# Logarithmic spiral used in rudder leverage

Discussion in 'Stability' started by Bahama, Jul 24, 2010.

1. Joined: Jun 2010
Posts: 85
Likes: 1, Points: 0, Legacy Rep: 20
Location: Minneapolis

### BahamaJunior Member

I'm in the middle of moving this week and so I won't have access to my packed up scanner for awhile, but here is the link of my cutter plans that I bought from Ted Brewer: http://www.tedbrewer.com/sail_aluminum/orca.htm

She's a large radius bilge with a moderate fin and skeg hung rudder and I've stretched her as described previously except for one calculation mistake on my part, the displacement is now 46500 Lbs., ballast of 16000 Lbs.

The draft of 6' is a bit shallow for this and so I'm including a 3/4" thick 14" wide horizontal fin plate on the bottom of the keel; its just the width of the fin at the leading edge, then it gets wider gradually as you go aft until the maximum width of 7" per side is reached (about 45% aft), and then it stays that width going back.

The mast is now 72' tall above the rail (76' 6" to LWL) and on center 20' 5" from the bow.
Foretriangle length 20'5", luff 72', 735 sf
Mainsail: foot is 20'3", luff 64'9", 655 sf
So the total square feet is 1390 and the S.A./Displ is 17.2

A 130% Genoa will give me 935 sf and for a total of 1590 sf with the main and 19.67 S.A./Displ.

The Staysail will be 350 sf.

The required moments of inertia of the mast will be: Itt 46 ins^4, Ill 114 ins^4.
That is based on a removable forestay and the Ill would reduce quite a bit if the forestay was fixed.

However, I am considering a fixed forestay.

All shrouds, headstays and back stay are 3/8" 1 x 19 type 304 wire. The backstay is split near the base to be two 5/16" 1 x 19. The upper part of the stay can then be insulated and used as a radio antenna. Both 3/8" and 5/16" wire use 5/8" turnbuckles.

I plan to use a small 60 sf backstay at night when moored to keep her pointed into the wind.

I hope this helps, if you need anything else, let me know and I'll get it. Thanks for the help--I'm surprised at how much work in involved in good rudder design.

By the way, I don't have to have someone do the work for me, I just don't know what the formulas are that help me to determine with the optimal size of the rudder should be, so even a formula is appreciated.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.