Guys,
What Charles says is true. The physics haven't changed, only the effect of them based on how we apply them. The boats are faster and lighter than they were 30 years ago, but the forces at work are still the same. I think anything that worked 30 years ago, in regards to turning dynamics, but doesn't work now, is due to reaching the limits of what a particular surface area will hold the arc we are wanting them to. We are ALL looking for that same balance point between the rudder and turn fin, it's just a matter of how we get there that varies. I firmly believe that there is a "perfect" size/shape/position/orientation/etc. I am also fairly certain we will likely never find it.
I don't really understand the nitty-gritty details, but I've got a fairly firm grasp of the basics, which is likely plenty for most of us.
The basic concept is that the boat follows the turn fin and the rudder controls the vector, which is obviously also influenced by prop walk. When the turn fin is doing it's job perfectly, the rudder essentially throws the boat into a controlled tailspin, pivoting on the turn fin. While the boat is in the turn, the rudder is actually keeping the boat from spinning out, with more force on the outside edge of the rudder blade than the inside. I have found that the best set-up has the majority of the turn fin AHEAD of the CG, with the CG at the intersection of the trailing edge and the waterline. The angle of the trailing edge leaves some of the fin that is below the waterline behind the CG. Our rudder throw is only about 5-7 degrees. I've actually had guys in the pits ask me if my rudder servo was working. If you need more than that, either your rudder and/or turn fin isn't wide and/or deep enough, or you've got some flex in one or more of several parts, which may be the fin, rudder, steering linkage, booms, whatever. One thing to look for is that "wall of water" coming off of either the fin or the rudder. It looks cool, but it's a dead give-away of lost speed in the turns. A boat that slips through the turn with little to no water tossed in the air is running VERY efficiently. The less water in the air, the better. I have also found that NOTHING flexible is good, except fuel line tubing
. Anything that is flexible is going to behave differently at varying loads, as well as reaching toward a yield point.
I can only speak from the experience of the few builds I have done. I have not had the time to dedicate to building boats simply for the study of it. I build boats to race. Obviously, there are changes that we all make from one build to the next, improving on areas that need attention. When I started building my 21 boats, I started from a well benchmarked design, scaling it down appropriately, and went from there. There have been ZERO changes to the geometry of the hull and sponsons, although there have been some adjustments to AOA and rear sponsons depth, particularly after the first two boats were built, most likely resulting from errors during scaling. I also started with a well proven turn fin, resized for the application. It is your basic shape, .04" Ti, with a 1/8" aluminum backing plate above the waterline, vertically mounted, with about a 3/4" radius curl at the bottom. The initial shape and size worked plenty well enough for me, but when a few others started pushing boundaries, we found the turn fin was allowing the boat to drift. As per the advice from one such driver, I increased the width and depth of the turn fin. The additional horizontal surface area created a REALLY bad tendency to lift with hardly ANY left rudder at all. In fact, I found that if I let off the wheel too quickly at the end of a turn, the boat would lift off and cartwheel. I began dressing the leading edge of the fin back, resharpening it each time. I eventually arrived at a shape that would hold the turns at 75+, and would not be too eager to blow off. I did have to let off the throttle and wait for the boat to settle before making any left corrections, but I learned to live with it. I have since developed a better shape that seems to hold the turn at any speed I've been able to get it to, and has little to no tendency to blow off, even while making a concerted effort to do so. Going back to the second, oversized shape, I put a much tighter curl at the bottom, something like 3/8-1/2" radius, which GREATLY reduced the horizontal surface. The boat now handles like it's on rails and could go the wrong way around the course at a considerable speed with proper left rudder throw. This fin is fairly sensitive, as far as the horizontal surface, and must be at a nearly perfectly neutral attitude relative to the water line.
Hope this helps.
Thanks. Brad.
Titan Racing Components
BlackJack Hydros
Model Machine And Precision LLC
