Very interesting David.
It is interesting to note how a boat sails at anchor with chain vs chain/line. In the spread sheets at Al’s site, graphs are produced to show the cycling effect of the two systems. What I saw was with Chain you cycled (from one tack to another) more often than with line in the system and generated twice the dynamic loads.
After reading Dashew, I checked the strength of nylon and it would appear that using a little heavier nylon for anchoring is in order. I can see where using polyester might be the way to go for sea anchors. But the two situations are quit different. Huge seas and no lee from the winds for a sea anchor vs 2 anchors out, and much smaller, or no seas for anchoring. I wouldn’t sail in the neighborhood of hurricanes without mangroves close by. It’s the stretch of the nylon that absorbs a lot of the dynamic load and dissipates it. Thus putting much less load on your anchor. I think I will contact Al and see if he can do an analysis and spread sheet for polyester instead of nylon.
They speak of switching to polyester for moorings after an MIT study. But moorings are a different animal also. They basically rely on brute force to stay attached to the bottom and can take much higher dynamic loads than an anchor.
I think I will get 500’ 5/8" of polyester for my sea anchor now.
i looked at van beest website- the 9mm shackle you mention will fit into the 1/4 G4 link, but if i read the specs correctly the shackle is rated at .75 ton Working load limit while the G4 chain is rated at 2600 lbs. that will leave me with a weak link. am i missing something?
Boat’s are a compromise and the shackle and swivel connecting the anchor chain to anchor is usually the weak link. Check out “Red Pin” shackles at Hamilton Marine. These are USA made and meet Fed specs. They will still be the weak link but at least you can trust the quality and working load rating.
I looked at the spreadsheets but without knowing how the author derived the results it difficult to say if they are right or wrong. We have tendency to trust computer generated data without questioning how those results were derived.
Why not use 125 ft of 5/16 hi-test chain connected to 1/2" Dacron polyester double braid or 5/8" 3-stand nylon?
Boat’s are a compromise and the shackle and swivel connecting the anchor chain to anchor is usually the weak link. Check out “Red Pin” shackles at Hamilton Marine. These are USA made and meet Fed specs. They will still be the weak link but at least you can trust the quality and working load rating.
I looked at the spreadsheets but without knowing how the author derived the results it is difficult to say if they are right or wrong. We have tendency to trust computer generated data without questioning how those results were derived. If the calculation are incorrect, the results are still garbage regardless how good the input data.
Why not use 125 ft of 5/16 hi-test chain connected to 1/2" Dacron polyester double braid or 5/8" 3-stand nylon?
i abandoned the idea of 1/4" G4HT until gary mentioned there might be a company that made a shackle with a comparable working load limit to the chain. if i am reading the specs correctly and the van beest doesnt’ do the trick, i will abandon the idea of the 1/4"G4HT . an underrated shackle made in the usa doesn’t appeal to me any more than an underrated shackle made in holland. so, i will move on.
Jo anne,
The shackle is an interesting question. What has me wondering is the van Beest has a breaking strength of 9000lbs. 4.5T. They use a 6-1 saftey margin for the WLL.
where the 1/4 HT has a breaking strength of 7800lbs with a 3-1 safety factor for a WLL of 2600lbs. Did you get my PM re: ABI windlass?
Rod, those spread sheets are not computer generated. They are just programed with the math formulas Al uses. It has been awhile since I had my engineering degree, but his math looks sound. And if you read carefully he explains how he calculates all the factors of centenary, momentum, loads etc. which is all fairly basic stuff for an engineer.
We all realize that the weight of the boat is an important consideration. So when the boat starts moving from a gust and comes up tight on the rode, there is a lot of extra energy there than just the force of the wind, there is the momentum. Having the nylon line in the system dissipates a lot of the transfer of energy to the rode. Where as the all chain rode transfers the wind load and momentum of the boat right down the chain to the anchor. That is way you see in the spread sheets such a much larger dynamic load with chain vs chain/line.
The dynamic load is equivalent to F = M delta-v/delta-t - Force is equal to Mass times the change in velocity divided by the change in time. Mass times a change in velocity is momentum and is equal to Force times a change in time (impulse). The BCC or FC 26 has a significant amount of mass. When the boat pulls up tight against its chain-rode, the boat’s velocity changes from perhaps 1 to 3 knots to zero in a very short time. When you look at the equation, F = M X delta-v/delta-t, M is a very large number, the change in velocity, although not significant, does increase the Mass-load by 2 or 3, the change in time when the boat pulls up tight against its rode is very small, hence the Force (dynamic load) becomes significantly large. An example is when we stop a car. If we stop a car slowly, the force exerted against the driver is small, whereas, if the car is stopped instantly, as in a crash, the force exerted against the drive is significant. The mass has not changed nor has the change in velocity compared to slowly stopping the car but the length of time to stop the car in a crash, is significantly less (almost instantaneous), hence the force or dynamic load becomes very large.