check my math?

[Dan_Cousin]

This is basic physics but I'm thinking there is more to this than I am considering.

A 6lb hydro is running 70mph down the straight then makes a perfect 35' raduis turn without losing speed.

the centripetal force required to keep the hydro on that arc is

F = M x (V^2/r)

F is the force in lbs

M is the mass in slugs (6lbs / gravity = mass) = .1867

V is the velocity (70MPH) = 102.66 feet/sec

R is the radius (35feet)

F = 56.2 lbs

Now assuming that the pivot point is the turn fin, and the turn fin is close to the CG of the rigger it should carry most of this force.

Does the fin really see that much force during a high speed turn? Anyone else tried hanging a 50lb weight from their turn fin? not pretty.

 

[Don Ferrette]

Actually that almost sounds a little on the conservative side. I find myself replacing the turn fin on my twins twice a year as they lose strength & then won't hold the original shape. :blink:

 

[slsova]

Some of this force must also be carried by the rudder, as the rudder is the force vector changing the direction of the boat. object in motion and all the stuff. Steamer

 

[Dan_Cousin]

Actually that almost sounds a little on the conservative side. I find myself replacing the turn fin on my twins twice a year as they lose strength & then won't hold the original shape. :blink:

Don

I'll bet your twin is closer to 12lbs than 6.

for a 12 lb boat its DOUBLE!!! (112lbs)

for a 18 lb boat its TRIPLE !!!! (168lbs)

That would explain it beat up fins.

I couldn't imagine hang 168 lbs off a fin and it staying on the boat. Thats a grown man standing on the fin.

 

[Don Ferrette]

Actually that almost sounds a little on the conservative side. I find myself replacing the turn fin on my twins twice a year as they lose strength & then won't hold the original shape. :blink:

Don

I'll bet your twin is closer to 12lbs than 6.

for a 12 lb boat its DOUBLE!!! (112lbs)

for a 18 lb boat its TRIPLE !!!! (168lbs)

That would explain it beat up fins.

I couldn't imagine hang 168 lbs off a fin and it staying on the boat. Thats a grown man standing on the fin.

Amazing what we put these boats thru ain't it??

 

[AndyBrown]

I would say the fin only carries about 60% of that force (maybe less). The side of the sponsons (front and rear) carry a good amount of the force. Also the rudder carries a big force. After the rudder initiates the turn the force of the water actually excerts a force against the outside of the rudder blade. This is evident by the slight bend/set in the rudder blade that can be seen after it has been in use for some time.

Also the strut blade carries some of the load.

And we can't forget aero, there's probably a few pounds there also ( it could be more than we think).

 

[Mark Bullard]

No wonder I have to use spring steel for my turn fin on my Twin.

Mark

 

[Russell Bear]

This is basic physics but I'm thinking there is more to this than I am considering.

A 6lb hydro is running 70mph down the straight then makes a perfect 35' raduis turn without losing speed.

the centripetal force required to keep the hydro on that arc is

F = M x (V^2/r)

F is the force in lbs

M is the mass in slugs (6lbs / gravity = mass) = .1867

V is the velocity (70MPH) = 102.66 feet/sec

R is the radius (35feet)

F = 56.2 lbs

Now assuming that the pivot point is the turn fin, and the turn fin is close to the CG of the rigger it should carry most of this force.

Does the fin really see that much force during a high speed turn? Anyone else tried hanging a 50lb weight from their turn fin? not pretty.

Where did 70 mph come from?

 

[Mike G]

What about a 12lb mono running at 40-50 through the turns? :blink:

 

[AndyBrown]

What about a 12lb mono running at 40-50 through the turns? :blink:

That water has an amazingly good grip on the boat.

 

[Mike G]

That's what I was thinking Andy, but it still makes the turn in the required radius. Backs up what you said about not all the force is on the turnfin.

 

[Dan_Cousin]

Even half this weight is a significant number.

We're talking 80 lbs for a 18lb twin blowing around the corner at 70MPH.

Thats a bag of cement hanging off the turn fin. It had better be stout.

12 lb boat is 56 lbs still a nontrivial number.

What about a 12lb mono running at 40-50 through the turns? :blink:

Its also about 56 lbs for a 50MPH pass.

Where did 70 mph come from?

Chasing after your dead scale boat. :lol: :lol: :lol:

How many Gs during the crash that followed?

 

[jeff baham]

chasing after russell dead boat , but the day i saw you, your feet was in the air

and your hand was all around

the next thing you know

your ass just hit the ground.

 

[Mike Hughes]

Ok now lets think about the fin when it is bending. Do you think it is bending parallel to the ride of the boat? I think not. Most rigger fins have the front bottom of the fin rounded off. That puts the most force at the back of the blade. You now are changing the angle as the force is applied. This will cause a hydrafoil affect. Have you ever wonder why the boat comes unhooked from the fin in the turns? With the fin bending out it causes lift in the right sponson hydrafoiling off the fin. BOTTOM LINE. Fins should not bend. We make them in a certain shapes for a reason. If the shape is changing then it is no good in the turns. Brackets must also not flex in the turns as this only make the problem worst.

Mike

 

[Russell Bear]

chasing after russell dead boat , but the day i saw you, your feet was in the air

and your hand was all around

the next thing you know

your ass just hit the ground.

Man, it only took two months for you to bring that up. I thought Dan was in the clear. Yea, he scored a "10" on that bit of gymnastics. :lol:

 

[HugoIvans]

I would say the fin only carries about 60% of that force (maybe less). The side of the sponsons (front and rear) carry a good amount of the force. Also the rudder carries a big force. After the rudder initiates the turn the force of the water actually excerts a force against the outside of the rudder blade. This is evident by the slight bend/set in the rudder blade that can be seen after it has been in use for some time.Also the strut blade carries some of the load.

And we can't forget aero, there's probably a few pounds there also ( it could be more than we think).

Fill in the number belov and I will try to calculate, drag water/air on the boat (or any boat) SAW. h34r:

Displacement lbs

length of planing surface: In

planing wetted surface area: SI

rudder length: In

rudder wetted surface area 1 side: SI

fin cord length: In

fin wetted surface area 1 side: SI

max cross sectional area of vessel: SI

angle of attack of planing surface:

Desktop racing :unsure: <_<

 

[t_hedlund]

Very interesting, what program are you going to use to figure this? I have always been very interested in the turn fin. i think a lot of the speed lost is on the turn fin> i have some ideas that i would like to test, out.

Tom

 

[HugoIvans]

I'm using the full version of racemate.net

 

[t_hedlund]

cool, i have to check it out, Thanks