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[adventuren1]

Hi all,
Not sure why I didn't join this group sooner but happy to be here now. Plus I dumped Fakebook and lost the access to the valuable MacGregor pages there. I am a retired boat bum that spent 2 years living on a 1999 Mac 26X. I cruised solo, living on the hook from Puget Sound to Desolation Sound for one summer, and then trailer-ed it to San Diego, living full time at Pier 32 marina. I now live in Port Townsend, WA and am land locked while I search for a trawler to live on. I currently have a 1984 Mac 21 I picked up for "free" that I am rebuilding. The hull is in great shape except for one potentially big problem. But I haven't been able to put it in the water yet to see if it is even a problem. Having the weight of the boat on the trailer causes oil canning at the tips of the bunks on these boats, and others. I had a Catalina 22 that did the same thing, but it was harmless to the hull on that boat. My Mac 26x also did it, but very mild (but no heavy keel). The bad thing on this boat is that the oil canning pushed so far into the hull that it cracked the floor liner on the inside. Just above each aft bunk end, the glass is cracked inside the boat. There is no visible damage to the hull. Getting it in the water will be the test I think. Has anyone seen this? My thought is that it is caused by not lowering the keel onto the trailer while towing. That would transfer all that weight directly to the trailer and not use the boat to do it. Someone attempted a repair to reinforce the area inside the bilge, but it is a poorly done repair and prevents better inspection of the hull. (apologies if you know the boat and did the work).
So just wondering if I should expect water to seep through fine cracks in the hull I can't see, or if the cracks inside are common but the boat survives? Hoping for the later! Everything else with the hull is in really good shape.

Looking forward to learning from this group and sharing what I know.

[NiceAft]

First off, welcome.

To be honest, I had to look up “oil canning.”

The bunks on Mac trailers (at least X & M) form a vertical “V” to the length of the trailer. I was always told that this was to protect the thin hull of a Mac. Just wondering if the bunks of your trailer are like that, or do they parallel the length of the trailer?

[adventuren1]

The bunks on this trailer are parallel. Also, this only applies to boats with cast iron keels. The X, M, D, and S do not have this issue.The weight of the keel is hanging from the keel cable, which is directly over the aft end of the bunks, Resting the keel on the trailer will transfer the weight from the cable, directly to the trailer. I can't say if this is the best way to tow. But certainly for long term storage it will help prevent the crushing of the hull.

[BOAT]

I did not know what Oil Canning was until Captain Mike Inmon told me about it around 10 years ago. I guess metal boats that dented were called "oil cans" in the Navy. but then it became common in boats with the thick hulls made with a chopper gun. The hulls were thick and rigid and brittle with little flex so when pressure was applied to a small area over time it would put a 'dent' the hull that did not spring back. Squeezing out as much resin as possible from the layup helped solve the issue and made a better hull that was lighter and more flexible.

But, then the issue became rigidity.

Captain Mike did say changes were made over the years to make the hulls more rigid.

The ballast tank resolved all rigidity issues below the waterline. A spline was added later that is approx. 101 inches (almost 9 feet), ahead of the transom. This 7 inch wide spline rests right on the main bunk of the trailer directly over the axle. You can see the spline in the dinette. Here is a picture of it in the yellow circle:



The ballast tank was a long deep 'canoe' of a box that was pretty much indestructible. The only thing left flexing was the sides of the hull, (being thin and flexible on purpose), so a square transom, wider rub rail flange, and deep chines were designed to stiffen the whole thing up. The boat is still very "flexible" by design and yet sits on a trailer with very little support and zero reports of oil canning.

The entire rear of the boat including the motor hangs approx. 41 inches (almost 4 feet) behind the rear bunk of the trailer! Sometimes boats would sag, bend, or oil can in those conditions. Dry berthing thick hulled boats can cause oil canning too I'm told. I imagine the only reason a 21 foot MAC oil can is because it was stored on a non factory trailer or dry berthed.

[adventuren1]

It is on a factory trailer. I really don't think the hull is damaged at all because of the flexibility of these boats. But the opposing point on the inside liner just happened to be at bend in the glass, so no flexibility. Thus the cracks on the inside but none outside. But these boats are not water ballasted so the construction is different. It actually looks like the inner liner is the stringer/support for much of the hull. Beside the keel trunk, there isn't much else there.

[BOAT]

Yeah, if your talking about the liner those are still made the old way with a chopper gun and they are really brittle. If the hull flexes it can put cracks in the liner because the liner is not as flexible as the hull. In the M boat some of the upper liner is a chopper gun but all the lower liner is a layup so the lower liner is way stronger and more flexible than the upper liner.

You can see the lower liner in this picture next to the upper one.



The lower liner is ALL ONE Piece hand layup. The upper liner is three different pieces of varying quality - lower liner has structure and upper does not. In fact on the M I have a picture of my boat without the head wall in it and me jumping up and down on the deck proving that the upper liner does nothing to support the deck. That head wall is unnecessary.

Wait - your talking about the VENTURE with the single window? I was lost there - is the crack in the cockpit?

[Sheppie62]

Does anyone have similar picture of a 26x?

[NiceAft]

I just photographed this. It is of a photo on a page in an X sales brochure. I hope it helps somewhat.

[Jimmyt]

It is on a factory trailer. I really don't think the hull is damaged at all because of the flexibility of these boats. But the opposing point on the inside liner just happened to be at bend in the glass, so no flexibility. Thus the cracks on the inside but none outside. But these boats are not water ballasted so the construction is different. It actually looks like the inner liner is the stringer/support for much of the hull. Beside the keel trunk, there isn't much else there.

First and foremost, welcome to the forum! Glad to have you here!

On to your post.

Your boat, your rules.

If mine had a dent around the bunks while on the trailer, I'd be alarmed and taking immediate action. First, I'd install a keel support on the trailer to take the weight of the keel, and I'd relax the lift cable while the boat is on the trailer (as it seems you are considering). Second, (especially if taking the keel weight out of the picture didn't fix the problem), I'd increase the bunk areas to spread the support loads, and/or change support points to more rigid points in the hull.

I don't know about thin hulls, but a thick hull can take a set if you dent it from improperly supporting it for an extended period. Then, you have a hull repair issue.

My $.02 is; correct it as soon as you can. Hate that it already fractured your liner.

[BOAT]

Gee whiz I'm sorry adventuren1 I think some of us were confused about your boat. I thought the X boat made a crack but I think your actually talking about a Venture 21 with the single window and the swinging keel, right?

Gosh I remember that boat from when I was like 15 years old? I think it was the first Venture I ever saw at a boat show and it had a pop top I think (or maybe that was the Venture 25).

Any-ways, It's obvious you know your way around a sailboat but it's hard for some of us to imagine where the tips of the bunks are. In the picture you posted is the cracking in the area I circled in red in this picture??



Sorry I got everyone mixed up on X boats and power sailors - your trying to fix a REAL sailboat. There are guys on this site that know such things - they can help.

[Sheppie62]

Thanks Niceaft for the picture, sure looks like the ballast tank goes all the way forward.

[NiceAft]

According to the diagrams in the brochure, the ballast goes right up to the bow.

I didn’t realize you wanted info specifically about the ballast tank. My bad.

[Sheppie62]

That’s really interesting the way it spilts then comes back together. I always assumed tank was under floor also, but that makes more sense for head room. Does anyone have a picture of the ballast tank being installed?

[NiceAft]

You're really pressing it now.

Please add onto your post request, the winning lottery number for tomorrow.

If anyone has such a photo, it's got to be BOAT.

[BOAT]

I saw the M boat get built. I was at the factory and Mike gave me the tour - all bays.