Fin keel swap to shoal keel

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blueberry

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Jul 26, 2009
3
2 31 Repentigny
I have Irwin 31 feet 1982. When the boat was sold new , Irwin offer two keels, fin and shoal keels. To reduce draft, instead to put lead bulb, I would like to replace the entire fin keel by a shoal keel with the same mounting keel bolt. Is-it a good idea ?? Any body tried to do this on Irwin or other sailboat ?
Thank you
 
Ross

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
How good is your moment of inertia and strength of materials engineering? There are probably other changes that Irwin made that don't get talked about in the literature.
 
Rick D

Rick D

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Jun 14, 2008
7,204
Hunter Legend 40.5 Shoreline Marina Long Beach CA
Can't possibly be worth the expense and commitment. The boat isn't worth the effort. Lots easier to sell it and buy a shoal keel boat if it is that critical. Don't mean to be curt, but a rational examination of the cost and effort vs. resale and expected time of ownership should reveal a disproportionate investment.
 
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Steve Dion

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Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
A better option may be to have the existing keel cut off to the proper (you determine) length and have a couple of half torpedos made by Mars Metals to bolt to your existing keel.
 
Hermit Scott

Hermit Scott

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Sep 25, 2008
2,288
C30 Event Horizon Port Aransas
Steve Dion said:
A better option may be to have the existing keel cut off to the proper (you determine) length and have a couple of half torpedos made by Mars Metals to bolt to your existing keel.
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I'll do anything when it comes to modifying things, to the point where people think I am crazy. But I wouldn't try cutting off the keel to add some bulbs unless I was copying a job that someone already did on a boat like mine.
You can play around with some static moment equations and maybe get the righting moment clost but there is an aerodynamic factor to a keel that would be more difficult to get right. Unless you can put your hands on a boat like yours that it's been done to, you risk makning you boat not want to point well into the wind.
 
Ross

Ross

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Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Hermit Scott said:
I'll do anything when it comes to modifying things, to the point where people think I am crazy. But I wouldn't try cutting off the keel to add some bulbs unless I was copying a job that someone already did on a boat like mine.
You can play around with some static moment equations and maybe get the righting moment clost but there is an aerodynamic factor to a keel that would be more difficult to get right. Unless you can put your hands on a boat like yours that it's been done to, you risk makning you boat not want to point well into the wind.
Click to expand
Hermit,
It's gotta be a hydrodynamic factor not aerodyanmic factor on a keel. ;)
 
RichH

RichH

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Feb 14, 2005
4,773
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Blueberry -
Mars Metal can provide you with a new shoal draft keel or provide you with bolt-on 'bulbs' that you can apply to a keel that has been shortened/cut down. They have the calcs. to keep the righting moments equal to the OEM design. This is not an inexpensive project. http://www.marskeel.com/

The most commonly used sailboat keel shape is an aerodynamic design: NACA010-xxxx which is an airfoil shape designed by NACA the predecessor of NASA
 
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