Turn fin

[JimClark]

Looking for a turn fin for my Vac-U-Pickle and was wondering if this fin is flat?
http://www.offshoreelectrics.com/pro...od=dh-aqub9515

[carlcisneros]

The fin, if you look real close is pre-bent (the 2 lines) at the bottom.

This is needed to properly hook up in the turns.

Carl

[JimClark]

well that brings up another point Flat or Curved which is better. i not sure curved is truely needed to hook up the boat in a turn as the big unlimiteds are straight.
So I added a poll

Jim

[BakedMopar]

Jim heres a link to my old thread. Some good reading in there.

http://forums.offshoreelectrics.com/...Fin+discussion

[JimClark]

Thanks for the thread

[Rex R]

curved, and thinner material than what came off my UL-1. in my limited experience curved fins are sensitive to ride attitude.
rex

[AndyKunz]

Like Rex says. It's a whole lot easier to get a straight blade right. I'd rather have just one thing to worry about than making sure the bend is in the right plane (which can change based on power).

Andy

[Fluid]

Having tried both flat and curved in many iterations on many boats, I find that most of the spec and P hydros I've run make better, smoother, sharper turns with a curved fin. Yes the plane of the curve/bends is critical, but that's easily taken care of by marking the turn fin bracket and checking before each heat. The curved fin also allows an additional tuning feature that on a few hills is very important in keeping them on the water. The fact that full-scale hydros use straight fins means nothing, water doesn't scale particularly well. Look at the fins on gas and nitro hydros and see what they use...

But fin shape is only one critical feature. Too many fins are thin and flexible, making any turn an adventure. Some brackets are more suitable for hanging an antenna on than a turn fin. And some fins are not aimed parallel to the boat's centerline, forcing the boat to crab through the water. So a flat, thick, stiff fin on a strong bracket is always superior to a curved fin made of foil on a whimpy bracket.



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[detox]

There's a fine line how much a turnfin needs to pull you down in the corners. Too much and it will make you go slower. You just have to experiment.

[Dangerous_Toys]

From the Home page..................

ABOUT HYDRO TURN FINS

I think that a turn fin should ideally be vertical with the water I say this admitting that I ran bent or angled fins in model boats and canted fins in full-scale hydros.
I didn’t completely understand all that goes on with turn fins and rudder dynamics when I was running model boats. I came to gain a better understanding only after earlier experience with model boats and then full-scale hydros in the ‘80’s and ‘90’s.
I now feel that bent turn fins are just another easy tuning crutch. It’s an important crutch in that it can quickly correct unhooking turn fins in high G turns, by minor alignment changes.
Bent, canted or angled (same thing) turn fins MUST be very accurately aligned or they will lift or dig-in and slow down the boat drastically in the turns. They must have some means of adjustment and be able to remain rigid at the forces applied to it in the turn.
The fin should be of a material that will be rigid with the thinnest material, even if it must be heavier! Going to a lighter, but thicker material is kidding yourself. The very slight weight saving is far overshadowed by the increased drag of the thicker material!
I now feel that 1/16, 5/64, 3/32 or 1/8 hardenable tool steel is probably the best. Ed Kalfus knew this 40 years ago. All his rudders and fins were hardened & blued tool steel, very thin - - and WE thought we knew better!
The neat thing about angled turn fins is that you can tweak them by pivoting them on one mounting hole, the other one slotted, to make the boat turn lighter or heavier on the right sponson. But, all you are doing is inducing drag (lift or dig-in), which can be usually dialed-in with other adjustments, one of which is rudder caster.
However, angled turn fins are aligned optimally at only ONE angle of attack That means if it is dialed-in in the turns when the transom is lowered; then in the straights as the prop lifts the transom to the maximum, it is NOT dialed-in and is either lifting or digging-in.
I suggest that you don’t follow the pack and try for yourself a straight or SLIGHTLY canted fin along with a left rudder with a little negative caster. Start with a vertical fin. Don’t angle it unless it blows out. If it blows out, decrease the rudder caster until you feel the increased rudder drag has gone too far. THEN, cant the rudder, A LITTLE!
Optimum fore-aft turn fin location will vary slightly for different hydros. If you make provision for easily adjusting the fin location when building your hull, you will be pleased you did so when optimizing your racing set-up during testing.

Never run a bent turn fin on a timed straightaway run. Put on a shallow, vertical fin. You may have to run a deeper rudder because the straightaway prop will usually want to prop walk more than an oval prop.
Remember, my comments about turn fin spray. Design the placement and bracketry so that no spray induced lift occurs.
Outrigger turn fin placement usually necessitates bolting the fin to the inside of the right sponson, trailing the fin back and down to get it close to the boat CG. If you trim the top of the fin down close to the water line, it makes it more subject to twisting in the turn. If you leave it tall all the way back, the spray off the inside bottom of the sponson causes more unwanted drag on the outside of the fin. A happy medium compromise must be determined between flexing and drag. Again, a rigid material fin will really help with the flexing here.
To check if your fin is strong enough, consider this: If your boat weighs 8 pounds, and negotiates a turn between 3 and 4G’s (I think this is a good guesstimate. Let’s hear from the engineers.), then the force on the fin is 24 to 32 pounds, if it is on or close to the CG.
Make a block of wood to bolt your fin to just like it is mounted on your sponson. Mount the block in a vise with the fin horizontal. Mark the waterline on the fin. Mount a wire pointer to indicate the vertical position of the fin in the fixture. Place a 24 pound weight on the portion of the fin below the water line. It may be easier to make a wood clamp for the fin from which to hang the weight. I bet you will be surprised at the deflection of your stainless steel fin! Now, try a 30 pound weight! These weights may be incorrect. Maybe our engineers will suggest appropriate weights, if I am off the mark. Also, consider the force on the fin if your boat weighs 10 pounds!
I think you will find that your bent, stainless fin is bending severely in the turns. That’s why the stainless fins must be bent so much; because they flex and if not bent severely, become less than vertical under the above G forces.
If the above proves true for you, go make a straight hardened carbon steel fin, sharpen, polish and test it as above. If it is thick enough, it will be rigid. Now have it plated and mount it slightly canted.
ALIGNING THE FIN

A straight edge, parallel to the water line, held against the sides of a non-wedge fin, should be parallel to the boat centerline as viewed from the top. To properly align a wedge turn fin, the wedge angle must be bisected and the resulting centerline aligned with the boat centerline. A wedge shim, ½ the angle of the included angle of the wedge fin can be made as an alignment tool. This shim is then held between the straight edge and the fin when aligning as above.
Of course, with a bent or canted fin, the portion of the fin to be aligned is all portions in the water.
You never want to purposely misalign a fin. More drag. You only need ONE rudder!
You should not need to cant or angle a straight fin more than a degree or so more than the dihedral angle of the sponson.
Again, vertical or slightly canted, straight fins are much easier to align and do not induce as much drag as do severely bent fins, when the running attitude changes from the turns to the straights.

CONCLUSION

My intent here was to stimulate your thoughts and open your mind to possible alternate approaches to tuning your boat.
Most boat racers don’t think about some of the concepts discussed above.
If I have contributed one “Ah-Ha” to your understanding of rudder, fin and spray dynamics - - I am very happy.

If none of this works for you, I am sad.

[Fluid]

I think that most experienced hydro racers are aware of Glenn's interesting and valuable article on model hydro turn fins and rudders. (Note that Glenn is characteristically modest when he states that these are his opinions and are open to debate.) I have no issues with his opinions on rudder dynamics because they mirror my own experiences over the past twenty years. Over the past twenty plus years I've tried both straight and angled fins, then went back to straight fins for awhile after reading his article. But on the five different hydros I tried, none were better racers with a straight fin. I replaced the straight fin on my FE-30 with a curved one and the turning difference is startling.

Continued experimentation has changed things in the seven years since Glenn wrote his article, and all one has to do is look at what is on the winning gas and nitro racers to learn what is working now. Recent two-lap FE record holders have used angled fins too, mirroring their gas brethren.

Sometimes changes like this are merely fads, but in this case not so. Today angled fins allow the hydro rudder to be placed on the right side of the transom without compromising right turning ability - but there is more to it. With the rudder on the right side the hydro is much less likely to blow over if a bit of left rudder is needed. This can matter more than inexperienced racers believe.

For evidence witness the huge success of recent gas hydro designs like the Insane and Whiplash boats with their deeply curved fins and right mounted rudders. The ability to correct left without blowing over means the racer is more likely to finish a heat, and experienced racers know that finishing is more important than just being fast. As with most details on racing boats, there is often a lot more there than first meets the eye.




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