rudder design?

[Ian_Edwards]

I have a Southerly 46RS which has a very deep high aspect ratio keel with a draft (keel down) of 3.3m, and very low aspect ration shallow twin rudders, which allow the boat to take the ground. The draft when the keel is up is only 0.8m and the rudders are less than 700mm long.

When the boat is reaching in gusty conditions it can be very difficult to control the boat, in essence the boat develops a lot of weather helm and the rudders stall.

When I eyeball the rudders they look like they are far from a NACA 0012 cross section, the leading edge looks far too blunt. The shape is driven by the large diameter rudder stock which is placed well forward, so although the rudders are semi balanced, the area forward of the stock is relatively small. This in turn leads to heavy steering loads when the boat is hard pressed.

I'm thinking of dropping both rudders this coming winter principly, because they both appear to have water in them, and getting them rebuilt. This presents an ideal opportunity to reprofile them. However, I'm struggling to find information on how to improve the performance of the rudders. I'd like to change the cross section to a NACA 0012 and add some area forward of the rudder stock. The objective is to reduce stalling, i.e. increase the angle at which they stall and to reduce the steering loads by moving the axis of rotation back from the leading edge.

So a request for pointers to the optimum design of low aspect ratio rudders (technical paper, books or articles) and possibly to someone who understands the design and advise on an optimum design.

 

[lw395]

Is the weather helm a separate and bigger problem than the rudder design?
Adding more balance may just mean you can stall the rudders with less effort on the wheel.
Would lifting the keel part way help?
Or more fullness in the jib to bring CoE forwards?
Adjustable backstay?

 

[pmagowan]

It sounds like a sail balance problem to me

 

[Foolish Muse]

I've been looking into adding tubercles (bumps) to the leading edge of the rudder. There has been a lot of research into how it improves the stall angle by quite a bit. Pm me and I'll send you the research documents.

 

[lpdsn]

I have a Southerly 46RS which has a very deep high aspect ratio keel with a draft (keel down) of 3.3m, and very low aspect ration shallow twin rudders,...

You could try contacting Rob Humpreys. I believe he made improvements to the rudder design on the later Southerlys. The 46RS is not listed on his website, but even then he should have a good understanding of the trade offs on the rudder design.

He may even design you new rudders if you were willing to pay.

Another very well known designer quoted, I believe, 600 for the design of a new rudder for a one-off racer (not sure if that was Euros or Sterling).

 

[Ian_Edwards]

I appreciate that the problem of rudder design is separate from that of weather helm, I've already shortened the foot of the main by 350mm when I had new dyneema laminate sails made 2 years ago. The boat has a self tacking jib, which limits the scope for adding more sail area forward, the jib already has the maximum roach we could realistically achieve, supported by 3 vertical battens and has to be fairly flat because it's on a furler.

It's a Selden carbon mast with rod rigging and very stiff, I can wind a lot of tension into the backstay with the hydraulic tensioner and it makes very little difference to the weather helm, but it does bend the mast and flatten the main which helps when going to windward in a breeze.

The boat was designed by Jason Kerr, I've contacted him by email and he doesn't appear to be interested, we had one exchange then he didn't reply, bigger fish to fry I guess.

I can take the keel up a bit and that certainly helps, but the keel has a clever system which wedges the trailing edge of the keel on the centre line when its fully down, taking the keel up release the wedge and allows the keel to move, so it's not a good idea in anything other than calm water. The lift keel weighs 2.5 tons.

I'm hoping that by increasing the area of the rudders forward of the pivot points, I'll move the centre of lateral resistance aft a bit which will help with the weather helm, but I obviously don't want to over balance the rudders, I'd also like to reduce the steering loads. If I improve the rudders performance by using a better cross section and with luck reduce their tendency to stall, that would also be a positive.

I'm happy to pay for a design, so long as I have reasonable confidence that it'll improve the performance. That's one of the reasons I'm looking for more info' on rudder design, I'd like to know what the options are, so I can make an informed decision.

 

[lpdsn]

The boat was designed by Jason Kerr, I've contacted him by email and he doesn't appear to be interested, we had one exchange then he didn't reply, bigger fish to fry I guess.

That's a shame. I've raced on a Ker design and he did seem amenable to talking to the owner.

In the meantime it might be worth looking at adding more control of the mainsail shape. I don't know the 46RS. Do you have a traveller that you can use to adjust the twist? Could you fit a Cunningham?

 

[Lon nan Gruagach]

I appreciate that the problem of rudder design is separate from that of weather helm, I've already shortened the foot of the main by 350mm when I had new dyneema laminate sails made 2 years ago. The boat has a self tacking jib, which limits the scope for adding more sail area forward, the jib already has the maximum roach we could realistically achieve, supported by 3 vertical battens and has to be fairly flat because it's on a furler.

It's a Selden carbon mast with rod rigging and very stiff, I can wind a lot of tension into the backstay with the hydraulic tensioner and it makes very little difference to the weather helm, but it does bend the mast and flatten the main which helps when going to windward in a breeze.

The boat was designed by Jason Kerr, I've contacted him by email and he doesn't appear to be interested, we had one exchange then he didn't reply, bigger fish to fry I guess.

I can take the keel up a bit and that certainly helps, but the keel has a clever system which wedges the trailing edge of the keel on the centre line when its fully down, taking the keel up release the wedge and allows the keel to move, so it's not a good idea in anything other than calm water. The lift keel weighs 2.5 tons.

I'm hoping that by increasing the area of the rudders forward of the pivot points, I'll move the centre of lateral resistance aft a bit which will help with the weather helm, but I obviously don't want to over balance the rudders, I'd also like to reduce the steering loads. If I improve the rudders performance by using a better cross section and with luck reduce their tendency to stall, that would also be a positive.

I'm happy to pay for a design, so long as I have reasonable confidence that it'll improve the performance. That's one of the reasons I'm looking for more info' on rudder design, I'd like to know what the options are, so I can make an informed decision.

Stall is stall and profile wont do a whole lot to change the angle at which it happens, some, but not a lot (for a symmetrical section). What you need is greater lift for the angle you can use. That can be done either with a longer section (more than the 700mm you quote) so not feasible, or with a longer chord, or thicker section.

What you have said about the current ones looking a bit thick may be an attempt to address this, and a thinner section, NACA 0012 might be a step in the wrong direction.
If you want to lighten the steering then, as you say, extend the rudder in front of the stock, but at 1/4 th chord back from the leading edge is the balance point.

Without ooodles of playing in wave tanks or £loads-a-money on CFD then its guess time. One thing for sure, the greater the lift, the greater the drag...

 

[Neil_Y]

Find a student from the yacht and small boat design course at Southampton Uni they should be interested to use their software skills in trying options. We just employed two on our stand at Seaworks (one was studying ship science) both were really keen to put theory into practice.

 

[adwuk]

Is the helm ever balanced? If not, then the rudders could be at fault, and moving the stock back a small amount may help. If you do get a balanced helm from time to time, then you really need to be looking at the sail plan. Thinner foils won't help as they stall more easily than thicker ones.

I'd suggest that before you do anything, go for a sail and really experiment with the sail plan at various points of sail. Make notes as to how the helm feels and what sail setup you have, along with boat speed, wind speed and angle. You will need this objective data before a designer can do anything useful for you.

 

[Ian_Edwards]

Hi, I've got my head in a bunch of paper sent to me by Foolish Muse and others down loaded from the internet, so I've start to understand (re-remember!) some of the theory. I'm an applied physicist by training and use to do hard sums, but that was many moons ago, before I got dragged in to senior management in the oil industry, but its starting to make some sense.

I know some of the lectures at Southampton University, having worked with them before I retired, so I'll investigate a student project.

I'm away sailing on Friday for 2 weeks, so I'll makes some notes on the speed, wind speed and angle, rudder angle and heel angle, which I can get from the Raymarine EV-1.

The boat has a cunningham but no track, it's a mid boom mainsheet with a "German mainsheet" powered by a big electric winch. The main is about 500 sq ft. The lower block is on a dyneema bridle, so it's not adjustable, but there is a big Selden boom vang which I use to control the twist in the main and the boom is a very alloy heavy section and doesn't bend. The cunningham doesn't seem to do very much, the sails are dyneema laminate and the halyard is 14mm dyneema so there's not a lot of stretch in the system.

Thanks everyone for your input, it's helped a lot.

 

[anoccasionalyachtsman]

When you say that the rudders are stalling, does that mean that they give up and you spin up into wind?

It's a long shot, but have you looked at the 'toe-in' of the rudders? Tony Castro reckoned on two degrees 'in' for each rudder on his twin rudder designs (so four degrees difference). The idea being that there's some divergence of the flow as it comes up from under the boat - more at low speed becoming more or less zero once planing, and he took a stab at 2 degrees as being a happy medium. If yours are horrendously wrong that could give unpleasant symptoms. It may be that in the reaching conditions that you're having trouble with you have one rudder out of the water, so the foregoing isn't helpful.