Rudder Design

[Trident]

I have a 50ft catamaran withs keg hung rudders - Prout Quasar

At some point in the past the original fibreglass rudders must have died - probably water in the foam - and the previous owner decided to make a stainless plate as his new rudder - I assume on the original rudder stock, just 5mm plate cut to the same shape as the original rudder in profile, and with some webs at 90 degrees to the plate to provide more strength . It certainly turns the boat and being solid stainless is probably a good thing in orca alley but..

She's coming out in spring for a paint job and at the same time I'm changing the link bar between rudders (as this means cutting a hole in the side to remove it and best done before the paint) and this got me thinking about making up new rudders whilst I'm at it.

I'm thinking a carbon stock with torque tabs securing it to a foam core and a few layers of glass - all done with epoxy and completely sealed so as never to get water in the foam.

My question is this - with a skeg hung rudder how much difference will the shape make ? I think, that compared to blade designs they are inefficient anyway (but may or may not be safer but its what I have so that makes no odds) but I assume some aerofoil shape will improve on the flat plate I have but , should I pay a designer to come up with the shape (at no doubt some considerable cost) or should I just do my best to copy the shape of the originals from photos etc and make a generally even wing shape across the known height and width (and leading edge thickness is quite guessable from the trailing edge of the skeg) ? How much difference will it really make on a fast but cruising cat from 40 years ago? (15-18 knots , 8-9000 kg)

Any thoughts please

 

[rogerthebodger]

Having a steel boat with a steel rudder I would just modify the existing rudder by fitting /modifying the ribs to a hydrofoil shape the plating over the ribs

I modified my monitor self-steering by adding an auxiliary rudder in th way I describer and shown in the pic below

https://flic.kr/p/WooUTs

Once welded up pressure test and fill with old engine oil

I doubt any orca could chomp that

This is my main rudder built fro MS in the same way

https://flic.kr/p/VKo4si

 

[Trident]

Having a steel boat with a steel rudder I would just modify the existing rudder by fitting /modifying the ribs to a hydrofoil shape the plating over the ribs

I modified my monitor self-steering by adding an auxiliary rudder in th way I describer and shown in the pic below

https://flic.kr/p/WooUTs

Once welded up pressure test and fill with old engine oil

I doubt any orca could chomp that

This is my main rudder built fro MS in the same way

https://flic.kr/p/VKo4si

I had thought of plating it over the webs to provide a shape but the annual anode change and chance of corrosion on the stainless makes me prefer a composite stock and rudder - plus frankly the prep time to get my current rudder clean enough to weld nicely is less than making an entirely new composite one

 

[rogerthebodger]

What anode if you use 316 stainless or even duplex, I would not use an anode.

Mine is painted with epoxy str and I find the anodes waste very little as very little galvanic corrosion.

 

[Trident]

What anode if you use 316 stainless or even duplex, I would not use an anode.

Mine is painted with epoxy str and I find the anodes waste very little as very little galvanic corrosion.

I didn’t build mine so no idea but it’s not 316L so I guess A2 but who knows as it was built in the Caribbean I think. Definitely needs anodes every year …

 

[noelex]

A proper hydrodynamic shape will have a few advantages over a flat plate, including:

A much higher stall angle
Lower drag
Higher lift

The higher lift improves the turning force of the rudder and the upwind performance of the yacht.

I think it is worth the trouble to make this modification. Any of the symmetrical NACA sections should be fine.

 

[Tranona]

Flat plate is terrible as a rudder. It is normal to use NACA airfoil profiles in the 0012-0018 range. Google best airfoils for yacht rudders to get plenty of discussion on the subject. You maybe able to use your current stock as a base and probably easier to use a wood (cedar) core sheathed in epoxy. It is easy to stop water getting in with a cored and sheathed rudder as there are no joins unlike a moulded rudder, just the stock entry. On the rudder I made that was sealed with epoxy and was sound 25 years later when I sold the boat. Jefa use an O ring at this point on their rudders. Photo of your existing rudders would be useful and also how they are mounted on the skegs.

You should not need anodes on a stainless rudder and stock if it is only the stainless that is in the water.

 

[rogerthebodger]

Flat plate is terrible as a rudder. It is normal to use NACA airfoil profiles in the 0012-0018 range. Google best airfoils for yacht rudders to get plenty of discussion on the subject. You maybe able to use your current stock as a base and probably easier to use a wood (cedar) core sheathed in epoxy. It is easy to stop water getting in with a cored and sheathed rudder as there are no joins unlike a moulded rudder, just the stock entry. On the rudder I made that was sealed with epoxy and was sound 25 years later when I sold the boat. Jefa use an O ring at this point on their rudders. Photo of your existing rudders would be useful and also how they are mounted on the skegs.

You should not need anodes on a stainless rudder and stock if it is only the stainless that is in the water.

I used the NACA foil profile for my rudder not sure which one but as you say better than just a flat plate

I had my profiles laser cut to ensure a uniform shape and once welded up pressure test to ensure no leaks

Welding to the stainless-steel shaft also ensures no leakage ass you tend to get with GRP rubbers

 

[Trident]

Flat plate is terrible as a rudder. It is normal to use NACA airfoil profiles in the 0012-0018 range. Google best airfoils for yacht rudders to get plenty of discussion on the subject. You maybe able to use your current stock as a base and probably easier to use a wood (cedar) core sheathed in epoxy. It is easy to stop water getting in with a cored and sheathed rudder as there are no joins unlike a moulded rudder, just the stock entry. On the rudder I made that was sealed with epoxy and was sound 25 years later when I sold the boat. Jefa use an O ring at this point on their rudders. Photo of your existing rudders would be useful and also how they are mounted on the skegs.

You should not need anodes on a stainless rudder and stock if it is only the stainless that is in the water.

I was looking at the NACA 0015 based on some advice
As the current stocks are 40 years old and one needed a chunk of welding last time it was out as it had some pits from corrosion I think its time to go and replace it with carbon fibre and foam (I've done lots like this but not with cedar so I'll look in to that ) .

The rudders mount onto a shoe on the skeg - I built new ones in 316 when I rebuilt the boat - they are effectively a U shape which bolt onto the skeg (flush as the GRP is cut back to take the shoe ) and have a phosphor bronze bush in the back end for the stock to sit in . The only picture I have is from when I first looked at the boat on the hard so not very useful but here it is anyway

 

[thinwater]

... Any of the symmetrical NACA sections should be fine.

... but not behind a skeg, since the skeg is part of the foil. The rudder is more like the elevators on a plane. I'd Google that. Or rather you would somehow truncate the forward portion. I'm thinking a triangular section matching the skeg thickness would be a practical shape, particularly if rebuilding metal. If forming a new rudder, then yes, NACA for the aft portion.

I would NOT use carbon fiber. There simply aren't any upsides for this boat.

• Break not bend.
• No stronger than fiberglass, but far less tough (much less flex before failure).
• Galvanic problems with stainless are a concern, including parts up on the quadrant.

I'd be thinking about optimizing the keels for windward work. I modified them on my PDQ to great effect.

 

[Trident]

... but not behind a skeg, since the skeg is part of the foil. The rudder is more like the elevators on a plane. I'd Google that. Or rather you would somehow truncate the forward portion. I'm thinking a triangular section matching the skeg thickness would be a practical shape, particularly if rebuilding metal. If forming a new rudder, then yes, NACA for the aft portion.

I would NOT use carbon fiber. There simply aren't any upsides for this boat.

• Break not bend.
• No stronger than fiberglass, but far less tough (much less flex before failure).
• Galvanic problems with stainless are a concern, including parts up on the quadrant.

I'd be thinking about optimizing the keels for windward work. I modified them on my PDQ to great effect.

The plan would be carbon for the stock only and there would be a grp sleeves to insulate the connections to the quadrant and the shoe

I’d also use carbon for the torque tabs to ensure the stock doesn’t turn inside the foam core ever but the rudder would all be grp with epoxy over a foam core . I’ll look at whether grp could be made strong enough for the stock too but I figure almost everyone uses carbon for a reason

 

[Trident]

I'd be thinking about optimizing the keels for windward work. I modified them on my PDQ to great effect.

Can you elaborate

The Quasar is very good to windward already as she has very fine hulls unlike all other Prouts - it was designed to win the Ostar and was the first cat to do so.

We can point to 35 degrees or better and have done 14 knots in 22 knots apparent at 40 degrees so not sure how much better it can get but interested to hear what you did

 

[noelex]

... but not behind a skeg, since the skeg is part of the foil. The rudder is more like the elevators on a plane. I'd Google that. .

Yes, ideally the total shape of the skeg and rudder should form the NACA shape. Just like most aircraft rudders.

 

[Trident]

Yes I get that but appreciate the clarification- will have to be a best match of course as it’s unlikely that the skeg is a perfect front end to a NACA profile

 

[thinwater]

The plan would be carbon for the stock only and there would be a grp sleeves to insulate the connections to the quadrant and the shoe

I’d also use carbon for the torque tabs to ensure the stock doesn’t turn inside the foam core ever but the rudder would all be grp with epoxy over a foam core . I’ll look at whether grp could be made strong enough for the stock too but I figure almost everyone uses carbon for a reason

Stiffer.

But is it better than stainless, or is it fashion? Consider how long the original lasted. Both you and the boat will be long in the dirt before it recurs. Just sayin'.

 

[Trident]

The problem with stainless is it can corrode and embedded in a foam core seems exactly the sort of place - if it went wrong - that oxygen depleted water would cause corrosion which simply can’t happen with composites

It’s the same reason my bronze through hull will be replaced with TruDesign and my stainless chainplates are being replaced with carbon I’m laminating in to the hull etc I’m just building a new toe rail to replace the aluminium one that having lasted 40 years is corroding so I’ll laminate in a grp one (which also seals all those pesky screw holes for the toe rail in to deck) and laminate in grp stanchions at the same time … if I can make it not corrode or rust or need maintenance I just think I might as well

 

[thinwater]

Can you elaborate

The Quasar is very good to windward already as she has very fine hulls unlike all other Prouts - it was designed to win the Ostar and was the first cat to do so.

We can point to 35 degrees or better and have done 14 knots in 22 knots apparent at 40 degrees so not sure how much better it can get but interested to hear what you did

Maybe nothing. I see now they are long and making them deeper would be a lot of work. Still, they seem small in proportion to sail area, but then again, many beach cats have none. I was more familiar with other Prouts (neighbors). They are slugs to windward.

Look at the leeway and helm angles when pointing and see.

 

[thinwater]

The problem with stainless is it can corrode and embedded in a foam core seems exactly the sort of place - if it went wrong - that oxygen depleted water would cause corrosion which simply can’t happen with composites

It’s the same reason my bronze through hull will be replaced with TruDesign and my stainless chainplates are being replaced with carbon I’m laminating in to the hull etc I’m just building a new toe rail to replace the aluminium one that having lasted 40 years is corroding so I’ll laminate in a grp one (which also seals all those pesky screw holes for the toe rail in to deck) and laminate in grp stanchions at the same time … if I can make it not corrode or rust or need maintenance I just think I might as well

Yes.

The boat is 40 years old. We can do it better now, so do we really expect the boat to make ... 90 years? OK, but I doubt it. Maybe.

 

[Trident]

Yes.

The boat is 40 years old. We can do it better now, so do we really expect the boat to make ... 90 years? OK, but I doubt it. Maybe.

I think it’s more that it will be the last boat I own and each year I get older , fixing things will be harder so the more I do now to make it maintenance free the less work my older self will have to get sore doing

 

[Tranona]

Carbon has a mixed reputation and usually used as a rudder stock to reduce weight compared with stainless, but lots of examples of breaking suddenly. Really OTT for your application. Stainless is used in thousands of boats and crevice corrosion can be virtually eliminated by ensuring the point of entry of the stock into the blade is sealed properly. Your arrangement is very simple with a well supported stock and really does not need anything fancy - just careful construction. Do it properly and it will outlast you, just as the one I made in 1992 was still in perfect condition when I sold it it 2019.