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.