rudder length to keel depth ratio

[marina95]

I am planning on building a rudder for a small (24') lightweight mini-tonner yacht and although I understand the profile required (NACA0012 section, 12% as thick as its chord) without the plans for the boat I am having to guess at the length required.

Does anyone have any guidance on whether there is a correlation between keel depth and rudder length?

Also from a construction standpoint I plan to build the rudder from multiple sheets of 12mm marine play expoxied together, shaped and then sheathed in woven mat and faired smooth with an epoxy filler. The chord would have a maximum width of approx 30mm (including the GRP and filler). Can anyone suggest whether this will be strong enough?

Thanks in advance!

 

[Stemar]

From a point of view of almost total ignorance....

Most boats I've seen with blade rudders have the rudder slightly shorter than the keel. The longer it is, the more effective it will be, but too long and when you get it wrong sneaking across a shoal, you'll bend or break the rudder stock instead of just grinding to an ignominious halt. IMHO ignominy is preferable...

There is a rule of thumb regarding rudder area compared to keel area for fin keeled boats.

"A rudder area of between 8 and 10 per cent of the total lateral plane or underwater profile is the desirable size for a sailboat. F S Kinney" (http://home.clara.net/gmatkin/therules.htm)

 

[oldsaltoz]

G'day Richard,

24 mm of marine ply would make for heavy rudder on a lightweight craft, have you considered making this from fibreglass, it would be much lighter and stronger.

What size rudder stock are you planing to use? how will you fix the ply to the stock? What sort and how many tangs will you be installing? Do you plan to introduce any balance on this rudder to make it lighter to handle?

Does your design include a sacrificial lower section in case of grounding?

How do you plan to seal the top of the rudder to the stock to prevent water entry at the stock to glass interface?

Avagoodweekend......

 

[Blueboatman]

I have built a balanced windvane rudder blade to the same Nasa 12 section and.. I have built a replacement spade rudder for a 26foot fin keel jobby . Based on (only)these 2 experiences this is what I found that may help .
I assume that you are talking about a transom hung rudder extending up to deck level where the tiller slots over the rudder stock ,so for a start one practical consideration is to have enough ''meat'' (ie plywood fore to aft) at the top to handle the leverage of the tiller. For the immersed part from the waterline level down you may want to come forward of the pintles by say 10percent chord length ,to make the helm partially balanced and lighter.
And lastly ,your real question: NASA 12 develops high lift at small angles of helm,but stalls easily at greater angles ...So I would suggest that you make the rudder as deep as the keel ,less say 100mm as per STEMAR's advice.And ,the longer the chord fore to aft the greater range of tiller angle before stalling (but the greater the wetted surface area,and this is a racing boat) .Not the easiest thing to get right first time ...Lastly you will probably want to add lead weight strategically within the plywood area to achieve a)neutral bouyancy and b) a balance of weight between the mass behind and infront of the pintles,else the helm will feel very strange as the boat heels.
Maybe if you described the boat type here ,someone with a near identical hull form can advise more specifically ,good luck!

 

[marina95]

salty

Yep - I know its going to be heavy - presumably you're suggesting a foam core? I've considered this but the rudder is transom hung on gudgeons/pintles and I'm concerned about compression when I fix the gudgeons...

Sacrifical lower section, no, other than a heavy (thick) buid up of GRP...

I was going to sheath the whole rudder and stock (as one piece) and overdrill epoxy fill and then drill the holes for the pivoting rudder head to avoid problems with water ingress into the rudder stock section.

I think I've managed to ascertain the overall rudder dimensions now so any additional tips on construction very welcome.

 

[Blueboatman]

Re Strength.
''chord about 300mm''
So max thickness =36mm,less w/rovings and epoxy fairing,say 3 layers w/roving per side,thickness of ply therefore 30mm,should be fine .Do a test sample of 3 laminate layers to determine true thickness and then grind ply down to required t ,grizzly disc in a grinder ,belt sander ,and a well made profile template to offer up and work to,should be plenty strong enough.If on completion all is well you may be tempted to start reducing the rudders depth until you feel the wetted surface is minimal yet retaining good range of steering control.

 

[marina95]

[ QUOTE ]
Maybe if you described the boat type here ,someone with a near identical hull form can advise more specifically ,good luck!

[/ QUOTE ]

Good advice - appreciated.

The boat is a lightweight GRP hull (an SK700 by Stephen Jones) very flat forefoot and flat rear section, fixed iron fin keel.

Here are the lines...

 

[fdmarsden]

The drawing answers some of your questions--size/shape
It also shows that the pivot axis is raked and the leading edge is vertical, giving balance area.

I dicovered that my grp/foam rudder was seriously cracked last year, and did significant checking.

I then made a new rudder, using vertical grain Douglas Fir in strips, epoxy bonded and biscuit joined, all sheathed in glass cloth, epoxied. section to NACA 0012. I have coordinates/ drawings and some templates of the section if they are any help.
Final result weighs 44lbs; the original grp/foam (& water) weighed 98 lbs.

I think plywood would have inferior strength.

The Douglas Fir was obtained from Capricorn Timber, near Uttoxeter, who were extremely helpful in selecting the pieces with vertical grain, no heartwood. no sapwood. cost was £70.
Total cost for the rudder about £150

 

[scoobylee]

sk700

hello to everyone im new to this web site and thought id ask for some help as ive just bought a new project an sk700 and it has no interior or rigging and i wanted to know if anyone has any pics of one that might help me in restoring her to her former self!!!!! many thanks simon