[Fr. Frank]

We were running about 5700 rpms & the only markings on the prop are 21-M, it's a 3 blade.

If the pitch is 21", and you're really only turning 5700 rpms at that speed, that's a slip ratio of under 5%. ( my book says all the Yammie 200 2 strokes after 1997 had a 1.86:1 gear ratio.) I thought I was hot potatoes when I got my custom-made PowerTech which had 8% slip at WOT, trimmed.

Essentially, with those numbers, your prop is running at 95% efficiency. Industry average is 75-80%, and better than 88% efficiency (12% slip) is absolutely phenomenal. Many people pay $3,000 to $10,000 to get a single prop with only 10% slip or less.

I'd be interested in what a calibrated digital or computerized tachometer would show for actual rpms. Analog tachs can be off by as much as 10%, and average 3-5% error. A 5% error in the tach would make a difference of of 5% in slip ratio. Still phenomenal, either way.

It certainly reveals why SeaCraft is such a great racing hull.