Boat Flexing

[rik_elliott]

I have a 1979 avon sportsboat 340, recently purchased with a 15hp 2 stroke. Lovely bit of kit and really good fun. The only problem I seem to be having with it is that despite the tubes being well inflated to 2.5-3psi in any sort of chop (even really small stuff) when at speed the nose flexes up. The nose wooden floor plate seems to pivot on the fixed thrustboard (right name for the fixed plank?). This makes the nose plough in and knock the boat off the plane - really annoying! Ive tried screwing a nice strong bit of wood across the joint between the board and the thrust board that worked great for a few hours, before the screws pulled out!

The question is, am I doing anything wrong, or is this nose flexing a design feature (?fault), and is there anything I can do to remedy it?

Thanks for any help

[chris123]

I am not sure of the science of why SIBs are built like this, but it is definitely a deliberate design feature. Sorry I can't be more help.

Cheers

Chris

[m chappelow]

most sibs are prone to it ,,,,one reason for the concept of the r.i.b ,,and the older they are the worse they get ,the more boards the boat has and more joints there is the more there is to flex ,even with one /two piece floors there is still an amount of flexing .
with chris,s and my own ex y boats,,based on the standard avon 3.60, the rnli did away with multi boards and modified them like the larger D class, with a one peice main board one bow board tensioned with bottle screws up to the small fixed triangularboard in the bow ,this does away with the single thrust board ,
you may find that if the boat hasent been used for a while or it has been in storage that the wood floor may have shrunk/dried a bit ,leave some water in it a few days see if it tightenes it up .

[rik_elliott]

thanks very much guys. interesting suggestion with the bottle screws m chappelow, could be an avenue i explore if i get fed up with it nosing up. I did find the nose up was much better with a proper inflation of the tubes again after being on the water for a while - forgot the air volume dropped when cooled! Prior to rediscovering the laws of gasses the whole boat was flexing through almost 90 degrees with the tubes creasing on hitting even a small wave. Terrifying!

I recently remade the floorboards using the details i found on this forum so they should be bang on.

Thanks anyway, expect to hear of my adventures in the solent once i get a day off in good weather!

[m chappelow]

Originally Posted by rik_elliott

thanks very much guys. interesting suggestion with the bottle screws m chappelow, could be an avenue i explore if i get fed up with it nosing up. I did find the nose up was much better with a proper inflation of the tubes again after being on the water for a while - forgot the air volume dropped when cooled! Prior to rediscovering the laws of gasses the whole boat was flexing through almost 90 degrees with the tubes creasing on hitting even a small wave. Terrifying!

I recently remade the floorboards using the details i found on this forum so they should be bang on.

Thanks anyway, expect to hear of my adventures in the solent once i get a day off in good weather!

Dont forget the keel is more prone to post cold water immersion pressure drop than the main tubes or sponsons as its in the cold water more than the rest of the boat .
regards and good luck mart