>>I've been thinking about measuring the platform stiffness on my Nacra, but I have an issue with how the above test was performed. Several factors may influence the results

Lets run through them

-1- the ground may be slightly uneven

How would that impact on the results in a significant matter ?

The platform during measuring is layed up on a, what is called, three point support. It therefor WILL rotate untill all supports carry their portion of the load as determined by a simple balance equation. This means that all boats will look for an find a comparable loading sitaution as all boats are governed by the same balance equation. In addition, how bad is say a 1-3 degrees rotation of a platform when compared to a perfect level other measurement. It would cause an offset of less than 0.1 % and therefor be neglectable when compared to the manual measuring accuracy of about 1 or 2 mm of a 50 - 100 mm deflection.

But the measurements where not conducted on the ground but using 4 saw horses. Two supporting the sterns and two acting as a layup when resting the boat. During measuring the two front saw horses did not support the platform. Only the human lifting one bow and the two stern saw horses did. I trust the measurers made sure that their reference point was calibrated.

I know data of one person and he used exactly the same setup for his measurements of 3 boats so here the relative ratio between the measurements is accurate even when in absolute sense larger offset may be present.


-2- were the boats all tested in the same exact spot?

Does this matter ? Again during testing we have a point support situation forcing comparable loading on boats of same length and weight (and easily scaled loading on boats of similar length and weight). We may assume gravity to be constant over the earth surface when looking at our manual measureing accuracy.

Slopes or uneven ground below saw horse that are less than 5 degrees produce are neglectable offsets in the end results. Cosine (5 degrees) = 99.62 or the offset in loading is less than 0.4 %. And 5 degrees is a noticeable slope. A rise of 3 inches for every 3 yards. No body would use such ground for testing.

Then the way of measuring, lifting one bow till the other comes free of its supports, garantees that pre-flexing or such plays no role in the final measured flexing. Again think of the 3 point support situation.

Last point is that the centre of mass is always located behind the mainbeam and thus behind the halve way point along the hull when looking from the stern supports. Therefor the boat will always be leaning on both rear supports and a little less on the supported bow. This makes sure that the hull doesn't fly itself on just two supports at a perfect balance. The reason why the centre of mass is where it is is because of that 3 - 5 kg rearbeam.



-3- The boats themselves may not be perfectly flat and may have some pretwist in it -

See my comment at point -2- the way of lifting goes a long way in prevent ofsets with this cause. BUT yes the reference point for measuring must be calibrated.

I would say that the use of a plank or wooden beam and a waterlevel tool will do wonders here. I do not know where such a thing was used by Marcus. But you can when measuring the boats are tradewinds.


-4- were the measurements taken from each side and then averaged?

I don't know.


>>Truthfully, even before I seated my beams, 95mm (3.74 inches) of deflection simply by lifting an opposite bow sounds unrealistic on my nacra...it's SO much stiffer than my 6.0 was (I would believe it on my 6.0).

If you just lift your boat by the bow when it is on the beach you will get smaller deflection readings. Because the fulcrum can be significantly ahead of the sterns. Mast and stays will probably stiffen up the platform as well when pretensioned.

When laying on the beach I had rougtly 50 mm on my 1975 Prindle 16. When I lifted it with two people where one took the stern and another took the diagonally opposite bow then it did flex alot more.


>>Instead of doing the test as described above, I propose that we perform our own independent test. We should start by loosening the rigging substantially so the rigging doesn't help support the hulls and beams adding a variance to different width boats.

I understand. Just make sure we can still compare the data to eachother.

Also width will always be a factor in what ever way you measure platform stiffness. And absolute platform stiffness (at a the real width) is the only thing important while sailing. However, using theoretical means we can normalize the data to a fixed width. The formula's for this situation are not that difficult.



>>Then we support the boats by both sterns and one bow and measure the height of the unsupported bow from the ground.

You can do that and arrive at much the same situation as the way the Aussies measured the stiffness. Just make sure your measurement reference point is calibrated somehow.


>>Then apply, say, 10kg to that unsupported bow and measure again.

You will tip the boat of its supports by doing that and no longer have a stable 3 point loading.


>>That will give a more accurate indication of vertical axis platform stiffness and would also remove some of the ground level variance and most effect from any preexisting platform twist.


You can exclude pre existing twist more effectively by measuring on both sides and than averaging the results.

10 kg weight may well be relatively little and only produce a small deflection requiring very accurated flex measuring to get enough accuracy. What is the extra flexing is 10 mm ( alot !) than a measuring error of 1 mm (not alot) constitures a 10 % offset already. Alot worse than say 1 mm out of 50 mm (2 % offset)

You must think this one through very carefully.



>>Of course I probably won't have access to the Blade F18, or any Tornados soon, but would very much like to see the results there too. Can someone help with that?

Probably. We are all interested in acquiring good data. For designers such data is golden.


>>>Mary, if you're still putting up with this thread, can we submit a Catsailor article on the test results? Perhaps we can do some testing at Tradewinds. Wouter, want to co-author it?


Ohh yes, it'll be my pleasure. You know I'm a sucker for this stuff.

Wouter