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Old 01-02-2015, 23:29   #1
Marine Service Provider

Join Date: May 2011
Location: Cruising Mexico Currently
Boat: Gulfstar 50
Posts: 1,978
5/16" G70 in place of 3/8" BBB

Years ago I replaced the 5/16" HT chain that came on my boat with 5/16" G70. The HT chain does not feed very well while the 5/16" G70 feeds flawlessly. I always wondered why so this weekend I decided to figure it out.

The boat has an Ideal windless and after some careful looking I was able to find the chain size markings on the windlass. Ideal has stated that most of their windlasses from that time period were 3/8" BBB. And indeed I was able to locate a "3/8" stamped onto the wildcat. It took a tracing to bring out the numbers clearly.

The pitch of a chain is the inside length and the wildcat pushes against every second link so any differences in pitch between different size chain needs to my multiplied by 2 to get the length of the mismatch. For example if the inside length differs by 0.050 (fifty thousandths) then each cog of the windlass will be off by twice that. The first cog fits fine, the second is 1/10" off the 3rd is 1/5" the 3rd is off by almost 1/3 of an inch. It builds up.

So why does 5/16" G70 work on my 3/8" BBB wildcat and not 5/16" HT? It all lies in the pitch. Rather than rely on the published specs for chain I went to my local west marine and measured the chain they had in stock. I also measured the G70 and HT chain on the boat.

Attached is a spreadsheet of the chain measurements that I took. The differences are referenced to 3/8 BBB. 3/8" BBB has a pitch of 1.105".

The 5/16" HT has a measured pitch of 1.017" which is 0.088" shorter than the 3/8" BBB. 2 times that is 0.176" short on the second cog. And on the 3th cog the difference (0.352") is greater than the available free space inside the link (pitch - 2*wire dia = 1.017 - 2 * 0.349 = 0.319"). No wonder it does not feed very well it is overriding by the 3rd cog.

5/16" G70 has a pitch of 1.122" which is 0.017" longer than 3/8" BBB pitch. It is 0.034" longer at the second cog and 0.068" longer at the 3rd cog. The inside gap in the G70 is 0.452" (1.122-2*0.335=0.452") which is much more room. Plus it is longer so rather than overriding the cogs of the wildcat it rides further out on the cogs (they are radial so get wider the further out you are.) But physically 5/16" G70 is much closer to 3/8" BBB.

The spreadsheet shows some other chain sizes. I would suspect that 3/8" G43 ISO chain would feed fine in a 3/8" BBB wildcat as well.

I also suspect that having a shorter pitch that the one the wildcat is made for would cause problems with a smaller mismatch than those chains with a longer pitch.

Anyway, there you have it. Why 5/16" G70 works in a 3/8" BBB wildcat (at least on my boat)

Attached Files
File Type: xls Chain specs b.xls (20.5 KB, 77 views)
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