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Author Topic: Racing Sparrow  (Read 4075 times)

AlanP

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Racing Sparrow
« on: October 28, 2010, 05:56:49 PM »

This is my first attempt at a yacht, I normally do scale boats, so a couple of questions to the people that know  :-))

Apparently this yacht has a tendency to nose dive, so do you think it would help if I moved the keel back, say 20mm.  Also I have read somewhere that you should incline the lead bulb up a couple of degrees, the drawing shows it horizontal.

Thoughts from you gentlemen would be appreciated   O0

Alan
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Netleyned

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Re: Racing Sparrow
« Reply #1 on: October 29, 2010, 04:35:26 PM »

My first attempt at a yacht also.
The book was delivered today.
The 650 version has a lot more volume forward
which will be better in a blow and should not
bury it's bow.
Download the 650 version and compare with the 750
I haven't made my mind up which one to build yet.

Ned
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Islander1951

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Re: Racing Sparrow
« Reply #2 on: October 29, 2010, 05:19:59 PM »

Alan,

Which one are you building?  As said, I believe that this makes a difference with this boat.
Remember that by moving the fin you are changing not only the cg, but the CLR as well, so to  get a well balanced boat the CE should be adjusted as well......
CLR=centre of lateral resistance #### CE=centre of effort.
  My advice would be to build it as per plan where all the math has been done for you, and if you don't like it, then making a second hull shouldn't take too long, or the first could even be 'adjusted'   %)
  For the bulb, yes two degrees up angle is a good plan, it makes for better downwind speed.   :-))

Good luck with the build,

                    Edward
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Little Rascal

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Re: Racing Sparrow
« Reply #3 on: October 29, 2010, 06:12:39 PM »

Hi Alan,

As already mentioned, moving the keel will change the relative position of the CLR and CE. Moving it back will increase 'lee helm' - the tendency for the boat to slink off to leeward and not want to tack especially in light winds. Of course moving the mast back also would compensate for this but the precise amount wouldn't be easy to guess. (And doing so further changes the CG!)

Moving the CG back will just cause the boat to 'trim by the stern' until the Centre of Buoyancy re-aligns with the new CG. This could cause drag if the transom is immersed.

If the boat wants to play submarines then it's really a fundamental design problem and changing the CG won't be a 'silver bullet solution'.

The boat actually pivots around it's Centre of Flotation. This is the centre of area of the waterplane and moves a little as the trim changes. The way the boat responds to heeling and to the nose down force of the sails when running is affected by the way the CF moves when trim is changed. This is really a question of hull shape - as netleyned said, the solution is often more volume forward. (Which incidently means moving CG forward not aft!) Rig aspect ratio also plays a part.

The CG change you suggest will have some effects but you'll probably find you end up with more negatives than positives if you start changing anything fundamental.

I would go for a straight build as per plan - then you can tweak things as neccesary.

All the best on the project!
Jon
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AlanP

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Re: Racing Sparrow
« Reply #4 on: October 30, 2010, 09:16:58 PM »

Thank you gentlemen for the replies, very informative  :-)) so I will build it as per plan but inclining the bulb up a couple of degrees and see how it goes. Going to tackle the lead bulb this coming week ( another first )

It's the 750 that I am building.

While I have your full attention  :} and looking ahead, can I ask what material I need and where to get it from to make the sails, cheapish would be good in case I muck it up.

Thanks again Alan
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Little Rascal

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Re: Racing Sparrow
« Reply #5 on: October 30, 2010, 11:55:25 PM »

Not sure about this particular design but a lot of smaller boats use drafting film for sails. Footy's even use plastic bags! Anything that won't stretch too much really...
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Netleyned

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Re: Racing Sparrow
« Reply #6 on: October 31, 2010, 06:58:32 AM »

The book gives sail details using drafting film


Ned
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Islander1951

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Re: Racing Sparrow
« Reply #7 on: October 31, 2010, 08:23:16 AM »

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AlanP

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Re: Racing Sparrow
« Reply #8 on: October 31, 2010, 09:28:34 PM »

Thanks to you all, thats the sort of site I was looking for Edward  :-))

I will reserect this thread if I have any queries during the build.

Thanks again Alan
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Islander1951

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Re: Racing Sparrow
« Reply #9 on: November 01, 2010, 09:54:07 PM »

My pleasure,

see also:    http://www.nylet.co.uk/sails.html

and :  http://www.sailsetc.com/

also have a good look here:  http://www.magicksails.com/secondpageb.html       for lots of useful info.

Edward.
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AlanP

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Re: Racing Sparrow
« Reply #10 on: November 21, 2010, 09:01:13 PM »

Didn't think it would be long before I had to reserect this thread   :embarrassed:

The boat is almost finished ( photo ) but I have a problem with the main sail, the book says in order to create an aerofoil shape the main sail is cut into three pieces, then taped together overlapping on the LEADING edge by a few millimetres.  A chap at the pond today who makes his own sails says that it should be the trailing edge (thats the leech I think) that should be overlapped.
Now I dint know what to do so some input from you gentlemen would be appreciated

Alan 
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boatmadman

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Re: Racing Sparrow
« Reply #11 on: November 21, 2010, 10:20:41 PM »

Alan,

Looking good  :-))

Googled tinterweb and found this bit of free software for you to play with - design your own sails!

http://sourceforge.net/projects/sailcut/files/

The profile of the aerofoil shape will be dictated by the amount of overlap at BOTH front and back of the sail, some useful info can be found here:

http://onemetre.net/index.htm

Try cutting some thin paper and sticking bits together in different ways to get a feel for how the shape is developed.

Ian
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boatmadman

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Re: Racing Sparrow
« Reply #12 on: November 21, 2010, 11:11:01 PM »

This site might help a little as well:

http://www.seawindrc.com/sail%20making/Sail%20Making.asp
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