SoCal Jet Boats
Tech Talk => Hulls => Topic started by: beerjet on November 15, 2008, 10:30:27 PM
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I see a few boats that run the shark fin style . Tom Papp told Jim Rich to run that shape on his Sanger , I've also seen just about every other Sanger flat with the same size and style . I've seen most of the other flats run a more squared off type . Is it just trial and error on the fin type or a perticular hull requires a perticular style fin .
-beerjet-
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Last time i saw Jims boat run it didnt turn, my Sanger has the squared off fin.
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Maybe Tom didnt like Jim very much.
I'm not sure what type of fin is under my old flatbottom, but I will look and get a pic for you beerjet.
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I'm thinkin flats need the surface area of the bigger square fin, shit they still slide
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Yeah , your right Terry . Jim did swap it out for his squared off "Ginsu" as he called it . I'm guessing since more and more boats are being updated with the squared ones there is no real direct relation between fin shape and hull .
My theory is that the shark fins type being the earliest designed to have the water follow the trailing edge or tip and not disrupt water flow . Try to make it as efficent through the water as possible
(https://www.socaljetboats.com/proxy.php?request=http%3A%2F%2Fimg.photobucket.com%2Falbums%2Fv210%2Fflyin-t%2Fkentsantos1.jpg&hash=d3e3311c0e566e4e3055dcd9a9e3f0eafd544708)
The traditional squared/trapazoid shaped ones like mine , from what I've gathered are a knock off the outboard guys . I really think you'd be hard pressed to find a better group of guys to copy a fin from than them . I dont know specifics on leading and trailing edge angles but I'm sure a whacker guy will tell you .
(https://www.socaljetboats.com/cpg/albums/userpics/10034/Fininstall019.jpg)
Ultimately , I think the sacrafice in speed to surface area is minimal and now we're seeing the big rectangle shaped ones with only the leading edge angled . Of course the most surface area for more bite and even doubles on the racing flats .
Of course my theory's are based on conversations I've sat in on with a few folks and forum descusions as well .
-beerjet-
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Beerjet, here's the ones under my old flatbottom. Didn't even know it had 2 until I crawled under there today :-[ .
[attachimg=1]
And the one under the Daytona, and no I don't know why it has one. I haven't seen one under any others. :-\
[attachimg=2]
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Yup ,thats them big ol' meat cleavers . Rex even sells them as double drop-thoughs . I think the extra bite is worth whatever speed they scrub off .
(https://www.socaljetboats.com/index.php?action=dlattach;topic=5405.0;attach=20135;image)
-beerjet-
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OK another old story. Once we were trying fin shapes, originally for nozzel fins I think, and Bennett worked like a devil to make the ultimate aero foil shaped fin out of 7075 T-6 aluminum he could. When we ran it the foil shape was off just a bit and caused so much lift to one side it folded up 90 degrees to vertical and caused the boat to flip. Since then we always used Steel and a simple chizzed leading edge that caused the water pressure to break evenly to each side of the foil. Instead of thinking "cessna" think "X-15" The renolds number between water @ 100mph is much closer to the renolds number of water at high mach numbers. Long story short Meat clevers are just fine. One larger fin vs. two smaller ones I have no clue. My guess is one fin smaller in area but a bit deeper than the twin fin would work best without a drag penalty. But if anyone wants to do some real tests let us all know.
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Hay propless, What Daytona is that one. The only Daytonas I know that had fins are the ones Millet and I circle raced with but we put those in the center pod. Mine you could change for and aft about two feet. A bit larger than the one you show. Is that one on the left side only and do you know if it pulled the boat to the left? It looks like it might work like a turning skag on a APBA hydro
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Its my 21' (outboard), and yea its on the left side only with the trailing edge 38" from the transom. I don't know if it makes the boat pull to the left or not (have had it in the water yet), but I think it would. Maybe, the OB had a paddle wheel effect at speed and that helped ??? ???.
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FINS? I thought I got rid of fins when I sold my last sailboat!