Spudman wrote:
I guess my question is how do you know what does what when you are cutting it down? I'd like good performance with or without a sail, good clearance in the shallows, what else? Do you have a good idea for an all around performer? Reverse isn't a big issue since I'd be paddling. Any suggestions on shaping a cutdown are greatly appreciated.
From over 8 customized rudders, the best all around rudder is this one:
I used this rudder for about 2 years and built it specifically for shallow water. Depending on the boat, this will take you down to 7 to 8" of water and in most applications it is about as shallow as the Mirage Drive when pinned against the hull. (remember that the boats have rocker so the Drive sits in the deepest part of the boat and extends a minimum of about 1.5" below the hull bottom). It also has very good low speed authority for fishing and works well with the Hobie sail. Here are some considerations.
A rudder's effectiveness depends on size and location. With Hobie kayaks I consider 2 primary zones -- behind the boat and beneath the boat. The behind-the-boat portion sits in wake and burble, whereas the beneath-the-boat portion is exposed to relatively clear water. This is the part that is most important when considering options.
This next pic shows a small rudder superimposed on a large rudder. The red line roughly delineates the 2 zones. The white line shows where the cut is to be made.
Notice, even though the cut rudder is now shallower than the small rudder, it retains about twice the clear water area and substantially more burble area as well. The squared off bottom allows the maximum area below the boat with the minimum depth.
Can you take off more? Sure, but it will lose some of it's excellent authority. Here I reduced the area even more to make it more efficient by radiusing the leading edge. There was a loss in authority, especially noticeable with the longer boats. It's ideal for the Revo 11 though. I have not tried it sailing, so would be reluctant to recommend it in that capacity at this time.
To make the rudder, use your small rudder as a depth gauge and chalk a line across the large rudder parallel to the top edge. A hack saw makes a nice cut, fairly steraight. If you use a sabre saw or jig saw, use a fine tooth blade. A file rounds off any sharp edges and medium and fine sand paper (wet-dry if you have it) finishes it. Pretty simple operation.
Disadvantages? With large swells and following seas, rudder depth is helpful when the stern pops out and for broach control. Also, this design does not have an
efficient shape or aspect ratio as rudders go, but unless you're racing, it doesn't matter. Even then it's a minor point with our kayak speeds.
Here's a stern pic with the rudder scooting along in about 10" of water showing a Turbofin mud signature in a full flutter mode.
Customizing your equipment and technique can open up a lot of doors!