Motor Boat Stability

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Deleted User YDKXO

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Most of you will know the feeling. You're in a moderate beam sea and a big wave comes along. The boat leans over alarmingly and you start thinking 'f**k I wonder how far this boat can lean over before it goes all the way'?
Our scuttlebutting friends are having a lively slanging match about the stability of a US built Legend sailboat which, in case you didnt know, enjoys an image something like a Bayliner
So how far can the average motor boat lean over before it capsizes? Are sports boats better than flybridge boats in this respect? Is a round bilged Nelson type hull better than yer average planing hull? What difference does speed make? What about if both engines fail and your dead in the water?
Does anyone know of any incedents where modern motorboats have capsized?
 

jfm

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You are definitely thinking oh f**k long before there's any real danger of capsize.

For a motorboat the angle of vanishing stabil is invariably more than 90deg. In other words, if you turn the boat so it is lying sideways, it will self right (if the cabin doors are shut). Sports cruisers better than FB, but both will self right from that position.

Trouble is, this is all academic because angles of vanishing stability refer to a static state. If a motor boat is on its side it has been pushed there by waves not wind, and if the waves are that big they are capable of roling/capsizing the boat regardless of its stability angles. So start panicking if the boat is being pushed more than 45% imho.

The main reason motorboats are "safer" in this sort of weather than sailboats is that they can out run it at 28 knots, imho
 

zefender

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Re: Motor Boat Stability/safety

Assuming
1, There is (still) a safe place to return to
2, The engine doesn't pack up with all that movement
3, The forecast is listened to/early meteo signs heeds
4, Sea sickness doesn't spoil the escape plan.

Regardless of how quickly a MTB will self-right, presumably the lack of a keel means that it would take much less of a sea to cause an inversion. If a wave height of say 45% of sailboat length
on the beam is enough to invert, I wonder what % is needed to invert a MTB. Presumably most engines would fail on inversion anyway so the self-righting bit helps immediate drowning but that's about it, isn't it? Genuine comment, not trad stinkievraggie tussle.
 

jfm

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Re: Motor Boat Stability/safety

Yes, assuming the 1,2,3,4 but those are manageable risks (and should be managed by a competent crew). All other things being eual, MTB can outrun weather 4x better than a rag boat, which is a major safety factor imho.

The keel is a 2 edged sword. Undoubtedly it helps static stability. But where the forces causing capsize are big waves rather than wind, it can reduce stability by causing the "tripping over" effect.

All imho, but for safety I wd always take a combination of top-flight electronic and visual weather forecasting, and massive speed, rahter than a keel. Except when crossing an ocean, when I prefer Concorde. ;-)
 

kimhollamby

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Re: Motor Boat Stability/safety

Biggest danger in a 'typical' situation ie the kind of weather most semi-sane motorboaters tackle, is where the craft starts to take on green water and the water itself then has a destabilising effect a la water on the car deck of the Herald of Free Enterprise. Only forum reports I've seen of capsize (other than lifeboats) are where boats had taken lots of water and so were threatened with sinking anyway.

Worth imagining the effect of trapping a lot of water in often bulkhead free or only semi-segmented bilge spaces, or across a cockpit with deep sides and not much in the way of freeing ports or drains.

Too much weight on the flybridges of smaller motorboats can certainly affect stability, although I've never heard of one rolling over.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com
 
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Deleted User YDKXO

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Re: Motor Boat Stability/safety

As you know, Kim, I used to own a Sealine 410; this had a cockpit drain at the forward end of the cockpit. Whilst the boat was planing (at a bow up angle), the cockpit used to fill with water if there was spray flying about until there was 12-18" at the aft end and I always used to wonder whether that quantity of water did the handling any good. The first time this happened, we had left the saloon door ajar with inevitable consequences when the boat came off the plane
I did hear of a US fish boat which capsized due to poorly fitted lazarette hatches allowing water to get into the bilges so its something to think about
 

Medskipper

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Hmmm, I have been a motor boater for 12 plus years and my brother has been a yachtie for a bit longer. He has had several sized yachts and currently has a blue water cruising steel yacht 38'. Over the years I have been out with him in some god awful conditions but have always felt very safe, you see I know they are slow compared with our motor cruisers but I have to admit these thing are just built for the sea! I really mean rough seas! You dont have to run for cover because if you know what you are doing you ride it out! I have to say chaps that whilst my love still is with motor cruisers they just dont give you the same sort of confidence when you are in heavy seas and I would NEVER consider putting to sea in the average motor cruiser in the sort of heavy seas that I have been through with my brothers yacht! but of course you dont heel the boat over when you are up against it, you take down most of your sails and keep the bloody thing as upright as you can! I mean if you dont you spill most of your whisky!!!

Barry
 
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Deleted User YDKXO

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Not many motor boats could be described as blue water boats, thats true, but have you had experience of a motor boat in heavy weather? If so, you've lived to tell the tale so it cant have been all bad. I know a delivery skipper who's delivered many motor boats to the Med summer and winter without mishap and nobody seems to know of one single incedent where a modern motor boat has capsized in heavy weather
I am not saying that modern motor boats are designed for heavy weather cruising for clearly they are not but maybe the limits are higher than you think. I, too, would rather be in a heavy displacement yacht but I'm not so sure I'd want to be in some of the bubble boat yachts you see around either
 

hlb

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But its not really a motor boat V Yacht question. What we all mean is a planing hull as opposed to a keel and yacht shaped hull. It's about fast boats or slower boats because there shaped different and cant have both, its all a compromise.
Can always buy a nelson and do 18 knots through a F8.

Haydn
 
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