Forums: The Deben 5-Tonner: station numbers New Topic Post Reply
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GregH
Posted: Nov 3rd, 2011 at 21:53   |   Subject: station numbers
Good evening George;

I was going through the plans and just want to confirm some things.

As I understand it, the stations are 660mm apart?

If so, does it make a difference that each station then is 1mm numberically-wise off from a multiple of 660? I know a single mm isn't much, and over the length of the boat, that is about 10mm. In this day of computers and such, that seems like a "big" number, but in my mind, in terms of boat building, does it really make any difference? I will be happy if the joints and such I build will be 1/8-1/16"! =)

Also, you have a station "4168" listed. Should this be station "4620" instead? It is drawn at an equal interval from the other stations on several sheets.
debenriver
Posted: Nov 4th, 2011 at 22:20   |   Subject: Station numbers
Hi Greg

Hmm ... there is a typing error on the station (section) numbers s -4168 should be s -4618. I see I have managed to do it on all three plans.

The station spacing is actually 1/9th of the dwl – so 5938/9 = 659.78, which is pretty close to 660 as you say! But the computer program naturally has to get to 5938 so it rounds up and down appropriately.

Using 660 would make the dwl 5940. Not a big amount and certainly not enough to make the slightest difference to the boat. But it would make existing drawings have incorrect data and dimensions on them which might be confusing.

Nowadays, lines are rarely laid off manually. Instead I fair a design and calculate the hydrostatics etc. on various computer programs. So making the dwl a nice round number sort of drops out of consciousness. Hence 5938!

You don't really want the station shapes per se, except in the lofting process to check fairness and draw out the waterlines and buttocks, so that you can derive the shapes of the various bits that you do want (which may or may not lie on a station line).

Traditionally there are 11 stations, with the dwl being divided into 10 spaces. This suits the most common set of Simpsons Multipliers, which work on an odd number of stations.

For some obscure reason, on this design I decided to have 10 stations, so the dwl is divided into 9 ... There is a less common set of Simpsons Multipliers that work on an even number of stations.

When you set off the datum waterline on the loft floor, you usually make it the correct length (i.e. 5938mm) and then set of the stations to divide it equally (by 9 or 10). After that you don't really worry too much about what the spacing actually is.

Actually, if you haven't started the lofting at all, it might be sensible to revert to the more traditional 11 stations/10 spacings. There is something to be said for having a midship section (which you don't get with 10 stations) – quite a few traditional bits of data are based around that. 11 stations would give us a 593.8 spacing (near enough 594).

I could quite easily issue a new set of lines and offsets based on 11 stations. Let me know?

In general, if you are building to USA measurements (feet, inches etc) you would work to the nearest 1/16". If you are building to metric dimensions you would work to the nearest mm, which is a bit more accurate (1/16" is approx. 1.5mm). Because those are the smallest divisions on a standard tape or rule.

While the final boat may come out rather more than1/16" different from the theoretical boat (and it pretty much certainly will) it is best to try to keep each individual item close to 1/16' (or 1mm) tolerance. Otherwise you may get cumulative errors which turn out to be quite big in the end.

Let me know about a different set of lines (it's really the offsets that will be different).

George
GregH
Posted: Nov 6th, 2011 at 00:34   |   Subject: station numbers and other things
thanks for the info George. I had a feeling about the station number, but I really hate to assume too much! ;)

"Actually, if you haven't started the lofting at all, it might be sensible to revert to the more traditional 11 stations/10 spacings. There is something to be said for having a midship section (which you don't get with 10 stations) – quite a few traditional bits of data are based around that. 11 stations would give us a 593.8 spacing (near enough 594)."

Maybe it's just a comfort thing for me, but having a midship section makes a lot of sense to me, and a location to base various measurements from. I would like to do the 11 stations/10 spacings. So yes please. a set of lines based on this would be very appreciated.

Being from Canada, whether the measurements are in metric or imperial makes no matter overal - I use both! With most of my carpentry work I use feet and inches; keeping this all metric is good for me and in some ways easier.


Might I ask why you measure along the WL from the bow stern-wards, such that the station numbers are thus negative.From other study plans and such that "0" station has been typically at the stern. Just that way you do it, or is there a bigger reason? =)

You also mentioned in another post about being able to easily do up a laminated frame drawing as an example; for the general idea of proportion and the longitudinal components that will run through.
Could you, when you have some spare time (okok what is that eh? LOL) do that?

I like the 750mm spacings using the 25x45mm values. In regards to beefing up the frames in the area of the mast partners, would using the 35x60mm values be beefy enough or would it just need the wider siding value?

Hope you are having a wonderful weekend!
Greg H
debenriver
Posted: Nov 8th, 2011 at 00:29   |   Subject: station numbers and other things
Hi Greg

I will do the 11 station/10 spacing lines and offsets. It will take me a day or two.

I think that counting the stations aft from a zero point at the intersection of the forefoot with the dwl is actually pretty normal – most all (but not all of course) of the boats I have built have been this way round, and originally I was taught this way. Most data (LCB, LCF etc etc) are counted from forward. So although it may seem a bit odd to mostly be working in negative position numbers, it has a sort of strange logic to it. And "astern" has a sort of negative feeling to it ... doesn't it?

Some designers work from the forward perpendicular – the foremost part of the boat at the stemhead. Some work either side of a midships section – positive forward and negative aft. And some designers work their heights from a baseline, rather than the dwl.

It really is a matter of choice. No bigger reason. But I personally find the intersection of the dwl and the forefoot a pretty natural sort of place to start from. Of course there is no reason why the numbers should be negative – they could be positive and anything ahead of zero could be negative ...

Like you, I can switch pretty easily between feet and inches, and metric. We changed to metric in England in 1971, so it's been a long while. And all the calculations are much easier in metric terms. When I first moved to the USA I had real difficulty remembering where something like 63 13/16" was on a rule! But I have gradually got used to it again. I wouldn't want to try designing in non-metric units now though – just too difficult.

Something like 35 x 60 would be fine for the mast frame, if the standard frames were 25 x 45 at approx. 750 spacings.

I will do a typical frame drawing.

Best regards

George

GregH
Posted: Nov 10th, 2011 at 01:55   |   Subject: station numbers and other things
Evenin' George!
Just back from an overnighter at the local historic fort with a gade 7 class! And even managed to get a wink or two of sleep! LOL

I know you are busy with many paying things as opposed to this freebie, so I totally understand about the re-draws taking some time to get done. And very much appreciate it! In the meantime, I am working on some simple sketches; thinking about the cabin item layout. About how a keel stepped mast will affect the door to the head and little things like that. =)

"Something like 35 x 60 would be fine for the mast frame, if the standard frames were 25 x 45 at approx. 750 spacings."
- thanks I'll make note of that.

I will do a typical frame drawing.
And thanks, I'll keep an eye out for it.

CHeers
GregH
debenriver
Posted: Nov 12th, 2011 at 13:54   |   Subject: station numbers
Hi Greg

The mast doesn't absolutely have to be keel stepped. A gaff rig just gets much better support from a keel stepped spar, because there isn't a standing backstay – and as the boat gets larger, this becomes more evident.

A deck stepped mast generally puts more compression loads into the spar and the structures. So a deck stepped mast usually needs about 35% higher moments of inertia than a keel stepped spar.

If we can arrange for a keel stepped mast, then I think we would have a nicer, more stable rig. Specially if you are going to set a topsail.

But if it's going to ruin the interior, then we can arrange for a deck stepped spar. And especially if you are building a cold mounded hull and structure, we can arrange to take the compressive loads more easily.

If you are sticking pretty much to the internal arrangements as shown, then we might need to make the WC compartment a little larger, so that the mast could be accommodated in the forward corner – either inside the WC compartment, or possibly outside it, with a curved corner partition.

Best

George

debenriver
Posted: Nov 15th, 2011 at 11:27   |   Subject: station numbers
Hi Greg

The modified Lines Plans and Table of Offsets for 11 stations are available on the website now.

I'll work on some frame drawings.

George