# Deck construction under windlass / capstan / mooring winches

Discussion in 'Boatbuilding' started by saad, Dec 26, 2012.

1. Joined: Jan 2008
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good morning ,
can anyone guide me in calculating the under deck stiffener (carling) scantling which supporting a winch or windlass, the vessel is under ABS class

I am assuming the plate under winch is subjected to lateral force thatfor we should calculate the buckling strength for this plate with the attached carling, is it correct?

2. Joined: Oct 2008
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You have several issues to address

1) Bending moments.

The moment created by the winch at its max lever above the deck with its max SWL load.
So, assuming the winch casting is taken as satisfactory, how are the moments transferred?...they first of all go via the bolted connection. (Don’t forget to take the weight of the winch into account too if it is a large one).

So you need to check the polar moment of inertia of the bolt pattern to ascertain if there is sufficient stiffness. Additionally the distance apart of the bolts shall create a couple where the reaction loads shall be tensile or compressive loads on the bolts.

The what is supporting the bolts, and thus that is your next load path. If a thick base plate, make sure the thickness of the plate is sufficient for the diameter and thread of the bolt.

Once into the plate what supports the plate….frames/longitudinals??...if so, what is supporting them? Since the bending moment is now transferred to the surrounding structure. So if the plate is mid-span of a transverse frame or very close to it, you need to apply the bending moment to the frame, or longitudinals.

Assuming, again, the casting of te winch to be satisfactory the horizontal pull shall be transferred as direct shear too. So those bolts above must also withstand the direct shear as well as bending, so combined bending and shear.

Then this shear load goes into the base plate…so what is the bearing stress of the bolted connection..ergo..will the blots simply pull through like cheese wire through cheese?

Finally once you have done that, apply this shear/horizontal load into the plate and check for buckling of the attached plating and stiffeners.

Oh..if this is steel…then that should be fine. If it is Aluminium or composite, you then need to check the deflections at each stage too. Since low modulus materials are deflection driven rarely stress driven.

Good luck

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### akhilsvnairNaval Architect

under deck stiffening of capstan

which is welded to the deck.I would like to explain my procedure.please go through and comment )

a) calculated the maximum pull= 1.25 times the load

b) now I found out the couple acting on the base considering load on rope is aft to forward direction.

c) couple produce tension and compression on deck. check for allowable compressive stress. which is 235/k . k is material factor.

d)checking shear and bending

Load acting on capstan will be transferred to the under deck stiffening in the form of shear and bending.
check shear and bending separately with load is nothing but calculated couple.

finally repeat the procedure in athwartship if we are not sure about the load application.

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### T0x1cJunior Member

I don't know about ABS, but in CEN ISO 15084 the anchoring point has to resist a minimum of 125% of the ultimate tensile strength of the rope or chain (meaning the rope or chain is supposed to break before tearing the deck apart).

If bolted with 316L studs, minimal stud area requirement would be UTS/350 in mm2.

Then I would use a finite element tool to check structural arrangement, with a safety coef. 2.

5. Joined: Oct 2008
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Never use UTS values, always yield/proof stress.

No need for FEA, see above.

What's the justification for a FoS of 2?

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### philSweetSenior Member

Also consider the likelihood of user mods. Such as somebody stuffing a 4" pad under the winch because the handle interferes with a cup holder. For powered winches, look at the family of winches and motors that are interchangeable. Any chance a simple parts swap will ruin someones day? For industrial bolt-on junk in general, I try to design mounts to accommodate the class of machine, not necessarily a specific one.

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### T0x1cJunior Member

As mentioned, this is as per ISO 15084, the standard uses UTS. This UTS value is the one of the chain/rope, not the SS bolt.

Let's say it is an alternative way, doesn't take more time.

ISO 12215 asks for a SF of 2 for structure. Again, this is the way I would do the calculation for a CE certification, might be irrelevant for ABS.

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