Centerboard Trunks on Aluminum Boats Maintenance Questions

[UNCIVILIZED]

I have a few questions about the maintenance & coatings on aluminum boats which I could use some help with. Specifically, on aluminum boats with centerboards, I’m wondering how one deals with keeping up both the bottom paint, & especially the barrier coat inside of the CB trunks. Plus, whether or not it’s common to have dedicated anodes for the trunk walls?

The reason I ask is that I figure that the trunks get a lot more wear than does the hull, given that the centerboard is continually rubbing on one side of the trunk, or the other, or both. Especially when under sail. As then the board is being firmly pinned against one side of the trunk by hydrodynamic forces, but it's still moving slightly under a lot of pressure, via the lift it’s generating. So then the wear on any & all coatings would be greatly accelerated by this. Right?
And I reckon it wouldn’t be easy to monitor the condition of the coatings inside of the trunk either, unlike those on the hull. Which for me, is a cause for concern. Particularly with aluminum's fragility along those lines.

So, I’m wondering what kinds of preventative measures that one can take to ensure decent coating life inside of the trunk. As well as how to monitor the trunk coatings' health, if such is even possible. And then, when it’s time to renew the bottom paint, what’s the process for doing the trunks? As on the hull, you’d inspect things, both before & after prepping the old bottom paint for adding fresh antifouling. And touch up any barrier coat where things were at all in question. But as to doing this inside of the trunk... I'm at a loss on the procedures & techniques. Ergo, I need to borrow some wisdom & experience.

Thus, it seems that protecting the inside of a board trunk is another matter entirely. So I’d surely welcome any tips & insights as to the care & feeding of same. And as to CB trunks, are they typically constructed with thicker plating to compensate for reduced access for maintenance?

Any & all wisdoms on this would be greatly welcomed! Thanks.

[noelex 77]

I have never owned an aluminium boat with a centreboard case, partly for the reasons you describe. A fixed keel is much simpler and foolproof.

However, I don't think the problems are as great as you imagine. Uncoated marine grade aluminium does not corrode in salt water. It only needs a protective coating (paint) to isolate the antifouling paint. So the epoxy barrier coat only needs to be applied where the antifouling paint is applied. The top of the keel case is dark so will not get much growth.

A bigger problem is isolating the dissimilar metals in the pivoting/lifting mechanism, particularly when hydraulics are needed to control the centreboard/keel.

Aluminium is much more resistant to abrasion than fibreglass so I don't see this as much of issue, although the centreboard can make quite a clunk when sailing, making sleep difficult on some models.

I did meet an owner that had a sealed centreboard case and while at anchor pumped air into the case to drive out the water killing growth. It seemed a great solution, but is hard to retrofit and I have not seen it used on other boats.

Hopefully someone with first hand experience will respond shortly.

[UNCIVILIZED]

Originally Posted by noelex 77

I have never owned an aluminium boat with a centreboard case, partly for the reasons you describe. A fixed keel is much simpler and foolproof.
I love the idea of a good aluminum boat, period. Though finding one with a good CB setup would rock! As she'd be as shoal draft as a lot of catamarans, & still have the inherent strength of a metal boat. Both of which I'd love to have.

However, I don't think the problems are as great as you imagine. Uncoated marine grade aluminium does not corrode in salt water. It only needs a protective coating (paint) to isolate the antifouling paint. So the epoxy barrier coat only needs to be applied where the antifouling paint is applied. The top of the keel case is dark so will not get much growth.
Right. My big concern is the board abrading coatings off of the inside of the trunk walls.

Also, I posted this query over on SA Forums, & a gent there suggested that it would be simple enough to add HMWPE onto the sides of the CB's upper end. Which should cut down on friction between it & the trunk by a large amount.

And 2 hours ago, another poster added that I could simply contact the companies which routinely build these boats. As they'll have long since worked out solutions to these problems
So I'll probably do that, along with asking for lots of literature & info.

A bigger problem is isolating the dissimilar metals in the pivoting/lifting mechanism, particularly when hydraulics are needed to control the centreboard/keel.
I should think that it'd be fairly easy to isolate the pivot pin from the other bits via isolator sleeves akin to those used on keel bolts in aluminum bolts.

The bigger "trick" is having a huge chunk of more noble metal that's the CB, housed inside of the aluminum CB trunk (case). Albeit the board should be well barrier coated, prior to painting it with antifouling. Or in theory, one could encase it in epoxy & cloth.

Aluminium is much more resistant to abrasion than fibreglass so I don't see this as much of issue, although the centreboard can make quite a clunk when sailing, making sleep difficult on some models.
Yep, I hear you on the clunking issue. No pun intended. As all of our boats, growing up, had centerboards. And the sound always made my Dad worry (mildly) about them maiming their housings to the point where the ballast would tear out. It's quite funny in retrospect.

Anyway, it is possible to put rubber or plastic discs onto the pin, to shim things. So as to cut down on how much play there is in between the board & the trunk. But I'd wager that the designers have long since figured out how much play in CB's is acceptable.

I did meet an owner that had a sealed centreboard case and while at anchor pumped air into the case to drive out the water killing growth. It seemed a great solution, but is hard to retrofit and I have not seen it used on other boats.
I can't imagine that it'd be tough to retrofit, as the case is already WT. You'd just need to add a fitting to the case which you could attach an air hose to.

On my Searunner's (tri) CB trunk, it was super easy to pull off the top off of it, even with the boat in the water. And no water came into the boat when you did so.
So now I'm thinking that perhaps some of these aluminum CB mono's are built the same way. Or that they could have their trunks rebuilt to be similar in configuration. So that then you could pull off the cap, & use the access in order to both inspect the insides of the trunk (with the board out). And to do maintenance on them this way.
Thoughts?

Wish that idea had come to me earlier, but I appreciate your comments in helping to stir the thought(s), & to help it rise to the top of my consciousness!

Hopefully someone with first hand experience will respond shortly.

In what scenarios are you thinking that hydraulics are needed to raise the board vs. a pendant & a winch?
I know that having a hydraulic lock down mechanism, which is also designed to allow the board to kick up at a set loading makes sense. That way the board will stay in place in heavy weather, but be kicked up should you strike a rock in the shallows. Though I'm curious to hear other thoughts on this. Plus, I need to go & check out the systems on Skip Novak's boats again. Given that both of them have big CB's.

[noelex 77]

Originally Posted by UNCIVILIZED

should think that it'd be fairly easy to isolate the pivot pin from the other bits via isolator sleeves akin to those used on keel bolts in aluminum bolts.

The bigger "trick" is having a huge chunk of more noble metal that's the CB, housed inside of the aluminum CB trunk (case). Albeit the board should be well barrier coated, prior to painting it with antifouling. Or in theory, one could encase it in epoxy & cloth.

The centreboard is normally solid aluminium. If it is a lifting keel, it is aluminium with internal
lead ballast. The lead is sealed within the aluminium structure and does not come into contact with water so there is no dissimilar metal problem. There have been some aluminium boats built with centreboard/keels made of other materials (such as iron), but it is rare and would make me very nervous. It is a big lump of dissimilar metal.

Keel bolts on an aluminium boat are not a great idea. The keel is usually integral and welded. No bolts or dissimilar metal to worry about.

[Matt Johnson]

Ours is currently just barrier coated up to the waterline within the trunk and some bottom paint brushed in as far as they could reach from below. We need to apply the barrier coat again as it's chipped and the rest of the boat's bottom is stripped, but it's not happening at this haul.... we've got enough other things besides dropping the centerboard.

There were so many barnacles wedged in between the trunk and board when we purchased the boat, we had to use a sludge and hydraulic jack to get it to drop. So, the PO must not have ever cleaned the slot like everything else on the boat. It'll just get slathered with paint and we'll watch it until we have a chance to drop it in a year or two.

If you find out what Ovni or Boreal use, please update with the info.

Matt

[Snowpetrel]

Most aluminium CB cases have some sort of plastic (HDPE?) Strips attached to reduce friction, noise, slop and avoid alloy on alloy contact. I am not sure how they commonly attach them. As mentioned any shaft holes are bushed and the whole lot protected by anodes, so it shouldn't be any worse than any other part of the boat. Matt (funjohnson) is refitting a French trisallu with centreboards . Might be worth asking him how they are made. he has some issues with the aft ones due to them having been mistreated by the PO. http://mjsailing.com

Also look up Andrew Troup on here, he was on here a few years ago and had some great ideas about how to make a steel centreboard work with an alloy boat he was designing. Might be worth a search. One cunning plan was to make the case airtight and inject pressurised air into the case so less marine growth would grow while the boat was sitting.

[Matt Johnson]

Also, how would one even get in a centerboard slot to prep for barrier coat? Do they make blasting extension wands like used for power washers? Our slot is 3" wide and waterline is 3.5' up.... no way am I able to sand the surface, and I can't see the angle working for blasting from below with normal equipment.

[UNCIVILIZED]

Originally Posted by funjohnson

Also, how would one even get in a centerboard slot to prep for barrier coat? Do they make blasting extension wands like used for power washers? Our slot is 3" wide and waterline is 3.5' up.... no way am I able to sand the surface, and I can't see the angle working for blasting from below with normal equipment.

On my Searunner the cap on the CB trunk was only held on with wood screws & silicone, so access was pretty easy. Though the trunk was about 2" wider than yours, at 5". But vertically, the depth was about the same, as she had a shallow, hollow, fin keel which was part of the trunk. Making her draw 2 1/2', & the trunk ended 6"+ above the WL.

So if one were to build the board trunk on a monohull similarly, it would let you have access to things from the top down, as well as from below, upwards. And access from both ends would make both sanding & painting inside of the trunk much easier. Especially if it's built a bit wider. Though of course you'd need the cap to bolt on, in order to ensure that things stayed WT. Vs. the svelte attachment system that my tri had.

One other trick thing you could do if there was a removable cap on the board trunk, is to put an access panel into the cabin top. So that with it & the CB trunk cap removed, the board could be both installed & removed from above. Most likely with the boat still in the water. Much like I could do through the slot for the CB which was in the cockpit floor. And that feature made servicing things very easy.