twin rudders

  • 03 Nov 2018 20:46
    Reply # 6887267 on 6884837

    Thanks for your interesting reply Peter. It is interesting to see all these latest developments.

    My particular line of inquiry is not in search of high performance, and also is not asking whether twin daggerboards/rudders perform better than single.

    Just a very narrow question: If a skeg and rudder is placed out of alignment with a fin keel or board, would this cause more drag than if these appendages were aligned?

    A scow or any other shallow draft cruiser which has twin boards and twin rudders is already dragging quite a lot through the water. While plodding along at cruising speed, would it make much difference if, for each pair, the rudder and its shallow skeg are placed directly in the slipstream of the centreboard?

    I noticed from the Googling I did, after reading your reply, that boards and rudders are often aligned, as you suggest (but not always) - but nowhere could I find any reason given for this, or even a reference to it.

    Perhaps it is safe to assume that at plodding-along speed, it probably won't make much difference, if any. I just thought that maybe there might be at least a theoretical answer to this question, out there somewhere. Thanks again to those who gave replies.



    Last modified: 03 Nov 2018 21:01 | Anonymous member
  • 03 Nov 2018 13:40
    Reply # 6886805 on 6884837
    Anonymous wrote:

    I wonder if someone experienced with twin keels (or boards) and twin rudders can give me some advice.

    Is it desirable or undesirable to have the skegs and rudders mounted directly in line with (and presumably in the slipstream of) the (lifting) keels?

    Does it matter?

    I am looking at a choice here, and do not have experience of twin appendages. Any boat I have owned in the past has only had single keel and single rudder, aligned of course.

    Grateful for advice or opinions.


    Graeme

    I have had considerable experience building and sailing with twin rudders and twin daggerboards, as well as twin rudders with single daggerboard.

    If you look at the open 60 designs used in races like the Vendee Globe, until the recent introduction of foils most of the designs utilised twin daggerboards inline with twin rudders. This design arrangement was the result of extensive tank testing, and was considered the ultimate solution.  The boats of the '90's had a single daggerboard on the centre line in front of the keel.  As design was refined that became twin daggerboards either side on the keel and inline with the rudders.  I'm not a designer and can't explain why performance improved.  However the twin board arrangement proved to have significant upwind performance increases.

    I should include the caveat that these boats also have swing keels which no doubt was a factor in the performance gains.  

    There was a secondary benefit of the twin boards.  It provided the skipper with redundancy in the event of a collision with an underwater object.  The idea being that the redundant board could be transferred to replace the damaged board.  The other benefit was that the board took damage and saved the rudder.  

    The rudders are usually on a kick up system with a fuse that in the event of striking an object will kick up to minimise damage. 

    There is quite a lot of information available on the web if you search Mini Transat 6.5m.  

    Peter

  • 02 Nov 2018 22:31
    Reply # 6886276 on 6884837

    Off topic - if anyone else wants to enlarge on this, please reopen the appropriate thread on the JRA, Mag and website forum.

    Not sure though how to put it directly into the text rather than at the bottom.

    The instructions on how to do this can be found here. It is all rather more involved than using the "Choose files" button, because you have to have a photo album in your profile and move back and forth between your profile for editing and your public profile.


  • 02 Nov 2018 22:05
    Reply # 6886262 on 6886158
    Anonymous wrote:

    Thankyou David and Arne for your replies. It is interesting to me that you both focused on the question of rudder efficiency. From your replies I understand that while there may be a benefit to having the rudders offset, it is most likely that being well back from the (lifting) keels, effectiveness will not be an issue – especially as there are two of them.

    However there was another side to the question which I thought might also bring a response from you lateral thinkers. That is, drag.

    I had a notion that four foils, each ploughing their own furrow, might start adding noticeable drag, and I had wondered if aligning each pair might result in just two furrows of broken water instead of four, and hence less drag. 

    This is just my imagination – do you think that might be the case? From memory I think the SIBLIM design has the foils aligned, in pairs. Catamarans generally do, I suppose. I have never seen an Atalanta so I am not sure about that one. In any case, this does not tell me if drag was an issue in the minds of the designers. Now that I think of it, perhaps the various catamaran designs might provide an answer.

    Here is a model of a particular case. It is a scow hull, with boards and rudder blades which swing up. The rudder cheeks are an extension of shallow skegs, but the rudder blades (which are not on the model) – and the offcentre boards - are well separated and are in free water. It is a little unusual for a monohull in that the boards are mounted right out on the sides, similar to a catamaran. The model shows the skegs/rudders placed further inboard, well out of alignment with the boards.

    Would it be better from a drag point of view if the skegs and rudders were moved out to the aft quarters in alignment with the boards, as shown here? Or is this just my imagination and maybe it makes no difference?


    (PS can someone explain to me how to embed a small photograph directly into the post, rather than having to resort to a link? I can't seem to make it happen)

    One only needs to look at a lot of the newer yachts to see how different rudder and keel configurations are treated. A lot of yachts these days are being designed with twin rudders, but only a single keel. The rudders are usually at an angle so that when the yacht is heeled the leeward rudder is almost vertical in the water, and therefore I guess a lot more effective. Look at modern race yachts such as the Volvo yachts. As well as the keel they have dagger boards, and twin rudders. I think the dagger boards are there to provide leeway resistance because the keel, being often canted to windward probably does not do much to stop the boat going sideways. The RM yachts in France do modern twin, (bilge) keel yachts. They have only one central rudder.

    With regard to drag, that probably has a lot to do with how well the foil is executed, that is how cleanly the water passes over the surface, and then away from the foil at the trailing edge. I know the rudders I have built for my new catamaran seem to be pretty nice foils and I can see them providing minimal drag through the water. These were built from the tortured ply method.

    Regarding embedding a photo into your post. When you are working on the post in the dialogue box you should see just under the writing frame a line that says: Attachments, and then a button which says Choose Files  If you click on that button you can then look on your computer for files such as photos, and once you have selected the file it will upload to your post. Not sure though how to put it directly into the text rather than at the bottom.


  • 02 Nov 2018 20:07
    Reply # 6886158 on 6884837

    Thank you David and Arne for your replies. It is interesting to me that you both focused on the question of rudder efficiency. From your replies I understand that while there may be a benefit to having the rudders offset, it is most likely that being well back from the (lifting) keels, effectiveness will not be an issue – especially as there are two of them.

    However there was another side to the question which I thought might also bring a response from you lateral thinkers. That is, drag.

    I had a notion that four foils, each ploughing their own furrow, might start adding noticeable drag, and I had wondered if aligning each pair might result in just two furrows of broken water instead of four, and hence less drag. 

    This is just my imagination – do you think that might be the case? From memory I think the SIBLIM design has the foils aligned, in pairs. Catamarans generally do, I suppose. I have never seen an Atalanta so I am not sure about that one. In any case, this does not tell me if drag was an issue in the minds of the designers. Now that I think of it, perhaps the various catamaran designs might provide an answer.

    Here is a model of a particular case:


    It is a scow hull, with boards and rudder blades which swing up. The rudder cheeks are an extension of shallow skegs, but the rudder blades (which are not on the model) – and the offcentre boards - are well separated and are in free water. It is a little unusual for a monohull in that the boards are mounted right out on the sides, similar to a catamaran. The model shows the skegs/rudders placed further inboard, well out of alignment with the boards.

    Would it be better from a drag point of view if the skegs and rudders were moved out to the aft quarters in alignment with the boards, as shown below?  Or is this just my imagination and maybe it makes no difference?



    Last modified: 03 Nov 2018 04:09 | Anonymous member
  • 02 Nov 2018 18:34
    Reply # 6886057 on 6885996
    Anonymous member (Administrator)
    Robert Self wrote:
    Hi Arne--Something I've wondered about the folkboat-type rudder with the forward slanted hinge line. When the boat heels, if it wasn't for the water flow past the rudder, it seems like the rudder would tend rotate in the same direction as the heel and the tiller in the opposite direction, i.e. the weather-helm direction. A weather-helm self-correcting mechanism? Can you sense this at large heel angles say 15 deg plus?

    rself

    No, not really. I know there are some ideas about benefits and drawback with this kind of rudder. One is that this rudder tends to pull the stern down as one turns the rudder, which may be good when running before. 

    However, I couldn't say without trying the boat with different rudders, first. One must not forget the hull lines when it comes to make a boat easy to control. I have made this rough sketch for streamlined 'AUX' rudder for my boat, but will probably not bother with building it. Ingeborg is, after all very well-behaved as she is. However, with the position and the streamlined shape of it (NACA 0015), the aux rudder would probably 'out-turn' the main rudder quite easily.

    Arne



    Last modified: 02 Nov 2018 18:40 | Anonymous member (Administrator)
  • 02 Nov 2018 18:05
    Reply # 6885996 on 6885396
    Anonymous wrote:

    I have also been pondering this rudder issue, not least after I got my long-keeled Ingeborg with the rudder attached to the keel’s trailing edge.

    The area of Ingeborg’s rudder is 0.68sqm. That is plenty for that little boat, but compared to its area, the rudder still does not impress, and it lacks the precision of previous free-standing rudders I’ve had.

    Hi Arne--Something I've wondered about the folkboat-type rudder with the forward slanted hinge line. When the boat heels, if it wasn't for the water flow past the rudder, it seems like the rudder would tend rotate in the same direction as the heel and the tiller in the opposite direction, i.e. the weather-helm direction. A weather-helm self-correcting mechanism? Can you sense this at large heel angles say 15 deg plus?

    rself

    Last modified: 02 Nov 2018 18:09 | Anonymous member
  • 02 Nov 2018 12:29
    Reply # 6885396 on 6884837
    Anonymous member (Administrator)

    I have also been pondering this rudder issue, not least after I got my long-keeled Ingeborg with the rudder attached to the keel’s trailing edge.

    The area of Ingeborg’s rudder is 0.68sqm. That is plenty for that little boat, but compared to its area, the rudder still does not impress, and it lacks the precision of previous free-standing rudders I’ve had.

    Obviously, Ingeborg’s rudder works in a thick boundary layer of slowed down water flow. The question is, how far aft of the keel must one move to find a water flow of 80-90% the speed of the free water flow? When watching what Paul Thompson achieved by moving a new hydrofoil rudder further aft on his La Chica, my hunch is now that the water flow picks up speed  (helped by the surrounding water) quite quickly, after leaving the keel. As the photo of La Chica below shows, the gap is not that wide. It would not have surprised me if the new rudder would have been even more powerful if the ‘deadwood’ around the propeller had been removed, and thus widened the gap.

    What is an objective truth, free from any silly ideas of mine, is that thousands of boats sail around today with finkeels and separate rudders (but still in line with the keels). These steers very well with much better precision than boats with long keels and integral rudders.

    I therefore see no need to offset a rudder to any side, as long as there is a generous gap, say one, or better two metres between keel and rudder.

    Arne

    La Chica with her new rudder:


  • 01 Nov 2018 22:36
    Reply # 6884904 on 6884837

    I have not had any experience with twin rudders as you describe so this is just an opinion, but I would have thought it to be better to have the rudders operating in clean water, that is not in the slipstream of the keel, or lifting boards. If the rudders are flowing through clean water then they should be a lot more effective, just the way any foil works best in either clean, non turbulent air, or water.

    Last modified: 02 Nov 2018 04:34 | Anonymous member
  • 01 Nov 2018 21:26
    Message # 6884837

    I wonder if someone experienced with twin keels (or boards) and twin rudders can give me some advice.

    Is it desirable or undesirable to have the skegs and rudders mounted directly in line with (and presumably in the slipstream of) the (lifting) keels?

    Does it matter?

    I am looking at a choice here, and do not have experience of twin appendages. Any boat I have owned in the past has only had single keel and single rudder, aligned of course.

    Grateful for advice or opinions.


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