Hi Don, Looks to me like this should go on TWL. Would you like to post your preceeding email to TWL - as it has lots of important content? >I have long felt that the forgotten factor in anchor design is >the strength of the bottom material. Right - like load dynamics, an unruly topic. I'm really looking forward to your treatment. >article I am in the process of writing for Ocean Navigator on >this subject interesting. Since it hasn't been published yet a certain >amount of discretion is required. I would be keen to review draft(s) under confidentiality. >As far as the stress on nylon to reduce dynamic loads, I feel that the 25% of >breaking strength is too conservative. Agreed. I've been using a design parameter of 25% as the target loading for static wind load of 60kn. Guessing dynamic loads could easily produce real stress > 50%, plus gust loads can certainly exceed the 60kn bogie. I figure you have to design to some criteria - if you choose 70kn...+ you end up with insufficient elasticity for the most likely cases. So I just accept that there are unlikely load cases that we might encounter that will demand some imiprovisation, or where we are in the soup. But I'm hungry for definitive info on how to account for the dynamics. BTW, Our computer VPP model gives us a pretty good grip on the statics. The VPP has had extensive testing on a large number of multi designs, including Stars & Stripes, so Gino is confident of the wind drag numbers. But I agree with your concerns as to how the avg. boatie gets valid Cds, areas, etc. >Rarely have >problems. I dove on my 35 pound CQR in the Gilberts after laying 2 weeks in >steady 25 knot trades, the anchor was still on its side. ???? That didn't >make sense using conventional thinking. Hmm...I'm not sure what that tells me. It's common to find you've been tethered to nothing but chain and friction. Even a bloody Bruce will keep you in place until the night you lose your boat. Unsupportable generalization - the oceans are populated with cruisers who swear by their anchor because they have never been tested above 40kn and good sized waves. When the test does arrive at 0300, West Marine won't be handy to fetch the gear one will be wishing for. >As for Van Dorn editorial slip, I am not sure. He just made a math error - see the correspondence I forgot to send first go around. >He implies that nylon, is >better than chain in many short scope conditions. I actual ran some test and >found data that supported that claim. Neat - love to see that! I've actually not worried too much about this relative scope nuance, because the chain needed is dictacted by chafe (and convenience), and one doesn't want more chain than needed for that assignment. John Neal doesn't agree as he wants all chain, but I also don't agree that a pair of CQR's is the optimum complement. OTOH, John's done maybe 100knm on his CQR's, including Antartica, and hasn't come to grief yet. Is he right, or is it due to his particular sample? John's belief in all chain must just indicate that every skipper's certainties are conditioned by his own anecdotal history. John is unquestionably one of the best skippers out there, and I would sail anywhere with him. (I might sneak aboard a broken-down Fortress just in case we get blasted in some slippery stuff...or if I got assigned kedge duty <g>) >Shallow water coral sites. We anchored for three years in shallow water >coral >sites using a nylon rode with 30 feet of chain. The end of the nylon was >buoyed to keep it 6 or 8 feet off the bottom after setting. Worked well. I can certainly see how that would work. Though it definitely leaves you vulnerable to bommie wraps when it goes quiet, shifts, quiet, shifts, ... So it's a tradeoff - 60-ft gives more cushion, 120-ft yet more cushion, ... Then there's the operational convenience factor. If X-ft leaves you on chain 95% of the time, keeping the chain splice pristine, that has a value. Between +/- 30 latitude, 150-ft will usually cover it. Absent a windlass I would probably splice in 50-ft, add 25-ft sections as needed, stealing from the around-the-rocks shore line chain segments. But it's hard to even think about cruising without a windlass. >If your interested I can give you more details. You bet! >My article on dynamic loads will take you through how I got around that >problem. I'm sure looking fwd to reading. I've tried to tom sawyer Evans Starzinger into doing some load cell tests on Hawk, but I'm not at all sure I've made a sale! Afraid he's much to smart for that duty. >The 2/3 rule is too simplistic. Simplistic fur sure. However, I would argue that so long as folks remember it is idealized "everything else equal - nothing varies but weight of design X" it is a very useful approximator. And if one is to normalize published anchor tests, a weight correlator is required. The skipper designing his ground tackle can't be expected to do multi-variate optimization (esp. given the absence of hard data). He's doing well if he can classify the sediments and foul he needs to address, choose a suitable design for that class, decide what upper limits of wind and sea state range he feels safe accepting. Then decide how to cut the 7 ideal choices back to three or four he can actually carry. Given all that complexity and uncertainty, appreciating that the required weight of design X will need to increase about as cube of wind speed is a pretty good guideline. Then faced with a situation likely to significantly exceed what he has down, he has a good basis to decide he had better bail out - at least to try to find some nice sand or stiff mud that he can plant that FX-55 in, hooked up to 90% nylon <g>. >The sediment coefficient is a step in the right direction, but it is >inadequate and not too useful in my estimation. Unless charts would start >showing sediment coeffiecents. Agreed w/r/t on the spot tactics. However, to make any sense at all out of the published tests, I think it is pretty useful for normalization. As it stands, any anchor brand can easily choose the tests that prove theirs is superior (not even considering that no tests I've seen even attempt to look at how "big" differing designs must be for comparable results). The Puget Sound test went much further in trying to at least document what sort of bottoms each site exhibited. >Lack of soil strength is the overriding problem. CaptnWil would no doubt agree since he has to anchor in shaving cream over who knows what <g>. I guess I think the bottom line we all are seeking is a sort of basin classification (hopefully not stretching the geology analogy too far). E.g., in Easter Med sediments, design X, size Y for vessel properties Z; but for Western Pacific +/- 20, ... >By the way you should read the article in practical sailor "the Load on Your >Rode" Vol 22 Number 13 July 1, 1996 . Thanks - I just checked our PS archives. Almost every 96 issue is in place but that one. Could you snailmail me a xerox? Now, if we can just get you a really-big grant to fund all the testing and science we need! Cheers, Steve ------------------------------------------------ Stephen C. Darden Email: darden@xtra.co.nz Adagio Marine Ltd. Fax: 649/4037-213 P.O. Box 161 Voice: 649/4037-801 Russell NEW ZEALAND