As I have the Twister lines in PolyCAD (naval architecture software), I let it determine a Twister's centre of gravity. It puts it directly above where the forefoot of the keel starts to rise, which is naturally a sort of fulcrum:
The trouble is, I am not sure how PolyCAD calculates the COG. It doesn't know about weight distribution, where the ballast is and how much there is, how many anchors I have on the fore deck, how much chain I have in the chain locker, or where the main water and fuel tanks are, or how full or empty they are, meaning I take that COG position with a pinch of salt. Nonetheless, if by some chance it is close to the real COG, it puts a dried out Twister right on the tipping point. What this suggests to me is that any Twister may nosedive when dried out, whether a particular Twister will nosedive is down to how the movable weights on board are placed before drying out.
John - I've been wondering about the very same question. There are photos online of I think Moonshot dried out on a beach (Flushing/Falmouth?) using just two legs eg here and elsewhere. It is not possible to determine exactly where the legs are in relation to the likely COG, maybe just a bit forward, but certainly not a lot. The boat doesn't appear to have fallen over, but I can't help wondering how precarious is she with that setup? Were I wonder extra steps taken to lighten the bow and/or add weight aft?
I’ve only been aboard 1 Twister with drying legs - so a small sample. On that boat (Louise, for sale at that time) the holes for the legs to bolt through the topsides were located in the head/hanging locker, roughly level with the centre of the forward windows. I reckon that is well forward of the COG so having the legs there ought to prevent nose-dipping as well as falling over sideways.
Alastair - interesting. The yacht I mentioned earlier is for sale, with photos in the public domain on a site that shall not be mentioned by name, including this one:
Here the legs appear more in line with the fwd end of the middle portlight, much closer to the tipping point, though it is hard to be sure given the angle the photo is taken from. But visually, there does seem to be more 'mass' of the vessel aft of the legs than forward.
A few thoughts to save everyone’s worst nightmares about this subject…
We’ve owned Crionna for 30+ years. She’s a 1970 all GRP MkIIA with iron/steel ballast. When afloat she’s normally loaded and stowed for cruising and floats with her design waterline level, though often a bit lower!
We’ve dried out many times in different places, walls, quays, scrubbing piles and our own trailer. We’ve never shifted any items prior to drying and have only ever gone a bit nose down (15o) once on a softish level bottom in the Outer Hebrides. Even that didn’t reach the ‘alarming’ category.
We would always recommend reconnoitring the site at the previous LW and clear any nasty stones and rocks. When alongside fender appropriately and secure well with springs and breast ropes. Take a line ashore from the masthead, or around the mast to lean her in, also rig a ‘check line’ from forward, straight up, to discourage nosing down. Then tend all lines until she has settled down.
The attached drawing shows the position of longitudinal and vertical C/G (Centre of Gravity). The force will always apply be vertically below it.
The Vertical C/G was established by a) Our own summation of weights of hull, ballast, rig and gear aboard. b) Supported by professional calculations and inclining experiment carried out on a Twister when the original ‘coding’ requirements were being drawn up by the, then, Department of Transport back in the late ‘80s.
The Longitudinal C/G was established empirically by us tipping Crionna out of the water on her ‘hauling out’ trailer.
All this seems to have been borne out in practice.
The plot on the drawing, with waterline horizontal, shows the LCG vertically above the point where the keel starts its rise to the stem. Any ‘bow down’ will effectively move the LCG forward relative to original reaction/pivot point. This will tend to self correct as the pivot point moves forward too.
At small angles of ‘bow down’ the forces are small and easily contained by line vertically up from foredeck to quay
it is beneficial if the ‘bottom’ at your chosen spot slopes uphill. Even a few degrees is helpful. Beyond that think through your plan carefully, take care and sleep easy!
To end where this thread began … we have overhauled the heads outlet seacock whilst afloat, saving all this angst, but that’s an explanation for another day ….
John
I'd note that the COG position on John's PDF near enough matches the COG position on Chris's post at the beginning of this interesting thread. I'd also note that the COG is nearly above the 'rocker' point on the keel - the point where the bow rounds off into the keel.
John, all very interesting, particularly the theory and empirical data, and Peter thanks for your observation. I do think the fact the COG is above the fulcrum is of note, it is basically the physics behind why a Twister is potentially unstable when dried out, even if in practice Crionna has been OK, bar one (minor) occasion. I am for my part reminded of the definition of a minor surgical operation: an operation done on someone else.
I, and I expect others, would be very interested in hearing how you overhauled the heads outlet seacock while in the water. Some form of seal on the outside? Otherwise, the moment the cone comes out, then the boat has only one way to go, downwards.
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