Rudder sweepback angle

Hi guys,

I am designing a new rudder (in lifting cassette) for my 43' trimaran. Please see attached PDF.

Due to the aft rocker on the main hull the rudder sweepback angle is about 12deg.

To what degree will this adversely effect the efficiency of the foil?

The sweepback angle could be reduced. But this would necessitate a gap between the top of the rudder and the hull reducing the endplate effect.

So I guess the question is what's worse? A rake (sweepback) aft of 12deg or a gap above the rudder top under the hull?

Many thanks for your thoughts on this.

andyf

 

http://www.socasailboats.com/VARA_System/body_vara_system.html

http://gunboat.info/gallery_48/pages/_Y6L0008.htm

Pericles

 

It is MUCH better to keep the gap as small as possible. A little extra sweepback will not hurt as much as loosing the wash over the top of the rudder due to too large a gap.

 

Thanks to Pericles and Greybarn for your replies. I am a little confused though: Per sent me a link to the gunboat website which shows a really neat dagger-rudder arrangement set into the hull. This would be an elegant solution to my problem. However, with a dagger-rudder (unless making a VARA type system) there is no hull over the top - which picks up on Grey's point.
That said Morelli/Melvin probably engineer everything soo well that they can get a dagger-rudder to work and work well. I may not be so fortunate?

 

On the 48 foot Gunboat catamaran the rudder is transom hung in a cassette. (saw it up close and in person) There is a big v cut into the hull area around the rudder. So there is no hull surface over the top of the rudder.

Just an observation.

 

Nero,
that's exactly what i mean - thus no endplate effect on the rudder. But this goes against what Grey' said earlier in this thread.
It's nice the way the Gunboat rudder design keeps the aft end 'clean' by using a transom hung dagger-rudder set into a 'V'.

 

Tank tests have shown that any sweepback drastically reduces a rudder's effect. Donald Street has written a bit about this.
brent

 

would this also apply to keels?

 

Probably not all that much. The sweep will reduce the lift-curve slope by about 5%, meaning you'll need 5% more deflection of the tiller to get the same effect.

The effect on drag isn't so straightforward. The sweep will load the bottom of the rudder a little more than it would if the rudder were vertical. This may be a good thing, because, even when sealed to the hull, there is some free-surface effect and at high speeds, the lift distribution for minimum drag is shifted downward somewhat. There will be a little cross-flow in the boundary layer, but that isn't too extreme at 12 deg sweep.

Not necessarily. You could incorporate a stub fixed to the hull, and rotate the portion of the rudder below the stub. The stub would need to be long enough to allow the rudder to pass behind the sugar scoop, which isn't much for your design, due to the narrow chord of your rudder. This photo of a Chris White designed Hammerhead 34 has such a rudder:



When the rudder is neutral, or nearly so, the gap is closed and there's not much of a step in the spanwise lift distribution. And the slot in the hull need be only just the width of the cassette thickness.

I think I'd keep the rake and avoid gaps. A gap will result in a much larger impact to the spanwise lift distribution, and increase the induced drag significantly.

Actually, you may be able to reduce the rake and keep the rudder sealed to the hull. The requirement is the bottom contour where the rudder rubs must be a surface of revolution, taken about the rudder axis. You may be able to find an angle for the rudder post where the angle of the run lends itself to being used as the diagonal contour, taken through the rudder post and faired into the skin of the hull forward of that area.

 

No it doesn't apply to keels . The sweepback lets the rudder suck air down causing it to stall easily. Raking it the other way sucks water up it ,as long as you don't rake it too much. You could put a slight bustle on the hull to fill the gap. An outboard rudder in a V drastically simplifies and strenghtens self steering and inside steering options. Great way to go.
Brent

 

small amounts of sweep (+/- 5 deg) will not have much affect, larger sweeps reduce the l/d of any lifting surface. On a rudder it simply means it has to be slightly bigger, or you use more tiller angel, to get the same amount of turning moment. You will also get more wetted area with more sweep, and it will also likely have a small effect on the feel at the tiller/wheel.

Slightly higher drag on turns means you will slow down more during a turn, but if your goal is to have it flush tight against the bottom of the hull, the efficiency improvement from the "end-plate" effect will be much stronger than the drag increase caused by sweep. Many rudders of course hang off the transom without any end plate effect and work just fine (some even have sweep too!), the owners never even notice that their rudder would be more efficient if they were under hull mounted.

If your option was to have either sweep with end plate effect, or no sweep without end plate effect, I would suspect the end plate effect would be the better quality to keep. The differential pressure from one side of the surface to the other during a turn would be allowed to bleed through to the other side, losing much of the effect of the rudder (as much as one third depending on the aspect ratio), the sweep generally only increases the drag due to wetted area increase.

Besides the sweep looks fast and racy, everyone will be impressed with swept rudders, looks like a supersonic jet. That alone makes it the better choice.

 

I'm thinking of welding plates top and bottom on my rudder to get end plate effect.
Street said when he asked designers twhy they reduce the effectivenes of rudders by sweeping them back they rtold him"Looks fast and sells boats." Style over substance can get you killed , a piss poor , socially insecure reason to do anything.I'd rather impress people who know what they are looking at.
A friend went for an inboard rudder on his new 57 footer. His landlord calculated that when it comes to a far more expensive autopilot, hydraulics, self steering , inside steering, etc etc, that decision to go inboard on the rudder cost him an extra $10,000, and left him with a far flimsier and more trouble prone steering system.
Style over substance again. A trimtab on the trailing edge of an outboard rudder is as bulletproof as you can get.
Brent

 

Yes, but if you depend on only selling boats to people who know what they are looking at, you will go broke. It is an unfortunate reality of life, that if you build and design boats, you have to design and build what actually sells if you want to stay in business. Bells and whistles is what actually sells the product, you need them, and you need the ignorant buyer to make a living, let alone prosper. I learned that the hard way making outdoor equipment many years ago and lost a lot of money in the process.

If putting sweep in a rudder makes it looks sleek and sexy, and makes no other significant or noticeable difference, then I would do it if my income depended on it (because it does). The world if full of ignorant people with too much money to spend, I am happy to design them an excellent product, but if it takes swift looking details to get it sold, I would never do without them. Getting it sold is as important as having a rudder at all, that is if you intend to stay in business. This is an unfortunate reality of any business.

 

When people have asked me to build a boat with narrow sidedecks I've always told them" If you want a piece of **** houseboat, go find yourself a piece of **** housboat designer. I don't do anything I don't believe in. I refuse to prostitute my art, period. " Sure prostitution pays well, but at what cost? If I had been willing to design and build to every one elses whims my boats would definltly not have the reputation they have for being some of the best cruising boats out there, and the amount of work I do for experienced cruisers would drop off. Trust has to be earned. I'm proud of having earned a living in a way in which my gain is not someone elses loss. Perhaps the willingness to prostitute ones art explains some of the abortions out there, and why so called "modern" boats when they capsize , stay capsized.When someone tells me they only need a boat for coastal cruising in sheltered waters I'm not so naive as to assume that they will be the only owner , nor that some future owner will not go offshore in it.
With lives of families, kids included, being at stake , I refuse to build or design anything that I would consider unsafe for offshore cruising anywhere.
I've made a comfortable , anything but extravagent living , by living aboard full time and cutting my costs , which gives me 11 months a year free time to do full time product testing.
Perhaps those who prostitute the art of Yacht design, fail to connect their need to struggle with the lack of trust that such motives cause them to have .
When a rudder that is raked aft ,for style over substance reasons , causes a boat to get out of control downwind, it can be extremely dangerous for life and limb aboard. I refuse to endanger anyones life for the sake of mere money.
I will not knowingly cost anyone their life or the lives of their kids for prostitution's sake.
Brent