Carbonfibre keel/rudder?

[Robin Larsson]

Hi everyone!

As some might remember I´m buildning a modern version of an old Swedish production Wasa 55yacht, a decendant to the even older Skerrycruisers.
I´ve got an "old" hull and deck, and thats about where the similarities end
I´ve got a modern fractionrig, the overall styling will be more modern, and also the interior will be way better then original. I guess that atleast some of you knows aboat the Orustyachts we have here in Sweden, like Najad, Malö and Hallberg Rassy, lets say thats the level. I´m schooled at Orust boatbuilding School

But as the title says, I want modern underwater appendages to. It will be a deeper, bulbkeel, to reduce keelweight from 2700kg to about 1700kg in a bulb.
That will give me a draught of 2.5meters instead of 2.

So, I´m thinking maybe the keelfin should be made in composites? I´ve seen the keels on yachts like T-boats, and on one of Eric Spoonbergs designs. It seems to be a very good way to custombuild a keel. Woodcore and carbon/glasfibre.
The alternative would be welding up a steel spaceframe or something and laminate over it to make it fair.
The thing is, I work in a metal workshop, we can build almost anything in metal, we have an Abrasive Waterjet, bending machines and good welders. Forging(correct word?) is the only thing we dont make. But we also build composite part and boats.

Whould it be much more expensive to build the fin in partly UDcarbon? Dont think I would build the fin completly in stainless, even if it would ofcourse be possible. Then I could have tankage in the fin.

The biggest pro for me woth carbon is the fatigue strenght, I think atleast?

Oh, the keelfin would be about 1.7meters deep, about 30-40cm would enter in to the boat. Topchord app. 90cm, tipchord app. 70cm. Bulb app. 1700kg.

And the same with the rudder, would it really be worth it with carbon rudderstock? Was thinking to use a 80*80*3mm stainless squaretube, and watercutted foil sections welded to it. First I would of course calculate more exactly it the squaretube would suffice.

What do you say? Carbon or steel? Pros and cons?

The pic is a sistership, one of the really nice ones, the drawing is a approximate drawing of my boat, not complete though.

Best regards from Sweden!
Robin Larsson

[Wardi]

Why not use duplex 2205 or 2507 stainless steel as supplied by Sandvik, Outokumpu.
Many benefits including easy fabrication, lead moulded in keel ie:- no lead mould reqd, water tanks in keel - more space in hull & no extra tank cost.
I do not see any benefit in using carbon fibre, it is expensive and counter productive for stability.

Also, be careful applying a fin keel to this older design, it may not in fact improve performance, it can increase heeled resistance and adversley affect hull balance.

[Robin Larsson]

Wardi, thank you for your answer!
You mean I should build the fin completly in stainless? Like a framework inside and stainless "plates" around?
As you say, would give tankspace, altough not much, about 100liters.
Is´nt there downsides to having say watertanks in the keel, like changes in stability when one uses the water?

No, maybe carbon dont have so many advantadges, but as far as I know, it is much better when it comes to fatigue.
The rudder would only bve good to have light, but the keel, maybe better to make the fin heavy

I´m not exactly sure, but it might be more expensive to build a hollow stainless keel, then to build a composite keel with UDcarbon as main strengtmember. Stainless is rahter expansive, even for me how works with it.
Would guess that the sideplates in a hollow steel keel, would be 3-4mm thick, depends on how much framing there is inside ofcourse.

Well maybe stainless would be the best" option for me, considering the workshop I work in. But I´m a bit "worried" about how much difference it would make with keeltank full vs. empty.

Could it not work well with a framing in nonstainless steel, maybe high strength steel instead, and make the foilshape with GRP? Even easier, and cheaper.

Hmm, this boat is originaly finkeeled, but with a much older fin design, no bulb, topchord app 2300mm, tipchord app 1700, raked backwards.
I can´t see how a way more effective keel could be bad. Provided that the Centre of Effort is in the same place.

Would be nice with some input from peoble how´ve built carbon keels, surely someone here has? Maybe Spoonberg himself will read this

Best Regards
Robin Larsson

[Ramona]

The Van de Stadt 34 built in timber has a steel keel filled with lead. Check out their web site for pictures and diagrams. There are other production yachts in fibreglass that employ a steel keel as well, some have the steel covered in epoxy and Dynal.

[Robin Larsson]

Yeah, Ramona, I´ll check that out!
But I feel that a real bulbkeel, not a fin filled with lead is the way to go.

I´ve been reading about a few different T-boats now, almost all of them uses a carbon/e-glass fin, with a leadbulb. Most seem to have timbercore.
Is that a good way to build a keel? I suppose its the easiest way for DIY guys t build a good strong keel.
But why doesent boats like the VO 70 have carbon keels? Ah, because of the rule, but why does the rule state that?

Is it not true that carbon have much better fatigueresitans then steel? It should therefore last longer. But maybe dont take groundings as good?

Wouldnt it be possible to build a keelfin with just UD e-glass and epoxi and core ofcoruse. Sure, carbon is stronger and stiffer, but it just a bit more weight and you have atleast the same strength with UD glass.

Or would a steel frame, maybe square tubing and so, covered in grp be best and cheapest? A full stainless keel would probably be a bit expensive..
And also, maybe the differences in Rm with empty vs full keeltank would be a bit much?

The pics is from a Swedish Performance cruiser, the Performance 35, www.maxknots.com. Their keel is made of two steeltubes and a flatbar. And later covered with a kevlarskin. They have about 1100kg bulb. By the way, my rig is from one of those boats

No one here how´s built a carbon keel?

Regards
Robin Larsson

[Omeron]

Why are you particularly worried about the fatique strength of steel?
Surely, with a structure where there is no limit on weight,or in other words it is not a limit design, you can use as much steel as you want so that the
fatique strength treshold is absolutaly not an issue.
I havent heard of any keel failure because of fatique.
On the contrary, you would like to have the densest material going into your keel, so that for a given overall volume, you would have the heaviest keel, with CG as low as possible.
Any amount of fibre and epoxy to go into a keel, would be stealing away
precious little volume which can be filled by a denser material.
Since you are going to use fibre at the outermost surfaces, it will consume
a lot of volume, compared with what is left on the inside.
If you know the total immersed volume of the keel, you can guesstimate
how much volume is wasted for a given composite thickness.
It is probably much more than you think it is.
If by design, you have plenty of volume to begin with,than probably there are more gains in reducing it than using composites.
For a DIY keel, concrete over a steel frame, encapsulating some lead scrap may also be an alternative.

[Robin Larsson]

Omeron, thank you for your answer! No, I´m not really worried about fatigue. But still, composites are better It would never rust either

But as you say, I´m not designing to a rule, so I can have any weight I want to., but I still want to make the boat as light as possible/economic

If one have a heavy keelfin with a bulb, you will have a higher CG then if you have a lighter fin and the same bulb, but you will of course have mor RM with the heavier fin. But its better to distribute more weight into the bulb, for a given keelweght.

So, does it make sense to build a keelfin in 2205 stainless, only in steel, and with the main volume of the fin as watertank? Of course a lead bulb in the end

Any one with expirence of building keelfins? I´ve checked the steel that was suggested in the first answer, 2205, and its about 20% more expensive then 316 steel, but its almost 100% stronger, so it probably more economic to build in 2205 anyway! And 316 dont work well under water.

I´ve got the machines to work with stainless, so it would be smart to use them Also, not as much boring fairingwork with a stainless keel as it would be with a composite one. Propably faster construction to.

What does the expertise say?

Anyone have any good tips on safetyfactors for designing the keelfin? I´ve seen 3, but that was in the VO70 rule, maybe a bit to little? I´ll check ABS.

Regards Robin

[rob denney]

G'day,

Building it from carbon is not particularly difficult, the loads are not too different to those of an unstayed mast, with the obvious differences that the keel is end loaded, rather than uniformly loaded and it is in water. Built correctly the carbon will be lighter than the steel and won't rust. I would build a cheap and cheerful mould and there would be little or no fairing required.

The devil is in the details of how the bulb is attached and how the keel is attached to the boat. I would not use standard steel keel techniques (bolts) for a carbon keel.

To give you some numbers to work with, we sell standard modulus carbon tow for $45/kg plus freight and your local taxes. Epoxy you will get locally. There are a few tricks to reduce the carbon required.

Our engineer can calculate the laminate and/or the steel required if you wish. He engineers unstayed carbon masts for $1,000, suspect a keel would be similar, especially if it is not bolted to the hull or bulb.

I can advise you on the laminating procedure if required.

regards,

Rob

[Eric Sponberg]

[quote=Robin Larsson;185700]Maybe Spoonberg himself will read this/QUOTE]

OK, I'll bite.

If you are going to use stainless steel, the only alloys you can rely on are 304L or 316L stainless steel. The -L designation is important, which stands for "low carbon". Welded stainless, as you may know, suffers from carbide precipitation, which means that the carbon precipitates out of the alloy in the vicinity next to the weld. When submersed in an electrolyte, like seawater, that area where the carbon is gone will corrode fairly quickly. By using the -L alloys, there is little to no-carbide precipitation, and so the structure holds up much more in seawater.

Whether carbon or stainless (or aluminum for that matter, which I have done) the skin of the blade should be the primary structural member. It has to be there anyway, and it is in the perfect location to provide the best strength and stiffness. Therefore, with a stressed skin design you can go with otherwise relatively lighter internal structure--at least for keels.

Interestingly, on one carbon keel blade design of mine, for Bagatelle, the original keel was carbon fiber over wood, and draft was about 8'. The owner later hired me to redesign the keel for more bulb weight, less draft (6'), non-lifting instead of lifting, and to keep the width of the section that fit into the hull the same so that the same keel slot could be used. The keel blade section, however, was wider. We used stainless steel for the construction of the new keel--again stressed skin with internal structure sufficient to handle the loads. I can't recall the factor of safety we used as the files are tucked away somewhere, but probably I used FoS of at least 2 over the yield stress based on the full righting moment of the modified boat design. You definitely don't want the structure to take a permanent bend under load. So this was a case where on a single boat we used both methods of construction.

For rudders, it is a bit of a different story because you need to carry all the loads into the rudder stock, so the stock has to be heavy and an integral part of the rudder blade. That's if you use a stock. You can design a lifting rudder in a drum that does not have a stock, in which case the rudder blade skin becomes the primary structural member.

For further information on keel and rudder design and construction, see if you can get a copy of my articles in Professional Boatbuilder magazine, as follows:

PBB #79, October/November 2002: Cover Story: "Case Study in Lightweight Engineering." This is the story of Bagatelle and how she was designed and engineered.

PBB #96, August/Septenber 2005: "Keels and Rudders: Engineering and Construction". This discusses engineering and design in depth, and it includes some construction section drawings of Bagatelle's carbon/wood keel.

Carbon fiber has tremendous fatigue characteristics, so that should not be a problem. You would need a mix of fiber in a composite construction, unidirectional (about 60%) running up the blade, with the remainder of the laminate (40%) being off axis fiber, both +/-45 and 0/90 fabrics. I use the wood core concept on my keel blades so that I can keep the skin stable without having to build an internal structure. It is OK that the wood is heavy--you want heavy in a keel. Not so necessarily with a rudder, so foam works best in a rudder blade.

A word of caution--do not mix composites and metals. They have vastly different moduli of elasticity so that they don't necessarily work that well together. And forget about aramids (Kevlar) except is bulletproofing dep in the laminate. Aramids have very poor compression strength, and rudders and keels can be compression strength critical.

I don't know the area of your rudder, but an 80 x 80 x 3mm square tube for the stock sounds way too light. I recently finished a check on a 43' trimaran rudder with area of 1.5 M^2 and a proposed stock of 63 OD x 5mm wall round stainless steel stock, and my recommendation was for a tube 158.8 OD x 19 mm wall (standard 316 s.s. tube 6.25" x 5.50") based on first principles engineering (not ABS), and this was for a boat intending to do the Round Britain race reaching speeds of 25 knots. You won't be going that fast, but be prepared to go to a heavier stock. People usually have a tendancy to estimate too lightly on their rudder scantlings.

I recommend that you consult with a naval architect who knows something about the engineering of these things. You have a lot of variables at hand, which include your expertise in building. So you want designs for the keel and rudder that will work, not fail, and that are relatively easy for you to build.

Finally, I am designing a new sailboat now that is also derived from the Skerry Cruisers, called the Scandinavian Cruiser 40:

http://www.scandinaviancruisers.com/about_us

This design will be built totally in carbon fiber--hull, deck, appendages, and rig--with lifting keel, lifting rudder, and free-standing wingmast rig.

I hope that helps.

Eric

[Pericles]

Eric's article in PBB #96.

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

Pericles

[Robin Larsson]

Hi everybody, sorry to take so long time to reply, been way to busy.

Rob, thank you very much for the figures and your help! The prices sounds really good for the carbon, might just take you up on that! But the Tow, is it approxematly the same "size" as standard fibreglass tow? About 1cm wide?
Do you also sell carbon uni at a good prices?
Yeah, as you say, a keel is not at difficult thing to engineer really. But I see one big downer with carbon, or really any composite, if overloaded it will snap right of, wont it? Or maybe its not so difficult to make it withstand grounding loads after all?

No, standard bolt arrangement doesent feel right with a carbon keel, it would have to be a box I think, maybe even make it lifting, if only just like 80cm, then it could be contained in the rigid part of the saloon table.
Same with the bulb, but I have an idéa, make it almost like the composite chainplates that for example some Farrier tris have, and simply run a long bolt through the bulb and keel, lenghtwise. And glue. The way Eric Sponberg designed it on Bagatelle looks good also.

But I´ve "discovered" the steel Weldox, www.weldox.com a Swedishmade high strength steel. Much cheaper then carbob, almost a factor 10 based on weight, but of course carbon would be lighter, so the differens is smaller. it feels like an easy way to make two stressed skins and some internal members, both horisontal and vertical in weldox. I have the machines to watercut and bend them so. And also good welders. But it would need good rustprotection.

Eric, thank you for your long and very good answer! I´ve read your articles in the digital issues of Proboat(Pericles, thanks for the link!) and almost everything on your webpage. Lots of good info, thank you!
As you say Eric, one does not want the keel to bend permanetly!
Stressed skins is as you say the best way to go. I belive what you say about mixing composites and metals. But still, many boats have steelgirders laminated in to take keel, mast and shroud loads, like X-yachts. It seems to work good.

Yeah, the squaretube I mentioned feels to light, I agree! I will calculate a better one of course! But I must say, making the rudderstock in carbon is appeling, but more work the cuting some metallpieces and welding it to a tube/roundbar and covering it with foam and grp. Maybe the stock and structure could be in aluminium. Easy to work with. But composites feels much better... Any good tips on how to calculate a composite rudderstock?
Of course, since the loads are the same, one could use the ABS formulas for solid and hollow stock, and the backtrack it to find how strong the carbon needs to be.

Eric, the Scandinavian Cruisers looks like really nice boats! Have to take a look at one in real life some day, Denmark is close

And finally, I will have to rebuild some of the bottom/keel structure to fit the new keel, if I build a composite fin, then a composite structure inside the boat is the way to go of course, but could not a primary loadstructrure in weldox steel work good to, and "just" laminate the bottom watertight so to say?
I need a good strong frame to tie in the shrouds, as I said, backswept spreaders and no runners, so alot of load there. Steel would be much stiffer than glasfibre, but carbon would be even stiffer?
Or maybe a carbon/glass frame/girder construction would be "bad" when grounding in high speeds?
In some ways it feels like carbon is overkill on an old boat like this, its built from csm and polyester. But then again, why not?

Thanks again for your replys, and I appolgise again for taking so long to answer them.

Best regards
Robin Larsson

[Eric Sponberg]

Robin,

A couple of thoughts:

Weldox looks tremendous. The lowest grade, Weldox 700, is stronger than a typical carbon laminate. An average strength for a quasi-isotropic laminate in carbon would have a strength of about 65,000 to 70,000 psi (450 - 480 Mpa). In a good carbon UDR mast or rudder laminate, you can expect a strength of maybe 100,000 to 120,000 psi (690 - 830 Mpa). The Weldox 700 has a yield strength of 700 Mpa and an ultimate strength of 930 Mpa max, which eclipses carbon. The highest grade of Weldox, #1300, has strength values of yield = 1300 Mpa, ultimate of 1700 Mpa. This is way up there. Since they show examples of welded keels, presumably this steel has suitable corrosion resistance in the welded condition. Typically, the modulus of elasticity (stiffness) of steels is around 30,000,000 psi (207 Gpa). Presumably, Weldox is similar, since it is a steel. Carbon laminates of standard grade carbon and quasi-isotropic construction have a modulus of elasticity of around 8,000,000 psi (55 Gpa), and good UDR laminate can get up to about 11,000,000 psi (75 Gpa). Steels are way stiffer than carbon laminates. It is because they are so much heavier--6 times the weight--that carbon is attractive in many cases.

In a keel, you really don't have to worry about weight. You want weight down low, so steel is OK.

One of the drawbacks of steel is its ease of fabrication into a true aerofoil shape. This takes some real skill to get a good aerofoil shape for proper lift. This is very easy to do in carbon. And one of the reasons for making carbon keels with wood cores is that the wood is a good solid material that supports the carbon skin and it adds considerable mass and resistance to damage to the keel. A wood core also makes it easy to fabricate the keel.

Rudder design and construction is more complicated than a keel because the loads are harder to define--they depend on the speed of the boat and the angle of attack of the rudder at any given time. Also, all the loads have to transfer from the blade to the stock which is an incredible transition, and this is usually why composite stocks descend deeply into the blade. You have to determine the load distribution along the rudder stock for the combined bending and torque loads--each rudder is different, and there is no rule of thumb or easy engineering solution. Once you know the loads, you have to calcuate the diameter of each section along the rudder stock and determine its corresponding wall thickness for that diameter to give you the strength and stiffness that you need for the materials at hand. Once you get the diameter (actually, stocks tend to be trapezoidal shaped inside the blade, and round above the blade, so you have to calculated properties with these shapes) and the wall thickness, then you have to convert that into a laminate schedule for the materials at hand. Everyone uses different fabrics (weight, thickness, width, etc.) so each design is tailored to the design at hand and the builder so that he can build it right and come out with the diameter, thickness and properties that were intended.

That is why I say, with your situation, you have a lot of variables that you are dealing with and considering, and it is best to consult with someone who knows what they are doing to come up with a properly engineered solution.

Thanks for the link about Weldox--that seems like really neat material. I wonder if it is available in the US, perhaps under a different brand name.

Regards,

Eric

[rob denney]

Originally Posted by Robin Larsson

Hi everybody, sorry to take so long time to reply, been way to busy.

Rob, thank you very much for the figures and your help! The prices sounds really good for the carbon, might just take you up on that! But the Tow, is it approxematly the same "size" as standard fibreglass tow? About 1cm wide?
Do you also sell carbon uni at a good prices?
Yeah, as you say, a keel is not at difficult thing to engineer really. But I see one big downer with carbon, or really any composite, if overloaded it will snap right of, wont it? Or maybe its not so difficult to make it withstand grounding loads after all?

No, standard bolt arrangement doesent feel right with a carbon keel, it would have to be a box I think, maybe even make it lifting, if only just like 80cm, then it could be contained in the rigid part of the saloon table.
Same with the bulb, but I have an idéa, make it almost like the composite chainplates that for example some Farrier tris have, and simply run a long bolt through the bulb and keel, lenghtwise. And glue. The way Eric Sponberg designed it on Bagatelle looks good also.

But I´ve "discovered" the steel Weldox, www.weldox.com a Swedishmade high strength steel. Much cheaper then carbob, almost a factor 10 based on weight, but of course carbon would be lighter, so the differens is smaller. it feels like an easy way to make two stressed skins and some internal members, both horisontal and vertical in weldox. I have the machines to watercut and bend them so. And also good welders. But it would need good rustprotection.

Eric, thank you for your long and very good answer! I´ve read your articles in the digital issues of Proboat(Pericles, thanks for the link!) and almost everything on your webpage. Lots of good info, thank you!
As you say Eric, one does not want the keel to bend permanetly!
Stressed skins is as you say the best way to go. I belive what you say about mixing composites and metals. But still, many boats have steelgirders laminated in to take keel, mast and shroud loads, like X-yachts. It seems to work good.

Yeah, the squaretube I mentioned feels to light, I agree! I will calculate a better one of course! But I must say, making the rudderstock in carbon is appeling, but more work the cuting some metallpieces and welding it to a tube/roundbar and covering it with foam and grp. Maybe the stock and structure could be in aluminium. Easy to work with. But composites feels much better... Any good tips on how to calculate a composite rudderstock?
Of course, since the loads are the same, one could use the ABS formulas for solid and hollow stock, and the backtrack it to find how strong the carbon needs to be.

Eric, the Scandinavian Cruisers looks like really nice boats! Have to take a look at one in real life some day, Denmark is close

And finally, I will have to rebuild some of the bottom/keel structure to fit the new keel, if I build a composite fin, then a composite structure inside the boat is the way to go of course, but could not a primary loadstructrure in weldox steel work good to, and "just" laminate the bottom watertight so to say?
I need a good strong frame to tie in the shrouds, as I said, backswept spreaders and no runners, so alot of load there. Steel would be much stiffer than glasfibre, but carbon would be even stiffer?
Or maybe a carbon/glass frame/girder construction would be "bad" when grounding in high speeds?
In some ways it feels like carbon is overkill on an old boat like this, its built from csm and polyester. But then again, why not?

Thanks again for your replys, and I appolgise again for taking so long to answer them.

Best regards
Robin Larsson

G'day.

The tow is 50k, which means it is pretty thick. It is about 10mm wide. We use a wet out machine as it is hard work wetting it by hand. Uni is about twice the price. A decent size order, prices come down.

The fore and aft bolt idea is cool, could be fun lining it all up though. An oversize hole in the keel and lots of epoxy around the bolt.

Metal work in the boat is a great way to transfer loads. It is a boit heavier than composite, but the main problems are corrosion and transferring the loads from the metal to the composite. Both car fixable, but need a bot of thought and planning.
Carbon structures can be as flexible as you like (fishing rods for example), but the stuff does break instead of bend. Whether this is a problem or not is your call. The FI guys survive crashes in their carbon cars far better than they ever did in their metal ones, so it can be done.

regards,

Rob

[Robin Larsson]

Eric,

Yes, Weldox looks really great, and the 700grade seems to cost about 2.5Euro/kg so its alot cheaper then carbonfiber. I should thinks its available worldwide, but I dont know. But I have seen a good many raceboats on the web with Weldox keels, and we dont build much high performance raceboats in Sweden sadly..

Is´nt carbon stiffer then that? Okey, thought I´ve seen better figures, but thats probably for just the fibers, not laminates.

Yeah, getting the aerofoil right with steel can be a bit tricky, will have to test it in our bendingmachine, and I will also watercut horisontal foilshaped frames, with 0.1mm accuracy. And I can always fair the keel afterwards, but the better the steel is shaped, the better.
But Weldox is not a stainless steel, so rustprotection is vital I belive. Maybe a layer of thin glassfiber/epoxi will do the trick?

Even if its fairly easy to shape the woodcore for a carbon keel, and I also now how to laminate, it still feels like it would be more work to build in carbon then steel. Atleast when I have our CNC machines here.
If one builds a keel, or for that matter, a rudderstock in carbon, isnt it vital to get more or less the same tension in all fibers? How to do that easily?

Eric, it does seems like its alot of work to design a carbon stock, but it would be nice to have one.. Will have to think about it more.. A metallstock seems much easier to calculate. Maybe alu is a good choice?
But maybe a bit harder to get the rudder thin enough to be effective?
I remember reading about a Class 40 boat where they made a rudderstock(or was it a core in a sternmounted rudder, dont know) of some high strength steel, dont think it was stainless, and they came out pretty light, not much heavier than carbon. Anyone knows something about that?

Rob, thanks for the info! Yeah, can imagine that its a bit hard to wet out be hand. About how much uni is a decent order?

I must say that I feel that a keel and bottomstructure that bends and deforms under extreme overload is better than one that breaks into carbondust... In F1 racing they dissipate the loads by breaking the car into to almost nothing, dont want that on my keel really

Hmm, a few steelframes made in weldox would be nice, of course a bit heavier, but stiffer.. But then again, it needs to be laminated to the hull really good, even if I place both the mast, keel and chainplates in the construct. And then if I´m gonna do all that laminating job, why not put a bit more glass in it and get rustproof composites instead? Its that elusive stiffnes.... Would be a great way to achive(maintain) tension in the rig.

Thanks for all help so far!

Best regards
Robin Larsson

[rob denney]

G'day,

A decent order is enough for me to ship it direct from the USA. 10 kgs of tow or a couple of 300mm wide rolls of uni. Single rolls of either I can do from here, but they already have 10% tax and freight added to the price.

regards,
Rob