Rudder design

[2hulls]

I may have the "opportunity" to change the design of the rudders on my Catana 471. I lost my stbd rudder blade and depending on the health of my port rudder - not yet examined - I may be in the market for two blades. If I need two blades, I may as well try to make them better. The original blades are shorter and broader than I think they ought to be. Definitely NOT high aspect ratio. I suspect they are the typical compromise >> short to allow shallow draft and broad because they're so short. Anyone with recommendations or a general design guide?

Thanks,
2 Hulls Dave

[mikereed100]

Dave,

The only design guide I am aware of is that efficiency increases with a higher aspect ratio for a given area. You may be constrained by your hull and daggerboard design. On the other hand, if you don't plan on beaching the boat you may be able to go a bit deeper with your blades. You could probably just keep the same area and increase the aspect ratio as much as you dare, keeping in mind the limitations of your rudder stocks.

My own boat came with rudders that sport a 1:1 depth:cord ratio. Pretty inefficient. I am planning on eliminating the rudder stocks and building high aspect ratio daggers that will fit in a rotating drum. These will be retractable for beaching.

Best of luck,
Mike

[2hulls]

Thanks, Mike. The aspect ratio on the standard Catana rudders are a definite compromise. Surface area is .47m2. Most of that area is at the top. An acquaintance put rudders from an Olsen 911 on his boat and raved about the improved handling and feel. He reused the original stocks - these are pretty beefy. But this added 11" of draft, up to 4'6".

2 Hulls Dave

[mikereed100]

Yeah, there's that pesky compromise thing you mentioned. It seems that most cat designers opt for shallow draft over performance and that probably makes sense in most cases, particularly cruising boats. Rudders are supposed to work by creating lift which pulls the sterns around, but I suspect that many low aspect cat rudders work simply by pushing water to the side.

[gramos]

Yeah, there's that pesky compromise thing you mentioned. It seems that most cat designers opt for shallow draft over performance and that probably makes sense in most cases, particularly cruising boats. Rudders are supposed to work by creating lift which pulls the sterns around, but I suspect that many low aspect cat rudders work simply by pushing water to the side.
Mike , do you think extending the aspect ratio / length of rudders would give ,apart from more efficient steering, increased pointing ability ?. when i took delivery of my cat the rudders from a 42 ft cat were fitted .mine was 32 ft . this always bothered me so i re-made them and reduced the length , proportionally, and they work fine ... would lengthening them improve grip to windward ? the chord and depth would remain the same .

[gramos]

rudder pics ...

[2hulls]

Mike , do you think extending the aspect ratio / length of rudders would give ,apart from more efficient steering, increased pointing ability ?

I'm not Mike, but I think the answer is "yes" - theoretically. The entire underwater profile of any boat makes up the total resistance to leeway. So, ANY addition to the "side view" surface area theoretically should improve pointing ability. Whether this also improves overall windward performace is another matter. For example, additional wetted area would be a detriment to VMG.

As for rudders, higher aspect ratio should provide higher "lift". Your boat was probably better off with the bigger rudders - especially if they were no deeper than the keels (so your rudders wouldn't be the first to hit bottom).

I invite any REAL naval architect to correct me on this.

2 Hulls Dave

[Noddy]

How did you lose a rudder in the first place?

Too deep? Too much side force on rudder?

I would factor that into my design decisions.

Paul

[2hulls]

Very broad reaching in an increasing blow + Too much main sheet = increasing weather helm.

Add in likely delamination and water intrusion into the foam core rudder + slow skipper response to sheet out = blade separation from the stock.

It was fun while it lasted. :D

2 Hulls Dave

[Noddy]

You sailed the rudder right off of the bottom of the boat - very impressive. The stock is OK? Good

I guess you want performance then!

If I had the budget I would get a qualified designer to input on the design. Maybe the guy who designed your cat. He/she would have made design decisions along the way and would, no doubt, be able to help you to get the rudder you need.

Build it well - you don't want to be doing the job again: I'm in the process of replacing a centreboard on my 30 cat which I built 'on the cheap.' It broke! Doh! :(

Paul

[2hulls]

You sailed the rudder right off of the bottom of the boat - very impressive.

That's a very funny way of putting it, Paul. I may use that description in future sea stories........

But, don't consider this an impression of my go-fast abilities. I think it's much more an impression of what can happen when fiberglass covered rudders delaminate and the foam gets rotten and a skipper is too slow to balance the helm in changing conditions. :o This rudder was going to fall off sooner or later - maybe just sitting still on the hook. :D

My choice of a replacement rudder (or two) is being influenced by the usual suspects: availability, cost, target functionality (alternate design considerations). The former racer in me wants two new nice, deep, high aspect ratio rudders for better performance. The pragmattic cruiser in me wants to retain the shallow draft and keep the "acceptable" cruiser performance and procure only one rudder. I think pragmatism will win.

2 Hulls Dave

[Noddy]

Will the performance mods make a real difference? So much of this sort of thing is about perception:

I used to have a Kawasaki GPZ900R, an early superbike. I had the chance to swap rides with a Honda CBR900RR. The hottest production honda thing at the time. The reputation and specification of the Honda suggested that it would wipe the floor with my Kawasaki.

Apart from some subtle handling differences the Kawasaki was easily as good (fast) a bike as the Honda, but £3000 cheaper. The only time this sort of edge shows is when racing against a closely designed boat / bike with crew of roughly equal abilities.

I would keep the drying out ability and buy a beach cat for roaring about and breaking bits off. Or am I getting old?? :(

Paul

[2hulls]

Will the performance mods make a real difference?

Well, I think the literal answer for my rudder considerations, is "yes". Will this be noticable in boat performance? Maybe not. Add to the mix, I'm also in the process of getting a new main. So any improvements in boat performance will not be able to be absolutely attributed to either rudders or the new main alone. One might be worse and I might never know it. :D

Answering a previous question, I hope my rudder stock will be reusable. I'll be removing it and the intact port rudder this weekend for assessment.

Another 471 owner replaced his rudders with higher aspect blades and reported much improved "feel" in the helm. Any improvement in this regard would make a difference to me. But I bet the autopilot doesn't care....

I have no intentions of ever drying out by boat, so preserving that flexibility is not important to me. Raced beach cats for 25 years. I got that out of my system..........

[Sandy Daugherty]

I would like to propose another consideration: Like the fender 'feeler' springs on a '53 Chrystler, a sacrificial rudder tells you when the saildrive is about to take a hit. Given that the rudder will shed some fiberglass, but a saildrive leaves a pretty insistant 'leak', I would stick with the original designer's thoughts on the subject! As you pointed out, feel is nice to have, but doesn't have a lot to do with what your Catana is meant to do. If performance were the end all and be all, you could have bought "Green Flash" from Dave Calvert!

That said, your rudder is creating drag when you have weather helm. A more efficient, higher aspect ratio rudder will have the same turning force with less drag.

[2hulls]

That said, your rudder is creating drag when you have weather helm.

Hi Sandy - for the record, I don't make it a habit of enduring very much weather helm. ;) I'm usually successful in balancing the helm pretty dern good. - I want to take it easy on Otto. :D

Noting you're located in Annapolis, if you're bored this Friday, we'll be removing the bare stock and intact port rudder down at Vera's, just north of Solomons. An easy drive for you. Buy you a beer.

2 Hulls Dave

[Sandy Daugherty]

I didn't know you could haul at Vera's! I actually met her once, she was past 80 and still dressed to the nines!
I'll try to be there, (I would love to see your boat) but I've got to figure out how to squeeze some antifreeze into my A/C coils past a bum pump, before the next hard freeze.

[2hulls]

Sorry to confuse you, Sandy. Boat's in the water. Nobody can get hauled at Vera's except at the bar........

We're gonna drop the bare stock and push the port rudder out through the bottom with a line attached to the top. Then fish them out. Should be a hoot. It'll be cold, though.

We do have an appt. with an electronics geek at 1100 to plan our upgrades - we hope to do the rudder work first. So come on down! I'll PM you my cell number.

2 Hulls Dave

[oldsaltoz]

G'day Dave (2hulls),

I have completed a number of rudder repairs over the years and have noticed a few poor design faults and picked up a few tricks on the way.

It sounds like the tangs have broken off the stock in your case, I would almost bet they are flat bar on edge, one or perhaps two meeting at the back end.

This is a common failure in this configuration; adding gussets can help but turning the strips so they are flat and welding them to the stock after shaping the end to fit the stock is a lot stronger, also adding a strip to the trailing edge improves strength.

Tangs should be placed such that lower section of the rudder is designed to break away leaving you with some rudder, rather than bending or breaking the stock, the upper tang should also be set low enough not to damage the hull if the stock is bent, again the top is designed to break.

The area between the tang and the foil skin should be filled with Micro-Fibres for strength, the rest of the void is filled with closed cell balloons, this prevents water turning the inside into a foam sludge.

To preventing water entering via the interface between stock and the top of the rudder, glass in 2 or more "0" rings with a little Vaseline under them.

All internal and stock to rudder joints should be done with an epoxy resin.

I hope this helps.

Avagoodweekend......

[2hulls]

It sounds like the tangs have broken off the stock in your case......

Thanks for the advice, Salty OZ. In my next post on this topic you will see the wisdom of your prediction...........

2 Hulls Dave

[2hulls]

First - Mr. Sandy Daugherty is a fine gentlemen. He will work for a sandwich and he knows how to tie a sheet bend. A good guy to have around any boat project. Donna and I look forward to having him aboard for a sail in the spring. He can choose whichever helm he likes. :D

Thanks to Sandy's vital assistance, today we successfully removed my bare stbd rudder stock and intact port rudder. It went pretty much as expected. The stock sank like a rock and the rudder floated like a cork. I haven't yet closely examined the removed components, but on first look, the bare stock will be re-usable, upon restoring the broken tangs, and the port rudder seems dry.

Photo - Dave & Sandy with trophies (these are a bit tough to fillet....)

2 Hulls Dave

[gramos]

the rudders fitted to my boat were oversize and made from solid grp , with a solid s/s stock 1200mm x 40mmwhich made them very,very heavy . this played on my mind for years , until I dropped them out and fabricated replacements . the first set were made using the same stocks and were just 4mm s/s plate welded directly onto the shaft in the usual "spitfire wing " shape . these were 40% lighter than the grp types. these were interesting in that from 7 knots and upwards , they hummed , a satisfying sound . these were made this way as a pal who was involved with the Whitbread 60,s told me they were using flate plate rudders at that time , as they were so strong . but as i dont race around the southern ocean much I decided that i should change them for faired wing section as the flat plate design was giving no lift . now they are fully faired by folding 2mm plate around the posts with 2 ribs inside for strength .these are the best yet , but if i ever did them again I think it would be in carbon fibre ,stocks too . i made rudders on my previous cat in the past , beautiful faired shape in aluminium ,they had to be held down when fitting on a mooring ,they were so light and had positive buoyancy . the problem was in spite of all the protection applied ,etch primer , etc etc was that they corroded .quickly .

[Sandy Daugherty]

What a wonderful, roomy and well kept Cat, Dave! It was a pleasure to meet both of your Ladies, and a bit of fun getting the rudders off.

Gramos: Your rudders are already the most efficient foils on your boat, but not very significant compared to your shoal keels, which are not.

[gramos]

Gramos: Your rudders are already the most efficient foils on your boat, but not very significant compared to your shoal keels, which are not.[/quote]
Alas ,Sandy , you are right , I had no input with the keels , I am always trying to figure out ways to improve them . its not an easy task . I am all ears for suggestions though ! I have looked at F/Pajot fixed keels and they appear to be much finer with a sharp leading edge also cut off to vertcal on the trailing edge Iwonder if this makes much difference to performance ?

[Sandy Daugherty]

The planform (what it looks like from the side) is not particularly important. What is important is how true it conforms to the foil section chosen by the designer, who selected it for a specific set of characteristics; lift over drag ratio, stall characteristics, endplate effect, etc. I can't think of anything you can do to the keels to improve their efficiency other than to make sure they are perfectly fair, by sanding with a long board and filling slight voids. If they end with a flat bottom, you should assure that there is a uniform radius all the way around, and that has a smallish diameter, on the order of 2 cm to maintain the endplate effect.

I know of two cats that experimented with inward curving endplates or winglets, and there were no noteable advantages, probably because they added as much drag from increased surface area as they might have gained in lift. Monohull keel winglets have some performance advantage when the vessel is heeled, not a desireable condition in a cat!:D

[mikereed100]

Here's a good series of articles on rudders entitled "Steering System Fundamentals" by David Gerr in Profesional Boatbuilder, 3 issues beginning Dec/Jan 2006.
http://www.proboat-digital.com/proboat/200512/

[2hulls]

Mike - thanks, that is a good read. Now I gotta find my slide.., er, calculator........

2 Hulls Dave

[mikereed100]

Dave,

Not to keep you too busy, but check out the Jun/July issue for a good article by David Vacanti and the Aug/Sep issue where Eric Sponberg weighs in.

Mike

[2hulls]

Mike - I couldn't find those. I started in the "Archives" and selected what I thought would be the correct editions, but saw nothing in the contents listed with those authors. Clearly I'm doing something stoooopid. Advice?

Thanks,
2 Hulls Dave

[mikereed100]

Sorry Dave, I neglected to mention that they were the 2005 issues. (Doh!). Here's the links:

http://www.proboat-digital.com/proboat/200506/
http://www.proboat-digital.com/proboat/200508/

[2hulls]

No problem, Mike. Thanks. Thought I was really going nuts for a few minutes. Rather, more nuts...........

[BigCat]

Very broad reaching in an increasing blow + Too much main sheet = increasing weather helm.

Add in likely delamination and water intrusion into the foam core rudder + slow skipper response to sheet out = blade separation from the stock.

It was fun while it lasted. :D

2 Hulls Dave

"In the violent winds, the GUNBOAT actually flew a hull several feet in the air (the wind was so strong that the sails were pushed hard enough to lift the windward hull out of the water). Big cruising cats are NOT EVER supposed to fly a hull like a little Hobie beach cat! The GUNBOAT crew tried flying a spinnaker in the heavy winds, far exceeding its sail plan for such high winds. The GUNBOAT did not pull off that stunt – it broke a carbon fiber rudder! That rudder probably costs more than your family car!"

From: http://www.indigomoon.us/triplog/tr014.html

[BigCat]

I'm not Mike, but I think the answer is "yes" - theoretically. The entire underwater profile of any boat makes up the total resistance to leeway. So, ANY addition to the "side view" surface area theoretically should improve pointing ability. Whether this also improves overall windward performace is another matter. For example, additional wetted area would be a detriment to VMG.

As for rudders, higher aspect ratio should provide higher "lift". Your boat was probably better off with the bigger rudders - especially if they were no deeper than the keels (so your rudders wouldn't be the first to hit bottom).

I invite any REAL naval architect to correct me on this.

2 Hulls Dave

Higher aspect foils of any kind develop more lift per unit of square area-rudders, sails, or keels. However, they also stall sooner than their low aspect ratio counterparts. Some foil shapes (wider foil shapes with the widest point forward of the middle) are more resistant to stalling than others, at the cost of higher drag. Everything in yacht design is a question of trade-offs and priorities. NACA 0012 and NACA 0015 foils are often recommended. 0015 gives lots of room for a sturdy rudder shaft.

[BigCat]

This link will take you to a drawing of a NACA 0015 foil. It's a high resolution .bmp, so you can print it out with good accuracy. I also have this in a .dxf file for those who can use a .dxf file.

http://bigcatcatamarans.com/NACA0015.BMP

[Nordic]

Here is a neat piece of software that will let you print out templates of over 2000 foils. It even handles printing them so they are ready to glue together from smaller pages.

It is free to download, but send him a few euro and you will have access to it all.

http://www.profili2.com/eng/default.htm

Alan

[BigCat]

Here is a neat piece of software that will let you print out templates of over 2000 foils. It even handles printing them so they are ready to glue together from smaller pages.

It is free to download, but send him a few euro and you will have access to it all.

http://www.profili2.com/eng/default.htm

Alan

And best of all, it exports .dxf files. Computer guided equipment will follow .dxf files precisely - Waterjet, laser, and plasma cutting tables are examples of tools that will usually import .dxf files-and so will a lot of sign making software that guide routers or cutting knives for vinyl. I believe that there are even machines which will move a hot wire through foam to cut cores for making daggerboards and rudders. :cool: