NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: 7/8 Scale Sextants More
From: Robert Gainer
Date: 2004 Aug 2, 13:21 +0000
From: Robert Gainer
Date: 2004 Aug 2, 13:21 +0000
Joel, Be careful about using the word �everything� when discussing scaling a design. Speed varies as the square root of the waterline, sail area and wetted surface varies as the square of the waterline, displacement varies as the cube of the waterline, stability varies as the fourth power of the waterline and moment of inertia varies as the fifth power of the waterline. All the best, Robert Gainer >From: Joel Jacobs>Reply-To: Navigation Mailing List >To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM >Subject: Re: 7/8 SCALE SEXTANTS MORE >Date: Mon, 2 Aug 2004 07:53:04 -0400 > >Let me add something that I thought of after mailing. > >In boat and ship design when you scale something up, everything changes in >a >geometric progression rather than an arithmetic one. > >For example, one foot by one foot is one square foot, but if you increase >the size to two feet squared, the result is four square feet or four times >greater. > >George, you're the mathematician. Doesn't the reverse hold true when you >shrink something? > >Joel > > >----- Original Message ----- >From: "Joel Jacobs" >To: "Navigation Mailing List" >Sent: Monday, August 02, 2004 7:39 AM >Subject: 7/8 SCALE SEXTANTS > > > > Hello George, > > > > Some additional comments on reduced scale sextants (7/8): > > > > The small size mirrors collect less light to pass through the optical >system > > and consequently make twilight observations of stars and planets more > > difficult. There is another equally important disadvantage. When taking >high > > altitude LAN sights the small surface area of the mirrors allows the >object > > to jump off the mirror more easily than with large size mirrors. > > > > In respect to the telescopes on the reduced scale sextants, they have a > > small objective and ocular lens. The Freiberger scope is about 2.5 x >25mm > > compared to a normal 4 x 40 scope and passes less light through the >system. > > Again this makes observations of stars and planets more difficult. > > > > If there are any shooters amongst us, they may comment on the preferred > > weight of a rifle. Some people believe that light weight rifles are >harder > > to hold on target, and less accurate even though carrying them is >easier. > > The same holds true of a sextant. A very light weight one is thought to >be > > more difficult to use. > > > > I'm not sure of this, but I think Tamaya was the first to introduce a >7/8 > > scale sextant around 1975. It was the MS 933 Venus which, when sold >today > > used, sells for about the same as a NEW Freiberger of the same size. > > Freiberger's list price is $600.00. Here's a link to my listing of the > > original Venus sextant. GO: >http://stores.ebay.com/LAND-and-SEA-COLLECTION > > It is the fourth one down. I don't know when Tamaya discontinued >production > > of this model, but they appear on ebay fairly regularly and sell >quickly. > > > > Here is a link to a European Freiberger Yachtsman Sextant site since > > Claussen Instrument's is down. > > http://www.busse-yachtshop.de/dae_Yachtsextant.html Note that 600 Euros >is > > about $720 over there. > > > > If you do mostly sun sight navigation, the small size and light weight >might > > appeal to some, but the 7/8 scale sextant has never had much of a >following > > by celestial navigators. > > > > Joel Jacobs > > > > > > ----- Original Message ----- > > From: "George Huxtable" > > To: > > Sent: Monday, August 02, 2004 5:40 AM > > Subject: Re: Celestaire vs Freiberger Yacht Sextant > > > > > > > Joel Jacobs wrote: > > > > > > > > > >From my experience, if your going to do serious navigation relying on > > > > twilight sights, the 7/8 scale sextants are very lacking. Their >optics > > are > > > > not very good, and there small size mirrors are not anywhere as > > effective > > > > when taking star sights or high altitude sun sights. > > > =============== > > > > > > Something about that puzzles me. Why should it be so, I ask? > > > > > > First, I should make it clear that I have never even handled such a >7/8 > > > size sextant; and that observations on my own small boat have never >passed > > > beyond a plastic job, though I have used many "posh" sextants, >belonging > > to > > > others. So I make no claims to being an expert on sextants. > > > > > > Yet, it seems to me that if you were to shrink a sextant to 7/8 of its > > > original size, and shrink its mirrors correspondingly (in both >directions) > > > while preserving the same angular field-of-view of its telescope, then > > > (because the distance from the eye to those mirrors is also reduced to > > 7/8) > > > the patch of sky that the mirrors subtend would be exactly the same as > > > before. > > > > > > So, in those circumstances, why should the smaller mirrors present any > > > disadvantage? > > > > > > George. > > > > > > ================================================================ > > > contact George Huxtable by email at george@huxtable.u-net.com, by >phone >at > > > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy > > > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > > > ================================================================ > > _________________________________________________________________ Don�t just search. 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