Swing Keel on a Cruising Boat

[TomL]

I just started sorting through this thread and found it very interesting. I sail a Craig Walters designed 47' centerboarder...Draft is 2'6" board up 8'3" with the keel down...The centerboard a very nice elliptical shape, very fine foil and when fully lowered a cassette completely seals off the trunk very clean.
Construction is composite. Sailing downwind in heavy air retracting the board all the way is the way to go, she just slides on waves and acts like a dart...(Don't want to do this in close quaters racing because she only turns straight!) never has come close to broaching as there is nothing down below to trip over. The two main draw backs I have is the interior volume lost to the trunk
AND, I feel bad when other boaters assume there is deep water around me when I'm on the hook in 3' or cutting a corner...happens everytime.

[GaryHLucas]

I keep seeing mention of aluminum oxide being soft. what is the most popular abrasive used to make sandpaper? Aluminum Oxide, because it is extremely hard! Hard anodized aluminum is very resistant to abrasion. However if the load is high enough to move the aluminum underneath then it goes away in a big hurry. Aluminum is one of the best electrical conductors. Guess what high voltage power line insulators are made from? Aluminum Oxide, because it has among the highest electrical resistance of all materials, and is very strong. So the reason that aluminum oxide protects aluminum so well is that it is a very hard insulator.

[Deadeye]

Wow, I've only been here a week and I found a thread where my own experience is relevant. If I may, I've been building aluminum boats professionally for about 5 years. I just left off it a year and a half ago to start building my own boat.

We built water taxis and planing trawler-styled yachts to 45'. We installed Volvo D6s so I can't speak to bonding prop shafts and the like, but the zincs are different and, yeah you have to be very conscious of dissimilar metals and stray current corrosion.

The problem isn't with stainless (dropped screws, etc) but more with wire trimmings. ABYC actually requires that any electric terminals attached to aluminum must be tinned (they should be regardless...) We had threaded spuds welded on to the hull for thruhulls and threaded stainless nipples into them, so no brass or bronze and uber-strong. The zincs are specific to aluminum as well.

The biggest problem was getting the paint to stay on the hull, which brings me to aluminum oxide. Yes, it's damn near hard as diamonds and nonporous (corrosion doesn't escalate as it does with steel) BUT the oxide isn't bonded very well to the substrate (uncorroded aluminum underneath). Essentially, the paint bonds to the oxide and the oxide lets go from the hull, taking the paint with it. We wound up having to sand and etch prime it, and often there were still bubbles resulting from any scratches in the paint. In these waters, there's loads of deadheads so the paint shop was always busy.

The solution was to leave it unpainted, as we did with the workboats. Abrasion will rub the oxide off but it does re-oxidize really quickly. In areas of frequent abrasion like hawsepipes, the aluminum eventually gets polished to a mirror shine. As long as the boat is 'used', this polish is maintained and no corrosion forms (I don't know why).

Personally, I would never get an aluminum boat unless I was staying in waters near yards that had experience working on aluminum boats, including welding. That, and it sounds gawd-awful when you're on plane: we wound up spraying in 12" of foam to to try and kill the noise, but it came off in big chunks because it wouldn't bond to the metal.

Oh, and aluminum is a great heat-sink if you're in cold water....

My .02

[johnshasteen]

Swing keels are fine, just know that they do require maintenance from time to time. If you are in Souther waters where your boat only comes out of the water every two or three years, that's something to consider.

[Leither]

Originally Posted by GaryHLucas

I keep seeing mention of aluminum oxide being soft. what is the most popular abrasive used to make sandpaper? Aluminum Oxide, because it is extremely hard!.

Gary, I think you have got yourself confused here. The form of Aluminum Oxide used for sandpaper is corundum which has relatively big crystals and is, as you say, extremely hard. It is made industrially from bauxite (the main ore of aluminum). The oxide coating which forms on the surface of aluminium is an amorphous form of the oxide and does not have the same properties as corundum. The coating is thin and relatively easily removed. For this reason, it is often made thicker for certain applications by anodising or hard anodising.

Stuart

[Deadeye]

It is made industrially from bauxite

Leither, that makes perfect sense - I learned something new today. On the sailboats I used to build, we sent out the aluminum exterior trim to be anodized and it came back really hard.

[Leither]

Originally Posted by Deadeye

Leither, that makes perfect sense.

Thank you, Deadeye - that may be the first time anyone has said that to me on this forum!

Stuart

[camaraderie]

Let's try not to make a habit of it Stuart!

[Deadeye]

Sorry Stuart, new guy and all. I won't make that mistake again !

[Jim H]

We had a great time at the London Boat Show (spend seven hours there with the kids), and we learned a lot about the Ovni 395 on display there from the UK distributors and an Ovni owner. If I don't work 13 hours today, I'll post some pictures from the show and share some of the points we learned.

One thing I'll note-- the interior of an Ovni is practical. The woodwork is simple, and the access to mechanicals and hull are good. Stepping off a HR or Island Packet onto an Ovni could be a real culture shock to some, but so is the price difference, and personally I like the practical and simple interior (especially with kids and a son who is a serious fisherman).
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