Side fin question

[Ram4x4]

Something that came to mind as I was looking at the various boats in the pits during the Franklin event, and after looking at various websites at hardware for boat building...the side fin....... It seems they're all pretty much the same size for any given class.

I sort of wondered why that is. Trying to analyze it in my head, it seems to me that you have two points on the boat that help you to get around the corner. The rear of the boat with the motor lower and the side fin toward the front (of course there is the friction of the entire boat hull too). To me, it seems the size of the side fin can act to produce the equivalent in a car as over steer or under steer. Would that be more or less correct, or am I totally in left field?

Would a larger fin area give you more bite in a corner and allow harder turns, or do you risk spinning the rear end around (over steer)? Is the fin size regulated to a maximum size in a class?

[bmitch1]

There are different sizes and shapes, and fin size, position and depth all play a factor in how the boat handles and turns. You'll find that the smaller boats (J, AX, A, etc) use a little shorter or smaller fin than the larger, heavier classes (C, D, etc). Depending on water conditions and layout of the course racers may also choose to tinker with the fin. For example, large sweeping turns (like Dayton) may require less overall fin to navigate the turn while short, tight corners (like Lock Haven) require a little more fin. You may also see hydroplane drivers use the same fin on different courses but they'll tilt the fin fore/aft in order to get different depths which affects the amount of bite in the water.

The speed of the boat and the position of the driver in changing the center of gravity on the boat also play a factor in how the fin sets in the water. It is possible to "over" drive the fin - similar to how a race car understeers - by failing to have enough fin bite in the water at the time you need to turn. Example: when entering a shorter/tighter corner the driver needs to remember the throttle moves two directions and by letting off the gas and getting shoulder out over the left sponson they can 'set' the fin to facilitate the turn. It is also possible to create oversteer when the fin grabs well and the prop/skeg comes loose of the water (sometimes just a result of choppy water) and the boat will spin out.

You will see differences in hydro vs runabout fins and you will see runabouts that may have fins on the side or underneath which each require different driving approaches. Check out some examples of various fins at:

http://www.btmco.com/Sponson_Fin.html

http://www.portagebaysystems.com/marine/products3.htm

Most importantly the driver needs to have plenty of seat time to get a feel for how the boat handles under various conditions which ultimately determines how much depth and size are necessary....which is where boat racing becomes more art than science...

[Ram4x4]

Thanks Brian, that's exactly what I was looking for. So fin size and angle is to the boat as matching car tires to track conditions and driver preference.

[bmitch1]

Exactly - but also keep in mind that, like a car, the rest of the 'chassis' plays a significant part in how the boat carries and performs... so beyond just the fin everything in contact with water and air have to work in concert. The fore/aft weight load, size/shape of the planing surface, the size and configuration of the sponsons, the depth, size and shape of the decks, tunnel and air traps, and the shape and angle of the chines all contribute too....and not just hydroplanes...the runabouts share similar complexities.