Forums: Whisstock Designs: Metric to imperial conversion. New Topic Post Reply
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Posted: Sep 29th, 2012 at 11:36   |   Subject: Metric to imperial conversion.
Has anyone had experience converting plans from metric to imperial measurements or is it a simple manner of using metric rulers, tapes ect.? Would there be an issue with using standard wood thickness that approximates metric thicknesses?
Posted: Sep 29th, 2012 at 20:27   |   Subject: Metric to imperial conversion.
Perhaps US based builders will be able to give their input into this question. We have a lot of boats building in the USA and it comes up from time to time in emails, but so far not on the forum.

I have been based in the USA for 12 years now – learning (or rather re-learning) to use feet and inches and sixteenths (mostly for house remodelling) was a bit of a struggle at first, to say the least! So I am sympathetic to the problem.

The trouble with designing is that you really have to stick with one system or another - else you get ridiculous conversions (decimals of an inch which aren't on anyone's tape measure). Or you produce two completely separate sets of plans for the different measurement systems.

back in 1971 we (my boatyard in England) converted to metric - we did it in one hit – on metrication day we issued new metric-only folding rules and measuring tapes to everyone and thenceforward we only used metric.

So, after all this pre-ambulation, I would say: use a metric tape. Preferably one that doesn't have inches on one side (because they always seem to be on the side where you want to measure). The only value of a metric/inch tape is that it gives you some idea of the equivalents. But in reality they are pretty simple:

1/16" is almost exactly 1.5mm (so working to 1mm is a bit more accurate)
3/16" is near enough 5mm
1/4" is about 6mm
3/8" is almost exactly 9.5mm
1/2" is about 13mm (bit of a nuisance that one)
3/4" is about 19mm (again a bit of a nuisance – 20mm would be so much easier!)

1" is about 25mm
4" is about 100mm
12" is about 305mm

4' is almost exactly 1220mm (the width of a sheet of ply)
5' is roughly 1525mm
8' is almost exactly 2440mm (the length of a sheet of ply)
10' is roughly 3050mm
20' is roughly 6100mm

Don't use these for measurement purposes - just to check that you are in the ballpark and you boat isn't going to be 10 times bigger or 10 times smaller than you expected.

That is really why we use just millimetres (millimeters ...) on all our drawings, whatever the size of the boat. They are the smallest division on a metric rule, so mistakes are less likely.

Once you get going, buying US sized lumber is OK. You just convert it to metric dimensions (mostly for the thickness – width and length usually have to be machined anyway).

So - in summary - best to use a metric tape (one with no inches on at all). These are available online, probably not in your local hardware store.

Mostly they knock the last zero off the stated measurement to save space – or to pander to the French who insist on using centimetres (I love you anyway, France) – so "10" on the tape is 100mm, "20" is 200mm, "240" is 2400mm and so on

But whatever, the smallest sub-divisions are 1mm – hang on to that fact and you will be OK!

Hope this helps


Posted: Sep 30th, 2012 at 14:20   |   Subject: Metric vs Imperial
Thank you for your quick and comprehensive response.If you know of anyone with photos of a completed Plan 074 please provide the source.
Posted: Oct 9th, 2012 at 11:19   |   Subject: Metric to imperial conversion
I don't have any photos of completed boats to this design unfortunately. I had good photos of the first of these boats to be built, but a whole box of photos and other paperwork got lost somehow in a move to the USA – and at that time I hadn't scanned them or anything, so they were lost forever.

The only current photos are of one of these boats building in the UK, which are on the website. But I don't have any updated ones!