Transom towers and righting moment

[Dallasron]

So I have seen some really big transom arches on some monos with a lot of stuff mounted up on them. Rigid solar panels, radar, wind generators you name it. So I was just wondering what effect all this could have on the righting moment in the event of a knockdown

[Manateeman]

I’ve seen some catamarans with the helm so high up I thought it was a set for a new King Kong movie.
I’ve seen a monohull with a permanent Bimini and a manatee on top. It looked like a one ton Mary Poppins ...(the manatee crew posted this description of our boat.)
I saw a tee top fisherman with a radar so big I thought it was a helicopter.
Let turn this question over to boatdesign.net or make more popcorn.
The manatee crew on too much coffee.

[tkeithlu]

If you know the weights and the length of the arm, subtract it from the weight/arm of the ballast. Whether that number is significant to coming back from a knockdown is going to depend on the size/ballast of the boat.

I used to own a 13' sailboat similar to a Laser that could not safely be left in the water with its mast up.

In 1962 the Doubloon did two 360 degree rolls off Charleston. Didn't have much of anything above deck after that one.

The Andrea Doria capsized (look at the pictures) because she was in violation of international standards for metacentric height. Too many cabins up too high.

[Manateeman]

It’s a bit difficult to give a complete answer to the OP original question without writing a book. The short answer is yes. Not just in cases of a knockdown. These forces are present all the time. You can find a thorough explanation in most books on yacht design. Look under stability. Both static and dynamic.
If it looks “wrong” it probably is wrong. When ships are loaded or unloaded, the Captain and crew are suppose to do a lot of calculations on stability. It’s part of the test to get a license. But errors are made. Look up the Golden Ray. An almost new, hugely expensive, car carrier. Almost rolled over in an important ship channel. And this is not the first time errors like this have sunk big ships.
So I guess the answer to your question is what are the consequences of adding stuff above deck level. Well, the added weight could be really bad for the stability of the vessel. The designer did the calculations, but how would he know what weights you were going to add. This weight up high might not look dangerous when the boat is at the dock, but in a storm, it’s a different story. Wind pressure.
My grandfather had a wooden boat with two completely different rigs. The winter rig had much shorter masts and we would add a ton of beach stone ballast. An American fishing craft design. So you had two boats, one for much higher winter winds and waves and more canvas, a taller rig, and less ballast for summer.
They didn’t calculate center of gravity, righting arm, hydrostatics or metacentrics.
Designs evolved from the users, from the needs of working vessels.
So you don’t need a computer to build a seaworthy boat, you can find lots of existing designs but adding weight, especially above the waterline, is something to think about. So to is removing weight. That almost rolled the Golden Ray.
You can learn to do the calculations but my recommendation is simple. Try to keep heavy things low, out of the ends of the boat and avoid manatees as vessel designers. Good crew, good boatbuilders. But...their idea of fending off ice bears with a pizza bazooka might not work.
Happy trails to you.…
Captain Mark

[Don C L]

The righting moment is a function of the hull and displacement. I am sure it depends on the boat and displacement of course, and the weight and especially height of the attachments. The math is the righting moment (RM) of any vessel = Displacement (delta) x GZ (lever) and
GZ = GM (metacentric height) x Sin(ϴ) (heeling angle)
But the effect on the righting moment, for most boats, by an arch, won't be a lot, considering the tons of ballast opposing it and that the distance from the center of gravity is probably, what, 1/5 the height of the mast?. The rate of roll (below 30 degrees of heel) when motoring without sails up, may be noticeably affected though by a sensitive skipper. Now if the boat is rolled, capsized, it is just one more thing resisting a recovery from capsize. To my eye, more importantly, is the windage those things create, especially for anyone who actually wants to sail upwind.

[MartinR]

Originally Posted by Don C L

But the effect on tTo my eye, more importantly, is the windage those things create, especially for anyone who actually wants to sail upwind.

Exactly. Another problem with these is that the last thing you want when running downwind in heavy weather/big waves is windage aft. This increases the risk of broaching and ultimately capsize.

[sail sfbay]

Originally Posted by Dallasron

So I have seen some really big transom arches on some monos with a lot of stuff mounted up on them. Rigid solar panels, radar, wind generators you name it. So I was just wondering what effect all this could have on the righting moment in the event of a knockdown

Your boat has a modest displacement and pretty stable so adding a tower like Atlantic Towers would not expect any issue. I ownered a Hunter 386 for years with a dinghy davits and sailed with the dinghy and motor secured high on it all the time....you would not even know it was there.

[sail sfbay]

Might want to read this article on righting moment:
https://www.sailboat-cruising.com/righting-moment.html

[PineyWoodsPete]

The paradoxical thing at play here is the positive effect of high-up mass on dynamic stability in keel boats - eg. rolling over in storm waves is much more likely if you've lost your mast and it's inertia resisting rollover in a sudden broach. In the Queen's Birthday storm between Tonga and New Zealand in '94 a group of 35 or so cruising yachts were caught up in a devastating storm, monster waves, many knockdowns. The boats that lost their rigs were subject to whiplash-quick rolling motion, and repeated 360 deg. rolls much more easily than those with rigs standing. Once in the water, mast/rig drag add to the resistance also.

Interestingly, the two cats - one a conventional Woods design, the other a much heavier Catalac survived with little damage, but were abandoned in the rescue along with many others.

[Jim Cate]

Originally Posted by PineyWoodsPete

The paradoxical thing at play here is the positive effect of high-up mass on dynamic stability in keel boats - eg. rolling over in storm waves is much more likely if you've lost your mast and it's inertia resisting rollover in a sudden broach. In the Queen's Birthday storm between Tonga and New Zealand in '94 a group of 35 or so cruising yachts were caught up in a devastating storm, monster waves, many knockdowns. The boats that lost their rigs were subject to whiplash-quick rolling motion, and repeated 360 deg. rolls much more easily than those with rigs standing. Once in the water, mast/rig drag add to the resistance also.

Interestingly, the two cats - one a conventional Woods design, the other a much heavier Catalac survived with little damage, but were abandoned in the rescue along with many others.

I think that you are confusing the losses in the Fastnet race disaster with those in the QB storm. I do not recall any of the QB vessels reporting rig loss with subsequent 360 degree rolls. Destiny did a pitchpole capsize and lost the rig but didn't do rolls later. No one knows what happened to Quartermaster (the only boat completely disappearing). I don't recall any other dismastings in the fleet, but there was a lot of other damage reported.

In the Fastnet race there were quite a few dismastings with subsequent roll overs, just as you report above, and the loss of polar moment was implicated as a cause. When we were dismasted in our early IOR era ex raceboat, the motion was extremely twitchy and rapid, very different from her normal motion... and that was in relatively flat sea states with no breaking seas.

The two cats did indeed do well, but the Catalac didn't survive on its own at all. It was deliberately sunk by the rescuing Fijian merchant vessel at the insistence of the nutcase lady on board. Ramtha was recovered fairly quickly, repaired and continued to cruise with the original owners aboard.

(sorry for the drift... the QB storm was a big teaching event in our early cruising life and still evokes strong emotions... even though we experienced it whilst safe in Opua!)

Jim

[PhilC]

The Andrea Doria capsized (look at the pictures) because she was in violation of international standards for metacentric height. Too many cabins up too high.[/QUOTE]

Is there more than one Andrea Doria? I thought the famous one rolled over and sank because she was rammed amidships and opened up massively on one side?

[Searles]

Yes, the Andrea Doria was T boned by the Stockholm,cut down below the water line ,filled and sank ,no stability involved ,the Stockholm survived for many incarnations ,and has only recently been cut up .⛵️⚓️

[PineyWoodsPete]

Originally Posted by Jim Cate

I think that you are confusing the losses in the Fastnet race disaster with those in the QB storm. I do not recall any of the QB vessels reporting rig loss with subsequent 360 degree rolls. Destiny did a pitchpole capsize and lost the rig but didn't do rolls later. No one knows what happened to Quartermaster (the only boat completely disappearing). I don't recall any other dismastings in the fleet, but there was a lot of other damage reported.

In the Fastnet race there were quite a few dismastings with subsequent roll overs, just as you report above, and the loss of polar moment was implicated as a cause. When we were dismasted in our early IOR era ex raceboat, the motion was extremely twitchy and rapid, very different from her normal motion... and that was in relatively flat sea states with no breaking seas.

The two cats did indeed do well, but the Catalac didn't survive on its own at all. It was deliberately sunk by the rescuing Fijian merchant vessel at the insistence of the nutcase lady on board. Ramtha was recovered fairly quickly, repaired and continued to cruise with the original owners aboard.

(sorry for the drift... the QB storm was a big teaching event in our early cruising life and still evokes strong emotions... even though we experienced it whilst safe in Opua!)

Jim

Jim, you're right of course! Maybe I have repressed things Fastnet-ish since being in a violent storm spun off of the upper Midwest depression that raced eastward, gaining the coast in southern New England late Aug. 10, '79, then racing across the N Atlantic, colliding with another depression from the N, and intensifying in the Irish Sea several days later with the Fastnet Race.

That afternoon of the 10th we were about half-way from Cape May, NJ to Block Is., 30 or so miles S. of Long Is., NY, when a long, low dense, black cloud system approached our Endeavour 32 from the direction of NYC - wife, two daughters 11 & 14 below and life-jacketed, and 17 year old son on deck with me, recently returned from a year aboad a 150' tall ship in the Baltic and Med. run by Flint Schools Aboard, and a very able seaman.

He dropped the 135 "mule" and lashed it tightly to the lifelines while I tied a double reef in the main and started the diesel. My big mistake, of course, was not dropping the main. The front hit like a ton from the SW with our heading NE, pegging the anem. at 60kts, and before we knew it were on our beam's end, the flogging main losing the upper two battens but not torn. And those below on the weather settee thrown across, with one cut head. We wrestled down the main and lashed it, then pretty much ran off toward Long Is. with winds abating over several hours. Entering Shinnecock Inlet we saw a Morgan O.I. 51 high and dry on the E. side, and later heard of considerable damage in New Jersey and NYC.

We heard about the Fastnet disaster several weeks later in Newport, R.I, but had no inkling then that our experience was related. And it was twenty or so years later when I saw the weather front details in John Rousmaniere's "Fastnet Force 10" 20 and correlated it with our logbook that I saw the connection.

One practical fact stands out: If you're on the S side of an approaching weather front (N. hemisphere), you're generally in for the worst of it with strong SW winds as we experienced.

[Na Mara]

The extra weight aloft and aft of an arch with parafinalia won’t make a huge difference to the average cruising boat’s trim or ultimate stability. The windage of the thing will make a huge difference to how the boat sails, motors, maneuvers and anchors. Arches markedly increased the boats drag coefficient on all points of sail but particularly when heeled if they have panels and they do this in an unbalanced way aft. Powering to windward you’ll notice this as needing to reef the main a bit earlier to counter weather helm. With the wind abeam you will heel slightly more. With the wind aft you’ll find you’ll run a bit faster for the same sail set with a greater tendency toward broaching. At anchor you will tend not to swing around so much but your also going to have higher loads on the anchor in a blow. Similarly fuel economy will take a bit of a hit. Finally, you are going to suffer more from cross winds when coming into marinas.

For full time live aboard cruisers on a typical cruiser the gain in utility is probably worth all the above compromises, which is why you see them everywhere. But for most of us they reduce rather than enhance the pleasures of boating. Not for me.