New rudder design for 49ft performance cat

We just bought a new to us Ocean cat 49 catamaran last year. A lightweight (7t dry-9t loaded) and potentially fast boat. Currently doing a big refit to it which you can follow on youtube if interested. As these ocean cats were copied and stretched from a 42ft Erik Lerouge jeffcat design I don’t know how much engineering came into play when designing the rudders.

During the survey I already found that the port rudder had water ingress and the shaft and rudder had play in them. Yesterday we dropped the rudder while at anchor to inspect and create a plan of action. As always we like to do it properly and this project will cost quite a bit of money, but we are also looking for a solution that is cost efficient. We are not striving to spend 3x more to gain 5% more performance.

First things I noticed. The shaft seems to be a hollow 2 inch (actually 2.375 outer diameter because imperial makes a lot of sense ) schedule 80 probably 316 stainless. In metric terms 60.3mm OD and 5.54mm wall thickness. It has severe pitting right around the waterline mark on the shaft. Pitting is almost half of the wall thickness so definitely need to replace the shafts.
Our rudder dimensions are: Top 73cm, bottom 30cm, Top max thickness 9.5cm. Length of rudder along shaft 80cm. Total area seems to be about 0.4m2 following trapezoid area formula.

This opens us up to three routes we can take.

1. Open up the rudder, replace the shaft with similair shaft, maybe go for duplex 2205 for added Yield strength and corrosion resistance, refoam and reglass rudder.
2. Buy compeletely new rudders.
3. Buy new shafts and built rudders ourselves.

I'm leaning towards option three. The choice we then have to make is shaft material, 316l, duplex, aluminium or carbon. This will depend on the quotes we can get to have these made.

Then we have some other options to consider.

1. One thing to consider is the bearing situation, currently the "bearings" are just plastic bushings top and bottom with about 80cm in between them. Our ruddertube is 84mm internal diameter. So no place for Jefa self aligning bearings, only the Jefa needle bearings would fit but that would limit shaft diameter to 49mm.
2. When designing a new rudder and shaft, what are the loads you take into account? I have found the great articles of Dave Gerr on rudder design. But if I follow his calculations we would come to a 80mm stainless solid shaft.... Loads go up exponentially with speed of the boat. But what speed and rudder angle do you choose to determine the max loads. Our boat should be able to reach 20 knots in surfs and in the right conditions 10-15 knots sustained should be possible I think. And what kind of rudder angles. At these higher speeds rudder angles should be quite small.
3. Designing the rudder blade. Our rudder blade seems more trapezoidal than more modern blade shapes. From reading it seems like the Naca0012-0018 range are used often for rudders. I’m used to building model airplanes and we cut out naca profiles from foam using a Hotwire cutter. This was a really simple and accurate way to create efficient wings. I think this should be possible for our rudder as well based on divinicyll foam core and vacuumed glassfiber shell.
4. We could just copy the rudder designs Jefa has made for the Outremer 51. A boat that is heavier and has more sail area than our boat and should be able to do similair or even faster speeds. They use a 60mm Alu 6082 shaft and a 0.45m2 blade. So that could work with some new bushings. Or we could replace the whole ruddertube for a jefa system with self aligning bearings.

Hope to get some pointers in the right direction so we can get this project moving. We are currently in the US so we should be able to get anything here.

 

Re number three; two caveats. Divinicyll (sp?) can't be hotwired and some means of transferring torque between shaft and foam will be required.

 

Looks like existing foil is just about 13%
What is the current boat's sail area vs centerboard area vs rudder area? Does that align with other boats?
What are the centerline of stock to extents of blade dimensions A,B,C, and D

 

Thanks for the quick replies!
@Skip Johnson Thats a good point about not being able to hot wire divinycell. I didn't know that but that leaves getting it CNC cut or hand shaping/router shaping. Torque tabs will be part of the design and depend on rudder shaft material.

@Tops
Measurements are
A = 21cm
B = 8cm
C = 52cm
D = 22cm
Angle from rudder shaft to top of rudder is 85 degrees.

At the bottom it is a bit hard to get accurate measurements as the cornes are very rounded off..

Here are some pictures of the fixed keels, this boat doesn't have daggerboards.
Sail area is 66sqm main and 42 sqm genoa.

 

I see the outremer rudder drawings were marked as copyright and deleted. But Jefa has posted these publicly on their own website. https://www.jefa.dk/ftp/old_boat_spare_parts/Outremer/Outremer 51/Outremer 51 2020 rudder rev. A.PDF

 

Thanks very much for providing the location on their site instead -- just wanted to be careful not to violate their rights.

 

Hi Hugy,
You should consider a max load that takes into Account the 20knot surf. A CL of 1.2 gives you roughly 2200kg load for a 0.4m^2 area.
I would be very careful with your material choice since some of the dimensions above combined with some of the materials you mention give safety factors of less than 1

 

Is there any real reason for the changes? Are there some observed deficiencies that need correction?
If not, and since budget seems to be a consideration I would choose the easy way, buy two pieces of 316 tube, have tangs welded, shape some high density foam to the old shape and laminate glass over it. Plain bearings are just fine, the dimensions seem standard enough to be able to find off the shelf inserts.

I get the drive to update everything to the latest and greatest and its fine if that's your main goal. In that case full carbon shaft shaped to the blade profile (tangless), carbon skin with fiberglass cover, epoxy foam inner, and tube replacement to accept self aligning bearings.

 

These Ocean Cats have a bit of a funny history where the design was made by pulling hull moulds of an existing Erik Lerouge Jeffcat 415 and extending them. So these were not built according to the original design plans by Erik Lerouge. Overall the built quality seems to be very good, but the steering mechanism seems a bit under built with push pull cables normally used for outboard for example. Only 11 were ever built and not much is known about them. Thats why I want to check and reference the strength of the rudder design before commiting to coppying the existing ones.

@Rumars Well if we are going to replace the 25 year old rudders it isn't really that much more expensive to bring the design to modern standards as well. For example copying the setup of an outremer 51 which has proven to work really well and is properly designed. The shaft from jefa seems to run about 1000 euros and the whole ruddertube including top and bottom self alligning bearings another 1000. Then make the blades ourselve out of foam and glass.

@fastwave Thank you for giving us some direction. I have run the calculations through the great articles of Dave Gerr in BPO. And the results puzzle me a bit. If I take the 0.4m2 rudder area, 20 knots speed, CL of 0.5 as he says in the article and a Safety factor of 6 It gives me a 77mm solid 316 shaft

If I use your 1.2 CL I get to a 0.7 safety factor on our standard shaft...

It seems like my 316 2 inch schedule 80 pipe is similair strength to a 60mm 6082 shaft.
So when running the calculations on the jefa outremer 51 rudder with its 60mm 6082 shaft. I get to a SF of 1.2 with a CL of 0.5, or a SF of just 0.5 for a CL of 1.2...

As far as I know there are no stories of outremer 51s breaking of their rudders, and they are known to hit 25+ knots in surfs.

So my question is how do you calculate the loads on a catamaran rudder properly? Are there different formula's or factors to use?

 

Rudder design has to do with how the boat behaves or you wish it to behave. What you actuate the shaft with is imaterial to the underwater shape. If you use the bigger outremer rudder you change the hulls overall underwater profile. You need to know what the new rudder will do before you install it, that means determine how it affects the CLR. There's also the isse of balance area, and of course if the new rudder blades extend below the existing keels.

Gerrs calculations are useless for catamarans and ULD monohulls.
As for pricing, that's not my concern, you spend as much as you want. You asked for opinions, mine is that I would not spend 2k on metal shafts.

 

Thank you for your quick reply and I definitely value your opinion!
Me looking at the outremer rudders was more to get a comparison to the strength needed for the shaft, the rudders could be made 20% smaller to get the same size.

If Gerrs calculations are useles for this application, than how do you determine rudder loads and required shaft strength? Apparantly they can be built a lot lighter than for a monohull?

We still definitely have the option open to just replacing the shafts like for like and building a new rudderblade around that, or even reusing the old shells and refoaming them.

 

I forgot to ask how long the stock is outside the rudder.
Drawing foils in CAD is sort of a 'white whale' for me...

 

Hi Hugy,

You need to decide what is important for you. The calculations are relatively simple. What the correct extreme load case is you have to decide yourself. Important is to know what the consequences/limits of your decision are.

Some stuff to consider: The Outremers surf at 20–25 knots no issue, that is not the question you should be asking. How many of them have stuffed their bows down a wave and went to max steer at those speeds? Is this a case where YOU think it is important to you? If you get caught in the middle of the ocean, is this something you don't have to worry about?

Then there is the question if that is a realistic case... Surfing at 20 knots is not the issue, usually high speed means a lot of control. Stuffing the bows at the bottom of a wave at an angle will be the issue. What speed do you have at that time? The argument could be that you drop quickly from 20 knots to 18 knots before you can go max angle (17ish degrees). Can you even go at max angle? 2000 kg on a lever of 0.15 m steered by a 1 m wheel * your gearing ratio * 2 rudders means that the helm (person) might not be able to reach those angles. How much load can you apply as a human on the wheel? 50 kg? How much is it if it was a 2 m long tiller with you pushing on your legs? Will you flip over anyway at those speed and it’s pointless?

Whatever you decide is the correct extreme case, I would recommend to calculate over a range of speeds what the safety factor is using the CL of 1.2. Then you get the full picture on what your decisions mean. From when does it theoretically start being an issue?

You will not get another answer from anyone else. What you should be looking for is experience, experience in what happens out there in the ocean, what other people found is the limit etc. This will help decide what is important to you.

 

@Tops

Thats pretty cool! The shaft outside the rudder is 89cm. And the 85 degrees is the other way. The shaft is a hollow pipe so I don't think it slims down within the rudder itself.

@fastwave
Some great guidance again. I was looking at the outremers, as they do a lot of ocean miles, in all kind of different conditions and at speed. Many have completed a full circumnavigation and they seem to be one of the most reliable catamarans you can buy at the moment from the factory. So having their complete rudder design available gives some great guidance on what kind of strength has been proven to be strong enough. Apparantly a 60mm 6082 alu shaft is on that 0.48m2 rudder.

The stuffing the bow and breaching scenario is good one and possibly the worst case scenario. As we are long distance cruisers 99% of the time the hydraulic auto pilot will be in control of steering.

Here are some calculations I did comparing our 316 2 inch shaft, with 2 inch 2205, 2.5 inch 2205 and the outremer shaft and rudder.
Our 316 shaft is not nearly strong enough for the 15knot scenario.
A2205 2 inch pipe would be a little bit stronger than the outremer 51 shaft
And only a 2.5 inch 2205 pipe would be strong enough with a SF above 1.

 

As fastwave said above, you design to a scenario you decide upon, and then you look at realistic outcomes. You can build the shaft to survive a collision with a log at 25kn without bending, but the blade will shatter leaving you ruderless. So you reinforce the blade but then there are the pieces it's attached to on the other side, what good is a straight shaft and intact blade if it's going to rip your hull open like a cleaver. And if you reinforce the hull locally it could just break the entire back end of the hull. The more relistic scenario is that it's better to have the shaft snap saving the hull and have an emergency rudder on stand by.

If you need to apply full rudder at 20kn rudder integrity is the last thing on your mind. The hydraulic system will (or at least should) have a safety valve limiting the force it can take. My main concern for a cantilever rudder would be keeping the shaft from flexing during normal sailing loads.