>>With regards to the stiffness test - that's not a very scientifc way to go about it.

Huh, all boats were tested in the same way and in the case of the F18's all dimensions of leverage and weight were identical. How do you go more scientific ? By putting on a white coat ?


>>Were the beams on the Nacra seated?

What do you mean by seated ? If you mean glued in than that is fair as the Blade F18's weren't glued as well.


>>I removed my hulls and bedded my beams in epoxy (mostly to get rid of the squeeking noises! ). However, most of the flex measured by lifting the hull is generated within the beams themselves, very little the hull to beam joint, and practically none of it by the hull itself.


I partially agree with you here, as I indicated in my other post. The design of the beamlanding itself is important. I know the Cirrus F18 has laminated in large stainless steel plates under the bolts and this boat is the stiffest F18 by far. Its beams aren't much different from say a Tiger of Nacra. But again I agree that when lifting the hull the hull stiffness itself is indeed not the dominant factor. It is completely difference when looking at toe-in stiffness.

Jake did you glue your beams in ?


>>You should see practically no hull flexure on any cat by lifting the bow.

This is absolutely not true. Marcus didn't invent the measurements. They actually did the test and got the numbers they got.

Personally I use a different methode on the beach. I put a boat on it cattracks with the track right behind the mainbeam and then move the bow up on and this I find the resonance frequency. At that stage you can really see the hulls make significant movement in opposite direction of each other. The ratio between frequency and hull weight is a good rule of thumb for the vertical stiffness. It is not exact but gives a rough indication of stiffness. Note how resonance is not dependent on any leverage arms only on weight of the hull and stiffness of the platform. I found a similar situation using this methode.

The Cirrus boat could almost not be brought to oscillation as the frequency required for it is was almost beyond human capability. The Nacra and Tigers were easy to bring in resonance and my own homebuild F16 was a little harder to put in resonance.(higher frequency required)

You should do this one time on your boat you'll be amazed how independently your hulls can move from one another.

The way Marcus and others did the measurement will show noticeable flexing on any catamaran platform. Remember the boat is only supported under its sterns !

>>This kind of flex characteristic indicated by your test has to do with the length and type of cross beams. Impressive, yes...but still not the whole picture.

Sure it but the wider beamed 1992 Tornado beats one and two year old big builder F18's with their reduced width and the Blade F18 is of the same width as the same reference F18's. That leaves type of crossbeam, beamlanding design and hulls as the only explanations for the difference. We can cross out the hull itself. Now how much bigger beam do you need to come out a factor 4 better ?

Take a look at the picture How much bigger are the Blade beams ?



I can assure you that the wall thickness of the beams isn't 4 times the thickness of the nacra or Tiger beams.


Wouter