twin rudders for 8M trailer sailer advice

Hi all,
I'm in the process of renovating an 8M trailer sailer which I am intending to have a transom hung twin rudder setup in cassettes so I can lift the windward board out of the water to reduce drag.
Was planning on a naca0014 foil section.

do I need to have a flat plate on the bottom of the cassette to stop air rising up the rudder causing cavitation?

If it is a good idea to have this plate, should it line up flush with the bottom of the hull or higher or lower ie protruding down past the hull ?

being transom hung and liftable rudder won't be balanced. I have heard of some rudders like this being tilted slightly forward under the boat so the bottom leading edge of the rudder is forward of the hinge point to give balance. Does this work well and a good idea or not necessary or bad idea?

Any thoughts / ideas much appreciated.

Tim

 

I would argue that pivoting rudders are more practical than cassette rudders. Pivoting rudders can have some balance built in if necessary.

A little bit of weather helm is the normal and desirable set up. A lot of helm suggests that you may need to manipulate the rig to mitigate the problem. Also weight trim in the boat could be a factor.

Cavitation plates might be useful on a super fast boat but probably not worth the trouble on a small cruiser type boat.

 

I had an 8m performance trailer sailer with dual rudders.
I experimented with adding a cav plate, but can't say there was any noticeable difference. In theory they should help, but add drag. The twin rudder was pretty good at heel anyway. They far exceed a single rudder with or without a cav plate.

I never generally raised the windward rudder. When heeled it was clear of the water or just touching it, so not much more to be gained. I did sometimes raise it on a long reach.

I had swing rudders with some of the blade in front of the pintle. It was beautifully balanced and always light to steer.

 

It is not a cavitation plate but an anti-aeration plate. Cavitation and aeration are two very different phenomena.

Whether a plate is necessary or not, will depend on the boat speed. If you are talking about displacement speeds (max. 7 kts for a 8m boat) then it is very unlikely that any aeration will occur, even at high tiller angles.

If the boat is a high-speed dinghy, scow or a catamaran, then there is a possibility of aeration and the plate becomes an option.

So more info is necessary about the boat, in order to give a correct answer.

 

U got it.
On rudders the effect is named " ventilation"

After my experience the best way to avoid ventilation on a rudder is to sweep it forward, about 4Degrees.
With a transom hung rudder, swing or cassette, these 4 degrees additionally bring the right balance.

http://www.f-boat.com/Media2/09News1/rudder600.jpg

for a mono trailersailer all these gimmicks are not necessary.
Two normal transom hung swing rudders will do.
Normally courses aren' t long enough for lifting the windward rudder.

Twin rudders are not parallel to the boat's centerline !!!!
Looking from aft , they are watching about 3-5 degrees inboard.
That has nothing to do with the Ackermann-effect .
Sailing upwind the leerudder is angled about 5 degrees ( wheather helm).
The windward rudder, more or less parallel to the waterline , MUST have 0 degrees then, otherwise it would generate horizontal lift, heeling the boat much more.






pogo

 

Actually, ventilation and aeration are used as synonyms in this case:



(taken from Molland, Turnock - "Marine Rudders and Control Surfaces").

But now that you made me think of it, I reckon that:
- ventilation is probably more appropriate to describe the phenomena which happens over the blade surface
- aeration is more appropriate for describing what happens to the water, which gets permeated by air bubbles.

So, IMO - the rudder ventilates, the water gets aerated.

Cheers

 

Mmh, on, better in waves , underhung rudders also often tend to ventilate.
The backside of a wave is often full with air- bubbles , acting as bridges for ventilation.
But, for whom is this knowlegde ?
On my 35' racer/cruiser tri the understanding of what was happening there was essential. Loosing control, or in worst cases steering in the another direction than given by the the rudder , forced me to examine these phenomenons. I had to cure the boats behaviour.

Note the rudder , kick - up , bulletproofed against grounding with up to 20kn, variable depth AND balance, no ventilation.
http://www.multihull.de/markt/2014/nielsen1.jpg

I think on a normal mono you' ll never be faced with those " problems" .
If, there gonna follow no desaster.
So what ?

Schön das wir darüber gesprochen haben.

Bethwaite gave good explanations :
http://www.amazon.de/High-Performance-Sailing-Frank-Bethwaite/dp/0713667044


pogo

P.S.
The foils on my tri had been designed by Leif Wagner- Smitt.
Later hydrodynamic prof and designer , crewmember of the last european winner of the Little America's Cup, C-Class " Opus III" and following defender " Sleipnir" .
Very special foils for high-speeds up to 35knots , but in practice --our speeds up to 26kn in cruising trim-- they were disappoointing, Suffered on ventilation. I modified them.

 

Cheers for advice. Boat is a 8m trailer sailor very flat bottom chined hull skiff like shape.1.8M lifting bulb keel. 2.4 m beam . 2.3m wide transom so quite a wide bum therefore want to have twin rudders. Should easily plane downwind and be quite fast. 300 litres water ballast each side . Is a bit of an experiment, will post up some pics soon.

Decided there is no need for anti ventilation plates. Been looking at a few examples mainly the seascape 27 . they have fixed rudders, no plate, with a bit out the front to give balance, but being a trailer sailor would like to either kick up or cassette.
In light winds would like to be able to lift the windward rudder or on a reach also.

Does angling the blades forward say 4 deg to give a cassette rudder some balance have any detrimental effects to rudder performance. Anything else I need to be aware of.
cheers

 

anyone Help with a suitable foil profile to use. Been doing a bit of research and reading . was looking at a naca 0012 or 0014??

Any ideas or recommendations?

thanks

 

They were not made for hydrofoil applications, but nevertheless are both very good all-round foils. especially if you are not looking for cutting-edge competitive performance.
NACA 0014 has only a slightly higher drag than the NACA 0012, but has a distinct advantage of a significantly lower peak pressure coefficient for all angles of attack. It means that 0014 will have lower probability of both ventilation and cavitation. They both also have pretty simple shapes which make the creation of the mold simpler and allow a structurally robust design. Hence, assuming that we are talking about some 15-20 kts max speed - if I had to chose I would opt for a 0014 or, in alternative, a Wortmann FX 76-120 which shows a similar peak pressure behavior.
Cheers

 

rudder design for 8m trailer sailer

have started shaping up some foam today for the rudders.
Using a NACA 0014 foil shape
320mm chord
1200 overall, 800mm below waterline
600mm down blade stars tapering to 220mm chord at the tip
thinking of angling the whole blade 4 deg forwards at the tip to give some balance.
See attached pic. Any advice on shape etc would be great.

was wondering if I use a different foil profile at the tip or keep the NACA 0014 foil all the way to the bottom??
Also where the taper starts 600mm down is a sharp change ok or do I need to have a gradual change?

 

The sharp change in planform is OK. There's not much to be gained in way of drag reduction by rounding it off, and you're likely to gain as much by making it accurate and fair with the straight taper.

Using the same section all the way down is OK, too. I don't think you'll experience any difference in performance if you went thinner at the tip.

 

andyful20

i read somewhere shaving a small bevelling (V)of the trailing edge reduces HuMM at speed-plus while-still on topic I hope-- does anyone have a great casstte :"kick"up design in alloy or composite with a top hinge/--for similar size rudder as above 450mm chord 1.8meters deep THANKs --1st post Building- 9.4M/2.75B+drop bulb Fractional ULB 2.5T.