Any fellow hydro design enthusiasts out there?

[RFR_Aero]

The dimples on the golf ball reduce the total drag and allows the ball to fly farther. The drag is reduced due to the turbulence created by the dimples that reduces the size of dead air (wake) behind the ball. Dimple designers shoot for the place on the drag (Cd) vs velocity (Reynolds #) curve where the Cd is the least. Note the plot in this article where the curves dip to a low point.

http://www.grc.nasa.gov/WWW/K-12/air...ragsphere.html

Recreating this reduced drag effect with dimples on a flat surface draged over water at an angle of attack is a totally different dynamic and I do not know of any research of dimpled surfaces on a flat surface at an angle of attach to the water. But some amount of roughness on the bottom would prove benificial as I have noticed on the hydro I ran. Much trial and error and time would be required to determine the results of all the variables.

Zul8tr,
You have the lions share of this correct, the dimples do energize the boundary layer and allow the wake to close further than it would without the dimples. They also produce a stable separation point (the point at which the flow separates from the local surface) allowing the ball to fly straighter and more predictabily. But when it comes to hyroplane aerodynamics, I dont think dimples (or anything that manipulates the boundary layer) is going to work, particulary on the bottom.

Think of riding in a car, sitting on the passenger side and holding a bag out the window allowing the air to "inflate" bag. Once inflation is complete, there isn't much flow inside the bag, because the flow has stagnated and the pressure keeps the bag inflated. Now cut a few small slits in the bag and you do get a small amount of low speed air going into and out of the bag. This is kind of what happens under a hydro when it is aired out. It is the stagnation pressure (pressure from slowing the local flow to near zero velocity) that keeps the boat aired out. While I would agree that there is a limited amout of airflow under the boat. It is not as much as most people think. The flow under the bottom is limited and therefore low speed. If you combine this with the hydrodynamic surface tension effect of the water, you dont have much of a boundary layer to work with. And I have tested a hydroplane (scale BeZoat) in a wind tunnel.

[Sonny]

What do you mean by a greedy engine set up?
I assume you mean more "radical", but what would be an example of that?

I'm sorry, by greedy I mean radical. to high on the prop shaft and to far kicked out. With the mods there is no height rule and as most know, the higher you go with the propshaft the faster you go. BUT!! everything at a price. higher means faster down the straightaway, lower means faster through the corner.

[B Walker]

I had no idea, Use your body weight to pull it out of the water?
Can you tell me about that?

You want enough fin to pull you through the corner without breaking loose and skipping out but at the same time, not one that pulls the boat down all the way to the water and buries a lot the outside sponson (drag).

This is as much driving technique as fin design, but without the right fin, you cant learn the technique.

Here is the idea:

Because 2 stroke motors have a very narrow powerband you want to carry as many RPM into and through the turn as possible. The turn fin is a critical part of achieving this goal.

As you set up for the turn, you shift your weight forward to engage the fin about 50%. Then you add wheel to get the wanted line into and through the turn using your body weight to get more fin into the water and therefore turn harder or weight back to disengage the fin a little and slide out of the turn keeping the RPM up.

It takes a lot of boat time to get the feel of it.

If done correctly it all feels very very smooth.

Hope this helps

BW

[mdaspit]

As you set up for the turn, you shift your weight forward to engage about 50%. Then you add wheel to get the wanted line into and through the turn using your body weight to get more fin into the water and therefore turn harder or weight back to disengage the fin a little and slide out of the turn keeping the RPM up.

It takes a lot of boat time to get the feel of it.

If done correctly it all feels very very smooth.

Hope this helps

BW

Ya, I get that... So, how much fin do you have in the water in the straights?

[B Walker]

Ya, I get that... So, how much fin do you have in the water in the straights?

As little as possible. None ideally

If your fin drags a lot in the straights, try a fin that is shorter, but wider at the bottom.

Half an inch in either fin length or depth can be a dramatic change. Even a change as small as 1/4 an inch can be very noticeable.

BW

[csh2z]

I just try to make them look cool.

[blueskyracer]

I just try to make them look cool.

Thats what I do too John. If you are not fast look good doing it!!

[B Walker]

Thats what I do too John. If you are not fast look good doing it!!

The ugliest boat on the beach always looks great when it is out front

BW

[csh2z]

Ugly has it's moments too!

[mdaspit]

As little as possible. None ideally

If your fin drags a lot in the straights, try a fin that is shorter, but wider

BW

Why does length have anything to do with width?

[B Walker]

Why does length have anything to do with width?

It helps to think of it in terms of square inches of immersed fin where immersed fin is all the fin below the bottom of the sponson measured in square inches.

Example: If a square shaped fin is 4 inches deep and 3 inches wide (12 square inches) it will have the same maximum pulling or holding power in a turn as a fin that is also square and 3 inches deep but 4 inches wide (also 12 square inches).

And of course the shorter fin will drag less in the straightaway.

I used a square fin as an example to make the math easy. I dont like square fins as they tend to yank the boat down or not grab at all. You get all of it or none of it, typically. Others disagree.

Make sure the fin is perfectly in line with the centerline of the boat. I cannot overemphasize this. A laser level is a good tool to use to measure this.

BW

[sam]

Because area may be nearly as important as length. If you are too deep, shortening the fin and adding length may be an acceptable compromise.

[ZUL8TR]

Turning fin

It helps to think of it in terms of square inches of immersed fin where immersed fin is all the fin below the bottom of the sponson measured in square inches.

Example: If a square shaped fin is 4 inches deep and 3 inches wide (12 square inches) it will have the same maximum pulling or holding power in a turn as a fin that is also square and 3 inches deep but 4 inches wide (also 12 square inches).

And of course the shorter fin will drag less in the straightaway.

I used a square fin as an example to make the math easy. I dont like square fins as they tend to yank the boat down or not grab at all. You get all of it or none of it, typically. Others disagree.

Make sure the fin is perfectly in line with the centerline of the boat. I cannot overemphasize this. A laser level is a good tool to use to measure this.

BW

Although the correct alignment of the fin is needed in a turn under water load the fin will bend and depending on the shape and distance from the securing bolt center will determine how much the fin deflects inward toward the center of the turn. Really thick fins will bent the least but all will bend somewhat and be different than the original static alignment set. That is why testing is needed to get the fin set correct in the turn.

[B Walker]

Although the correct alignment of the fin is needed in a turn under water load the fin will bend and depending on the shape and distance from the securing bolt center will determine how much the fin deflects inward toward the center of the turn. Really thick fins will bent the least but all will bend somewhat and be different than the original static alignment set. That is why testing is needed to get the fin set correct in the turn.

Good points.

I have had good results with mounting the fin bracket with a 1 degree to 3 degree off 90 degree alignment to the inside when I mounted it to compensate for the bend under load.

BW

[blueskyracer]

I thought this thread was about Design and not set up. Seems Dave M had a post on Boat Set Up a few weeks ago.