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Okay I finally got some pictures of my Etap 26 keel since we took it off last week. This is a retractable bulb keel made from cast iron and weighs 1500 lbs. You can see the rusty portion which spends its whole life in the water inside the hull from the water line to the bottom of the hull, hence it can't be painted. It at first appeared very rusty, but subsequent scraping shows it is actually in very good shape under the rust.

You can see the fin is very thin, and the bulb is flat sided and foil shaped too. So the the keel should be pretty efficient.

The picture of the deck where the mast step sits shows how it is depressed. There is no coring here, the keel truck supports the deck, and has moved downwards about an inch at the front!

The picture of the keel trunk where it meets the hull at the forward end has a laser line projected onto it. You can see the tilted portion of the hull where it meets the one large stringer in front of the keel. This where the keel trunk has pusshed the hull downward nearly and inch.

On the outside of the hull you can clearly see by eye the bulge where the two stringers, one in front of the keel trunk and one behind have pushed out the bottom of the hull. In other words the designer of this boat clearly underestimated the loads imposed by a hanging keel and the mast pushing downward. Fortunately this design does take groundings well, because the long keel trunk bolted solidly to the deck means that the forces go straight into the hull at the bottom, with very little downward force against the weakened stringers.

I am now pondering how to fix it. From inside the boat I am very limited to what structure I can add, because of double hull and foam. I can beef up the stringer and fill the four inch gap between the keel trunk and the stringer, and stiffen the striger somewhat too. However since the stringers are showing signs of pushing right through the hull already, that probably won't add much strength.

An alternative would be to glass the bottom of the hull, builing up at least an inch around the keel trunk and smoothly tapering into the hull surface in all directions. This would essentially make the hull a plate thick enough to take the loads, and spread that load over a much larger area. A couple layers of kevlar or carbon fiber might add a lot of stiffness that regular glass wouldn't give. What a ***** doing all that work overhead! Build a round wooden cradle and roll the boat upside down?
 

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Telstar 28
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Gary-

Don't use kevlar if you want to add stiffness... it doesn't do jack for that... you can't push on a rope.. :) CF will add stiffness. What Kevlar does is it adds impact resistance...since it is very unlikely to tear in the case of an impact... it will often deform but remain water-tight.

You need to encapsulate the cast iron. Iron expands about 10% when it rusts...so any corrosion is going to cause serious problems.

What you'll probably have to do is remove the foam core material in a section around the keel and build it up with laminate... If there is any foam between the hull skins in that area, the hull won't be able to really handle the compressive loads it is going to see IMHO. YMMV...you will probably want a qualified marine architect to look at this though.
 
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Discussion Starter #3
Gary-

Don't use kevlar if you want to add stiffness... it doesn't do jack for that... you can't push on a rope.. :) CF will add stiffness. What Kevlar does is it adds impact resistance...since it is very unlikely to tear in the case of an impact... it will often deform but remain water-tight.

You need to encapsulate the cast iron. Iron expands about 10% when it rusts...so any corrosion is going to cause serious problems.

What you'll probably have to do is remove the foam core material in a section around the keel and build it up with laminate... If there is any foam between the hull skins in that area, the hull won't be able to really handle the compressive loads it is going to see IMHO. YMMV...you will probably want a qualified marine architect to look at this though.
I plan on sandblasting the keel, heating it to more than 250 degrees with a Tiger Torch to dry it out, then epoxy paint it. Went down this road with a Catalina 22 swing keel. It wouldn't dry out enough to epoxy without heating the hell out of it.

You misunderstood my reference to foam in this boat. There is no coring of any kind anywhere in the hull. There is also no bilge, you are standing right on the hull! The foam is about 100 cubic feet of foam between the double hulls. Enough to float the boat even with BOTH hulls breached! So access to interior spaces is essentially zero.
 

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Larus Marinus
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A liquid that really works as advertised is Rust-Away - Google finds it. I suggest you use a proper steel preparation undercoat. Epoxy is great afterwards, but it would be good to have a chemical protection for the steel too.

Have you tried talking to Etap about your problem? They are (were) very proud of their boats and may have some good solutions.

The down-force of the mast with tight rigging and an impact from a grounding together put a huge load on the area in front of the keel. What ever you do, it needs to spread that force over the largest area possible. CF + epoxy will give you a lot more strength than glass and polyester but it will be more rigid that the existing hull, so will need fairing nicely. Inside I would go for additional ribbing in CF+epoxy. Outside, you will need the boat on its back to do it properly.
 

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Telstar 28
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I plan on sandblasting the keel, heating it to more than 250 degrees with a Tiger Torch to dry it out, then epoxy paint it. Went down this road with a Catalina 22 swing keel. It wouldn't dry out enough to epoxy without heating the hell out of it.
Sounds like a good plan. Idiens idea to chemically prep the steel to prevent it from rusting prior to epoxying is a good one IMHO.

You misunderstood my reference to foam in this boat. There is no coring of any kind anywhere in the hull. There is also no bilge, you are standing right on the hull! The foam is about 100 cubic feet of foam between the double hulls. Enough to float the boat even with BOTH hulls breached! So access to interior spaces is essentially zero.
Technically, it is two thick laminates, with a foam core... They may call it a double hull... but in reality, it's just a really, really heavy laminate schedule IMHO. Yes, it makes access to the interior a PITA and it makes adding new through-hulls that weren't on the original manufacturer's design plan a royal PITA...
 

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Discussion Starter #6
Dog,
You are still not getting it. This is an Etap. It really does have an outer hull that is solid laminated. Then there is an an entire inner hull all the way up to the hull to deck joint! It has no openings in in anywhere except around the keel. Then they foamed the space between the two hulls with enough foam to float the 6000 lb boat. If you cut this boat in half with a chainsaw, both halves float!
 

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Dog, are you sure the deformity in the hull has been caused by normal load stress? i have experience of an Etap 26 in which the keel mechanism was used incorrectly and damaged the boat quite badly. In effect the keel was cranked down to far , probably while aground, causing damage to the tabernacle and rigging and also possibly the kind of bottom hull deformation you are concerned about.
 
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