Depth Gauge Test

One problem with the current IOM class rules is that proper event measurement requires the use of a cumersome and not very accurate measurement tank to check for the overall depth and hull depth. During the winter Olof Ginström came up with this jig to do the job - here are some pictures and comments from the first trials.


Here is the basic design of the gauge/jig. The hull is positioned between the vertical posts with the fin and keel in the middle slot.

 
Here is the crucial part of this measurement process: When the boat sits in the jig, the supports under the bow and the stern (wooden blocks) can be moved fore and aft. They should be positioned so that the waterline of the hull sits at the jig-waterline, indicated by the plexiglass edge. This jig can't determine where the waterline actually is, that has to be done by other means, perhaps at the time of fundamental measurement. This waterline posistion could then either be marked on the hull, or perhaps recorded in the certificate (XX mm from stern, YY mm from bow)

After this waterline position is found, using the gauge is pretty straightforward:

Check for overall length of hull. This homebuilt Lokki was actually very close to 1000mm, many of the commercial IOMs we put in the gauge were up to 7-9mm shorter (see below!).


Check for hull depth. There is a slider of plexiglass that can be positioned under the hull to check that the maximum depth from the waterline is <60mm.


Check for overall depth. The 420mm depth has been accurately trimmed by placing a piece of plexiglass where the maximum depth for a typical IOM is expected. We don't have a test for minimum depth right now - but competitive IOMs with anything but close to maximum depth are very rare...


One of the shorter boats we tested. This boat is actually sold as a 'meterboat', maybe the proper name should be '99cm boat' ! 😉

Finally, some examples of how different designs look when they sit in the gauge:

 
 
 
 

The designs from top to bottom and left to right: Ericca (light blue), TS2 (dark blue hull), Noux (translucent), Noux (yellow topsides), Italiko (green), Italiko (Black), Cockatoo (red), and Bagheera (Black).

To conclude, if at fundamental measurement the waterline can be found (with a tank or in the pond on a very very calm day) and recorded, then event measurement can be performed by placing the boat in a jig and aligning the previously found real waterline with a reference line on the jig. Measuring overall hull length, hull depth, and overall depth can then be done without a measurement tank and with better precision than is usually achieved with a tank.

We will trial this gauge at the upcoming IOM Nordics where we expect around 20 competitors, hopefully I can report back with more pictures and comments + any problems or issues found.

2 Comments

  1. Hi Anders,

    The gauge looks wonderfully designed and built!

    However... It seems to me (having spent a while ago some time pondering the waterless measurement) that the fundamental flaw in the whole procedure is that, as part of the FUNDAMENTAL measurement we are measuring a highly variable component that is under a direct (and easy) influence by the owner (waterline position), while at the EVENT measurement stage we are measuring components that are rather static (hulls don't grow on their own - so if length and depth are once found to be right than they should pretty well remain so).

    In fact, variances in the waterline position (inadvertant or as part of trim-setting) which are easily achieved by weight re-distribution (perfectly legal if done prior to the event, and with new RRS/E possibly even during event) are, in my experience, the leading cause of event measurement failure! You can easily see what would happen to any of those hulls (either in the gauge or in the tank) if, say, you shifted some weight aft causing a 1 degree stern-down attitude change (especially given a relatively huge lever provided by the keel)...

    So, in other words, the approach of let's agree that this is your waterline, and based on that I'll check the other dimensions is flawed (IMHO) - as it is, in fact, the waterline that SHOULD be the main thing checked at the event measurement (since it clearly has a key role in at least two of the three measurements that you propose to measure with the gauge). Or, conversely, if people feel that it is "OK" to use the "reference waterline" deteremined at the fundamental measurement stage - then why not do the other 3 measurements at the fundamental measurement stage as well?

    Cheers,

    Marko

  2. Administrator

    May 30, 2006 at 11:44

    Hi Marko, nice to hear from you.

    The gauge is not without its problems... it seems that at an international event we will have to use the tank anyway.
    But I think for local smaller events this gauge can serve as an quick check that nobody has a boat that is grossly out of spec. I don't think weight re-distribution does much to the overall depth of most boats, the one degree you mention is equivalent to about 17mm of movement up or down of either the bow or the stern which is way more than will happen by moving the batteries.

    Anders

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