Interpolation

[owendo]

I am sure many people, who are doing the RYA shorebased courses this winter, find the RYA method of interpolating the required drift by using the two angle lines and parallel lines method a pain.

I have been trying to remember how to do it on a calculator, having been retired for 12 monthes the brain seems to have gone to sleep. Some one with a better brain than mine please help.

EG the problem = Range of spring tides= 6m from tidal diamond X spring 3.1 kn
neaps = 2m neaps 1.2 kn
what is the flow when the range is (say) 4.2


Mike Woodhouse
Vertue V17

 

[bedouin]

1.2 + (3.1 - 1.2) * (4.2 - 1.2)/(6-1.2)

Which is the numerical equivalent of the graphical mechanism

 

[Twister_Ken]

I guess 2.2 would be close enough

 

[owendo]

You have missed the point not the answer how is it done on a calculator to satisfy the RYA in the shorebased situation, thnks anyway.


Mike Woodhouse
Vertue V17

 

[jimi]

without wishing to be pedantic should'nt that be

1.2 + (3.1 - 1.2) * (4.2 - 2)/(6-2) ?

 

[halcyon]

Draw a line across paper, at one end mark a vertical line mark upper edge 6m, pick a suitable scale and mark the upper end scale 2m.
Mark the lower LH mark 3.1 kn, and RH lower end 1.2 kn, devide between lines i.e 3, 2.5, 2.0, 1.5.
All you do is mark required range above line, read tide rate on scale below line.

Hope you follow

Brian

 

[bedouin]

Oppps - just shows how easy it is to make mistakes

 

[owendo]

Thanks Brian,
You also have missed the point. That is exactly what I do not want to have to do.
I asked for help with a formula to use on a calculator to work out the drift to 1 decimal point as required on the shorebased course. Both you and I know that in the real world this can be done in the head in half a second-- not good enough for the Yachtmaster exam.

Thanks anyway,



Mike Woodhouse
Vertue V17

 

[ianwright]

Guesstrapulation.

Does your instructor let you use a calculator? Crumbs!
Tell him that after five mins sailing the course your GPS will indicate the correction required.,,,,,,,,, and in any case it's the bits of water BETWEEN the tidal diamonds that you are bothered about.

IanW

Vertue 203, Patience

 

[halcyon]

Re: Guesstrapulation.

Does your gps do secondary ports, and depth of water as well ?

Brian

 

[halcyon]

Have they altered the Yachtmaster exam?

When I did mine we were not allowed calculators.
Then you have water depth for anchoring, entering leaving ports and secondary ports that you need interpolation for.

Brian

 

[ianwright]

Re: Guesstrapulation.

Unfortunatly not, but if the altitude function was as accurate as it should be then reduction to soundings would be simpler don't you think?
Don't you "cheat" a little by aiming off a touch and then adjusting the course to maintain the 'direction to way point' function? I CAN do it by real pencil navigation, but as a solo sailor I take all the help I can get.
Navigation by calculator, for those that can use one, might well be a help. In my case I find that my fingers are much too error prone. Though I continue to try,,,,,,,,,,,,,,,,

IanW.

Vertue 203, Patience

 

[jimi]

Perhaps abacii though? ;-)

 

[Twister_Ken]

Bl**dy Silly

With all respect Mike, if the shorebased yachtmaster exam requires you to do this, then it's so out of touch with what happens in any small boat, that you have to question its real-world relevance. I've only ever sailed on one boat that had a calculator aboard, and that was a super sophisticated one used for sight reductions.

Why should you have to do ashore something which you would never do abaord, just to get a tick in a box? This sounds like a suitable subject for the RYA interview next week.

 

[jimi]

Re: Bl**dy Silly

Och man .. but you need to understand the principles before you do it in yer heid .. thats why theory is shorebased and not boat based IMHO!

 

[Twister_Ken]

Re: Bl**dy Silly

But with a calculator?

if the shore based is to train for the afloat, then surely it should be training you in the afloat method.

 

[david_e]

Re: Bl**dy Silly

Good point, I am midway through year 2 and sit there thinking about when am I going to use all this stuff for pootling about peelweely, but Jimi is right I'm afraid!

 

[owendo]

Re: Bl**dy Silly

I agree, did you do latin at school that was pretty daft but here you go.

Mike Woodhouse
Vertue V17

 

[owendo]

What a hornets nest!
I only asked for a method of doing the calculation. Thanks to all that have contributed I now have the method.



Mike Woodhouse
Vertue V17

 

[ean_p]

Re: Guesstrapulation.

Mmmmmm now the altitude function sounds like that could be real good......what kinds of accuracy are obtained with and without wass.....???