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Author Topic: Turn Fins?  (Read 3313 times)

john54

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Turn Fins?
« on: January 06, 2009, 03:56:19 PM »

Hi All
Can anybody advise on the ideal (if poss) position for turn fins (a pic will do) Shes a hor 25 from astec running a surface drive 3000k.v.
She slides on turns at speed makeing for a lot of prop slip {:-{ have been told 90 degrees from deadrise angle ?
Cheers
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andyn

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Re: Turn Fins?
« Reply #1 on: January 06, 2009, 05:13:09 PM »

Parellel to the sides of the boat, with the fins themselves as close to the sides of the boat as possible, without overhangin the bracket.

Andy :-))
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ids987

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Re: Turn Fins?
« Reply #2 on: January 07, 2009, 01:51:08 PM »

Sorry Andy, gonna have to disagree with you on the angle - especially if it's a Deep-Vee design. I'd go with 90 degrees to the deadrise. At that angle, normally, the end of the turn fin should be just above the keel line (when the hull is flat horizontally). That way, it is clear (or nearly clear) of the water when the boat is going flat out in a straight line, and enters the water when you turn - and the boat leans. As the boat leans, the turn fin will rotate towards a vertical position. If the turn fin starts off vertical, it will rotate underneath the boat.

Ian
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john54

  • Guest
Re: Turn Fins?
« Reply #3 on: January 07, 2009, 02:47:33 PM »

So when she is fully on the plane the fins should be just in or just touching the water and when she starts to bank over in the turn the fin should dig in and help her track round?
My main concern is that she might trip over herself and Flip over :(( Being She Is Not Fitted with a flood chamber This Might Be Putting It Mildly a Bit of a Nuisance!
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ids987

  • Guest
Re: Turn Fins?
« Reply #4 on: January 07, 2009, 06:33:47 PM »

In the great world of theory, the turn fin doesn't touch the water when you're going straight ahead. The idea being (as you pretty much said) that it digs in as you turn, and acts as a pivot (the boat basically pivots around the turn fin).
In my minds eye, I would have thought that; if the turn fin is at 90 degrees to the dead rise - and therefore shouldn't go beyond the vertical, the forces acting on the turn fin should try and push the boat level as you straighten the rudder. To look at it another way, as you start to turn, and the turn fin starts to touch the water, the lift created by the turn fin touching the water will try to level the transom, as well as pushing the nose down.
Not sure whether this helps - mostly for consideration and comments.........

Ian
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ids987

  • Guest
Re: Turn Fins?
« Reply #5 on: January 08, 2009, 01:01:47 PM »

I cut down my last post - to avoid trying to rewrite War and Peace. Also, there are a lot of interdependencies between the rudder and turn fin (length, blade area and placement of rudder and turn fin), which are both acting in more than one dimension - as well as factors in the hull design, speed, torque reaction, prop lift, trim tabs if present etc etc. I'm definitely not an expert on any of this either, so it's difficult to give hard and fast answers. I meant to be a little bit ambiguous, as I didn't want to give something as statement of fact without being certain myself. What I meant to allude to was that I believe (in most cases at least), that adding a turn fin should reduce the tendency to flip - rather than increase it. To some extent though, it may depend on the balance of the hull itself (in all dimensions), as well as the interactions between rudder, turn fin, torque, and others. The way I see it, some of the highlights are:

1) When you turn right, the rudder will start to turn the boat. The rudder will generate a sideways force, and also a rotational force (caused by the rudder acting as a lever) - rotating the hull, and the rudder clockwise (as viewed from behind the transom - this effect having a large dependency on the length and area of the rudder, as well as the rudder offset, and torque). The rotational effect on a right turn is usually in the same direction as torque reaction - which increases the effect. The rudder - just by it's presence in the water, will generate drag and lift. The drag and lift will increase when you turn the rudder. As the boat leans into the turn, and the rudder becomes non-vertical, it also starts to act as a trim tab. The more the boat leans, the more the rudder acts as a trim tab, and the less it acts as a rudder. The rudder as a turn fin effect has been written about by John Finch. He has used angled rudders to enhance this effect, and remove the need for separate trim tabs.

2) As the boat leans into the turn, the turn fin enters the water. The lift from the turn fin should help to couteract the rotational effect of the rudder. The force from the rudder is now trying to drive the transom sideways, and the turn fin is acting as a brake in the sideways direction - creating the effect where the boat pivots  on the turn fin.

3) As the rudder straightens out, the transom is still trying to slip sideways. The resultant rotational forces from the turn fin and rudder will help to right the boat.

From these visualisations, I believe that the turn fin will reduce a tendency to flip side over side as the boat leans into a turn, but there are effects in other dimensions. Especially lift (from the rudder and turn fin), and diagonal lift (from the rudder), at the transom, possibly causing the keel to "dig in" at the deepest point (maybe causing a rising spin, or an end over end flip - transom going up and over diagonally), or the possibility of a side over side flip in the opposite direction as you straighten up - especially if you overcook the turn, and need to correct. This should be lessened because you will come out of the turn faster than you go into it, and this is also (usually at least) in opposition to torque effect. This counterclockwise rotational force as you straighten up though, is presumably increased by the addition of a turn fin.

Please take these as thoughts and observations - designed to invoke thoughts, comments, additions - if anyone's interested enough (I might learn something that way).

Ian
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omra85

  • Guest
Re: Turn Fins?
« Reply #6 on: January 08, 2009, 02:41:45 PM »

War and Peace indeed Ian - but some very good points well put!
Something I noticed from the video of my Magnum crash was that before the crash, every corner was being taken virtually flat. Now this surprised me as I expected to see some lean into the corner. I didn't have a turn fin on that boat, even though it is a very big HIGH boat which (theoretically) would make it lean more. I think I will be putting a vertical fin on the inside corner just to see what happens.
I've also modified the spray rails under the hull which have a significant effect on turning and sliding, but lets not get into the heady world of hull modifications and "wedges" yet .....   %% %%
Cheers
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john54

  • Guest
Re: Turn Fins?
« Reply #7 on: January 08, 2009, 03:01:13 PM »

Thank you for some very comprehensive advice and observations I am very grateful.  I think I got the gist of it now :-)) I do not use her from racing only fun and recreation. Basically all I Wanna do is make her turn better and quicker ;)  I think now I shall experiment with with fin depth etc until I am happy with her.

Again thanks to all who contributed to this thread

Regards

John
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ids987

  • Guest
Re: Turn Fins?
« Reply #8 on: January 08, 2009, 06:24:27 PM »

Good luck John, experimentation is the key to frustration learning.

Thanks Danny for those kind words.
Someone who had quite a lot of success with Magnums (before he and his brother went surface drive), reckoned that a fin behind the rudder (with the normal Magnum rudder placement), made a big improvement to the handling. I guess an inside turn fin would be the nearest equivalent for your outboard rudder setup. Interesting to see how yours works out - it seemed to work for him
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omra85

  • Guest
Re: Turn Fins?
« Reply #9 on: January 08, 2009, 07:21:08 PM »

Hi Ian
In fact the great man himself (DM) started putting a small vertical plate set into the hull behind the submerged rudder on his multi boats years ago just to stop spin-outs - and it works - usually!
Mine is (as usual) a 'belt and braces' solution - hopefully  %)

Danny
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tmbc

  • Guest
Re: Turn Fins?
« Reply #10 on: January 13, 2009, 09:06:29 PM »

dont get involved on here much but ive built a fair few boats ! and turnfins should be roughly 90degrees !
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ids987

  • Guest
Re: Turn Fins?
« Reply #11 on: January 19, 2009, 04:49:40 PM »

Hi Danny (if you're still watching this thread), I may be completely off base here, but wouldn't the "body roll" element try and make the boat lean out of the turn - opposing the lean into the turn. I'm thinking of inertia and centrifugal force, but as an example, in much the same way as a car cornering at speed. If so, could that be why the high sided Magnum was cornering almost flat (as opposed to a lower sided boat with less body roll element leaning into the turn) ?

Ian
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omra85

  • Guest
Re: Turn Fins?
« Reply #12 on: January 19, 2009, 10:39:18 PM »

Hi Ian
It could well be the case as I was expecting to see the deck tilting to the left in the video. I assume it may be something to do with the angle of the V, even though the high sides must raise the C of G which would make it lean left (in a right hand corner). The slight right hand lean was fairly consistent on each corner - so it wasn't the wind.  It could be torque generated ...... but then, I had put a slight 'kicker' wedge on the front right ......   {:-{
No - I've no idea  :embarrassed: :embarrassed:

Just have top have fun racing it and trying to keep upright!

Danny

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andyn

  • Guest
Re: Turn Fins?
« Reply #13 on: January 19, 2009, 10:44:44 PM »

A bigger engine will sort it  ;)
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omra85

  • Guest
Re: Turn Fins?
« Reply #14 on: January 19, 2009, 10:57:30 PM »

Andy - it just so happens ........    :-X

Danny
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