I have a Beneteau 310. The backing plates have rusted and need to be replaced. I want to replace them with stainless. Does someone know what type of stainless would be best; gauge or thickness and the torque that the bolts should be tightened to?
Keel backing plates
- Thread starter behill
- Start date
Practical Sailor did a bunch of testing on this.breaking samples several ways in a test rig.
As a general rule, the plate must be at least 1/2 the bolt diameter thick and at least 5 bolt diameters radius for solid glass. But if the layup is light, the requirement could be greater. It isn't less. Round the corners and bed them.
As a general rule, the plate must be at least 1/2 the bolt diameter thick and at least 5 bolt diameters radius for solid glass. But if the layup is light, the requirement could be greater. It isn't less. Round the corners and bed them.
Typically you would want 316 stainless if available, but most metal suppliers stock type 304 and special order the 316. 304 is stronger but less corrosion resistant than 316. If your bilge isn't constantly full of salt water then you shouldn't have any issues using 304.
My local metal supply places stock 304 stainless in flat bar from 1/8" to 1/4" thick, 1 inch to 4" wide @ everything from 12 inches to long to full 10 or 12 ft lengths. The best deals are the cut off bins where you will short pieces of what you might need for a good discount. Stainless isn't cheap, but buying by the pound just for keel bolt backing plates is going to be much.
It is really important to make accurate templates. You'd be surprised at how much of a pain it is to accurately place the holes, especially if you make a plate that backs two or more keel bolts. For torque, most OEMs publish the correct spec for the size keel bolts they use, but you can also reference an standardized chart that will list torque values by fastener size and material.
Paying a fabricator to do it for you will be expensive in labor unless you have the right tools to do it yourself. You'll need a really good metal cutting saw and/or plasma cutter for stainless. SS is very hard on tools, my blades cost around $200 each and don't last long cutting 304 SS. I make most cuts with a plasma cutter, but going through 1/4" SS is a pain, so I hope to upgrade my unit to a higher amp version. A drill press is critical, it must have adjustable speed to turn a high quality (carbide) bit at slow speed with cutting oil to effectively drill SS. Also, you need a nice big belt sander or drum sander to clean up and shape the pieces for finishing. I know I make it sound like a lot, but if you already have these types of tools it really isn't bad. Depending on the boat and size of the keel bolts (drilling 5/8" to 3/4" holes in SS is a beast), I can typically do this job in just one day.
My local metal supply places stock 304 stainless in flat bar from 1/8" to 1/4" thick, 1 inch to 4" wide @ everything from 12 inches to long to full 10 or 12 ft lengths. The best deals are the cut off bins where you will short pieces of what you might need for a good discount. Stainless isn't cheap, but buying by the pound just for keel bolt backing plates is going to be much.
It is really important to make accurate templates. You'd be surprised at how much of a pain it is to accurately place the holes, especially if you make a plate that backs two or more keel bolts. For torque, most OEMs publish the correct spec for the size keel bolts they use, but you can also reference an standardized chart that will list torque values by fastener size and material.
Paying a fabricator to do it for you will be expensive in labor unless you have the right tools to do it yourself. You'll need a really good metal cutting saw and/or plasma cutter for stainless. SS is very hard on tools, my blades cost around $200 each and don't last long cutting 304 SS. I make most cuts with a plasma cutter, but going through 1/4" SS is a pain, so I hope to upgrade my unit to a higher amp version. A drill press is critical, it must have adjustable speed to turn a high quality (carbide) bit at slow speed with cutting oil to effectively drill SS. Also, you need a nice big belt sander or drum sander to clean up and shape the pieces for finishing. I know I make it sound like a lot, but if you already have these types of tools it really isn't bad. Depending on the boat and size of the keel bolts (drilling 5/8" to 3/4" holes in SS is a beast), I can typically do this job in just one day.
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If you don't have any luck with purchasing pre-fab plates from AYS, or you are shocked by the price... Post pics of your bilge/keel bolts. It is not hard to make these.
In case anyone is interested, below is a link to a Yachting World Article from Jan 2015 regarding keel bolts and backing plates. The pictures are noteworthy because they are some of the most hardcore backing plates I have ever seen. the reason for this is because you'll notice the keel bolts come through the hull laminate (skin) but not through the structural grid (frame). This is not uncommon and the massive backing plates effectively distribute the keel loads over a much larger surface area... These types of reinforcements were probably heavily influenced by the Cheeki Rafiki tragedy.
http://www.yachtingworld.com/featur...-not-expert-can-keep-track-changes-keel-61200
http://www.yachtingworld.com/featur...-not-expert-can-keep-track-changes-keel-61200
I think you are spot on, that would be a good rule of thumb to consider even if the keel bolts passed through a frame member rather than just the laminate... more/thicker/larger area never hurts unless you are restricted by the dimensions of the bilge.
My only reservation is that 1/4" thick plate for 1/2" keel bolts would be over kill, mostly because SS is so painful to drill.... In general, my thought is that 1/4" thick plate should be more than adequate for keel bolts sized 5/8" up to 7/8". It would be the surface area I would want to maximize, and notice how the examples in the article are also welded into box channel... Metal fabricators often 'box' classic car frames to beef them up in a similar manner. If I had a situation where I had limited bilge area that would result in an inadequate surface area of the plate, I would box it in as such while also high plating up the side nearest a main grid or frame member and through bolting it. The more 'Harcore', the better.
Oh, BTW, when looking at the thickness of the plate material you have to consider what you have for available thread on the stud. that super thick plate would do little good if you got anything less than 110% of nut threaded down on the stud. High-pressure waterproof grease! (I love TEF 45)