rudder design

[josch]

Hello all,

I am looking for a report and/or Graph about rudder design and rudder area estimation for sailing yachts.
It shall be used within a diploma thesis
I know that there are some articles and good graphs published for instance by "Dalzell" and A.F. Molland in 1978, (both having lectured and/or attended the Southampton Institute`s Yacht and Powercraft Design Course) but they can all not be accessed by internet.
Does anyone have something reliable perhaps scanned??

Any link or advise would be helpfull
Thanx

josch:confused:

[Eric Sponberg]

Dave Gerr just wrote a series of three articles for Professional Boatbuilder magazine on steering and rudders. They appear in the issues: Dec/Jan 2006, Feb/Mar 2006, and April/May 2006. You can see these issues online at www.proboat.com for free. This is probably the most concentrated material on rudders ever written, and it should answer your questions.

Eric

[Guillermo]

Gerr's articles mentioned by Eric are excellent. Maybe you can find also some extra useful information at:
http://www.sailboat-technology.com/links/online_articles.php
And in these forums:
http://www.boatdesign.net/forum/archive/index.php/t-937.html
Cheers.

[JPC]

Dear Josch;

Perhaps these links would be interesting/useful to you as well (discussions of rudder size/strength requirements in the context of building an emergency, replacement rudder)

Best regards,
JPC

http://www.pineapplesails.com/articles/e_rudder/index.htm
http://www.well.com/~pk/PCrudder.html

[josch]

Thank´s all for the replies,

I am using XFoil already and have though designed a pretty well section and I found some reasonable information about estimating the rudder area of modern sailboats in the professional boatbuilder link mentioned above as well, but that was more a rule of thumb formulation.

As far as I know the rudder planform and rudder area has much more effect on the rudder behaviour as the section profile, due to frictional resistance effects and so on.

So the aim of that post was more to get information about estimating the rudder area and planform. Especially in terms of rudder area I find no good data, graphs or formulation other than rules of thumb, which is not very sophisticated......

But anyway thanks a lot for the answers

[RHough]

If you know the force the rudder has to create, it should be very easy to design a foil to create that force.

For a sailing yacht, with rudder area as the only variable you should be able to resolve the force that the rudder needs to create. There is a yaw moment due to heel from the lateral displacement of the sail effort that creates a moment that acts to turn the bow into the wind. There may be a windward or leeward yaw moment from the hull due to heel. There is a windward or leeward moment created by the longitudinal displacement between the sail effort and lateral resistance.

The relationship between these forces is not constant. The force from the sails varies as the square of the wind speed with a wide range. For a displacement hull, there is an upper limit to the velocity of the water. You may find that a rudder that is large enough to work at low boat speeds is too large at high boat speeds, thus creating excess drag. You may find that the "perfect" rudder area for 20 knots wind and 8 knots boat speed is too small to work well at 6 knots wind and 2 knots boat speed.

Or you could use a rule of thumb. :)

[Eric Sponberg]

You can also try two other articles in Professional Boatbuilder in issues 95 (June/July 2005) and 96 (August/Sept 2005) which David Vacanti and I wrote, respectively. The first one is "Keel and Rudder Design" and the second one is "Keels and Rudders: Engineering and Construction". I believe these articles can also be viewed on line, the first issues in the on-line series.

Principles of Yacht Design by Larsson and Eliasson also has a whole chapter devoted to keels and rudders. Generally, they say that rudder area is typically around 1-2% of sail area. Another guideline is that rudder area should be about 5% of the underwater hull profile. There really are no other guidelines than that, at least for the amount of area. The shape of the area is something else, and the articles and book mentioned above also discuss that.

Finally, read C.A. Marchaj's book, "Aero-Hydrodynamics of Sailing" where he discusses sail plans and keel shapes. He has a whole section devoted to 3-D shapes for lifting foils. What works for keels generally works for rudders, but without the ballast feature.

I hope that helps.

Eric

[josch]

thank you very much those articles help indeed ;-))

[Crag Cay]

Now they are beginning to haul the boats for the winter, have a walk around Hamble Point / Mercury / Swannick marinas, etc with a tape measure, camera and note book. A days measuring will get you you loads of data. It's not that much effort to get boat type, draft, keel area, rudder type, area and planform. Rudder section is harder, but with a little 'jig' thickness and position of maximum thickness are both easy. A search on the internet will usually find the other details (displacement, Sail Area) of production boats.

You will see trends but will also be amazed at the variety, which suggests either 'the' perfect answer has not been found or there are a number of trade offs to be made at the design area.

You are sitting close to some of the answers you want.

[Karsten]

I have seen a rudder with almost twice the area. Both were for 42' racing yachts and the yachts were designed to the same class rules just by two different designers. With the large rudder you only need half the rudder angle to produce the same force assuming you also travel at the same speed. Therefore the boat might feel quite twitchy. With the small rudder and the larger rudder angles the rudder will also more act like a brake. The friction will be smaller but if you need a large rudder angle to keep the boat on course the total drag might actually be larger.
I'm sure you could figure out the optimum quite easily with a spreadsheet but it will depend on the boat speed and course as mentioned before.

[JEM]

Another guideline is that rudder area should be about 5% of the underwater hull profile.

To be clear, you mean 5% of the hull wetted surface area?

[Eric Sponberg]

To be clear, you mean 5% of the hull wetted surface area?

No. The profile area of the rudder should be at least 5% of the profile area of the underwater profile of the hull. That is, these are the projected areas of the rudder and hull as seen in a side-on view.

Eric

[JEM]

ok that's better. ;)

Didn't think I was reading that right so I wanted to be sure.

[teoman]

if you use two rudder, would that be 5% + 5% or 2.5% + 2.5% ??

thanks, just working on mine, but I will have 2 rudder for dual engine...

[Eric Sponberg]

if you use two rudder, would that be 5% + 5% or 2.5% + 2.5% ??

thanks, just working on mine, but I will have 2 rudder for dual engine...

For total turning power, you want a total 5% of area. If both rudders are working simultaneously, that would be 2.5% each. If one of the rudders is retractable so that only one rudder is working, as on some sailing yachts, then that's 5% per rudder. Again, these are minimums.

Eric

[BobBill]

I wish to replace a barn door type rudder with a high aspect, kick up version.

Would it be prudent to measure the square area of the old one and size the new one similarly, to keep it simple for me?

I have no clue how to measure as noted above...

[Eric Sponberg]

I wish to replace a barn door type rudder with a high aspect, kick up version.

Would it be prudent to measure the square area of the old one and size the new one similarly, to keep it simple for me?

I have no clue how to measure as noted above...

Maybe, it depends on the geometry of the current rudder and its relation to the hull profile. If the current rudder is inboard, ahead of the aft end of the waterline, and the new rudder is going to be further aft, it can maybe be smaller.

To measure the area, you have to take measurements of the current rudder as best you can and try to draw it to scale on graph paper. You can subdivide the rudder profile into rectangles, triangles, and/or segments of circles, calculate the areas of all the parts, and add them up. Or, you can count the number of squares in the graph paper within the area of the rudder drawing, which would be to the scale of your drawing, and including partial squares, and add up all their areas.

If you have a picture of your rudder and a sketch of what you want to do, we on the forum can give more advice.

Eric

[BobBill]

Here are the pics...boat is Kite Dinghy.

The first is the original non tip up barn door I have.

I have Laser II rudder and head I wish to use similar to pic of blade here.

The advert gives idea of boat...seems a simple task.

[Eric Sponberg]

Here are the pics...boat is Kite Dinghy.

The first is the original non tip up barn door I have.

I have Laser II rudder and head I wish to use similar to pic of blade here.

The advert gives idea of boat...seems a simple task.

OK, this is a dinghy. Most of the discussion above applies to much larger boats and does not necessarily apply to dinghies, whose rudders and daggerboards are generally much larger proportions of the underwater profile area. The Laser rudder may be a bit bigger, but so what. I wouldn't worry about it, and would not try to reduce the size of the Laser rudder just to make it equal in area to the Kite rudder. Just go ahead and switch the rudders.

Eric

[BobBill]

Thanks, Eric.

That was my plan. Thing is, I figured I would have to increase its size, that is, make the Laser II a bit longer and a hair deeper laterally, to match the older rudder.

Still, was going to set it up first and try it, with no alterations, save some custom work on the head to raise it above stern, but should be able to do it easily enough.

Here is pic of Kite with kick-up. Obviously, the difference is the head and the filler to butt the rudder, which is also barn door.

The more efficient Laser II rudder should do the old 73 boat well.

Bob H

[Petros]

keep in mind, that while the higher aspect ratio rudder is more efficient (i.e. less drag for the same amount of turning input), the higher aspect ratio rudder is more prone to stalling. The low asspect ratio rudder maintains better control of the boat in rough conditions, and less sensitive to over controlling (too much tiller input). When you put too much tiller input into a high aspect ratio rudder you will stall it, and cause drag and sluggish rudder response.

So if you plan on reducing the drag of the rudder for performance reasons, you will have to use a more cautious hand on the tiller to take maximum advantage of it. Eventually you should learn use it properly and it becomes more automatic, but until than keep your rudder inputs careful and deliberate.

[BobBill]

Good points, thanks. I had forgotten about that. Maybe I might be able to set it up for both applications. Either will do, but I may also be using a more up to date (mast boom sail) configuration due to scarcity of old Kite spars...I have Force 5 spars and might use Laser or other similar sail. Very close in SA. It will be a sort of "turboed" Kite.

[pagodino]

I have a similar problem. I own a Jongleur, a very fast boat built 1977 with wingmast, fullbutton mainsail, anit draft-keel (4°) but with a rudder that is a brake in the water. The keel is retractable (140-55cm) so the rudder has to be it too. I am thinking about a system like varaRudder to modifie my ship. Anyone has another Idea?

Best Pagodino

[DennisRB]

Pagodino. You boat sure does look fast and has nice lines. All besides the rudder which looks agricultural compared to the rest of the boat. They haven't even bothered to fair it.

Nice boat.

[BobBill]

Pretty nice boat. Reminds me of a trailerable Yngling. Need more of them. No info on WWW.

The Vara rudder is nifty, but looks expensive. Is it worth it, besides getting rid of the rough stuff?

Fair it up and it should improve? Seems the easiest way to go for a small increase in speed.

And found this on Vara problem: http://forums.oday.sailboatowners.com/showthread.php?t=54574

[Eric Sponberg]

Interesting story on the Vara rudder. Technically, it should work. I am surprised that Hunter was surprised that Nylon bearings absorbed water--duh! That's been known for a long time. Torlon is a better material and has been used many times in marine bearings. UHMW (Ultra High Molecular Weight) plastic is also good. Tivar 1000 is a popular brand name and choice.

I had a similar rudder built on Saint Barbara--the owner designed and built it himself. He cautions that the rollers or balls should be caged so that they remain separate from each other. If they are not caged, then the balls and/or rollers will rub against each other and that greatly increases friction. Here is a link: http://www.sponbergyachtdesign.com/SaintBarbara.htm.

I also used a rudder-in-a-drum for Project Amazon, back in the mid-1990s before the Vara rudder was patented. Link: http://www.sponbergyachtdesign.com/ProjectAmazon.htm.

I also have a rudder-in-a-drum on the Scandinavian Cruisers and the new Scandinavian Dory. Link: http://www.sponbergyachtdesign.com/SC40News.htm.

Interestingly, I have just posted a new story about a rudder retrofit to a Spencer 42 called Copernicus, on which I had done a mast conversion. Here is the story about her new carbon fiber spade rudder. Link: http://www.sponbergyachtdesign.com/CopernicusRud.htm.

A rudder-in-a-drum certainly works when built properly. You can see that I use the type a lot. I agree that the rudder on your Jongleur looks a bit agricultural, and the rudder stock looks way too small. Just about anything you do will improve the rudder, but you have to be aware that it will cost money--it will be a major modification. Do it right and don't skimp.

Eric

[Raggi_Thor]

Vara and Vario rudder, they look very similar.
http://www.boatdesign.net/forums/open-discussion/how-long-does-patent-last-17158.html

[pagodino]

Some informations about the Jongleur (7.5 x2x140/55, 700kg_280keel, 30qm with auto-fock, 40 with genua, spi 40): the boat was designed by H.Stoeberl, first presented on the Interboot 1976 where I saw it (I had 10 Years) the first time. We had a Trias (Stoeberl, Soling-like) at that time, 2 years later he designed the Akros which is nearly the sam, but streched. This boat was winner of many Lake races around euope but with the same caratteristics of the Jongleur. The Monas (singlehand small yacht), Dyas, Fighter, Quartas (Gustav Gans) are from the same hand.
In any case, after 10 years(former laser-contender-laser 5000, but without family) a boat I saw it for sale and I decided to buy it in the same moment.
They made 30 of the Jongleur, mine is number 28. The wing rigg is not easy in trimming, you have to overrotate a litte so in top the things working well. The pivoting keel (4°) works good. Trapez is on and the boat has the strange pilot-seat like the Monas (confort!, I am getting older).
Easy to sail because everything with fock can be made from the seat, hard to sail if you want to be fast, because of the different trimm-options and the wing-rigg. I tried it the last 3 weeks on the lake of konstanz and from 2 BFT with fock I was faster than the 30qm Schaerenkreuzer (Reimers). So I want to make the boat as fast as possible and here the rudder system is the first thing to change (other things planed and that will be a new thread next).
I thougt of another option like double rudders (Fighter has this) but the ton system is more interesting. The magior problem is the bearing I think, perhabs several bearings with about 2 cm (8-12) on the outer drum will reduce risk of frictioning (like the tons on the playground)? Build somthing like that is now problem for me so the costs are not to high.

best pagdino

[tspeer]

I have a similar problem. I own a Jongleur, a very fast boat built 1977 with wingmast, fullbutton mainsail, anit draft-keel (4°) but with a rudder that is a brake in the water. The keel is retractable (140-55cm) so the rudder has to be it too. I am thinking about a system like varaRudder to modifie my ship. Anyone has another Idea?

Best Pagodino

Why don't you just put a deeper blade on the existing rudder head? It looks like it already kicks up. The deeper rudder will put more load on the head, so you might have some problems there. But it would be the fastest way to experiment with different possibilities.

Instead of the Vara, I'd just go with a transom hung rudder of the daggerfoil type. Much lighter and simpler, and you can pull it up with the keel. You could also revert back to the original rudder very quickly to be class-legal for one-design racing.

[pagodino]

Well, the existing rudder head is the major problem, the most drag giving part of the whole system.
I want a system which is hydrodinamic and functional.
I think on a telescopic system with a central carbon-tube (4 cm) resistened for the load and to pull it up and block it down. The naca-foil around it only for the hydrodinamic work without static and dynamic forces. The 2nd deeper part of the blade could be made like a normal eps/cabon blade with the max diametre of the cabon-tube (inner diametr of the existing system on the boat). The upper part of the blade without anything in so the deeper blade can come up and when going down it will fix it with a minimal difference in shape. The upper part will fixed on the existing system witout the current rudder head.
I hope I was able to explain the concept, anyone tried something similar before?

Best pagodino

[Brent Swain]

Comparison with other similar boats will give you an idea of what works.
Comparison is a good starting point in any aspect of yacht design.

[larry larisky]

Comparison with other similar boats will give you an idea of what works.
Comparison is a good starting point in any aspect of yacht design.

since you never show the plans of the boats you pretend to have designed how do you know the first thing about yacht design since you are not a yacht designer (you own admission).