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The-easyrider
21-10-2010, 10:11 PM
Hi Guys I picked up a new, 2nd hand 4 blade it is a 15"D x 19"p ss and replaces a 3 blade 15" x19"P ss this dropped my WOT RPM down so that it just makes the bottom end of the recomended WOT rpm range. My question is will this have any long term effect as I have read that you are better off at the high end of the WOT rpm range. It also knocked about 3kph off the top speed and made no real effect on the economy at cruise speed.
Thanks Azza
fisho64
21-10-2010, 11:39 PM
what needs to be taken into account is that when the boat is fully laden the WOT rpm may well be below spec, thats the reason why higher in the range is better.
What sort of outboard is it?
The-easyrider
21-10-2010, 11:46 PM
Its a inboard V6 220hp MPI mercruiser I like the extra grip the 4 blade has and I am trying to get a bit of stern lift as the boat is bum heavy
fisho64
22-10-2010, 12:29 AM
No worries, just make sure that when you are full of fuel and a few bodies on board, that it still performs inside its parameters, especially if under warranty. Over propped is like driving your car in 5th at 50kph.
gofishin
22-10-2010, 06:06 AM
what needs to be taken into account is that when the boat is fully laden the WOT rpm may well be below spec, thats the reason why higher in the range is better.
... WOT 8-) he said.

The amount of transom lift you get depends a lot on the type of 4 blade prop you have Azza. Mostly Rake, but also diam and blade area also play a part. I have had 4 4-bladers on my tub now, all different, and the difference in transom lift (and performance) between them is/can be significant. There has also been a big difference in grip (and/or the lack of), which is sometimes hard to fathom - but then I am far from an expert.
cheers
trueblue
22-10-2010, 07:33 AM
the blades of the prop can be trimmed by an expert to let your WOT rpm come back up, but might need fine tuning with a few step by step modifications
cormorant
22-10-2010, 08:21 AM
Going from 3 to 4 blade you need to drop 1-2 inch or more of pitch to keep same revs as you have a extra blade to tuen and teh water it pushes.

3 or 4 blade arguement is all advertising as it all depends on shape and style of each and what they are powered by. A good 4 will be better than a average 3 blade and a good 3 will be better than a average 4. Just gotta do trial and error and every prop is a compromise at some speed or load uless you are flat out all th time and prop accordingy