Vertical turn fins on a mono do they work ?

[lenny]

Hi,
I have a 36"Shockwave with stock hardware and the turn fins are vertical.
Should they be changed to the normal angle on a mono ?
The boat is not running bad, 42.5mph gps,
Its running pretty loose and at top speeds,
It start to go into a death wobble were I have to back off the throttle or flip it.
I may try to move the cells forward towards the bow some to get it to lay down.

This is the setup,36"Mono, All stock hardware except for coupler and prop, Motor is a Feigao 9xl 1898kv,
Esc is a Turnigy marine 120a and programed setting are 2 4 3 3 with card,
Cell are Turnigy 4s2p 40c to 50c 3000mah and 5.5 bullets all the way.
Ran today with a p230 for 4 minute runs and gps at 42.5mph,
All temp cool 98f esc, Motor 115f, Cellls 95f, Just the caps got hot and were 150f.

Thanks for any help.

Lenny D

[steveo]

turnfins should be 90 degrees to the bottom of the boat and some trim tabs should help with the wobble and ose has a pair that are 3" wide split to act like two fins that should help, i cant believe that 9xl is pushing that boat at those speeds and staying cool good job

[lenny]

Thanks,
The hull is ruff and needs some tlc, but I had the setup parts lying around,
So I set it up to see what it would do on what I had to work with.
Yes the turn fins should be at 90 degrees,
But the stock ones are vertical,
And I want to know what they were thinking when Proboat did that on this hull.
http://www.proboatmodels.com/ProdInf...2050-5-450.jpg http://www.proboatmodels.com/ProdInf...2050-4-450.jpg http://www.proboatmodels.com/ProdInf...2050-2-450.jpg

Thanks for any help.

Lenny D

[CornelP]

Some hulls actually need the vertical turn fin. My Hopf Twenty Nine Mono2 has one and it's also pretty large: without it, the boat leans on the right and cannot be controlled properly in a straight line.
Usually, vertical fins will always be in contact with the water.

[Fluid]

You have the "death wobble" because too much of your hull is out of the water. It is trying to balance on the sharp keel of the hull - tough to do with a deep vee. Sometimes adjustment of trim tabs out near the chines will help, but the real solution is to lower the hull into the water. You do this by raising the strut until the wobble stops. We did this with a club member's boat last weekend, the wobble was bad at first, but after raising the strut about 1/8" the wobble was gone and he could hold full throttle.

Different hulls will behave differently depending on their lift and the vee angle. Lowering the hull may slow it slightly, but if you have to back off the throttle now to keep the boat on the water.....


,

[lenny]

Thanks for that great information Jay.
I was looking at a post about hull speeds and you talked about adding weight,
To the hull to make it faster and still fun to drive,
Just a good balance of speed and handling in a boat all around.

Do you think think I should try some weight and see if it helps to get a balance with this boat ?
And the vertictal turn fins should I make them into 90 degree ones or leave them be ?

Jay thanks for sharing this info. http://forums.offshoreelectrics.com/...44-Hull-speeds

All FE hulls require proper aerodymanics to run successfully. They generate more lift the faster they go. When they are designed, the maker has to compromise between lift and weight and speed. Drive a boat designed to weigh 5 pounds and 40 mph to 55 mph and it will likely become unstable.

Air compression hulls like hydros and cats are easy to understand - if they pack too much air and generate more lift than the weight of the hull - the hull flys off the water. That is the maximum speed for that hull in practical terms. Water conditions come into play, but since most of us don't have the luxury of running on glass smooth water very often, we have to set up our boats to stay on the water in our own water conditions. This is even more pronounced in oval racing.

The same holds true for monos. An example is my Tital 40. It was designed to race in the 40s with a 7.5 engine. When I installed a 4S setup and hit 50 mph the boat would not stay on the water reliably. To make it a reliable performer I had to add 1.5 pounds of lead, and now I can run the boat faster for longer. It is the most stable and reliable boat I race, and it wins much more often than it loses. If I wanted to go much faster I'd have to add even more lead, but then the mono would start to bog in the turns because it would be too far out of its design performance envelope.

The famous Dark Horse hydro was designed for speeds in the 40s and 50s, but I watched its designer run 80 mph with it. He added a lot of weight - 18 NiMH cells - and an air dam, and he moved the CG ahead of the sponsons. Yes, it ran 80+ mph but it was almost impossible to launch and it didn't turn well; it also didn't run well at under 40 mph. Even with all the modifications it still self-destructed in a high speed crash.

So when someone says that a certain hull can't go faster than xx mph, he really means that it is not a practical design for higher speeds. Modifications can add some additional speed, but eventually so much weight is added that the boat is no longer practical at lower speeds. Adding 6-8 ounces to the SV27 is one of the best mods you can make, but add 2 pounds and it will be little fun to run at anything but WOT. A successful R/C model is always a balance of speed, weight and lift.

[Fluid]

Lenny, I forgot to recommend adding weight. A boat that size may need a pound or two - reall. That is an easy mod too, as long a you secure the weight very well. A lead projectile bounding around inside your hull during a flip is not a pretty thing.....




.

[lenny]

Hi,

Well I modded the stock strut so it is adjustable now and added one more Rubycon zlh 35v 1000nf, cap to the esc to help with temps.
I moved the strut up 3/16" and it ran a little better, Than I moved trim tabs tips down to 1/16 from bottom of hull and it got stable,
Ran today with a x646 prop and got 38.5 gps for 4 minutes and temps were 125f on caps and Esc 89f, Cell 90f, Motor 100f,
It ran pretty stable at full throttle,
I may try a bigger prop and see what happens with speed and temps,
Also will try to add weight and move trim tabs up some tomorrow and see if speeds go up any.

Thanks every one for the help on this.

Lenny D