Effect upon stability of adding weight at boom height

[Kelpie]

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?

 

[Tranona]

It will, of course have an effect, although by today's standards your boat has a relatively high ballast ratio of 36%. The total of the bimini and the panels will be a bit more than that, although the bigger issue may be windage. The key thing is whether it will make any difference and I guess you are highly unlikely to get in a situation where it will make any material difference.

When you see the extra top hamper on many cruising boats from adding radar, mast steps, gantries, biminis davits with RIBs etc you sometimes wonder whether the fears about weight aloft are overdone as they seem to get around OK.

 

[Moodysailor]

To add to the above, you probably have added more than enough weight below the centre of axis in terms of "owners kit" to offset the weight aloft. I wouldn't worry about the weight, as Tranona said - windage would be a bigger concern for me.

 

[Graham376]

We've added radar and reflector up the mast, a bimini, gantry with solar panels and dinghy, to a CC 38 ft boat with 40% ratio. No noticeable difference in stability but usually only two of us on board above C of G anyway.

I think what isn't considered sometimes, is the accelerated weight of heavy bits added, when rolling violently.

 

[Blueboatman]

I think the benefits will far outweigh any concerns
And arguably solar panels will outlast the stitching on a Bimini
Graham is right about inertia or ‘ moment’ at least nominally but boom height is not like oversized standing rigging and wet halliard stretching waay up to the masthead

In a hurricane ( threat) just make sure you can remove them , remove all appendages really IME

YouTube channel Sailing Uma have experience of what you are designing , I think the did one full redesign before doing a N -E Atlantic passage @36ft, you’ve prob come across reference to some of their electric propulsion, all electric , all solar stuff ?

 

[sarabande]

It's less than the weight of a small crewmember standing on deck ! It is highly unlikely that there wiill be any quantifiable difference in stability or roll period.

As noted, any structure for panels must be strong enough to cope with gales, and people grabbing hold of it in lumpy conditions.

 

[Graham376]

Graham is right about inertia or ‘ moment’ at least nominally but boom height is not like oversized standing rigging and wet halliard stretching waay up to the masthead

Maybe I wasn't clear enough. When I mentioned accelerated load it was concern for the load on the mounting, not boat stability. Our bimini frame for instance would be a bit flimsy to add panels to, others are plenty strong enough.

 

[Kelpie]

Thanks for the replies, reassuring.
I am building the frame for the bimini out of 25mm stainless. I think I will make it a rigid (not folding) structure for strength and easier construction.
The windage is surely going to be the same no matter what the bimini is made of, but obviously it will be much harder to pack away a solar bimini than a fabric one.

 

[Kelpie]

duplicate post deleted

 

[Neeves]

Why not flexible panels that will be lighter?

You raise an interesting question specifically and in general - yacht builders know that owners are going to add real estate, usually in and around the transom, that might include davits (+ dinghy) as well as a bimini - knowing these devices are going to be added - do they design accordingly. You are adding weight centrally - but for a aft cockpit - it would 'all' make a lot more difference.

Jonathan

 

[Graham376]

Why not flexible panels that will be lighter?

You raise an interesting question specifically and in general - yacht builders know that owners are going to add real estate, usually in and around the transom, that might include davits (+ dinghy) as well as a bimini - knowing these devices are going to be added - do they design accordingly. You are adding weight centrally - but for a aft cockpit - it would 'all' make a lot more difference.

Jonathan

Obviously something to consider but, as sarabande points out, panels would be less weight than a small crew member. We have a 7 berth boat but on offshore passages there are only two of us, which is the usual case for most of those doing extended cruising. From that point of view, we're below the design load by weight of 5 crew members.

 

[Moodysailor]

You raise an interesting question specifically and in general - yacht builders know that owners are going to add real estate, usually in and around the transom, that might include davits (+ dinghy) as well as a bimini - knowing these devices are going to be added - do they design accordingly. You are adding weight centrally - but for a aft cockpit - it would 'all' make a lot more difference.

Any reputable builder will ensure the vessel design incorporates the anticipated additional weight & buoyancy distribution for items that can affect stability - but only if the added weight is integral to their offer (i.e as a factory option). This is required for the design approval. It gets less clear in the case where options are dealer or owner fitted - in the former, there is then no requirement for the manufacturer to consider this in their design, and in the latter they may have no idea what is being fitted, and importantly will have no control over it either.

Higher quality designers/builders of cruising boats will make assumptions around additions, but this isn't always the case and is less likely to occur in cruiser/racers.

 

[Kelpie]

Why not flexible panels that will be lighter?

A number of reasons.
Firstly, I already have the rigid panels. Swapping them for semi-flex would be a pretty massive cost.
Secondly, semi-rigid panels really need to be laid on top of a rigid surface to avoid flexing. They seem to work quite well when laid on a curved deck, for example. Adding a plywood or grp bimini top would negate much of the weight savings.
Thirdly, why do I insist on not just putting semi-flex panels on top of the canvas? Too many instances of premature failure caused by the flexing. And one case of such a failure leading to a short, and a fire, which ignited the sunbrella and dripped molten flaming plastic in to the cockpit.

 

[RupertW]

I think this is a legitimate concern which is almost always overlooked when people build solar arches as the assumption is that the amount of extra kit below more than compensates for the weight aloft. But down below most kit is stored low so we need to remember that 120Kg of weight 3m above the centre of gravity needs 1200kg of fixed kit 30cm below the COG to achieve the same stability.

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.

 

[Moodysailor]

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.

I'm no naval architect, but I suspect you have hit the nail on the head there. The AVS might decrease by 0.5 or so, and that would take a huge amount of weight - and by the time I'm at 135 degrees of heel, I'm not counting single digits... ?

 

[Laminar Flow]

I think this is a legitimate concern which is almost always overlooked when people build solar arches as the assumption is that the amount of extra kit below more than compensates for the weight aloft. But down below most kit is stored low so we need to remember that 120Kg of weight 3m above the centre of gravity needs 1200kg of fixed kit 30cm below the COG to achieve the same stability.

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.

You are correct, it is a legitimate concern.

A few years ago an Oceanis 40, I believe, capsized in the Biscay in a F8 and one crew drowned. The subsequent inquiry determined that the, rather standard, add-ons, furling gear, radar, arch, etc, etc, had dangerously lowered the vessel's AVS.

In a "modern" hull, it should be noted, every single piece of cutlery that is added to the drawer, every book on the shelf, is above the C of G. While adding "gear" below deck and to a degree above, does increase initial stability at low angles of inclination, it, in all likelihood, will have a negative effect on the AVS.

The overall problem will be the cumulative effect of all the individual bits added. I know of a perfectly lovely R. Clark that was ruined, IMHO, by all the modern add-ons that (some) Mr. Clark had never heard of: mast steps, radar, headsail furling and behind the mast furling, massive teak and glass dodger etc. It seriously affected her stability and roll behavior.

Since most folk don't go out in "big waves" or extreme conditions it rarely has any adverse effect on the normal use of their boat.

 

[johnalison]

I was told many years ago that Lloyds would only give A1 classification to a yacht with no more than one of furling jib, furling main, and mast-mounted radar. I would expect that newer boats would now be designed with more expectation of add-ons, though the low ballast-ratio of current boats looks like a step in the other direction.

 

[Graham376]

I was told many years ago that Lloyds would only give A1 classification to a yacht with no more than one of furling jib, furling main, and mast-mounted radar. I would expect that newer boats would now be designed with more expectation of add-ons, though the low ballast-ratio of current boats looks like a step in the other direction.

But many leisure boats such as mine have a Lloyds certificate which just says the hull construction is OK, what happens to it during or after fitting out is irrelevant as far as the certificate is concerned.

 

[RupertW]

I was told many years ago that Lloyds would only give A1 classification to a yacht with no more than one of furling jib, furling main, and mast-mounted radar. I would expect that newer boats would now be designed with more expectation of add-ons, though the low ballast-ratio of current boats looks like a step in the other direction.

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.

 

[Tranona]

I was told many years ago that Lloyds would only give A1 classification to a yacht with no more than one of furling jib, furling main, and mast-mounted radar. I would expect that newer boats would now be designed with more expectation of add-ons, though the low ballast-ratio of current boats looks like a step in the other direction.

The old Lloyds rules were often based more on educated guesswork than on calculations, probably on the basis that all the new fangled things were add ons (and probably bad). You only have to read the old yacht design books to appreciate how imprecise calculations for ballast and stability were before the days of computer software. Just been re reading Harrison Butler, arguably one of the more thoughtful prewar designers on the difficulties of manually calculating just the weight of a ballast casting of the shapes commonly used, never mind even attempting to predict the actual displacement of the complete boat. Made the point of saying that he had to guess how the builder would actually build the boat and what materials would be used. I was doing this because I was looking at one of his designs that was for sale but built over 50 years after it was designed. Whereas the original specified mahogany carvel planking on a mixture of frames and steamed oak ribs with a pine and canvas deck, this one was built of triple planked iroko on laminated frames and a teak on ply deck. On the other hand it had a lovely Sitka spruce mast and boom whereas the original would have been solid pine.

As noted earlier, stability calculations now are done for each standard specification so if a boat has furling jib and in mast main then the stability calculations will reflect that, whereas the Lloyds thinking would have come from a time when such things were being added onto boats that were never designed for it. This thinking persists at times here when people deride in mast by saying it is bad as it reduces stability because of the weight aloft, ignoring the fact that the designer was aware of this and would have taken any additional weight into account in his design.