Cheeki Rafiki loss report

[ben373]

Off thread a little, van keel bolts be tightened in the water? And what would be a typical torque setting.

Sent from my G630-U251 using Cruisers Sailing Forum mobile app

[mstrebe]

Torque depends primarily on the size and alloy of the threaded rod used, for which there are standardized tables engineers use as a baseline. Secondarily the compressibility of the material being clamped may require the torque to be increased or decreased a little, but you can't go too far from the standard torques for a rod so you'd have to change the rod size and then increase or decrease the number of rods used.

Going by memory, 90 lbs. for 1" threaded rod of 316 stainless is about rIght. The clamping force of a screw is 10X the holding torque, so that's 900 lbs. of clamping force per rod. So with 6 rods you get 5400 lbs. of clamping force, which in general is 2X the weight being attached to provide tolerance for shock loads.

Reverse all that math and you can see how the number and size of keel bolts is determined.


Sent from my iPhone using Cruisers Sailing Forum

[mstrebe]

There's another potential issue with bolt-on keels.

My boat delivery from the dealer and receiving boatyard was performed very well and according to Beneteau's guidance. This includes mast stepping and keel stepping. My boat is tight with no leaks at the mast step in heavy rains, and the cables going through the mast were properly covered between the deck and the mast at the mast step.

However, a number of other owners of my model report leaks at the mast step and uncovered cables with open entries into the mast. This is the result of the receiving boatyard not following either Beneteau's guidance or even industry standard procedures.

I wouldn't have predicted such a simple kind of failure, but given that some of these boatyards aren't following basic bedding procedures for mast stepping, it makes me wonder if they're following the manufacturer's guidance for keel bolt torques or just "winging it".

Improper delivery could lead to keel loss down the road, so if owners have any other reason to suspect a sloppy delivery, they should have their keel bolt torques checked after delivery.

[Snowpetrel]

To revive an old thread for any engineers out there. Seems like correctly torqueing keel bolts is a big issue. I was talking to an engineer in a different field and he mentioned superbolts as a great way to get good clamping forces easily with simple hand tools. Could this be an option for keel nuts http://www.nord-lock.com/superbolt/m...lt-tensioners/

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[Snowpetrel]

Same concept should work for prop nuts?

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[JPA Cate]

Hi, Ben,

Yes, they look interesting, but would/could not pounding vibration get those little screws to un-do? And are all bits s/s, and if not, titanium?

Old fashioned keel bolts, you can just by golly tighten, with a big enough socket and a long enough lever arm.

Ann

[Jim Cate]

Originally Posted by Snowpetrel

To revive an old thread for any engineers out there. Seems like correctly torqueing keel bolts is a big issue. I was talking to an engineer in a different field and he mentioned superbolts as a great way to get good clamping forces easily with simple hand tools. Could this be an option for keel nuts Superbolt multi-jackbolt tensioners from the Nord-Lock Group

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

G'Day Ben,

What a cool idea! I never ran into such devices before. If available in s/s or better material, I don't see why they wouldn't be a big improvement over conventional nuts. For most boats, access to keel bolts is awkward at best, so that applying huge torques is difficult in practice. Putting much smaller torques on a number of small jacking screws might be time consuming, but much more do-able in many locations.

I like it... perhaps some of out resident MEs could comment more learnedly.

Jim

[Neptune's Gear]

Here is a link to torque specs for bolts Bolt Torques - Stainless Steel, Brass Aluminum Bolts - Engineer's Handbook
According to this, SS 1" should be 259 ft/lbs. Pretty hard to do that in limited space often available. If your bolts are bigger, it will be about as tight as you can get them. If the bolt fixing can crush the structure, then IMO it is not strong enough - the keel will put more stress on it than you will. The plates and washers should distribute the load sufficiently to prevent crushing in a correctly torqued and designed system. With bolts over 1 1/4 inch, you probably cannot over tighten them with normal hand tools...

[Snowpetrel]

Ha, not sure if they come in stainless, but apparently vibration isnt an issue. Saw them on the massive bow lifting foil on an incat hi speed ferry doing over 40 knots. The normal bolts couldnt be torqued down properly and either kept loosening, or broke. The superbolts worked. Maybe use steel ones and epoxy and replace when ugly? It would be interesting to get a NA input.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[hellosailor]

I would expect an aerospace-style budget is needed to go with those Nord bolts, as each one requires more parts, more machining, more fitting and manufacturing efforts than any standard keel bolt, way more.


Mass-market pleasure craft are not built on the same price points as one-off high speed ferries.

[transmitterdan]

If you are worried about the nuts backing off accidentally there is something made by Lok-Mor called an Anco PN-LOC that can be had in stainless. They will not back up by themselves no matter what and require no lock washer. The idea is pretty simple and not wildly expensive. But you have to be able to torque them. Most people that use these use air operated torque calibrated impact drivers. You can't really trust a manual torque wrench for critical applications like bridges, television masts, etc. where these nuts are used by the tons.

Anco PN-LOC | Lok-Mor, Inc

[mstrebe]

The Nord-Bolts are very clever, but I imagine expensive. All they are really doing is replacing a single large bolt with a number of smaller ones capable of delivering the same clamping force. There's a nice video illustration of exactly how they work that shows how they hold tension.

A boat maker can do the same thing by using several smaller threaded rods and bolts rather than a single large bolt, although they strength of the large threaded rod in deflection may be a requirement, in which case these nord-bolts are an excellent solution.

As for "crushing"--it's normal compression of the material under clamping pressure, not "crushing"--that is the problem. Compression occurs with most materials under pressure but does not result in structural changes resulting in failure as the term "crushing" would imply.

Aluminum, FRG, wood, and in fact most materials compress under clamping pressure normally. Even slight compression causes a loss of bolt torque, so keel bolts do need to be re-torqued on hulls made of these materials shortly after they're first exercised. The only reason steel doesn't is its inherently springy nature. A single re-torque will suffice, as the materials will only compress so far. Of course annual inspections are still wise but should never show a difference in torque after the first re-torquing.

For the same reason, you need to re-torque the lugs on your aluminum wheels on your car to ensure that they're not coming loose after about a month of driving on them.

[Snowpetrel]

Originally Posted by hellosailor

I would expect an aerospace-style budget is needed to go with those Nord bolts, as each one requires more parts, more machining, more fitting and manufacturing efforts than any standard keel bolt, way more.


Mass-market pleasure craft are not built on the same price points as one-off high speed ferries.

The guy I talked to reckoned they where not much more expensive than the normal bolts.

Sent from my HTC_0PCV2 using Cruisers Sailing Forum mobile app

[cwyckham]

John from Morgan's Cloud has just posted a great article with his analysis of the report. He feels pretty strongly that the report was excellent but that the recommendations didn't go nearly far enough. I don't disagree.

[robert sailor]

Originally Posted by cwyckham

John from Morgan's Cloud has just posted a great article with his analysis of the report. He feels pretty strongly that the report was excellent but that the recommendations didn't go nearly far enough. I don't disagree.

He had a common sense sort of approach which is hard to find fault with