Best set-up for best cornering speed

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Joined
Jan 14, 2006
Messages
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I've been racing boats since 1977 and seen much slower, heavier less powered boats that handled poorly. Thing is back then it was considered the way to go because no way could you put that 60!engine in that 10 pound 40 boat. Now today boats are smaller/sleeker, faster and better handling at weights that used to be thought of as way too light to stay in the water.

What I've also seen improve is the cornering speeds of many boats. Boats, especially hydros used to slide around the corner, slowing significantly. Now the boats hook up like they are on rails and in some cases look like the speed up around the corner.

I've seen many hull styles achieve this too with different configurations and hardware styles so I could never really see a particular method of achieving high speed cornering boats other than buying a design that possesses that ability. But I was wondering is their a formula in set up to achieve this and does it differ for hull types? I'm most interested in hydros like sport boats and riggers and wondering if it's different from nitro to gas boats.

Any answers?

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It wouldn't surprise me if the sponsons themselves had something to do with it. The Jones asymetrical sponsons on the full sized boats improved cornering and, ultimately, lowered lap times when they first appeared. Obviously, there were other refinements in the hulls that probably helped increase speed but, when you look at the 1988 season, the Madison team gained 20mph going from the 15 year old Pak hull to their new Jones hull while using the exact same engines.
 
The placement of the turnfin and the design of it has a lot to do with it..Turnfinalogy is what Larry Johnston would call it..
 
The P limited riggers are a great test bed for this issue. They all have very similar power so props and setup are everything. There are several schools of thought on turn fins:

Stiff, straight angled fins

Stiff curved fins

Flexible, curved fins like the Eagles

The size also is important. The bigger the fin, the higher the drag, especially when generating lift for cornering. It is also important to generate down force to counteract the tendency to lift the inside sponson. Think of the fin/rudder system as an airplane with one side in the water.

Angled, straight fins generate both side and down force. The side to side angle determines the proportion of side force to down force. Adjusting this angle to give the best ratio is critical.

I believe that a fin that is straight with a curved lower section is the easiest to set up to accomplish these things. If this fin is angled fore and aft, adjusting this angle varies the down force generated by the curved lower section. The fore and aft position of the fin determines the portion of the cornering force that is carried by the rudder. There will be an angle of both surfaces that is the point of best lift to drag. Setup involves finding this point.

There is also the issue of minimizing drag when the fin and rudder are running straight. The flexible fin may do this by twisting to a higher angle in response to cornering loads while straightening to a low angle of attack in the straight. Andy Brown may have a lot more to say about this system.

At one time a dihedral angle on sponson bottoms was thought to be necessary. These days flat bottoms with their better lift to drag ratios seem to work well even without a non trip angle. It's probably due to better turn fin designs.

Lohring Miller
 
I had a Ira Cotton Chisel nose sport 40 II with a straight "angled-at-the-mount" turn fin. The shape I would say is conventional. Tried a smaller size you'd expect for a 40 boat and a 60 size or closer to it. The smaller size slid around the corner in much the way you might expect from this design. With the larger one, with some throttle maintenance and the input on the stick/wheel would hook up and zip around the corner. It was touchy when turning left though. It rolled over a lot easier. But with that set-up an Ira Cotton Chisel nose set the record for Sport 40 II with a 1:25. A time still competitive even today. It held until Mark Anderson broke it with his awesome Mutt boat. I couldn't put a finger on why that turn fin worked better other than it being bigger so it held the boat around the corner better. Only sport style that we've had that turned just as well is my Dad's Charlie's girl which turned a 1:24 best time and the boat wasn't ballistic in the straights but it cornered like it was on rails. This boat used a tear drop type turnfin. Now to me it seemed the tear drop provided the best cornering speed but the design on Eagles, JAE boats and Crapshooters seem just as comparable but have all different designs. But in this case I'm most interested in Sport boats.

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So would you say rudder on the left or right. I looked at Insane Gas Sport boats and The Backlash for example and the have the rudder on the opposite side of each other.

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Roger,

The teardrop design is a really good one.

I am with Rudy a larger turnfin will get you through the corner

faster with a better overall lap time. With the new ABC propellers

a lot of boats will need bigger turnfins and run them deeper also

to hold the faster boat in the corners. Testing is the only way to

prove if you are heading in the right direction, the stopwatch does

not lie.

Enjoy Testing New Designs,

Mark Sholund
 
Using the geometry is more what holds in good boat setups and better performance in most platforms now i believe, Andy browns mongoose was very similar to the jae principles used currently.
 
Roger; Mr. Charles Perdue had this figured out some 30 years ago on his Boss boats. Maybe he will chime in with some info. J.
 
Some boats are just poor designs and will never handle well. My first boat was an utter pig. You had to hold you breath if you turned to the left. It would just spinout or roll. I just thought that I sucked as a driver (turns out I only kind of suck
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) There are lots of good designs out there. I think we all see guys in our clubs that struggle a lot. In most cases I think that the equipment they have has more to do with their lack of success than driving skill. Nobody can make a POS work. It really helps the new guys if we can steer them towards decent equipment rather than having showing up with some freakshow they bought on ebay and try to make it work. Better for them, better for the hobby if they get something that is a proven design.

Lighter boats turn better for the most part. Three bladed props also add corner speed and stability but you give up something on the straightaways. I think you have to have a boat that is straight and doesn't flex to hang all the geometry off of. There is a reason why race car chassis/tubs are so stiff. Hanging a lot of exotic suspension (read sponson/bottom geometry in our case) on something that isn't rigid is just a waste of time and money.

Set up is huge. If you have to set the rudder trim anywhere but zero you are giving up a lot. Better to trim the turn fin or bracket. Clamp a stiff straight edge to the bracket then measure from the centerline of the boat and the straight edge all the way back to the transom. If the measurement gets greater or less shim the turn fin or bracket until it zeros out all the way down and you have a great starting point. .

I find straight versus curved fins to be a matter of driving style. I have seen guys run curved fins with a lot of success but they have to be set exactly. A straight not so much, especially the forward/aft position. If a curved fin is rotated too far back it's adding drag, too far forward and it's a hydrofoil lifting the sponson. I have tried curved fins but I find them to make a boat twitchy and it doesn't suit my driving style. I like smooth and predictable. I set straight fins with about a 5 degree tuck and the bottom of the fin parallel to the bottom and test it. If the right sponson is lifting in the turns I keep adding tuck until it just stops doing that and I'm done.

The turn fin cannot have a rounded edge. I made some like when I started that were rounded and effectively created a wing that caused the boat to pull to the right as I was creating a low pressure area on the right side of the turn fin. Turn fins can be exotic. Making breaks on the side is worthwhile if you are really looking for a small edge. The fin with breaks has less drag as it is running straight because you have less drag area and as the boat turns the aspect ratio opens up and the fin in reality will appear wider to the water for more grip. There is a school of thought in hydrodynamics that making the inside of the surface running in water dished because it creates a bubble that rotates and since air has less density than water there is less drag. The U8 crew actually was doing this some years ago and their boat would actually speed up in the turns. Villwock got nosy and put a straightedge on the inside of the fin and they outlawed it.

All this science doesn't really compensate for a well set up boat, sensible driving and the reliability to finishes heats.

When I coach new guys I try to get them to practice setting an arc through the corner. Set an arc at corner entrance and just let it run through the corners and get the rudder input out of it as soon as you can. A lot of steering inputs in the turns will get even a good handling boat to do something stupid as well as slow you down. Pretty much everybody sees how a boat picks up speed once the rudder gets straight coming off the turn. Having a rudder with any angle other the zero when the boat is going straight is just killing the speed.

The bottom line for me is if the boat won't hold lane 1 wide open in race water I have some work to do.
 
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Roger

My bother built and David Frank sport 40 back in 1988 and ran it the summer of 1989 and the turn fin was a angle one and that boat had a problem going in the corners and when you would give it a little left the boat would be upside down.He didn't run the boat that much untill 2016 we got a Dick Tyndell turn fin and that boat drive like it's on rails and you can turn left with out a problem now.

Dave Roach
 
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Sorry for that comment. It was a generic comment. The ideal size is a trade off between the needed lift and the drag it causes. it takes testing. I also agree that the curved fins are less forgiving, especially of left turns. I still think it's faster in the right hands. There are fins with slight curves that have characteristics close to straight fins. I run straight slanted fins on many of my boats, curved fins on my Mutt sport 40, and a slightly curved fin on my P limited hydro. They are the fins the boat's designers used in most cases. I find stiffness is good. The gas sport hydro is an example of what is needed with really high forces.

Lohring Miller
 
Avoiding people crashing. They call this hobby radio control but sometimes I think we are a little shaky on the "control" part. When are you going to make some more 60 size mufflers?
 
Mike Hughes

Some times some one will die right in front of you and it's call quick thinking and mite have to turn LEFT.LOL

Dave Roach
 
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