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Author Topic: Shortening a fin / keel  (Read 2299 times)

g4yvm

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Shortening a fin / keel
« on: August 21, 2013, 07:57:25 PM »

The Dragon force I just ordered has the now usual  long fin to keep her upright.   Someone on this forum has stated that he shortened it by 5 inches, and presumably though unstated, compensated by adding lead pro rata.


In a model yacht does chopping the fin down change the handling much? (assuming the righting moment is all kept constant).


David
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tiaki

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Re: Shortening a fin / keel
« Reply #1 on: August 21, 2013, 08:11:08 PM »

There is a huge change in lopping off that amount of  keel. The yacht will become tender; that is wind will effect it greatly unless a smaller sail plan is adopted. Generally weight can not be added as this will change the water line and slow the speed response of the yacht. It takes more wind to get moving but will drift further when the wind drops. You also need to consider the amount of surface area of the keel which stops the yacht drifting sideways and replace this with a broader keel to compensate.
Cheers
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g4yvm

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Re: Shortening a fin / keel
« Reply #2 on: August 21, 2013, 08:26:15 PM »

Thanks

Ill leave it as designed then!

D
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knoby

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Re: Shortening a fin / keel
« Reply #3 on: August 21, 2013, 08:28:51 PM »

Standard Df keel is 385mm, but DF are now selling a short keel  of 240mm as a spare part.It uses the same bulb as the standard keel. From what I read, they are using this short keel with the A rig in light winds to reduce drag & increase speed. They cost around 7, so might be worth buying 1 to compare with standard keel & if its dosent work you still have your standard keel in 1 piece.
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tiaki

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Re: Shortening a fin / keel
« Reply #4 on: August 21, 2013, 09:54:16 PM »

In using a short keel with the standard A rig one can only surmise the long keel version is under rigged! Once a yacht is optimised then with any change there is a trade off or compromise. Generally manufactured kit boats are under sailed to make them more manageable in a wider wind slot. Manufacturers don't want negative feed back on their products which would take the edge off the sales. The reality is better and faster yachts can be hand crafted cheaper if you put the effort in.
Cheers
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mrpenguin

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Re: Shortening a fin / keel
« Reply #5 on: August 22, 2013, 12:44:34 AM »

I would be cautious of shortening the keel - as suggested, if you need to reduce the draft (for a shallow sailing spot for example) then I would suggest you should get the standard short keel. That way you have the longer keel for when you have the chance to sail deeper water.

With the shorter keel with same ballast weight, the boat will be more tender (i.e. heel more easily) - this not an issue in light conditions but sure is as soon as the wind comes up a bit. Adding more weight to the keel will drag  the hull down in the water and make the boat sluggish. If you wanted to experiment with this, try taping a 100 gram fishing sinker on the bulb - the effect is VERY noticeable even if you streamline it...

Most of the performance of the DF comes from its light, deep keel design. A 385mm keel is typical for a much larger boat (eg 900mm to 100mm)
 
 
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slug

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Re: Shortening a fin / keel
« Reply #6 on: August 23, 2013, 06:31:09 PM »

I cut off 5 inches after a friend in the club cut his,our lake is only12-14-at the edge.it still sails very well-hasn't made a lot of difference.slug
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georgo

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Re: Shortening a fin / keel
« Reply #7 on: February 21, 2014, 11:48:28 PM »

  :-))  nice to know, guess its the only way to find out...
I cut off 5 inches after a friend in the club cut his,our lake is only12-14-at the edge.it still sails very well-hasn't made a lot of difference.slug
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rickles23

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Re: Shortening a fin / keel
« Reply #8 on: February 22, 2014, 07:54:59 AM »

Hi,
To quote the Late and Great Ben Lexon:
"If a keel is designed for a boat don't muck around with it".
Regards
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tiaki

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Re: Shortening a fin / keel
« Reply #9 on: February 22, 2014, 07:13:50 PM »

Having just purchased a second hand DF. I would say cutting the keel shorter will make the yacht tender. Standard the yacht sails very well with a good turn of speed. Nothing but shallow water would induce me to shorten the keel and then I would be tempted to compensate with down sizing the rig. I have siliconed both of the keel and rudder joints to give a be laminar flow. I discovered the keel is painted without under coat so have addressed that and repainted it. As I'm the only one locally with a DF I have changed the way she is rigged to a simpler setup which makes setup a doddle. I have epoxied in all the attachment points and have a dry yacht when sailed. Last outing I was giving a Fairwind a chase up.
Cheers
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