The Artful Dodger, or the answer to the universe

In response to a previous post on The Artful Dodger

” the arch installation became the standard and the design still stands in production today. It is a tough fabrication to pull off as the mounting points in the deck moulding are not ‘level’, as they could have been if I had thought of it. “

Chris writes:

Very much enjoy reading about the thought process that goes in.

I noticed the static load is only 250lbs. By the time you have a tender and motor, there isn’t a lot of load left. If you wanted to safely increase static load to 300lbs to add a wind turbine and still leave a safety factor, would limiting factors be arch construction, mounting points, none of the above, or something else entirely?

Answer:

The 250lb. suggested tender load limit is rather like the answer to the Ultimate Question of Life, the Universe, and Everything (see;Hitchhiker’s Guide to the Galaxy), (…42 btw).

The truth is that an engineering calculation wouldn’t have much validity, especially with the application of the customary ‘three times’ margin of safety.  This would produce a assembly vastly over-constructed for a pleasure boat and consequently heavy and ugly.  At some point, it would become necessary to apply some kind of judgment to the whole affair and cut it back to a being a practical, apparently adequate proposal.

Using the benefit of vast perspicacity and some experience (the observation of failure, hopefully not your own) ahead of time, a lot of complex, costly and boring engineering analysis may be precluded by just giving the design your best guess in the first place.

This may sound somewhat cavalier but the true load factors are quite variable and impractical to define, the actual static weight of the dinghy and motor being of scant application to a real world dynamic situation.

Very pretty, but tubes have to be close to their critical bend load at the bases, and what happens if it rains? Or some kid jumps up and down in the boat? At least side loads are unlikely.

The effects of a seaway and the mother boat’s response (dependent on its own loading) will create G forces due to acceleration that will multiply the hoisted tender’s effective weight and exert horizontal forces that are not apparent in the static situation at the dock.  Exacerbating this issue is the effect of resonance, (rhythmic bouncing, swinging etc.) that is really unpredictable in a practical sense.  In ship design practice, davit equipment becomes extraordinarily heavy and usually outweighs in itself the designed lift capacity.

Such an approach, engineering calculation plus a substantial margin of safety, is not something we can afford with a boat like the 44 where every item undergoes a weight/benefit (appearance) rationalization.  We do not adopt the racing rationalization, ‘if it didn’t break it was too strong’, but we are closer to that than to ship design methodology.

The use of an arch design (only really practical for a catamaran) at least precludes the necessity for cantilevered davits.  Such davits exert enormous torque loads on their foundations and are more likely to augment resonance due to their own elastic properties, (like two guys rolling over a small car by resonant rocking).

Cantilever davits held up by stern rail and lifelines, until the vessel leaves the dock anyway

The arch exerts most of its foundation loads in a vertical direction however and doesn’t have much elasticity, making it inherently a lot more stable.  There will always be some sheer and bend loads due to dynamic horizontal forces but these are relatively low and easily absorbed. The arch does have to contend with some bend loading, but nothing like that exerted close to the base of a cantilevered davit.

The penalty to pay with an arch is its much greater presence.  This is offset by making it perform additional functions, like carrying seats, lights, domes, antennae, solar panels, canvaswork, BBQ, even wind generators.  These relatively lightweight items do not represent the same magnitude of load as does the tender.  They are also fixed in position unlike the tender which may be free to swing (bad) or even drag (really bad).  The presence of the arch is not reduced by the addition of accessories but at least it may be excused on account of its utility; indeed it has become a cruising boat must-have.

In the absence of some kind of scientific destructive real world testing representing every possible contingency for vessel loading, tender weight, securing method, seaway, etc., we are left with the old standbys; how does it look? how does it feel? how does it stand up?  Prototypes sent to sea are always somewhat experimental, owners are more effective problem discoverers than any scientific testing or engineering study.  After many thousands of sea-miles and harbour incidents, the design of the 44 arch apparently enjoys an uncalculated ‘margin of safety’ having never displayed any structural shortcomings, and I am certain that the 250lb. weight limit is taken lightly by the owners; I witnessed one sitting in his hoisted tender to conveniently organize the stowing of several cases of beer therein, another complained about the inadequacy of the tackle to lift the tender when it was full of water. (Don’t do that.)

So why 250lb.?  At some point you have to gently suggest a practical limit on tender weight consistent with the vessel design as a whole.  Could the arch support more? Apparently yes, but really folks do you have to?  Will it stand up to sea conditions carrying substantially more?  Who knows, how much?  Will the vessel stern be depressed or react reluctantly to the seas as weight is added?  Yes.  The company is obliged to limit its liability, but its primary responsibility is to direct its owners towards circumstances it knows will be safe, satisfactory and enjoyable; 250lb. is their answer.

The answer to the question you posed is therefore the “Something Else Entirely” option.

As it stands, the arch will support whatever you will practically resolve to carry in way of equipment when your particular vessel is specified, but the tender target weight number remains, winks notwithstanding.  You don’t want your boat’s ass to sag, after all.

Regards,
Ted

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