45 Hydro in a turn

[Andy Brown]

The face of the fin must have a certain degree of Angle of Attack in relation to the direction the boat is traveling in order for the boat to turn.

Meaning all the pressure is on the inside of the fin.

I do believe the AOA is changing at a high frequency creating more load, less load as evidenced by the "stripes" in the wall of water.

So what is causing the AoA change? Is high frequency, micro "fish tailing" at the transom, possibly caused by the prop forces on every revolution?

Or Fin flexing? Or something else?

Brett "Ace" Ward's Twin turns phenomenally! Maybe Ian has gotten photos of it in the past?

 

[Tom Foley]

fellas i think you are over thinking this ,I personally think its a straight line trying to do a curved radius , if you wanted to stop this simply make your turn fin curved lol an then see it happen when you try to drive the boat in a straight line , the boat is turning a perfect curve an is angling the straight line in perfect jumps ,pulses what ever you want to call it . draw a perfect curved radius an then get a straight line an try driving it around the curve what do you have to do to get it round , another thing to me is the side pressure is hitting that turn fin perfectly in the middle for that to happen my 2cents . Cheers Ace

So ...you have never driven a hydro of any kind i take it !! LOL

 

[Ian Inverarity]

The face of the fin must have a certain degree of Angle of Attack in relation to the direction the boat is traveling in order for the boat to turn.

Meaning all the pressure is on the inside of the fin.

I do believe the AOA is changing at a high frequency creating more load, less load as evidenced by the "stripes" in the wall of water.

So what is causing the AoA change? Is high frequency, micro "fish tailing" at the transom, possibly caused by the prop forces on every revolution?

Or Fin flexing? Or something else?

Brett "Ace" Ward's Twin turns phenomenally! Maybe Ian has gotten photos of it in the past?

Andy, I wonder if it is just the flow on the outside of the fin that is unstable rather than the fin AOA changing? I would think the vibration from that would rip a boat apart!

I will check if I got any pics of Brett's twin turning, but I doubt it as I usually pit for him if we are at the same race!

 

[Andy Brown]

The face of the fin must have a certain degree of Angle of Attack in relation to the direction the boat is traveling in order for the boat to turn.

Meaning all the pressure is on the inside of the fin.

I do believe the AOA is changing at a high frequency creating more load, less load as evidenced by the "stripes" in the wall of water.

So what is causing the AoA change? Is high frequency, micro "fish tailing" at the transom, possibly caused by the prop forces on every revolution?

Or Fin flexing? Or something else?

Brett "Ace" Ward's Twin turns phenomenally! Maybe Ian has gotten photos of it in the past?

Andy, I wonder if it is just the flow on the outside of the fin that is unstable rather than the fin AOA changing? I would think the vibration from that would rip a boat apart!

I will check if I got any pics of Brett's twin turning, but I doubt it as I usually pit for him if we are at the same race!

That is a good point Ian. I am curios to see the back side of Aron's fin. We sharpen the fin with a straight angle that breaks clean. Some guys polish and/or round off the sharp break which does alter (negatively) the performance of the Eagle SGX fin.

 

[El Zorro]

Very interesting topic here. I have to think that when the boat is going around the corner the turn fin is "loaded", meaning that there is a lot of pressure on the inside of the fin. This water is exiting at the rear of the fin at the water line. While this is going on there would seem to be a "low pressure" area on the outside of the fin and water is also coming off of the fin at the leading edge. This could explain the pattern of the spray coming off of the fin in the pictures. Keep in mind when the boat is going around the corner, the rudder is in line with the arc of the corner..........the turn fin IS NOT. If you know anything about aerodynamics picture the profile of an airfoil and the way the air flows around it. Tilt the airfoil in a positive attack angle and a low pressure area is formed on the top of the airfoil, which is where the lift comes from. The air passing over the wing does not come back together until it is PAST the trailing edge of the wing. I think this is where we get the "twin roostertails" coming off of the turn fins on our hydros. No vibrations, no harmonics ( although the flexing booms could be causing other problems ) no smoke and mirrors, just simple hydrodynamics. At least that's my story and I'm sticking to it.

Dick Tyndall

 

[Aron Alleva]

This is how all of us Alaska guys go fast in the north. Little known Eskimo speed secret back when we ran our boats off whale oil in 30 below.

I've been busy these last few days, I'll be in the shop tonight working on my boats. I'll grab some pictures of the turn fin.

I haven't altered my turn fin in any way (bending, sharpening, polishing) since I got the boat from Jeremy late last season. There is a video of Charlie running my boat on my youtube channel that I will also post for you guys to analyze.

 

[Elias Vazquez]

This is how all of us Alaska guys go fast in the north. Little known Eskimo speed secret back when we ran our boats off whale oil in 30 below.

I've been busy these last few days, I'll be in the shop tonight working on my boats. I'll grab some pictures of the turn fin.

I haven't altered my turn fin in any way (bending, sharpening, polishing) since I got the boat from Jeremy late last season. There is a video of Charlie running my boat on my youtube channel that I will also post for you guys to analyze.

Good running boat!

 

[Aron Alleva]

Thanks Elias, I wish it ran down at the race like that but hey it was a learning experience and I had a great time with everyone down there.

 

[Joe Petro]

I think Mr. Tyndall is on to something.

Looks like airflow separation.

The inside is pushing a lot of water, causing an air bubble (cavitation) on the outside of the fin.
I think the bubble is collapsing, causing the waves. This can happen very quickly.

The frequency may be based on speed and load (push).

A heavier boat going about the same speed should have a lower frequency wave.

Just my opinion...

YMMV...

Joe

 

[bermudadude]

i thought of 3 really simple answers(prob overly so) [ ive been purposely trying to oversimplify things to try to get a handle on my o.c.d ]

the water streaming up the inside of the fin is disturbed by the bolt heads and/or the fin terminating before the top of the sponson.

horizontal oscillation - this would b high frequency and also may work like a drain/toilet and only manifest in one hemisphere ,combine that with the prop rotation direction and prop blade orientation in relation to the piston position. -would explain why only rarely seen. [ok that one was deep but it jus evolved there,i didnt mean for it to go that way lol]

the ignition/explosion or “hit” from the pipe/pipe resonance may be transmitting through the hull or air under certain conditions.

ok,,,,,maybe i did go a little deep back there somewhere.

 

[Aron Alleva]

Let me know what else you guys need. Hope this helps.

Aron

 

[Ian Inverarity]

This is how all of us Alaska guys go fast in the north. Little known Eskimo speed secret back when we ran our boats off whale oil in 30 below.

I've been busy these last few days, I'll be in the shop tonight working on my boats. I'll grab some pictures of the turn fin.

I haven't altered my turn fin in any way (bending, sharpening, polishing) since I got the boat from Jeremy late last season. There is a video of Charlie running my boat on my youtube channel that I will also post for you guys to analyze.

Looks good Aron!! Seems to turn just fine too!

 

[JamesShields]

I have been following this thread and wanted to post a picture Ian took of My 67 Road Runner running with stock Road Runner turn fin and aluminum boom tubes. I see little or no flexing in the tubes.

 

[Ian Inverarity]

I have been following this thread and wanted to post a picture Ian took of My 67 Road Runner running with stock Road Runner turn fin and aluminum boom tubes. I see little or no flexing in the tubes.

James, certainly your boats looks rigid in the picture! The picture also shows the AOA that Andy Brown mentioned (also called slip angle) as the difference between the direction that the boat (including fin) is pointed and the direction it is travelling seen by the path of the wakes.

 

[Tim van Schyndel]

Some designers of sailboarding fins years ago used slots in the fins to reduce cavitation and therefore in essence allowing water to pass through the fin and fill the void.

The second fin was commonly called the footy fin had a majority of the fin area at the bottom. My understanding of the concept was that the horizontal battens acted as dams and therefor reducing the amount of air that could pass down the back of the thin part of the fin to the area of most area and load at the bottom and deeper in the water.

 

[Tim van Schyndel]

Sorry - the ideas were obviously to reduce cavitation and allow the fins to hold the water under greater load and speed at the same time allowing the fin to cut cleaner through the water.

It would be interesting to see the effect of a slot in one of our turn fins and whether that would remove or reduce the phenomenon?

The below are the current designs. Long and thin. Remember that wind surfers in speed competitions are now hitting the 60mph.

Very interesting thread!

 

[El Zorro]

I think we are talking about different fins for completely different purposes here. Not that I know anything about wind surfing ( I have seen them in action ) but their fins are designed for going in a straight line. I can understand how the long ( deep ), narrow fin you pictured would be helpful on such a board. I am guessing that the depth of the fin would add to the stability of the board, while with the fin being narrow would decrease the drag somewhat. About the question of trying a turn fin with a slot in the middle of it on an R/C hydro; I don't think I would want to try that. My guess is that it would cause extra drag going down the straightaway and REALLY be a disaster when you went to go around the turn. Maybe some small holes in different places on the fin? New ideas are always worth trying.

Dick Tyndall

 

[Tim van Schyndel]

Thanks Dick

I appreciate that the applications are different but the hydrodynamic concepts between the two are the same. Essentially our turn fins are hydrofoils just like fins on a windsurfer or keel on a yacht.

The only difference is that the keel/fin on a yacht/windsurfer are converting a perpendicular force to forward movement whereas our rigger turn fins are converting forward or parallel force to perpendicular movement. No Keel/fin on a yacht, you would generally go sideways. No turnfin on a rigger then generally you would only go strait. Apply some rudder and the turnfin gains angle of attack causing the front of the rigger to go right and allow the centre hull to pivot (turnfin Aoa also counters any centrifugal force generated by the turn)

Naval architects have determined that long and thin is more efficient than square or short and fat.... no pun intended

...... and this concept is now being demonstrated (as per below images) in many facets of boat racing. Note that sailing craft are far more reliant on hydrofoil technology than powerboats.

Look at the foil on the current world sail speed record older (sailrocket). That small thin foil is attempting to convert the perpendicular power generated by the aerofoil into forward movement and lift. There are some massive aerodynamic forces being converted by a very small hydrodynamic foil in ratios far greater than I believe our model boats are experiencing.

I have merely suggested that if it is true that the phenomenon being witnessed in this thread is caused by a cyclic form of cavitation on the back of the foil (turnfin), then possibly our foil designs are inefficient and maybe we can take some learnings from some modern multi million dollar concepts that are just as fast as our model boats.

I know the photos aren't sexy as they are of "sail boats" but I still feel there is much that can be learnt.

I'm going to give it a go and experiment.

Cheers

 

[Tim van Schyndel]

Sorry but the images have not uploaded. I am trying again??

Here are the links just incase it doesn't work again and you are interested.

http://www.sfgate.com/bayarea/article/America-s-Cup-catamarans-built-for-speed-4504387.php

http://www.yacht.de/sport/news/60-knoten-mit-segelrakete-nr-2/a46967/fotostrecke/1200863/1198901.html

http://gearpatrol.com/2012/11/28/design-spotlight-the-sailrocket-2/

 

[dwilfong]

there is a trade of in the long fin and short fin. a short fin has less frontal area for drag in the straits but needs to be wider to achieve the same forces as in pressure area when AOA is changed in the corners. Along fin will add more drag in the straits but it will have a shorter foot print front to back this will cut down on turbulence making the same area more efficient

Then there is the flex of a long thin fin? good thing or bad?

Then there is no fin in the water at all..........just a modified sponson................

Sorry can't go there with ya......have to sign a release form first................LOL