41 keel bolt and mast shoe / step set up Thank you to Matteo for giving me a login.
I'm evaluating a 1974 41 for purchase and have identified that the mast shoe is badly corroded (but overall the boat appears solid - full survey to come). So I have to anticipate replacing or repairing the shoe. I have read virtually every post on the forums. Here is my question:
I know Lars has recommended that if the shoe cannot be reconditioned /repaired, an option is to cut it out and rebuild using g glass (G10/G11). What I have not been able to see on my inspection of the boat, or find on the forum, is if the keel bolts are fastened through the bottom of the mast shoe. Is this the case?
The forward most keel bolt - the one exposed through the large hole in the shoe at the centerline, appears to be. Are the others?
I want to understand if replacement of the mast shoe would require unbolting the keel and re-fastening the keel bolts through the new shoe?
Alternatively are there any specs that identify what kind of downward / compression load the shoe has to take? (The boat has a short mast, mechanical backstay set up.) I ask, because just maybe there is enough integrity in the old shoe for it to be reinforced verses removed. The forward keel bolt nut is badly corroded as well, and I have yet to inspect the boat out of the water to see if there is any separation between the keel and hull.
Any insight, or information is greatly appreciated.
we had similar problems on our 48 -- I posted a thread starting Feb 7, 2013, to describe the problem and the way it was solved. I am not sure if the 41 has the same setup but think so, so if you are interested I have more pictures than the ones I posted.
Two things went through my mind. First, what I refer to as the mast shoe is a stainless steel plate that the mast stands in (we had corrosion here, but of course it was on the mast and not on the shoe!). Second, the mast shoe is bolted to a massive steel frame that continues aft throughout the next bottom sections.
Replacing the shoe (in our case with aluminium) is a relatively minor operation. The frame is a different story. In the cross-section, the frame looks like a double T, and the keel bolts are bolted through the lower plate which works as a giant washer. As the corrosion in our case only affected the higher parts, we left the bottom part in place, including the keel bolts -- everything was glassed in anyway and free of corrosion. The replacement part was then bolted in to avoid welding so close to the hull.
Let me know if you need further detail.
Here a few comments in addition to the very helpful ones from Martin.
The mast step has several functions
- Receive the keel bolt nuts as mentioned, and suspend the weight of the keel.
- Resist mast compression, which is about the same as the weight of the boat. The steel web right below the mast shoe must be able to take this load with a good safety factor. Both compression and buckling to be considered
- Resist the sideways bending moment from the keel, for this there are transverse steel webs integrated with the I-section.
- Resist grounding loads, usually the aft end of the keel is forced up, and the forward end forced down.
If you can find out the original steel thickness in an uncorroded area, as well as the remaining thickness in corroded areas some advice can be given based on this.
The keel bolts are glassed over, if you open the nuts it is not necessary to glass them in again, but then the nuts need to be locked securely in some other way.
You say that one nut has corroded badly, this is surprising, maybe it is just covered with rust from the surroundings?
If the keel is well attached to the hull over most of its area there is no need to drop it even if you replace the mast step.
Lars, thank you so much for adding these points and I will include them to my project scope as I talk to thel yard about the repair and will, of course, contribute any useful info.
I need to take a closer look at the structure and do some poking around now that I have a better idea of whats involved. I'll be back soon!
A really significant factor in choosing this boat is the existence of this engaged owners group and your contributions over the years. These are amazingly designed and built boats. It will be a pleasure to own one.
Having been intrigued by the thread about the keel bolts and nuts, I wish to check few aspects with you. let me first describe what I can see under the mast step. I have also included some pics. Questions:
1/ is the mast shoe/step attached to the steel web through which the bolts and nuts appear? It is very difficult to assess this. If such is the case, replacing the mast step will necessitate the unscrewing of the keel nuts. I hope that i am wrong.
2/Looking at the attached pics, one can see that the keel bolts and nuts are on top of 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.
4/ I have tried to tighten the nuts without any success. There seem to be no way of moving them at all.
Summarising: is there an embedded plate under the nuts or is the steel web of the mast step the only thing there is?
Thanks a lot for any comments and help in understanding this issue.