Ian you sound like an expert
Ha ha, well certainly if an ex is a "has been" and a spirt a "drip under pressure" that's me.
For normal mass produced average boats it's not the black art that most people imagine it to be, however switch to Formula 1 or offshore power boats and the ball park changes. I've had some very interesting conversations with race engineers who are constantly trying to manage the outside of the envelope when it comes to drives etc. So it doesn't surprise me when Ianh got his props back from Steel Developments and found they had altered them and then had the audacity to blame the sudden lack of speed on the engines, which, on their own had miraculously decided to lose power........ha ha as if? The bigger you go in terms of production pleasure boats the fewer choices there are; the engine manufacturer will specify propeller size.
I had a very interesting chat a few years ago with one of the Quicksilver Prop Engineers, he said all blade sizes and diameters are different depending type you chose. For example, if a High Five had the same blade size and diameter as an average 3 blade say 14 1/4" x 21p it would probably spin at half the revs (a slight exaggeration but a good illustration). Laser11's are raked for bow lift and primarily designed for high speeds and outboards. Although, I have a Laser 11 on my boat, its not the best for that type, but its great for towing and going reasonably fast.
Most manufacturers will build a propeller suitable for certain types of engines and power range and adjust the diameters and blade sizes to suit, maybe even giving a small selection of diameters depending upon the speed at which the prop spins. So you can narrow it down to one or two in order to get what you want.
I was surprised when I was told that remarkably a propeller doesn't slip as much as you think once moving, assuming its the right one of course. For example, if we knew NickT's gear ratio,(I guessed it, but was probably way out
) given that his engine was revving to 4,000rpm at WOT and he was travelling at 30knts, we could calculate the slip, easily. Assuming the 21" pitched prop was in a solid, it would move 21" on a full rotation. Therefore at 30knts or 34.5mph, that is 2,185,920 ins an hour, divide this by 60 , 36,432, then divide by the prop pitch and you get 1,734 revs per minute at the prop assuming its in a solid.
Betty Boops Alpha One Gen 11 drive should be 1.81, therefore at WOT his 4.3 is something like 4,500-4,800 rpm, which means his prop is spinning at 2,486 rpm at 4.500revs. That's 52,206 ins per min assuming his prop is in a solid, 3,132360 ins per hour, divided by 63,360 ( the number of inches in a mile), then theoretically his boat would be doing 49.37mph. If its only doing 40mph then his slip is 19%. I'm not entirely sure but I think this should be closer to 10%.
Have you fallen asleep yet?
A called a prop main dealer in wales somewhere and he told me to stick to what I had on there as a safe option obviously he then tried to sell me a prop at £400 BUT as pure luck would have it there was a stainless laser 11 for sale in Torquay on ebay - 10 min from ones father - so he's been dispatched to pick it up tomorrow. £60 = a bargain.