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| Keel Bolts and General Topics on Keel - Keel bolt inspection -Swan 36 |
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Join Date: 18 May 2007
Posts: 5 |
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Keel bolt inspection -Swan 36 I own a Swan 36 (hotspur - hull 26) at a recent inspection by a naval architect for insurance purposes, he wanted to extract a keel bolt for inspection. He said because the boat is so old. there are no issues with the keel cracking etc. I am wondering if this is necessary. Have there been any issues with them? Has anyone tryed to do this? I am not sure where they are as I do not have a drawing of their location. I suspect they are under the engine and freash water tank, any help appreciated. Geoff
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Join Date: 30 January 2007
Posts: 461 |
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I own a Swan 36 (hotspur - hull 26) at a recent inspection by a naval architect for insurance purposes, he wanted to extract a keel bolt for inspection. He said because the boat is so old. there are no issues with the keel cracking etc. I am wondering if this is necessary. Have there been any issues with them? Has anyone tryed to do this? I am not sure where they are as I do not have a drawing of their location. I suspect they are under the engine and freash water tank, any help appreciated. Geoff
My humble opinion: change naval architect and, possibly, insurance company! Daniel, 411/004 Luna Menguante |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Geoff Even if there is not a history of keel bolt problems it would be appropriate to extract a bolt - they are close to 50 years old.
The bolts are fairly evenly spaced along the keel. Easiest to access is the bolt pair just aft of the mast step. The bolts on Swan 36 are peculiar as they are not cast into the keel, but have nuts at their lower end located in keel pockets.
You can find the pockets for the mentioned bolts about 200 mm down from the Top of Lead. The pockets are filled with pieces of lead, these need to be dug out to access the nuts.
The records suggest that the bolt thread size is M20, if you could confirm this a recommended tightening torque can be given.
Kind regards
Lars
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Join Date: 30 January 2007
Posts: 461 |
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A question for Lars: is there a reason for using or not using the bolt pockets you mentioned before? In a previous sailboat I owned that was dated 1973 there were indeed the pockets and I thought they were a good idea for inspecting the bolts. My present boat which is newer (1979) does not have them. Is there a rationale under this kind of choice? Thank you for your time in answering, of course! Regarding my previous reply: sometimes I tend to be too impulsive and generally I do not like very much the attitude of the insurance companies... forgive me! Daniel, 411/004 |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Daniel and Geoff I should better have added "in a corrosive environment" in connection with the 50 years:-)
The naval architect has good reasons for his requirement. Unfortunately stainless is not always stainless, and keel bolts may suffer unnoticed, this has been pointed out earlier on this Forum, pls see post 03 June 2015.
First a comment about the present bolts:
As long as these bolts are completely bedded in mastic where they pass though the keel and end in the pockets, corrosion is unlikely.
But, if there are some narrow slots where water can get in and stand due to imperfect sealing around the bolts this creates an inherent risk of crevice corrosion to the stainless material, caused by a lack of oxygen in the quiescent water.
From this it should be clear that when these bolts are withdrawn for inspection it is very important that they are properly sealed when put back.
An alternative route would be to eliminate the risk of crevice corrosion completely, by replacing the stainless bolt with a material immune to this type of corrosion, for example bronze.
Then to the production differences:
Manufacture of keels with precisely located long bolt holes and pockets is quite challenging. As far as I know the keels were cast without them, and these features were then added manually. Drilling long holes accurately in lead is very demanding.
When the bolts are cast in, they are fastened into a jig before the casting begins, and no additional operations are needed. Further only the part above the lead needs to be well bedded into mastic. On newer models the keel bolt upper ends are not matted over. As there is plenty of air and moving water in the bilge, the top ends of the bolts above the keel plank do not suffer from corrosion.
Cast-in bolts can not be removed. For inspection either the mastic has to be dug out completely around each bolt right down to the lead surface, or the keel dropped.
Would like to use this opportunity to ask Swan 43 owners to report whether they have drilled-in bolts with pockets, or cast-in bolts without pockets. It is likely that later hulls have the the cast-in bolts.
Kind regards
Lars
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Join Date: 24 April 2007
Posts: 6 |
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Dear Geoff Even if there is not a history of keel bolt problems it would be appropriate to extract a bolt - they are close to 50 years old.
The bolts are fairly evenly spaced along the keel. Easiest to access is the bolt pair just aft of the mast step. The bolts on Swan 36 are peculiar as they are not cast into the keel, but have nuts at their lower end located in keel pockets.
You can find the pockets for the mentioned bolts about 200 mm down from the Top of Lead. The pockets are filled with pieces of lead, these need to be dug out to access the nuts.
The records suggest that the bolt thread size is M20, if you could confirm this a recommended tightening torque can be given.
Kind regards
Lars
Mr Professor, Are there other S&S Swans with the same keel bolt construction as the Swan 36? Brgds Carsten Swan 37, Bnr 18 |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Carsten As far as I know the only other model with keel bolt pockets is Swan 43, early in the series.
Kind regards
Lars
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Join Date: 02 February 2007
Posts: 202 |
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Dear all, Crevice corrosion is a rather frightening process especially if there is no indication it is taking place. Is this indeed the case? For instance, in the case of keel bolts I would have thought that if water is going in through cracks, or improper sealing, there would be at least some rust weeping through them, showing that something wrong is happening. I have noticed rust on some of the screw heads fastening the pushpit of our 41, and indeed corrosion was happening underneath. If this is not the case I doubt that extracting only one bolt would be sufficent to be sure everything is ok. A a matter of protection against this problem would zinc anodes on the keel create some protection against such corrosion? Many thanks for any information... from dear professor! Kind regards. Philippe 41/022 |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Philippe Thank you for the observation.
You are right that corroding keel bolts would cause rust weeping from the keel-hull joint, and there is reason to look for this when the yacht is ashore.
It is surveying practice to require at least one bolt to be extracted, with the assumption that they all are affected in the same way.
Lead is less noble than stainless in the galvanic series, and therefore works as a sacrificial anode when the metals are in good contact electrically.
The amount of lead is very big compared to the bolts, and this reduces the galvanic action on the lead. Both stainless and lead have tough oxide skins on their surface, and this further slows down the process.
A lead keel brings some additional protection to the bolts even without anodes, but it should be noted that the keel or an anode protects only the area "within sight" and not for example around the corner or in deep crevices.
The recommendation is to avoid crevices around keel bolts by careful installation, and if there are signs of rusting this needs to be thoroughly investigated.
Kind regards
Lars
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Join Date: 18 May 2007
Posts: 5 |
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Dear Lars and all, Thank you for the helpful information, I have sign off on the boat this time but will need to look at this in the future. Just so I undersand the process- dig out the pockets that are 200mm down from the top of the lead. I assume the pockets go all the way through the lead keel and are accessed from both sides? I would also expect to see bolts on the top of the Keel, I can only see two bolts under the water tank and an not sure if these are keel bolts, others are not visible, are they buried in Mastic , glass or something else? Are these the most forward bolts, about 500mm back from the mast step. To get to them I would need to cut out the water tank which seems drastic. Are there two rows of bolts, at defined spacing? Thank you once again for the helpful information and discussion. Geoff |
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Join Date: 18 May 2007
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ps thought you might like to see the rudder post, this has no been replaced Geoff
 corrosion in rudder post |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Geoff The pockets are accessed from both sides.
It is likely that the first and last bolts do not have pockets, because the vertical dimension of the lead is insufficient. These bolts are inserted from the bottom of the keel, and have recessed heads at that end.
The bolts in the bilge are glassed over. Their positions from the saloon fwd bulkhead are: 0.27, 0.86, 1.29, 1.74, 2.15, and 2.55 m. All are pairs except the aftermost one
Referring to the photo of the corroded rudder stock - it appears it has been welded to the strap fitting. If the welding was not done correctly there is a risk of weld decay, causing serious corrosion. Has there been a keyway connection originally, later reinforced by welding?
Kind regards
Lars
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Join Date: 18 May 2007
Posts: 5 |
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Dear Lars, Thank you for the information regarding the keel bolts I will look for them when I next take the boat from the water. Regarding the rudder, as far as I can tell the rudder was original and not modified. No welding in this area and no sign of keyways etc. I noticed some extra movement in the bearings which lead to taking the out rudder and replacing the lower bearing, on inspection of the rudder stock there was small amount of corrosion around the top where the shaft entered the fiberglass of the rudder. Exposing more of the shaft by cutting the fiberglass away showed further corrosion as seen in the photo above. I think this occured because the fiberglass steel joint developed small cracks over time the steel was in close contact with sea water in the cracks in a confined anoxic enviroment and the oxide coating on the stainless steel was damaged allowing the corrosion over the past 50 years. attached is an other photo of the rudder top with the shaft exposed. A new 316ss rudder post was installed and it was re glassed and works well, hopefully for another 50years! Thank you for your interest Regards Geoff
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Geoff Thank you for the information.
I would expect keyways inside the two sleeves connecting the rudder blade to the stock.
Or is there another way of transmitting the torque?
Kind regards
Lars
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Join Date: 18 May 2007
Posts: 5 |
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Dear Lars, Re rudder-This was done a few months ago and It looked like it was welded, but there may have been keyways. I agree that the welds will change the metallurgy and could be susceptible to corrosion. Re keel bolts - the first ones (0.27m) are under the mast base and the second (0.86m) are under the water tank others are under the engine, this will be a difficulty job. Thank you for your help Geoff
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