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Keel Bolts and General Topics on Keel - Keel Bolts - Swan 411 -023
16 April 2020 - 19:43
Join Date: 12 February 2020
Posts: 4

Keel Bolts - Swan 411 -023

Lars / Forum

My boat is now over 44 years old and I have owned her since 1997, so over half her life. I am contemplating a major refit and was wondering if anyone had ever removed the keel bolts from the keel of a 411 and replaced them. There are no apparent issues at present but feel that it is time to undertake some preventative maintenance in this area. My boat is one of the deeper draft versions.

The keel was removed in 1997 to correct some original build issues where the two halves of the hull were slightly stepped and the top of the keel was not totally flat along its length. The excessive tolerances were filled with a glass mulch which had suffered bad osmosis. (We rebuilt the bottom of the boat to a perfectly flat surface and machined the top of the keel to match the hull, before re-attaching the keel - better than new!)

Before undertaking the replacement of the keel bolts I have a number of questions

a) Has anyone undertaken this work before and what are the pitfalls and challenges we might face?

b) How are the bolts fitted / attached to the keel structure?

c) How long are each of the bolts?

d) How are the keel bolts removed from the keel and the new one attached?

e) Is there anyone in the US (east coat or UK that has undertaken this work?

I would be grateful for any comments / thoughts

Thank you


17 April 2020 - 10:38
Join Date: 02 January 2008
Posts: 1461

Dear Paul

Here short answers to your questions
a) As far as I know this has not been done. The reasons are mentioned below
b) The bolts are cast in and have a nut and washer at their lower end. The nut is secured to the bolt with a tack weld. When the lead shrinks around the bolts during the cooling of the keel, the bolts are held very tightly, and it may be impossible to turn them for this reason.
c) The bolt length inside the lead is on average 500 mm
d) It is unknown if the bolts can be forced to turn by applying torque at their upper end due to the firm grip in the lead. The torque needs to be sufficient to also break the tack weld. If you cut windows in the keel at each bolt lower end the tack welds could be removed from the outside.
e) Not known to me - members fill in please
I could attempt to calculate the shrinkage holding the bolts
The bolts do not corrode inside and near the lead, because lead works as a sacrificial anode around the stainless bolts. It is sufficient to inspect them above the lead.
Kind regards



17 April 2020 - 16:18
Join Date: 02 January 2008
Posts: 1461


Calculated the radial interference around the bolts after the lead keel has cooled from 327 C to room temperature. It is 0.09 mm, and this creates a pressure of 76 MPa in the joint. This exceeds the lead SWL by a factor of about 4, lead is a soft material and will yield until acceptable compression is achieved. 
It appears that the bolts are held so tightly even by this reduced compression that applying a high torque at the bolt upper end will just twist the upper part.
The bolts are AISI 316, which is a soft material
Kind regards

18 April 2020 - 08:34
Join Date: 02 January 2008
Posts: 1461


MarsKeel uses a peculiar method to remove bolts needing replacement. Note that they do not use a nut and washer at the lower end.
If the bolt is corroded near its upper end like in the photo to the left, and there are enough undamaged threads left, it would also be possible to put a threaded sleeve on the bolt, and not go to the trouble of replacing it.
Kind regards

18 April 2020 - 15:32
Join Date: 26 January 2018
Posts: 5


Thank you for the detailed information.

As all appears to be fine I will leave well alone.

Best Regards


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