Rudder design for 14ft catamaran

Hi all, I have just purchased a 14Ft fibreglass catamaran but it has no rudders. I am an engineer and have access to heaps of different types of stainless steel, but I am having trouble trying to track down some info on what size and shape to make the rudders. Any help would be much appreciated. Cheers

 

There are lots of Hobies being thrown away due to hull separation. Every one has rudders with no real issues. Just keep looking on Craigslist or E-bay (or the equivilent down there- just looked at your location). You could even get a set for a 16 and make smaller blades to match the Hobie 14. Too bad you are not in Texas, I know just where to go about 10 miles away.

 

On a cat that size it is not likley super high performance anyway, so there is nothing really critical about size, larger ones give you better low speed control but contribute to drag at higher speeds, smaller less drag but poor low speed control. So to "properly" size them there are a lot of trade offs and optimizations that can be done. you can just pick a size based on scaling pictures of similar sized boats and be close enough. you might just make one out of laminated wood to tray it out, it would be easy to shape it and to change size to try out a smaller one. When you are happy with it fiberglass it and paint the outside to make it more durable. Or just varnish them.

Typically for a small sail boat they might be about 2.5 percent of the sail area (I would do that for each rudder since often only one rudder will be in the water at a time when on a strong reach). you will also want them relatively low aspect ratio (2 or 3 would work fine), higher aspect ratio has less drag but is easy to stall and takes more skill to operate properly. If you want to get fancy you can shape them into a profile shape of a NACA 0004 or 0005 profile. Otherwise just round the leading edge and make the trailing edge sharp.

Make sure they are strong enough to take the loads imposed by the wind force on the sail and hull. Consider, if you are so inclined, that lateral loads could be as high as about 1 lb for each square foot of sail and hull profile area, plus a safety factor two or more. consider that the mounting hardware and hinge or pintle will take these loads. If you loose a rudder you will have a hard time controlling the boat to get it back to shore, so make everything fairly strong (but do not over kill the strength, that will just as weight and cost).

Goof luck, post pictures for us when you are done.

 

:idea:Thanks for your help. I am a good way through making my rudders out of stainless steel. I have made them 600mm x 200mm out of 2mm S/S sheet, I angled the bottom edge slightly, onto one side I tacked a piece of 10mm OD S/S tube 100 mm short of the bottom and 70mm in from the leading edge. I then clamped & tacked along the front edge, then clamped down & tacked the back edge, then bottom end and fully welded. I was really surprised how easy and evenly the curve formed. This has made them extremely strong, ridged & aerodynamic. I could probably have got away with 1/1.5mm sheet. I will post some pics shortly.

 

Here are the 1st couple of pics, I am also building rudder boxes based on the Dotan kickup rudder system and should have them finished by the end of the week. So I will post some more pictures next week.

 

That looks like a really strong rudder! I hope it does not weigh too much. You can make them sealed up and they might even float! they look a bit large for a 14' cat, what size are they and how big is your main sail?

The LE looks like it will work, but you will want to make the TE a small flat cut at a 45 degree angle (to prevent flutter), leave the TE corners sharp for clean flow separation. A TE that is sharp like a knife would be best, but it could cause injuries when handling it and is easliy damaged, so putting a small flat at an angle is a simple and effective compromise.