Kick up rudder design on Tri

So this is my 1st build and I have also modified the plans to suit what I am looking for in a small multi-hulled boat.

Basically it is a scaled up Ulua designed by Gary Dierking, and I have turned it into a trimaran. Attached is a drawing with rough dimensions so you can get an idea of some of the forces that she will be under. Also the sail will be approximately 120 square feet.

The original design uses a rudder mounted on one of the beams, but since that was based on an outrigger canoe and my boat will have 2 larger outriggers I thought that the rudder should be placed at the stem of the stern.

Only problem is that the stem is tapered without a flat spot to mount a kick up rudder. So, after a lot of considerations I have come up with a design that I think will work. You can see from the attached .pdf, that the rudder is attached to a tube, which is fitted inside of a larger tube that is welded to a metal support member.

I still have to run the numbers to see if the metal tube that attaches to the wood rudder can be a "reasonable size" in order to resist the moment produced by the water on the rudder under maximum speed.

So any comments would be appreciated as to what you think of this design.

 

Go have a look see at some sailing cats they have twin rudders and they are all kick up and in all sizes and shapes .

 

I wasn't asking that. I asked for comments concerning the design. I realize that I can walk down to the harbor and look at many kick up rudders but that is not my intention.

Not sure why you would post a reply like that. It seems that you are trying to say something else, with the you added at the end of your comment. Why don't you just come out and say what you mean?

 

Erik,

I do not really see any questions in your original post, so if you want feed back you might actually put in some specific questions.

I did look at your drawing and my first impression is the rudder mount is a bad idea. the rudder produces large lateral forces against the sail and other forces. There will be very large torsion loads on the rudder mount. It can be made strong enough, but it will be heavy and likely springy that will have poor handling qualities.

I suggest redesigning the stern to it ends in a plumb vertical line, and than you can have two rudder pivot attach points, one low in the water, one up near the top. This will reduce the torsion loads into side loads on the pivots. by making a plumb stern the rudder will likely be more effective as well since there will not be a gap between the rudder and the end of the hull.

 

Petros, thanks for your reply. I guess when I asked for comments on the design what I realy meant was , "does anyone think that this is a poor design and why?"

I see your points and since I didn't do the calcs yet to size the tube & connection mechanism for the tortional moment due to the resultant of the water pressure on the rudder I realy have no idea what size of tube to use.

Only problem is that I am trying to not have to redesing the stern.

The other option that I thought about was duel kick up rudders mounted on the aft beam or aka.

 

the only reason to put duel rudders is if you expect to heel over enough to lift one central rudder out of the water. That means it would bring all the weight of the boat up on one ama, and still having control.

If you do not ever see that has happening, there is no reason to use duel rudders. Keep it simple (stupid)...KISS. ONe rudder is more efficient, fewer parts to make, fewer things to break or malfunction, fewer things to mess with when you launch. the lower the part count on your boat the better.

 

I agree that the rudder mounting is insufficient. It will not be able to handle the loads placed on it in normal conditions and will probably twist completely off in any significant wind and may tear up the hull in the process.. The shape of the stern is just not compatible with a rudder as shown.

 

The sort of rudder you show needs to pivot between two cheek plates, not run in a tube. The tube thing has been done, but only where the rudder can be raised far enough to clear the bottom of the hull when up. Your suggestion is so odd to me that I agree with the general sentiment here. I think you shoud go buy a rudder such as a Hobie Cat rudder and work out how to mount it to you hull. Copy stuff for your first few tries. Saves much time and misery. The prodution boys really have figured out the easy way to do things.

 

Rudders like a flat stern to be mounted on. Make the Ulua hull without the rear deck, drop in a plywood flat transom fwd of the current stern. Cut the curved stern off and fiberglass before adding pintels and guegons.

If you dont like the way that looks, build up the as designed stern with about 2-3" of solid timber well fastened to the side skins. Bore a hole along the centerline high and low and bond in steel rods. Drill and shape to take the rudder hardware. Use a beach cat rudder.

Either way should not be very hard.

 

this worked on a 27ft high speed tri for the 11years I sailed it and I hit bottom at speed many times

 

The system you have drawn will work but it is unnecesarily complicated. The disadvantages are several. The overhung load on the rudder shaft, the bearings in the pivot block and the necessity of imbedding the rudder shaft in the rudder body.

Best advice is to do what the other guys have said. Use a pair of cheeks to hold the upper part of the rudder. Pivot the rudder on a simple pin or bolt through the cheeks. This is the simple, practical, and proven way to do it without a lot of work. The rudder can be held down by clamp pressure from the pivot pin, with a bungee, or with the slip cleat that you have drawn.

You could use an inboard rudder if you are determined to leave the aft end of the boat as drawn. In that case the kick up is accomplished by housing the rudder shaft in a pivoting box, the box itself comprising the aft end of the run. Check out the way some of the Bolger boats have done the pivot box scheme.

 

I think you should make a bit of a gantry that fits over the stern. Mount a tube horizontally across the stern some distance forward of the stern. This will be the axis about which the rudder kicks up. Run a tube through this one, and then run tubes from the ends of that to the ends of the tube holding the rudder shaft. This will make a tetrahedral shaped truss to transfer the loads from the rudder. A line run from the bottom of the tube holding the rudder stock to the hull will keep the rudder down and can be released to allow it to kick up. The whole thing would be like a gantry mounted rudder on , say, an I14, but wouldn't be sticking way out behind the stern.

 

You could make a simple tube frame that mounts the rudder on the back of your hull, bracing off the sides, kind of like this foiler moth rudder mount (it would not have to be quite do fancy for your boat, this is just for illustraton purposes).

 

Wow, thanks all for your input. After consideration I think the easiest and simplest method would be to adapt a beach cat rudder and block off the stern as mentioned.

I am aware of the common expression, Don't re-invent the wheel, but there is something about coming up with a unique solution to a problem that no one else has done before.

I guess sometimes it is best to use a tried and true method or system for a solution.