats use successfully the TH rudder. Almost all the small French designs of the 50 and 60ties in hard chine plywood used the TH problem without problem. Easy to make, easy to maintain, cheap and efficient.
    All the racing dinghies and sport catamarans use it.
    The Mini Transat boats monos 6.5m, the best are able of 18 knots, and maintain average speeds of more than 7 knots during days while crossing the Atlantic, use TH rudders. The application of the Occam rasor and the TH concept seem to be efficient...:D
    Keeping it simple is the best way. It's just a small sport boat, so cost is an important factor. One TH rudder will be probably the solution.
    The kick up type like on the sport catamarans will be the most versatile solution for experimenting on profiles, compensation, angles, fences etc.

    Attached Files:

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