Freedom 21 Lifting Rudder Design

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Wilfred934
Posts: 19
Joined: Sat Dec 13, 2008 4:52 pm
Location: North Yorkshire, UK

Freedom 21 Lifting Rudder Design

Post by Wilfred934 »

Hi Guys,

This write-up describes the new lifting rudder I made for my UK-built twin lifting keel Freedom 21. Although this is probably only of interest to other owners of this UK version of the F21 the general approach may be of interest if anyone has to tackle the replacement of a fixed rudder for one of the smaller Freedoms?

Best regards,

Wilf Bishop
F21 - DAYDREAM
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F21 Lifting Rudder Design.docx
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Michel
Posts: 546
Joined: Sun Dec 07, 2008 5:48 am
Location: Zaanstad, the Netherlands, EU

Re: Freedom 21 Lifting Rudder Design

Post by Michel »

Wilf, nice bit of design and construction, and very well written down.
Michel Capel, Freedom 44 #4 1981 'Alabama Queen', NED8188, cat ketch with wishbones, home port Enkhuizen, the Netherlands, 52*42.238'N 005*18.154'E.

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Castaway
Posts: 286
Joined: Sat Dec 13, 2008 8:40 pm
Location: Lerwick, Shetland Isles

Re: Freedom 21 Lifting Rudder Design

Post by Castaway »

Wilf,

A very impressive piece of work! The outline of the rudder looks very similar to the one I have, but that is a flat aluminium plate with a tapered leading edge and square trailing edge; no suggestion of an aerofoil section. The aluminium is also quite heavy, especially noticeable when lifting it in and out of the boat. No downhaul is necessary, although there is provision for one. How much does your new construction weigh? Will the blade stay down without the downhaul, or does it float up? Are you going into production?

Kind regards,
Gerald Freshwater,
s/y 'Castaway', (UK F35 cat ketch, centreboard, 1987)
Lerwick Boating Club
Shetland Isles, Scotland

Wilfred934
Posts: 19
Joined: Sat Dec 13, 2008 4:52 pm
Location: North Yorkshire, UK

Re: Freedom 21 Lifting Rudder Design

Post by Wilfred934 »

Hi Gerald,

Apologies for not picking up your message & kind comments earlier. Regarding overall weight - the total weight of the finished rudder (excluding the tiller) is about 26lb which is considerably lighter than the old badly made fixed rudder that we originally fitted to the boat. Also I have calculated that the superior lift/drag characteristics of a good aerofoil section like the NACA 0009 section I used should give the followiwng comparison to a flat plate rudder - assuming 4kts speed through the water:

Flat plate lift at 5 degrees - 60lb
Flat plate drag at 5 degrees - 10 lb
NACA 0009 lift at 5 degrees - 88 lb
NACA 0009 drag at 5 degrees - 4 lb

Flat plate lift at 10 degrees - 116 lb
Flat plate drag at 10 degrees - 25 lb
NACA 0009 lift at 10 degrees - 175 lb
NACA 0009 drag at 10 degrees - 10 lb

If you are at all interested I can tidy-up my estimated lift/drag graphs & send you a copy. All of this data has been based on readily available information so I am confident about the figutres. Regarding your final question about "production" of my F21 lifting rudders I'm afraid that with so many competing jobs around the home where I have an ongoing task of renovating a 250 year old Yorkshire Dales farmhouse any such thoughts would be grounds for immediate divorce.

Thanks for your help and interest Gerald - maybe one day I can make it up to see you and get a sail on your F35 Cat Ketch?

Wilf

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