AeroMarine Sprint Cat 21

[RCUK John]

Can anyone confirm the position of the balance on the 21 series boat?
With an overall length of 29¼", I would assume it to be at 7 5/8" to 8¾" from the transom. There is no indication on the drawing supplied with the hull.

[Ron Olson]

Typical Aeromarine blueprints. They're probably figuring the flywheel or glow plug as where the CG ought to be in the drawing. Measure on that as it should be a full-sized drawing that came with it.
Your numbers should be real close.

[RCUK John]

Thanks for the reply Ron.
Measuring the supplied drawing gives a figure of 10¾" and to the same position on my fitted CMB engine that has become 11 3/8".
The hull is stripped for painting at the moment so I cannot get an approximate balance point for the initial setup.The figures that I used initially were based on 26% to 30% of the overall hull length.

[Ron Olson]

Which CMB? The slightly older ones had problems with the high-speed needle leaking around the O-ring. If you start having that problem, look for a remote needle to replace it. The CMB TT tuned pipe is quiet and cheap too, one of the quietest that I've ever heard.
If you got yours from JoTiKa Ltd., they might be able to help you out.

[RCUK John]

The engine is a CMB 21 RS (longstroke) - remote needle - with a 180º manifold and a Valvola pipe. It is setup as its last run earlier this year before being "transplanted" in the Cat. The cat top was a "Classic Cowl" which I have modified to cover the pipe. I have also modified the pipe to have a water jacket. The point of balance has now been checked out as the leading step in the hull (or about 11" from the transom).
If you are interested, I will send you a couple of pictures - just send a message.
The engine was new last year and removed from another boat to make way for a 7½cc engine - that particular boat now performs well. The 3½cc was too small.