Keel Bolts

The required torque  depends on more than just stud material and size.

Normally the required torque also needs to take into account...

A. The material strength of the material holding the stud. In this case we know it is cast iron, but we don't know what strength grade that is.

B. Compressive strength of the intermediate  material. This will depend on whether it is solid fibreglass, or cored, and whether polyester/epoxy/vinyl ester etc.

C. The required preloads to ensure  anticipated static and dynamic loads are achieved.

As the only people who know the answers to A, B, and C are the yacht designers they should have calculated the required torques on the keel bolts, and they shoukd specify the torques.

I can warn you that you should NOT use published values for bolts of a specific grade and size, as it will be too much for sure and possibly cause damage. These published industrial values are generally based on threads into steel, with steel in compression from the bolt. I have designed many hundreds of compression joints, (some transmitting 1,000,000+ Nm torque and massive bending moments at the same time), so I advise caution.

My memory is telling me the bolts and nuts are galvanised steel, not stainless steel.  For a given required pre-tension the torque required varies a lot, depending on whether precision marine threads/commercial grade/ Zn plated/hot-dip galvanised  and whether they are old or not. If checking joints which are now dry, the torque will be much higher than if they were oiled at assembly.  A few quick calculations show the M42 required pretension is less than about 80kN, to resist righting moment and weight on that short keel. With oiled and galvanised bolt probably 1200Nm torque would be enough.  But, the yacht designer should do an accurate calculation and advise suitable tightening torques.

 

Cheers, DJ

But it should also not be nessesery because the complete system is in polyester resin. I have seen how once a keel was remover from a Hanse in Norway ( YouTube) and it lasted a few days before it was separate from the hull.

So I will not do anything with it , only wenn I have to because i have hit something underwater.

Gr Marcel

Those torque seem to be quite realistic, and achievable using hand tools.

They are a little lower than I expected, but not a concern.

I had a shipwright try and undo one of the nuts to my 400e a few years ago and with a 2m long bar, and all his strength he couldn't undo it. From his strength I'm guessing that was well over 1500nm, but that is the nature of bolts which are tightened already. We left the keel in place...