Liquid Asset said:
Can yall explain the theory of props on inboard boats. Why 3, 4, or 5 blades. In the outboard world, flats fisherman go to a 4 blade propeller for hole shot, but loose top speed which they make up with horsepower.
How do you go about determining what is best for a specific inboard boat? I have 3 blade props and my extra set is 3 blades. Just curious as to the advantages of different styles and where the downfalls are.
I've sniffed around some and read some articles, and it sounds like every boat has its solution. Even the so-called experts in the mags will tell you that there is no reliable way to scientifically predict what will be the perfect prop for a given boat/motor(s)/gear(s) setup. All of the articles I've read say to find a prop shop that will take props back with no questions asked until you find the right ones.
I haven't seen too many in our crowd that even think of 5 or more blades, so I'll confine this to 3 vs. 4. You mentioned hole shot being one factor. With size, pitch and cup being important variables, it's too easy to say a 4 will give you more shot than a 3; what is pretty much an uncontroversial given, I believe, is that 4 blades will generally give you more lift at the stern. I can tell you in my boat (45c ser.II), going from the original 26x28 3-blade nibrals to 26x27 4-blade bronze wheels gave me more hole shot, but it wasn't a huge difference...not that it really mattered much to me. The big win was smoothness, and my original 3-blades were in very good shape and balanced. Plus, I think I DID pick up some slight pickup in cruise speed, likely owing to the added transom lift and there being less 45c in the water. But just for that silky smooth feeling, I would make the 3-to-4 change I made. It's really nice.
The other huge factor is DAR - disk area ratio. In other words, if all those blades were hammered flat, what % of the whole circle is taken up with metal. It generally (but not always) follows that 4 blades will be a higher DAR than 3-blades, and thus will also have higher drag. This is where the top-end speed coming down becomes a factor. But then again, who stays at WOT for more than the occasional test?
And I'm sure you know the biggest RULE #1 when playing with props -- whatever you go to, make sure your engines make their full rated WOT with realistic loads.
