small boat self righting

[ProDave]

As a kid I used to invert our cat quite often and then drop it very close to the floor. It never failed to self-right and always landed on all four paws

... but back on topic ...

http://www.youtube.com/watch?v=cCKeURrQp6M

and

http://www.youtube.com/watch?v=1wyCYqG4XZo

Interesting comparison.

So with the keel down she self rights. With the keel up, she just stays on her side, but only needs a little effort to right her. At least she doesn't invert.

I guess with a lift keel, just turning on your side the keel will stay put and self right. I think the problems would begin if she inverted, then the keel may lift.

Again it's interesting that the cockpit doesn't even get wet.

 

[Lakesailor]

More interesting is this one. What is virtualy a dinghy with a chubby lad up the mast.

and this

​

 

[LittleSister]

It apparently has a ballast ratio of only 17% though. Hmmm.
(that is according to http://www.ybw.com/forums/showthread.php?t=15305 which may not be the most accurate source...)

The ballast ratio is not the most pertinent figure, the ratio of ballast to buoyancy, and their relative positions is more so. In the case of the Mirror Offshore you have a relatively vast amount of buoyancy in its matronly body and tall cabin, and high up too (i.e. a long lever even though the keel is shallow), so I imagine its righting moment is more than adequate.

The other side of the self righting thing is the chances of it getting inverted in the first place. The Mirror Offshore has a tiny rig, so that would be most unliklely to overturn the boat. The keel is very shallow (under two feet), so wind and wave pressure on the hull will tend to push the boat sideways rather than over. The hull is very beamy for its length, again militating against an inversion. Of course in heavily breaking seas any vessel can be rolled, but I would imagine a Mirror Offshore would not be the most vulnerable.

A Mirror Offshore was sailed from mainland US to Hawaii and was reported to have coped fine (it did have to be modded for blue water - er, rudder pintles beefed up!). If it handled the Pacific I'm sure it'll be fine in UK home waters.

 

[snowleopard]

Interesting comparison.

So with the keel down she self rights. With the keel up, she just stays on her side, but only needs a little effort to right her. At least she doesn't invert.

I guess with a lift keel, just turning on your side the keel will stay put and self right. I think the problems would begin if she inverted, then the keel may lift.

I was told that when coding a boat with a lifting keel you can only measure the stability with the keel down if there is a mechanism to lock it down, otherwise you have to assume it will retract in a knockdown and test with keel up.

 

[finbarr]

The ballast ratio is not the most pertinent figure, the ratio of ballast to buoyancy, and their relative positions is more so. In the case of the Mirror Offshore you have a relatively vast amount of buoyancy in its matronly body and tall cabin, and high up too (i.e. a long lever even though the keel is shallow), so I imagine its righting moment is more than adequate.

This makes sense. If you look at the mirror head on it is the shape of a ships lifeboat, lots of buoyancy in the cabin, would have to affect the ability to right.

I am not really worried about it's capabilities around the coast here (South West Ireland) except for it's possible inability to beat off a lee shore, though the inboard should take care of that situation....

 

[ProDave]

I was told that when coding a boat with a lifting keel you can only measure the stability with the keel down if there is a mechanism to lock it down, otherwise you have to assume it will retract in a knockdown and test with keel up.

Interesting thought.

MY keel is lowered by winding with a winch handle, which winds a nut along a screw thread, gradually lowering the keel on a stainless steel wire.

Emerging from my keel box is also a rope. The rope pays out as the keel is lowered and retracts again as the keel is raised. It seems to serve no immediate purpose other than perhaps a "tell tail" to the state of the keel.

Would it be worth adding a cleat that this rope could be secured around once the keel is lowered, to try and use this rope to prevent the keel raising in the event of a knock down?

Not having the original owners manual I don't know the exact intended purpose of this rope, and there is no provision for cleating it off at present.

 

[Signed Out]

Perhaps it's for tugging on when it gets a bit sticky.

If you do cleat it, maybe a weak link is a good idea, albeit one that'll hold when inverted.

 

[ProDave]

Perhaps it's for tugging on when it gets a bit sticky.

If you do cleat it, maybe a weak link is a good idea, albeit one that'll hold when inverted.

My thinking is it probably wouldn't hold if completely inverted, probably the rope would snap, but it might have enough hold to keep the keel down if just knocked down onto your side.

 

[LittleSister]

I am not really worried about it's capabilities around the coast here (South West Ireland) except for it's possible inability to beat off a lee shore, though the inboard should take care of that situation....

Beating off a lee-shore is not what a Mirror Offshore is about - it ain't gonna do it, in my opinion. That's why it's generally classed as a motor-sailer. Yes, the motor should get you off, but more crucially avoid getting into such a situation in the first place - you have no plan B to fall back on if the motor stops for any reason (or rope round the prop, etc.).

That's no great constraint, you just have to be as cautious as a (sensibly skippered) motor boat. Don't get too close to the shore (fantastic though it is in SW Ireland!) unless you're confident there's no way the wind is going to shift to make it a lee shore before you're out of the way at your slowest speed. Make sure you have a reasonable anchor and decent length of warp, and know how to use it, if push comes to shove,

It's good to be aware of potential dangers, but don't get too anxious about scenarios that are usually so easily avoided with a little aforethought they rarely occur. Happy sailing!

 

[snowleopard]

Would it be worth adding a cleat that this rope could be secured around once the keel is lowered, to try and use this rope to prevent the keel raising in the event of a knock down?

Apart from the effect on stability, the sheer force of a keel retracting under its own weight unrestrained during a knockdown could cause serious structural damage. Imagine righting after a knockdown to find a whacking great hole letting the water in. It would spoil your whole day.

 

[Signed Out]

Apart from the effect on stability, the sheer force of a keel retracting under its own weight unrestrained during a knockdown could cause serious structural damage. Imagine righting after a knockdown to find a whacking great hole letting the water in. It would spoil your whole day.

Usually heftily engineered around the mechanisms though, and probably more of an issue with daggers than swings, like ProDave has on his. Torsion on the pin and surrounds, but doubt it would rip the hull, or the keel end strike. And daggers'll usually have a mech at the top that would take some blow. More of a problem when doing the righting, really, when the dagger drops back down, if undampened in any way.

 

[trapezeartist]

Anything with a ballast keel should be self-righting, even with it up. Water ballasted (Mac26 et al) is less likely to be so if tanks are empty or only part filled. Dinghies & most open boats with centreboards, lee boards, dagger boards etc probably are not. Motor boats probably are not.

So true. Our dinghy (skiff) was self capsizing. Float it off the trolley - let go - splat!

 

[aquaplane]

Apart from the effect on stability, the sheer force of a keel retracting under its own weight unrestrained during a knockdown could cause serious structural damage. Imagine righting after a knockdown to find a whacking great hole letting the water in. It would spoil your whole day.

I thought that, I think it'll have to go inverted though and that is going to be difficult just with the wind as demonstrated by that Legend video.

There was no way to fix the Fisrt 18 keel down, it was on a long screw and swung up underneath the boat. I once wondered why it was being a bitch to winch onto the trailer and it wasn't untill I went inside I realised I hadn't lifted the keel. The screw mechanism had pushed up into the boat no problem. A good featrure if you clout the bottom.

The Copland keel lifts vertically on a winch/cable, if that fell when inverted I suspect it would go through the roof, or certainly make a big mess of it. I have a couple of bits of GPR that fit on top of the lowered keel to keep it in place, I think it's a neat system and it works well.