Fin stabilsers at anchor - underwater video

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jfm

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Apropos nothing really, here is a vid shot yesterday showing Sleipner fin stabs in "at anchor" mode, in Mallorca. The anchorage was not that rough but there was a bit of swell and some wakes from passing boats, so the stabs were moving but nothing like as fast and furiously as they operate in a rougher anchorage. This was shot in Cala Mondrago, yesterday - lovely anchorage and water 28degrees. Sorry about my slightly scruffy antifouling :D

 
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mjf said:
Fab footage jfm - esp where you get the fin and waterline together so its easy to see the affect on the 'roll' as w/l stays quite stable whilst the fin is dancing away.


What sort of HP is need to drive a fin?
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and to add to this question, how long before you run out of juice (battery wise) and need to switch on the gen? I vaguely recall somewhere a value of 20A being mentioned, could be wrong of course...

fab system, would be interesting to have a rough cost for retrofitting a system to an existing mobo. [probably a range of systems vs prices for ppl to consider]

cheers

V.
 
Very interesting John, and as you say, a lovely anchorage. I guess you deserved the Pims after that dive!

How vulnerable are the stabs to grounding, or clouting something floating beneath the surface? From a techie perspective, what is the failure mode if the worst happens and you catch a stab on something immovable?
 
Strangely peaceful footage. I couldn't help wondering if they'd work so well without the stripe, though.
 
In answer to the questions:

The stripe is v important bjb! The boat was lifted a month or so ago, just for a couple of hours to clean the stern gear before this 1000nm trip around Balearics, and as Sleipner were doing a video the next day (which I still haven't seen) we painted the stripe thinking it would make the fins easier to see

Power wise, they run at anchor off a 10hp 3 phase 230v motor driving a Rexroth hydraulic pump. There is a big accumulator in the system to provide instant battery of energy if the system demands a fast sweep and the valves open wide. It all runs at about 140bar/2000psi. When the swell is big and they're working hard, the motor demands most of the 10hp ie most of 30amps and if you're in the engine room you can hear/feel the genset work a bit harder to maintain its 1500rpm. But normally they are absorbing about half that power. At these power levels there is no way they can run off batteries and 24v dc. They need the gensets. Underway they run off hydraulic power supplied by PTOs on both main engines (for duplication/back up)

Retro fitting is generally possible but not a small job. You have to reinforce the hull, fit the things in midships, fit a hydraulic pump pack, and run all the wiring and hoses. Serious job. In the order of £100k all in I'd guess, and that's the approx option price to have them fitted on this boat as an in-build option

Yes they stick out so could clunk debris. So could your props and rudders of course, so it's one of those risks you accept in boating. Sleipner tell me the hull reinforcing is stronger than the stabs so that in a major impact the fin would smash off and the (65mm dia) stab shaft would bend before a hole is ripped in the hull, but I don't plan to test that and of course it would depend on the details of the impact and its forces. Ultimately you could say they could hit something hard and fail, but I consider that way less likely than one in a million chance and in the meantime they take out rolling and seasickness and transform ride into a magic carpet experience so I consider the trade off worthwhile.

Pete I don't think they're dangerous generally. Only bad scenario is if someone gets an arm in the top gap twixt top edge of fin and the hull. you'd have to be a bit daft to do that, and there are warning decals, but I suppose it's always possible. To be honest in Mondrago it got so busy with loads of swimmers round the boat that I did turn them off, and it wasn't that rough anyway, but that's the first time I've felt I should tun em off for that reason. All that said, in a rough anchorage they move way way faster and with more angular stroke than in that video. The fastest sweep eaves a "hole" in the water that you can see if you look down from the side deck, and then a big whirlpool as it fills. But people generally don't swim in those conditions so it's not a problem. I'll try to vid it next time I get the chance
 
Great Video JFM .... I had never appreciated the 'motion' they would make. I always assumed they were very skittery ( if there is such a word) all the time.

Shame about retro-fitting difficulties ... looks a real winner.
 
Hey jfm, you must have got up very early to get a spot in Mondrago during August:) Excellent video of the stabs; surprised how fast acting they are
 
jfm said:
in a major impact the fin would smash off and the (65mm dia) stab shaft would bend before a hole is ripped in the hull, but I don't plan to test that and of course it would depend on the details of the impact and its forces. Ultimately you could say they could hit something hard and fail, but I consider that way less likely than one in a million chance and in the meantime they take out rolling and seasickness and transform ride into a magic carpet experience so I consider the trade off worthwhile.

Pete I don't think they're dangerous generally. Only bad scenario is if someone gets an arm...
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I fully agree, the trade off is well worth the risk.
Otoh, I would neither dismiss the additional risk as a one in a million chance, nor trust the idea that the shaft would bend.
In 2006, a relatively minor grounding was more than enough to tore off the shaft of a Nordhavn 62, which is probably the best and stronger boat they ever built (arguably also the last, but that's another story).
The hole left in the hull was big enough to give no other choice to the helmsman than try to ground the vessel on what he believed/hoped to be a beach (the accident happened at night, along a Mexican coast with poor chart coverage). But as it happened, the coast was rocky. Anyway, long story short, the aftermath of that shaft hole was one casualty and a total vessel loss.

Re. the danger of using stabs at rest, aside from the motion there's another thing to consider, which is often underestimated: while a genset is running (which can happen also for other reasons, but stabs at rest require it anyway), swimming nearby its exhaust can be EXTREMELY dangerous.
I was thinking of that while watching the video, J: were you snorkeling or scuba diving? If the former, be very very careful, because when breathing with your mouth you have even less chances to perceive any poisonous emissions. Besides, from what I saw in the video, the boat was not swinging at anchor, which means that the wind was not necessarily flowing bow to stern, hence "cleaning" the hull sides where you were swimming while shooting.
Call me paranoid if you wish, but on my own boat when the genny is running nobody is allowed to swim, period.
 
Deleted User said:
surprised how fast acting they are
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Oh, they can be much faster than that!
They're not working at 2k psi for nothing...
Though I suppose that's the central equipment pressure, to maintain a constant 500 psi or so for the fins.
Happy to stand corrected from jfm on this, though.
 
MapisM said:
If the former, be very very careful, because when breathing with your mouth you have even less chances to perceive any poisonous emissions. .
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Mmm, I ought to be dead then. I've spent many happy hours paddling about at the back of my boat scraping things whilst the gennie is running. In fact I quite like bathing in the nice warm water flowing out of the exhaust:)
 
MapisM said:
Did you try piXXing in it, as a greener alternative...? :D
Anyway, as the acronyms maniacs would say, YPYMYTYC!
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Blimey, I've swum in the 'death chamber' loads of times. Are there any statistics to back this up? Exactly how many swimmers have died as a result of CO poisoning under the bathing platform or is this safety nazism gone way over the top?
 
Well, TBH I think the risk is much higher with gasoline engines, whilst your genset is obviously diesel.
That said, I'd rather breath clean air than diesel exhaust anyway, thanks! :)
Just a thought: you might keep onboard a diving cylinder and a regulator for scraping u/w gear. Aside from avoiding to breathe the exhaust, that's also convenient for scraping those parts which can't be reached while snorkeling, as the props/rudders, transducer(s), water intakes.
'course you can dive back and forth also while snorkeling, but making a boring job even worse ain't my idea of pleasure boating...
 
MapisM said:
Well, TBH I think the risk is much higher with gasoline engines, whilst your genset is obviously diesel.
That said, I'd rather breath clean air than diesel exhaust anyway, thanks! :)
Just a thought: you might keep onboard a diving cylinder and a regulator for scraping u/w gear. Aside from avoiding to breathe the exhaust, that's also convenient for scraping those parts which can't be reached while snorkeling, as the props/rudders, transducer(s), water intakes.
'course you can dive back and forth also while snorkeling, but making a boring job even worse ain't my idea of pleasure boating...
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Actually I quite like diesel fumes. I dont know why but I do. Its a boaty smell. I can't be arsed with all that heavy diving gear stuff but there is some lightweight stuff on the market. What do you think about that?
 
LOL, are you sure that diesel fumes don't remind you of your business, so you feel less guilty for taking some days off while you're boating? :D
Re. your question, well, if you wish to try also some "real" dives, I'm afraid that there's no substitute to the traditional diving gear.
Otoh, if you're just interested in scraping the boat now and then, you might consider either a small emergency cylinder, or a surface air supply which you could keep onboard, as the following ones:
Sea Breathe
Third Lung
A tad expensive, but they allow you to stay under the boat as long as you wish, with no equipment at all aside from the mouth piece and the connecting hose.
Of course, you could as well turn off your genset and continue to enjoy snorkeling... :)

With apologies to jfm for the o/t.
 
jfm said:
All that said, in a rough anchorage they move way way faster and with more angular stroke than in that video. The fastest sweep eaves a "hole" in the water that you can see if you look down from the side deck, and then a big whirlpool as it fills.
Click to expand...

Very impressive. I hope Sleipner get their video out as it would be very interesting to see them in action in a rougher anchorage (or bigger swell). As far as I can see, the stab only has one swing at counteracting the list caused by a closing wave crest. If the waves are longer, so is the motion to counteract, so the sweep would have to be long and gentle (and this would reduce the effectiveness). Do you know if the algorithm moves both fins separately so that the port fin can 'extend' the effect of the starboard one, ie. pick up when the other has already used up it's useful motion?

Sorry if this has already been discussed earlier. It was great to see them in action, seeing is believing :)
 
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