Need advice: Very thick fairing over cast iron keel

[Adelie]

I have acquired a Cal20. The keel is cast iron with bulb at the bottom with most of the mass in it. The trade off is that the fin portion of the keel is very thin relative to the chord. Class rules indicate the fin should be 3/4" thick. To get to a decent shape like NACA-0010 or -0009 it needs to be 4.5-5.0" thick.

To get that I need to build up about 2" of thickness on each face of the fin.

My thought would be build up most of the way with Bondo the a couple of layers of glass and mat saturated with polyester.

Anybody have experience with that amount of build up?


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[Jim Cate]

The usual caveat: "I ain't no expert..."

If I were faced with that problem, I'd generate the foil shape in structural foam, bond the foam pieces to the fin and then glass over the area for strength. Or one could use timber instead of foam.

Shaping the foam or timber is much easier than doing it in Bondo or other filler material, and should give a stronger structure, not prone to cracking, etc.

But, I ain't an expert...

Jim

[leftbrainstuff]

+1 for foam. Bondo is not a great 'marine' product.

Muxh easier to fair foam. Then overlay glass.

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[Adelie]

Suggestions on what foam to use and how to bond it to bare metal?


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[minaret]

Originally Posted by Adelie

I have acquired a Cal20. The keel is cast iron with bulb at the bottom with most of the mass in it. The trade off is that the fin portion of the keel is very thin relative to the chord. Class rules indicate the fin should be 3/4" thick. To get to a decent shape like NACA-0010 or -0009 it needs to be 4.5-5.0" thick.

To get that I need to build up about 2" of thickness on each face of the fin.

My thought would be build up most of the way with Bondo the a couple of layers of glass and mat saturated with polyester.

Anybody have experience with that amount of build up?


Sent from my iPhone using Cruisers Sailing Forum

2" either side is madness. If you use filler, it will certainly crack, even after being glassed over. Unless you glass the hell out of it. If you fair it in foam, you will also want to do a pretty substantial layup. If you use a filler, make sure it's a good epoxy with high density filler ($).


Here are a few pics of how I make changes in keel shape-this prevents the keel ending up with a twist in it or any other asymmetrical feature. I use this method for very accurate keel shaping to barely fit within class rules. You can do a much faster and cheaper version using ply for templates.



In these pics I am actually going the other way, ie removing lead via power plane and then final fairing to exact shape. But the same method can be used to build up. Set up the framework of templates, and then fill the gap between each template and the keel using filler with dye in it (I use blue steel dye). Then pull off the framework of templates, and fill in the gaps between the filler stripes with filler of choice (in your case I might think outside the box and go with something like Portland cement-maybe). Obviously if you set the templates up so that they are as far apart as your biggest trowel, you can screed fair without using a large screed. Or, further apart with a big screed for less templates required (better for big keels).Float your fill a little high and board fair till you see the blue stripe-stop immediately when you see this. You could do this with foam as well as filler, or practically any material at all. It's a big time saver if you want a fair keel that doesn't have any twist or assymetricalities.

[Azul]

https://www.youtube.com/watch?v=yGFzgYfbjBg

Use the blue foam at Home Depot, as in the video. It shows you how to shape it etc.

Foam Lok Adhesive Spray | Demand Products

[toddedger]

I don't like foam under the water unless something like Clark foam, but that really expensive stuff. I might consider some G10 epoxy sheets glued on edge to the keel. Filling in with 3m high strength vinyl Ester filler. In fact you could use the 3m to glue hold the G10 on the keel. You can run a straight edge across the G10 strips to level out the filler. You can put multiple layers vinyl Ester filler on top of it's self and it will stick to itself. When you have the basic shape switch to regular strength 3m vinyl Ester filler, it sands easier and you can put it on smoother. Remember building up with small smooth layers means less sanding. I'm assuming 2" is just at the maximum chord. You won't need to glass over it when done, and there will be very little shrinkage unlike epoxy.

[roverhi]

Micro-balloons and epoxy are what they used to add all those bumps to modify boats under the IOR Rule. Probably still will crack but probably about the best system for a straight filler. Wonder if that flexible epoxy that West markets might work for a permanent fix.

[toddedger]

Epoxy is a bad idea. There's going to be to much mass. Think heat, plus every time you want to build on top of it your going to have to abrade the surface first. Vinyl Ester filler is far superior for this. The 3m high strength has glass in it so it won't crack. It's much easier to work with and it's made to be used under water. You would definitely want to rough up the metal first for good adhesion. Some screws or welded on metal sticking up would help give some mechanical adhesion. The only chance of failure would be it falling off in one big piece from not sticking to the steel well enough. I've made some big high volume fairing parts out of the stuff and it's held up really well.

[Delfin]

Originally Posted by minaret

2" either side is madness. If you use filler, it will certainly crack, even after being glassed over. Unless you glass the hell out of it. If you fair it in foam, you will also want to do a pretty substantial layup. If you use a filler, make sure it's a good epoxy with high density filler ($).


Here are a few pics of how I make changes in keel shape-this prevents the keel ending up with a twist in it or any other asymmetrical feature. I use this method for very accurate keel shaping to barely fit within class rules. You can do a much faster and cheaper version using ply for templates.



In these pics I am actually going the other way, ie removing lead via power plane and then final fairing to exact shape. But the same method can be used to build up. Set up the framework of templates, and then fill the gap between each template and the keel using filler with dye in it (I use blue steel dye). Then pull off the framework of templates, and fill in the gaps between the filler stripes with filler of choice (in your case I might think outside the box and go with something like Portland cement-maybe). Obviously if you set the templates up so that they are as far apart as your biggest trowel, you can screed fair without using a large screed. Or, further apart with a big screed for less templates required (better for big keels).Float your fill a little high and board fair till you see the blue stripe-stop immediately when you see this. You could do this with foam as well as filler, or practically any material at all. It's a big time saver if you want a fair keel that doesn't have any twist or assymetricalities.

Very nice, and sound advice.

[boatbrain]

You did not ask this question and please don't be offended but I think you should carefully consider if it makes sense to put a lot of money and time in the boat when you can get many small boats for free down in San Diego and LA areas - I am sure many would need work before being ready to race but starting with the keel seems like you might do a lot of work, spend a lot of money and then not be able to sell it if you wanted to.

[reed1v]

My old Ranger 26 had a cast iron keel(actually made by the same folks that made the Cal keels. Grind down to the iron, ospho if you can find the stuff, then apply epoxy puddy to fair(more you fair, the less you will need to sand later), then cover with a good poly one part or two part paint. Then cover with bottom coat. We used Trinidad Red and never had a problem with the iron rusting or the bottom paint falling off. That also solved water migrating up the keel flange. Inexpensive and about a permanent fix as you would want. BTW: Cal 20s have crossed the Pacific Ocean several times. Very durable, if wet, boats.

[ramblinrod]

Originally Posted by Adelie

I have acquired a Cal20. The keel is cast iron with bulb at the bottom with most of the mass in it. The trade off is that the fin portion of the keel is very thin relative to the chord. Class rules indicate the fin should be 3/4" thick. To get to a decent shape like NACA-0010 or -0009 it needs to be 4.5-5.0" thick.

To get that I need to build up about 2" of thickness on each face of the fin.

My thought would be build up most of the way with Bondo the a couple of layers of glass and mat saturated with polyester.

Anybody have experience with that amount of build up?


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I'm surprised nobody has yet asked the question, "Why?"

Why would one defy class rules to change the shape of a keel?

Are you competitively racing at a national level? If so, you don't want to defy the class rules.

Are you not racing at a national level? Then you are wasting your time on an old boat, that you could be out sailing.

If you enjoy glassing more than sailing, you could go work for a fibreglass tank manufacturer and get paid for it.

Don't use buoyant foam as a structural filler on a ballasted keel.

It will raise the COG (centre of gravity) and reduce the righting moment.

PS, the filler in Bondo is talc and it doesn't like water very much.

If you wish to proceed just for the fun of experimenting to see what difference it makes on performance, go for it. (But I expect you would need a pretty sophisticated test approach or a lot of before and after boat on boat comparisons to draw any meaningful conclusions based on reality.)

Sorry to be a naysayer, but IMHO, the likely outcome is a lot of time, sweat, and money spent for negligible performance change and reduced resale value.

Ramblin Rod
About Sheen Marine

[reed1v]

Bondo will come off in sheets so need a waterproof barrier between it and the water. Fat chance of that happening unless you use some type of vinyl esters to seal it. Even then, water will migrate into the glass and under the bondo to pop it. People tried that back in the 70s.

[Cadence]

Originally Posted by Jim Cate

The usual caveat: "I ain't no expert..."

If I were faced with that problem, I'd generate the foil shape in structural foam, bond the foam pieces to the fin and then glass over the area for strength. Or one could use timber instead of foam.

Shaping the foam or timber is much easier than doing it in Bondo or other filler material, and should give a stronger structure, not prone to cracking, etc.

But, I ain't an expert...

Jim

Jim beat me to it. Bondo is good the fairing not build up. I would laminate the thickness with wood and then glass it. I used foam for strings but only as a form for a lot of glass, multiple roving and matt layers. I could care if the foam dissolved, in your case it could be catastrophic.