Effect upon stability of adding weight at boom height

[zoidberg]

Hmmm.

I recently examined a pair of ORC Rating Certificates for the same boat, one year apart. Admittedly, this was a 27' fin keeled boat. There was a 'huge' difference in the AVS, from 173 degrees to 101 degrees. The one Certificate was for the boat 'stripped out'. The other was in full cruising trim, with a range of bolted-on addendums - and 496kg heavier.

No-one can guarantee they won't encounter a 'knockdown' breaking sea. No-one can guarantee that if knocked-down beyond, say, 90 degrees, the next wave will knock them back upright again.
One is then probably significantly more stable upside down.

Most will just guess that they'll 'probably be all right'..... but they won't know. That's what the calculations are for, and the qualified people who interpret them. - if you can't ask the designer of your boat.

 

[Gary Fox]

Luckily I can suffer no such angst; my beautiful old wooden cutter has such elegant lines, an arch is inconceivable!
Others may take a reciprocal course, having little to lose in looks...

 

[Laminar Flow]

There seems to be some confusion how designers calculate displacement, righting moments and what particular elements are considered and which are not.

I highly recommend a read of Larsson/Eliasson, Elements of Yacht Design, (1994) for anyone interested in the subject. The methodology of calculations, apart from the use of more up to date materials is, btw, little different from that used in Francis S. Kinney, Elements of Yacht Design, published in 1962. In that sense not much has changed.

Calculations are done typically for half load and empty conditions. They include, in Larsson/Eliasson, navigation instruments, galley equipment, personal gear, bedding etc, etc, half full fuel, water and sewage tanks and crew. For a 12.5m boat of 8.1 t displacement they reckon with 980kg at (half) loading. The combined VCG for this load is, incidentally and for those who believe that loading their boat with below deck gear will compensate for all and any variety of sins when fitting out, 0.46m above CG. For interest's sake: the cumulative VCG for the propulsion system and all other onboard systems and technology, including batteries, tanks etc, is also above CG.

Sails are considered as raised, as one would expect.

None of the calculations include radar or communication antennae, arches, wind generators, solar panels, outboards strapped to the pushpit or rows of jerry cans with water or fuel strapped to the rail nor dinghies in davits.


A radar antenna weighs about 10kg. Mounted some 10m above deck or 12m above CG, this equates to a loss of 120kg/m of righting moment. To compensate for this you would have to add 83kg to the ballast or 2.6% of total, assuming a VCG of -1.45m for the keel (using the above boat as an example). This does not include the weight of the cable or the mounting bracket.
And, so it adds up.

 

[boatiebert]

You raise an interesting question specifically and in general - yacht builders know that owners are going to add real estate

That's a good point. I know for a fact the designer of mine designed for 4 people in the cockpit whereas I've only been in it myself, & I know other owners of the same who sail them solo having removed the original 160kg inboard motor which surely adds ballast. Small boat though, I'd be more worried about it being picked up by the wind.

Kind of like this? Then with flexible PV panels? I wonder if there was a way to allow it to fold down for storing/storms? Essentially blocking access when you did want there to be any.

A friend in the US did something similar using Coosa Board. I'd never heard of the product before.

 

[Moodysailor]

It's threads like this that make me incredibly appreciative of our 19 ton ketch deck saloon with deep encapsulated keel. If we ever decided to fit an arch, the chance of it making a significant impact to the AVS is minimal, especially as she is rated for 6 crew and we usually sail with 2 or 3. And if we are ever concerned, I can just put some extra lead down into the keel cavity

 

[Neeves]

That's a good point. I know for a fact the designer of mine designed for 4 people in the cockpit whereas I've only been in it myself, & I know other owners of the same who sail them solo having removed the original 160kg inboard motor which surely adds ballast. Small boat though, I'd be more worried about it being picked up by the wind.

Kind of like this? Then with flexible PV panels? I wonder if there was a way to allow it to fold down for storing/storms? Essentially blocking access when you did want there to be any.

A friend in the US did something similar using Coosa Board. I'd never heard of the product before.

I thought along similar lines.

Use flexible panels and fabricate the roof from thin foam and thin glass, even make the supports in a similar manner, maybe thicker foam and thicker glass. It can be designed to be demountable and stacking, such that the roof panels nest into each other. This allows you to design the roof to fit and match the cockpit and the solar panels around which the roof would be designed. You might not want to stand on such a structure - but that was never the intention.

If you, someone, can design a nesting dinghy - you can design nesting roof panels - and surely it is lighter than 25mm stainless tube and rigid panels.

An option is to consider the flexible panels to be consumables, prioritise stability and place flexible panels on the coach roof, deck or the flat of the aft transom.. Maybe the final option his to forget the solar panels entirely and think of a WattnSea - as long as you have wind the yacht will move, and if the devices provide power to circumnavigate the world - they seem proven.

There seems an unhealthy focus on solar panels (the focus seems unhealthy to me) and to totally ignore other power generation options, specifically hydro. Eclectic have one option, WattnSea another - there is more than one way to skin a cat.


I keep having this recurring thought - if the designer had considered real estate as pictured by Gary, or described by Zoidberg or Moodysailor and seconded by Geem - might he, the designer, have had a bigger keel in the first place.

Adding real estate 'might be right, Mate!' if its only you - but as skipper you might have crew to consider.

By all means have sun protection - but are there not other ways to keep the electronics running?


Jonathan

 

[lustyd]

But practically I never heard any first hand accounts of a boat with top weight going over and killing everyone mid-ocean in big waves so it may not be an actual issue.

Realistically if you're in weather that might cause that kind of issue, would the solar bimini stay attached long enough to be a real issue? Probably not. Might be a case for not building it too strong too, once you heel enough that solar installation becomes a sail which will add to the force pushing the boat over.

Well a similar comment of mine recently on a “Solar on a boat” mostly US forum has been ridiculed beyond measure as most posters boast rigs with huge high solar arches full of 1000W plus of rigid panels and often a dinghy on strong high davits too and many do go ocean sailing. They see “math“ as a thing for theorists not sailors.

Of course the US also has huge yards full of hurricane damaged boats, many of which have had solar panels fly through the deck/window/hull

 

[doug748]

Ok actually bimini height. I'm adding a bimini and want to mount rigid solar panels on it. In fact I am seriously considering using panels as the actual bimini, in full or in part.

My only concern is weight. Adding up the panels I would be using gets me to 66kg, which sounds like a lot.

But then again, it's a 9t 39ft centre cockpit boat. When I imagine the effect of having a lightly built person standing on the coachroof, it seems laughable that that could have any effect on stability.

Any thoughts?

The approximate effect will be of removing 66kg off the keel, which is probably bearable and a small price to pay if it makes for comfort.
As all the others have said, the creeping danger is of adding other stuff, piecemeal. It's temping to get the liferaft off the deck, dinghy in davits or on deck, full cockpit enclosures, barbie, anchors, an array of stuff up the mast, steps, wiring, aerials, spinnaker poles; plus all the cruising gear below decks which, on a modern hull will tend to be above water, rather than below. Knock 500kg off the ballast weight and see what the ratio look like.
Still people do it even on lightly ballasted boats - so it can be done but most people are sailing in benign conditions, they plan to cross oceans but may not be actually doing it.

There is an interesting piece in Heavy Weather Sailing where Peter Bruce looks a design trends. He cites the case of a production 27ft yacht which with a conventional rig had an AVS of 127 deg, fitting it with a roller genoa and in mast main reduced this to 96 deg. Am extreme example but it illustrates the point.

An often overlooked matter is the effect on sailing ability which is twofold, first the loss of ability to stand up to her canvass and then windage. There is no doubt that the addition of stuff above deck cripples sailing performance but, as this is not the primary reason for cruising, a compromise has to be struck.
I might, on a centre cockpit boat, put panels on deck or find a way of reducing windage if things looked bad.

.

 

[RupertW]

The approximate effect will be of removing 66kg off the keel, which is probably bearable and a small price to pay if it makes for comfort.

It is many multiples more off the keel as the keel is closer to the COG than a Bimini arch.

 

[RupertW]

Realistically if you're in weather that might cause that kind of issue, would the solar bimini stay attached long enough to be a real issue? Probably not. Might be a case for not building it too strong too, once you heel enough that solar installation becomes a sail which will add to the force pushing the boat over.

Of course the US also has huge yards full of hurricane damaged boats, many of which have had solar panels fly through the deck/window/hull

Bimini frames should be no more than screwed into the deck so sacrificial but once you add in the structure needed for side loads of rigid panels and even a dinghy hanging then they are solid bolted through structures which will stay in place even with the boat on its side - with massive leverage.

 

[lustyd]

That was kind of my point. You could still make the solar bolts a weak link though

 

[RupertW]

That was kind of my point. You could still make the solar bolts a weak link though

The side loads of weight mean that once you put rigid panels up the weak links would tear through in non-extreme conditions

 

[doug748]

Not my idea of fun, wondering at what point the whole bloody lot lands on my head.

 

[Blueboatman]

To be fair , once the wind gets up to whistling speeds , those wind generators are quite good at keeping up with electrical demand .

I’ve always favoured solar panels slung off the side dodgers ( and then you faff around with cantilever sticks to get max charging angles etc ) but it does all start to look like a overloaded camel!

Next time around I would go the OPs route for 2in 1 Bimini /awning . With wing nuts

And perhaps foresake radar on the mast .

I am quite a fan of hankon foresails too , forestay and staysail both, especially to be able to get the windage down off the forestay in a blowy beat to windward in even 25kn plus the apparent .. writes one aging purist ?

 

[Kelpie]

There seems an unhealthy focus on solar panels (the focus seems unhealthy to me) and to totally ignore other power generation options, specifically hydro. Eclectic have one option, WattnSea another - there is more than one way to skin a cat.

It's too do with the proportion of the time that the boat spends moving. A hydro generator (and actually I do have an Aquair) only works underway, which by all accounts will be less than 10% of the time.

 

[lustyd]

Yes all good until you're the poor shmuck on the foredeck wrestling a sail down in a F9 with waves crashing over you!

 

[lustyd]

only works underway

Strictly speaking that's not true. A boat doesn't need to be underway to have speed through the water. It would be unusual to be anchored with enough current to be effective, but not unheard of

 

[Blueboatman]

Yes all good until you're the poor shmuck on the foredeck wrestling a sail down in a F9 with waves crashing over you!

Yup
Netting , downhaul , staysail ready to go up at F6, turn downwind to tie Genoa to rail or stuff it down below .. ( I’ve done it your way too!)

 

[Laminar Flow]

It's threads like this that make me incredibly appreciative of our 19 ton ketch deck saloon with deep encapsulated keel. If we ever decided to fit an arch, the chance of it making a significant impact to the AVS is minimal, especially as she is rated for 6 crew and we usually sail with 2 or 3. And if we are ever concerned, I can just put some extra lead down into the keel cavity

With all due respect and admiration for your boat, displacement does not necessarily equate to RM or license to add stuff at will. The Robert Clark, I mentioned earlier, was built in steel and reportedly displaced some 16 t; she sailed at 50 degr. heel at the drop of a hat and rolled through 100 degr. going down hill.

On the up-side, it is really tricky to add an arch on a ketch, or davits, or vane steering for that matter - don't ask me how I know.

 

[Laminar Flow]

An often overlooked matter is the effect on sailing ability which is twofold, first the loss of ability to stand up to her canvass and then windage. There is no doubt that the addition of stuff above deck cripples sailing performance but, as this is not the primary reason for cruising, a compromise has to be struck.
I might, on a centre cockpit boat, put panels on deck or find a way of reducing windage if things looked bad.

I agree, though windage is not the primary concern. Yes, it impacts windward performance, which in practical cruising terms could be up for debate, it might also cause some docking discomfort. But, wind per se does not capsize boats, waves do. Unless the first breaking sea washes the lot overboard there is still the imparted momentum in a roll-over. Bearing in mind that a 10ft wave is sufficient to capsize a 30' boat; 12' for a 40 footer, smaller boats are at a disadvantage. Ironically. it is also the smaller boat, often with moments at the mast head in the double digits, that are likely to be adding arches and other real estate.

So, in this context: size does matter.
I have always wondered about people preparing, often quite small boats, for offshore passage by adding arches, mast steps, nav gear, wind generators, ah yes, solar panels and then a row of jerries along the rails.
The review of historic voyages in small boats is interesting in this respect and often quoted by those adding the offending bits to their conveyance in preparation for such a voyage: almost none of their successful predecessors had any of these mod coms.