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Brian H24/03/2018 18:04:28
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2312 forum posts
112 photos

I don't believe that temperature compensation in a sextant is relevant as far as everyday use is concerned. Especially on a small boat, if you can get within 10 (nautical) miles you're doing well!

Brian

roy entwistle24/03/2018 18:53:45
1716 forum posts

John Haine A clock with a recoil escapement actually does go in reverse every beat. Though not much

Roy

Samsaranda24/03/2018 20:02:44
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1688 forum posts
16 photos

A few years ago we had a modern clock which resided on our mantelpiece, it kept very good time until one day it suddenly stopped and refused to restart. I dismantled it to see if it was a simple fix, when I opened it up I found one of the wheels had disintegrated, it transpired it was made of carbon fibre, apparently in the early days they hadn’t fully mastered carbon fibre technology hence as it age hardened it failed. Hopefully with all the composite components used in aviation today they have fully mastered the processes. Sorry to sidetrack the subject.

Dave W

richardandtracy24/03/2018 20:13:27
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943 forum posts
10 photos

Oh dear. I seem to have stirred up an unexpected discussion.

I think some of the yotties knew how to use a sextant. It could be made from anything. And several have been made, the makers were kind enough to send me photos. Makers ranged from Australia, S.Africa, Namibia, to Alaska (winter anti-boredom project) to Europe. One Spanish lady got her shipmaster's certificate using the making of the sextant as the project that's required as part of the Spanish certificate. I started making mine from 1/8" brass, but having had my boat plans sunk by a huge mortgage I've never finished it.

I think that unless the temperature differential is huge between cabin & outside world, there is unlikely to be a significant deviation coming from distortion from temperature. I calculated a deviation of 0.000063mm from the theoretical light ray path at 60degrees for every 10C temp change for a brass one. Manufacturing errors would exceed that by rather a lot, which is why I considered them insignificant.

Regards

Richard.

Meunier24/03/2018 20:30:44
448 forum posts
8 photos
Posted by John Haine on 24/03/2018 17:35:11:

Clock B has dural wheels and steel lantern pinions with the pins on small ball races. By the look of the photos even the escapement wheel is dural - but with a grasshopper escapement there isn't the same wear issue.

Many thanks for that link John, it led to an interesting diversion through Frodsham's site.
Now, if I only could win a lottery......
DaveD

John Haine24/03/2018 21:16:54
5563 forum posts
322 photos
Posted by roy entwistle on 24/03/2018 18:53:45:

John Haine A clock with a recoil escapement actually does go in reverse every beat. Though not much

Roy

Indeed, but the gear teeth always bear on the same surfaces since the reversal is forced by the pendulum.

Gareth Jones 924/03/2018 21:48:37
23 forum posts

Hello Brian, if you're expecting an error of 10 Nm when using a sextant, then something is very wrong. Last time I used one was on a crossing from West Wales to Ireland and comparing sextant derived latitude to the GPS latitude gave a difference of less than 1.0 Nm. I accept things might be different in a gale but this was in a force 5 to 6.

Gareth

I.M. OUTAHERE25/03/2018 07:13:36
1468 forum posts
3 photos

Any thoughts on electrolysis and oxidation or the effect it could have on the bearing surfaces ?

Bill Pudney26/03/2018 06:06:12
622 forum posts
24 photos

Clock B was/is stunning, thanks for the link. The al.alloy, in my opinion, (for what that's worth) looks beautiful. In fact that has decided me, that should my clock project go ahead, it's going to be al. alloy., not brass.

In a former obsession/hobby I spent about 20 years making model aeroplane bits out of carbon/epoxy. For about a nano second it was considered for "my" clock. I'm sure it would be possible, but the thought of all that epoxy and bits of carbon fibre convinced me that the concept should be left to someone else to enjoy! Why should I have all the fun after all?

I had a friend who used to own a business making dental implants from Titanium. Yes threads are really excellent and the material is not hard to machine, as long as you set up correctly for it, in terms of sharp cutters etc. A big advantage is that it looks beautiful, and can be anodised gaudy colours, which would exclude me from the Hallowed Halls of Horology forever!! For me, the material would be a bit of a leap too far though, maybe for my second or third clock, should I get that far.

As far as electroysis was concerned I have a picture in my head of a small pocket in the base with some silica gel there, hidden from view by a mesh screen. As for oxidation, developing my own anodising process shouldn't be too difficult, and as others have said might improve the wear qualities. In any event up here in South Australia the atmosphere is predominantly dry, unlike other parts of Australia where extreme humidity must be considered.

All I have to do now is get on with it!!

cheers

Bill

jaCK Hobson26/03/2018 08:35:28
383 forum posts
101 photos
Posted by John Haine on 24/03/2018 17:35:11:

I've often thought that it's odd that clock wheels have symmetrical teeth - it's not as if they go in reverse is it? There's probably a better tooth form if you remove that constraint.

A justification for the superiority of the dead beat escapement is that it doesn't drive the train in reverse. Other escapements do. Harrison argued that cycloidal gears driven in reverse gave very different resistance compared to driven forward - the screwdriver argument in the link (but nothing to do with coefficient of friction, just levers). Harrison used lantern pinions because they don't exhibit this difference (and the grasshopper isn't dead beat).

John Haine26/03/2018 09:46:01
5563 forum posts
322 photos

Bill, Dick Stephen a UK clockmaker has used titanium for escape wheels, on the basis that being much lighter than brass its stored energy, all of which is wasted, is less and so the clock should need less driving weight. I'm not sure if it was successful. I think that there are places where carbon fibre could be used, especially the pendulum rod, as if the layup is longitudinal its tempco can be very low. It has been said that cf rods are susceptible to absorbed humidity, which varies the mass and therefor the effective length, but I checked with a cf manufacturer and the material can be sealed with polyurethane lacquer to minimise this. Anyway the rod mass can be made very low by using thin walled tube, taking advantage of the material's strength and rigidity, so the effect of rod mass variations should be small. It would be great if you decide to go ahead with your aluminium allow clock, do share some photos.

Sam Stones28/03/2018 03:06:50
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922 forum posts
332 photos

Hi Bill,

I sent you a message.

Sam

Martin King 228/03/2018 08:24:42
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1129 forum posts
1 photos

Hi All,

Re sextant position accuracy, having done 5 Atlantic crossings in small sailboats (43-66 feet) I got fairly good with a sextant but never ever got better than 3-5 or so miles, usually earer 7-10 miles. I was able to sometimes check positions on the early trips with the odd passing ship and later against GPS 1 mile out in a 5/6 is fanatastic accuracy!

Cheers Martin

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