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Keel Bolts and General Topics on Keel - Bottom Plate under mast foot/step
14 September 2014 - 18:13
Join Date: 16 February 2007
Posts: 199

Bottom Plate under mast foot/step
Dear All and dear Lars

Having been intrigued by the thread about the keel bolts and nuts, I wish to check few aspects with you, hoping to clarify my understanding of the issue.

The issue is about the plate under the mast fot/step (sorry for the terminilogy). First, the mast foot does not show any corrosion or very little. Some years ago, both the steel web supporting the mast fot, the mast shoe and the base of the mast have been treated. No corrosion so far.

My present worry concerns the bottom plate under the steel web. This bottom plate seem to act as a giant washer though which the keel bolts and the nuts are emerging. These are not glassed and have not been as far as I know.

The bolts and nuts seem fine (as far as I can see) and the little rust on them is very superficial.

The bottom plate however is/was rusty. I cleaned it thoroughly and treated it with inter protect only. I do not want to do more until I have consulted you for advices.


1/looking at the attached pics, one can see that the keel bolts and nuts are emerging through the bottom plate. This plate seems to be attached to the rest of the steel web constituting the mast step construction. Is it so?

3/If such is the case, and since the bottom plate is partly corroded, I am asking if there is another plate embedded in the joint section hull/keel? I hope so. If not, how can I assess the integrity of this bottom plate in order to decide if it must be replaced or preferably re-inforced?

4/ Instead of replacing it, would it be possible and enough to re-enforce it by insert substantial washers under the nuts? How can I assess the necessity of such measures?

4/ The above being said, I must confess that I have not been able to move the nuts at all, despite a lot of efforts.

5/ Finally, I wonder if there is another plate embedded in the thick material under the plate? In the joint section hull/keel? I would be happy if such is the case. It would prevent me from worrying about the partly corroded plate that I see.

Pics attached!

Thanks a lot for any comments and help in understanding this issue.

(Farouche 47/50)

Mast foot/step. Not corroded

Bottom pate (corroded) and keel bolts and nuts. Not glassed

14 September 2014 - 20:36
Join Date: 02 January 2008
Posts: 1428

Dear Philippe
1) The mast step bottom flange through which the keel bolts emerge is a part of the welded mast step assembly. The keel bolt nuts as well as all horizontal bolts need to be removed before the mast step can be taken out.
The holes in the top plate are for providing socket wrench access to the nuts.
3) There is no additional plate embedded. The thickness of the bottom flange under the nuts needs to be determined, and compared to the other plating of the mast step. Originally they had the same thickness.
4) If the bottom flange is less than 3/4 the original thickness it would be advisable to add a large washer under the nut. Where the nut is close to a vertical web the recommendation is to use a piece of angle bar, possibly with triangular supports added, put one leg under the nut and bolt the other leg to the vertical web. Recommended washer diameter 4.5 x bolt diameter, thickness 0.4 x bolt diameter. Angle bar legs to have corresponding dimensions.
If only a few threads are showing above the nut the added washer thickness may be restricted by that. Nuts to be locked.
4) The keel bolt thread sizes are M24 and M30, and the recommended torque is 260 Nm and 510 Nm respectively, assuming ungreased threads. If the nuts do not move with these torques it is suggested that you try CRC or similar thread lubricant.
5) See point 3)
Kind regards

15 September 2014 - 11:48
Join Date: 16 February 2007
Posts: 199

Dear Lars,

Many thanks indeed for your very helpful clarifications. I will soon measure the remaining thickness of the bottom flange - from what I could see when cleaning it, it looks quite fine - and get back to you with the result. I will also measure to see how thick the washers can be. If there is enough thread I will go for 10 and 12mm thick washers and a diameter of 120 and 160 respectively for the different bults. Right?

I guess that the procedure of adding washers under the nuts on the bottom flange is best done with the boat on the hard resting on the keel. Right?

The reason for this bottom flange being corroded is because the water coming down from the mast obviously partly stays in the area under the mast step. Thus, all of it, does not run down to the lower part of the bilge.

This part, the lower one, is very difficult to reach and I do not seem to have a viable possibility to investigate the bottom flange there. The access is extremely limited and quite obstructed by the centre lift divice.

Is there a feasible way of investigating this pat of the bottom flange?

(Farouche 47/50)

15 September 2014 - 20:38
Join Date: 16 February 2007
Posts: 199

Dear Lars,

I have now measured the bottom flange and find it to be about 7 mm at least and in places thicker. The corrosion is not uniform. The thickness of the steel web constituting the mast step/foot and shoe is not corroded and shows 9mm.

1/Attached you can see a pic showing a nut from near. I observe that the flange is thicker under the nut and immediately around it. Similar observation is made with the other bolt/nuts. Sometime, this thicker area is not as smooth as the rest of the flange. However, it is a very solid steel. Question: is this assessment enough to decide about the washer under the nuts?

2/The visible thread of the emerging bolts are of a maximum 10mm. In many bolts, it is merely 3 to 6 mm. or there about. Question: would washers of - say 2-3 mm - do the job? Is it a worthless effort?

3/ The nuts and bolts directly under the mast step/foot ensemble are possible to reach. Those in the lower part of the bilge are almost impossible to reach especially as they are obstructed by the single lifting device. Question: is it worth it to do the job with the washers for part of the nuts only? Those on the forward end of the keel?

4/ In any case, I guess that such work can be carried only with the boat on the hard and resting on her keel. Right?

Thanks in advance for any update and advice.

(Farouche 47/050)

Thick material under the nut and the surrounding area.

16 September 2014 - 12:40
Join Date: 02 January 2008
Posts: 1428

Dear Philippe
Thank you for the photos.
The recommendation is to have big washers, but this may be wishful thinking, as there might not be enough room for them.
Suggest you measure the space before manufacture. Square or rectangular washers can be used if there are limitations.
You are right that it is preferable to have the yacht standing on her keel when it is attempted to open the keel bolt nuts, this should make it easier.
If there are bilge recesses collecting water it is suggested that you check if they can be filled so the water drains aft.
It is suggested that the lifting lug be removed.
The average remaining steel thickness in the area around the bolts should be considered here, even if there is more thickness locally under the nut.
If there are not enough bolt threads for added washers it is possible to use custom washers with a recess in the middle.
The bolts in the forward part of the keel are loaded more than the aft ones, but it is advisable to do the same to all bolts.
Kind regards

16 September 2014 - 13:28
Join Date: 16 February 2007
Posts: 199

Hello Lars,

Many thanks indeed for the advices. I feel that I now have some of the necessary insight for undertaking the job of reinforcing all the keel nuts with appropriate washers as each individual bolt and nut may permit.

As to removing lifting lug...I wonder how feasible it is! Given the lack of space and other things, it must be an ordeal!

This is a project that I am not looking forward to accomplish. It is in the back of my head and will stay there until the late spring 2015. I will then gather my courage, if any left, and will tackle this thing. I will then also take pics and let you know about the progress, if any.

Million thanks so far//Philippe
(Farouche 47/050)

28 December 2015 - 19:57
Join Date: 01 February 2007
Posts: 234


Removing the lifting lug is achievable by applying heat to break and soften the sealant holding it down. I tried brute force but failed, only a good amount of heat via a blow torch worked.

John B
411 010

30 December 2015 - 15:21
Join Date: 16 February 2007
Posts: 199

Hello John,

Thank you for the advice. It will not be necessary though to lift the thing because after close examination there were substantial washers in the aft section. I needed to add only 4 washers in the front, and that was done relatively easy.

Happy new year//Philippe
Farouche (47/050)

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