Originally Posted by Frar
First RIB been used 5 times now. Got a Verado 175 on a 6.8m (rated to max 275hp). All is fine and fast until we try to get on a plane with passengers in any kind of chop. Feels like the prop is losing bite and takes a good while to get up. Once on plane all is fine and fast again.
Prop is a bit dinged on the leading edges and one of the vanes looks a bit out of shape. Biggest dent probably us rather than previous owner.
It's a 21P Vengeance prop. Mercury website recommends 17-19. Thinking new prop and keep current as spare
Are the dings on the prop likely to impair performance that much?
Think it's a no brainer but dropping to 19P seems sensible given top speed is not an issue (easily runs at 40kn plus with 2 up). Sensible?
Everyone seems to have a 3 blade prop but a few 4s came up on Mercury website. Any reason why everyone has 3?
Any recommendations of brand?
Yes, damage will make the propeller more likely to cavitate.
If the engine is set up quite high it may be ventilating under high loads - where air is drawn down into the propeller. I'm suspecting if the boat has a 21" prop with a 175hp on a 6.8m rib a previous owner may have set it up for max speed rather than acceleration or general benign handling.
What max RPM do you achieve with this propeller? If it's within the max recommended range for your engine then 21" is the correct pitch.
4 blade propellers have more blade area and are less likely to cavitate under high load conditions. However, the increased blade area means more drag, less efficiency and therefore a lower top speed in many cases.
I'd recommend having the propeller repaired and going from there - Steel developments will do a quick mail order turnaround.
FWIW I had a vengeance propeller on my rib when I bought it and it was prone to blowing out after re-entering the water. Replaced with a Laser 2 and all was good.
Almost always a 19" laser 2 for sale secondhand somewhere. Would also be a good place to start.