Tool suggestion for grinding deep bilge areas.

[BAD ORCA]

Im getting ready to haul out in a few weeks and drop keel and install some good backing plates. In preparation for new backing plates i need to be able to grind out a bunch of the old crush filler they added in the bilge around the bolts. My bilge is a bout a foot deep and about six or so inches wide, but there are areas in the aft end of bilge that tapper to only about 1.5" wide that would need to be cleaned up as well (see pic). I need to get down about 1/4" or so into this substrate, so it need to be fairly aggressive ground out. An angle grinder is simply too wide to get to most of the areas. Whatever i use must be fairly narrow. As of right now. the only tool i can think of that will be able to work is one of these minus the handle.

Makita USA - Product Details -9031

1-1/8" x 21" Belt Sander

www.makitatools.com

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I'm wondering if anyone knows of a tool or attachment that might work better/faster? I would like the final surface to be relatively flat so i can properly bed under the stainless backing plates with epoxy and finally sika flex.

 

[sailcapt340]

Have you thought of using a series of different width Forstner Bits. You only want to go a 1/4” deep, they drill a round flat bottom hole. You control the depth. They’re used in cabinet making to inset hinges to a certain depth. You would need a set of carbide tip bits, which should be available.
Just a thought you might want to look into.

 

[Calif. Ted]

Are you dropping the keel just for backing plates and are you sure that's necessary?

 

[Jim Legere]

I had to do something similar when I replaced the shaft log on my full keel boat in adeep, narrow section at the back of the bilge. An oscillating tool (e.g Fein Mutimaster) with a carbide saw blade did the trick for me.

 

[BAD ORCA]

Have you thought of using a series of different width Forstner Bits. You only want to go a 1/4” deep, they drill a round flat bottom hole. You control the depth. They’re used in cabinet making to inset hinges to a certain depth. You would need a set of carbide tip bits, which should be available.
Just a thought you might want to look into.

Thats a fantastic idea. Thank you, no that never crossed my mind. I have some on the boat. I dont think they are carbide, but this filler material i dont think has any glass additive but im not 100% certain. I'll do a small test this weekend, but i dont see why this would not work. I can use the grinder just to tidy up the edges so should save me a lot of time.

 

[BAD ORCA]

Are you dropping the keel just for backing plates and are you sure that's necessary?

No. I suspect my forward most keel bolt might be severely corroded. I have a 'Catalina smile' with rust weeping out of the leading edge. Its a lead keel so not sure where the rust is coming from. I plan to do a lot of offshore sailing so I just want to be 100% sure. Its painfully expensive to do though.

 

[dLj]

I think rather than remove material for a backing plate to sit flush - that's what I think you are trying to do? - I would use structural epoxy to make a nice flat surface, then mount the backing plate on that. It would feel to me safer than trying to level that surface with mechanical tools. If course if you feel there is degraded material, you'd want to remove that, but the work of making it flat seems easier and safer to do with structural epoxy.

dj

 

[BAD ORCA]

I think rather than remove material for a backing plate to sit flush - that's what I think you are trying to do? - I would use structural epoxy to make a nice flat surface, then mount the backing plate on that. It would feel to me safer than trying to level that surface with mechanical tools. If course if you feel there is degraded material, you'd want to remove that, but the work of making it flat seems easier and safer to do with structural epoxy.

dj

I feel like that material is crushing from the years of the keel sitting on it with tiny washers. There are lots of spider web cracks around most of the bolts now. It certainly is in the forward part of the keel. I'm going to have to dig down about an inch on that one and add a piece of G-10 under the backing plate. Also, i need to grind into it a bit so i have space for the nuts to still be able to thread onto the keel studs. I'm adding 1/4" thickness backing plates plus probably another 1/4" piece of G-10 under that.

 

[Project_Mayhem]

If all else fails, make a hole in the side of the keel stump for better access?

 

[dLj]

I feel like that material is crushing from the years of the keel sitting on it with tiny washers. There are lots of spider web cracks around most of the bolts now. It certainly is in the forward part of the keel. I'm going to have to dig down about an inch on that one and add a piece of G-10 under the backing plate. Also, i need to grind into it a bit so i have space for the nuts to still be able to thread onto the keel studs. I'm adding 1/4" thickness backing plates plus probably another 1/4" piece of G-10 under that.

What will hold the G10? It won't crush, but what is the structure under it holding this whole construct? If you remove the material, it seems to me you are removing structure. How are you making the G10 part of that keel stub structure? That's what I'm not understanding.

dj

 

[BAD ORCA]

What will hold the G10? It won't crush, but what is the structure under it holding this whole construct? If you remove the material, it seems to me you are removing structure. How are you making the G10 part of that keel stub structure? That's what I'm not understanding.

dj

The keel stub bottom is thick fiberglass layer like any keel. I have no idea how thick though. On top of this layer of fiberglass (inside the bilge compartments) i was told once by someone who knew a lot about these boats, that they poured a layer of a composite filler (unknown depth) to absorb the impact from a grounding. I have no way of confirming this though. I was told that the intention was that the keel bolts would crush into this layer of composite and help prevent any structural damage from a grounding. I dont know what the material they used is, but its like a very hard gel coat type of material. I suspect my boat did suffer a light grounding at some point possibly because the forward most bolt looks like it might have compressed a little into this filler material. Also, after 40 years the small washers they used seem to have caused hairline cracking in this material around the bolts.

Once the keel is off and lowered, i will have a better idea of how thick it is and will be able to better assess how to approach this. For now, my intention is to grind back 1/4" or so to fresh material, epoxy in a big flat piece of G-10 on top of this freshly exposed layer and then put my backing plates on top of that. My thinking is that the wide piece of G-10 will sit on a wider aarea of the bilge and this layer of material and distribute any loads a bit more evenly.

 

[dLj]

The keel stub bottom is thick fiberglass layer like any keel. I have no idea how thick though. On top of this layer of fiberglass (inside the bilge compartments) i was told once by someone who knew a lot about these boats, that they poured a layer of a composite filler (unknown depth) to absorb the impact from a grounding. I have no way of confirming this though. I was told that the intention was that the keel bolts would crush into this layer of composite and help prevent any structural damage from a grounding. I dont know what the material they used is, but its like a very hard gel coat type of material. I suspect my boat did suffer a light grounding at some point possibly because the forward most bolt looks like it might have compressed a little into this filler material. Also, after 40 years the small washers they used seem to have caused hairline cracking in this material around the bolts.

Once the keel is off and lowered, i will have a better idea of how thick it is and will be able to better assess how to approach this. For now, my intention is to grind back 1/4" or so to fresh material, epoxy in a big flat piece of G-10 on top of this freshly exposed layer and then put my backing plates on top of that. My thinking is that the wide piece of G-10 will sit on a wider aarea of the bilge and this layer of material and distribute any loads a bit more evenly.

If the explanation is correct, and that softer layer and small washers were designed to allow absorption of energy in the case of a grounding, then you are redesigning the expected failure mode in such a case.

Bunch of if's here: If that's the original design intent, and If that is a legitimate design benefit then you should give consideration of how you could recreate that failure mode.

There is a tendency in the marine world to have a mind set of "forever" fixes that will "never" break. But that is a fallacy. Nothing is forever, and everything can break. In a well designed system, looking at how it will break is important. You want the way something breaks to lower the risk of loosing the boat, and to make fixing it easier.

Just some food for thought.

dj

 

[BAD ORCA]

If the explanation is correct, and that softer layer and small washers were designed to allow absorption of energy in the case of a grounding, then you are redesigning the expected failure mode in such a case.

Bunch of if's here: If that's the original design intent, and If that is a legitimate design benefit then you should give consideration of how you could recreate that failure mode.

There is a tendency in the marine world to have a mind set of "forever" fixes that will "never" break. But that is a fallacy. Nothing is forever, and everything can break. In a well designed system, looking at how it will break is important. You want the way something breaks to lower the risk of loosing the boat, and to make fixing it easier.

Just some food for thought.

dj

Thank you. Those are great points i had not considered. I guess my biggest problem is that I have no way of confirming what this material is and if that truly was its original intention or not, and if so, what was its intended lifespan? I have tried every search combination i can think of and nothing comes up on Google about any type of filler material for boat bilges that absorb impact. Also, the boat builder has long since been out of business. There is an owners forum for my boat but i have not been able to make any further determinations there. I need a proper Naval Architect but that's not a financial option unfortunately.

 

[dLj]

Thank you. Those are great points i had not considered. I guess my biggest problem is that I have no way of confirming what this material is and if that truly was its original intention or not, and if so, what was its intended lifespan? I have tried every search combination i can think of and nothing comes up on Google about any type of filler material for boat bilges that absorb impact. Also, the boat builder has long since been out of business. There is an owners forum for my boat but i have not been able to make any further determinations there. I need a proper Naval Architect but that's not a financial option unfortunately.

There's only so much you can do, we often are over-thinking these things. If you are going through all the work of dropping the keel, then just keeping these things in mind as you do the work may help keep things in perspective.

I'm assuming the main reason you are dropping the keel is because you want to inspect the keel bolts over their entire length - you mention rust somewhere else - and come up with a way to make that interior surface have less cracks. Ultimate goal is to keep the keel tightly attached. Essentially, you want to make sure your boat is seaworthy.

Dropping the keel is whole lot of work and from the pictures posted, seems a bit over the top. But, none of us are at your boat so we can't really say a lot.

dj