Originally Posted by
MarsPA
Hey Dharr,
I'm about to ask a really dumb question.... so are you saying that a 21 pitch is a lower pitch than a 19 pitch? So the pitch is inverse to number? The higher the number, the lower the pitch?
So the manual for my 2016 merc Fourstroke 20hp says that the WOT rpm range of my engine is 5400-6100. I took it out to the lake yesterday and found the following:
1. I don't believe my prop is spun, but I'm not completely sure. I marked the hub and prop, and the lines did move from each other, but only minimally...maybe 1/8". I'm assuming that's within normal range...but it is movement. Then again, it could have spun 3000 times and just landed really, really, really close to it's starting point. My guess is on the former...but I'll monitor again next time out.
2. My RPM's are lower than I was guessing. The lake was quiet and calm yesterday...so on a smooth day with minimal wind I was at:
5.35mph at 2800rpm
6.9mph at 4200rpm
8.05mph at 4600rpm, WOT
Everything I'm reading is pointing me towards a 4-blade prop. I've been looking at a 4 blade 9.25" 5 pitch or 7 pitch
Are you saying I should be looking at a higher # (lower pitch?) than my current 10p, like a 15-20p?
I know that going to a 4-blade will supposedly lower my rpm, but get me to speed faster. I'm wondering if prop Diameter should be changed to counter affect the loss of RPM (smaller prop) or to increase surface area (larger prop).
The Solas website says in switching to a 4-blade prop, if my rpms are good to stay with the same pitch (which seems counter intuitive to me...doesn't necessarily say it won't lower my RPMs, just says to keep the same pitch). My RPM'S are definitely low, and the four blade will lower it even more (I think).
Do you have any insight into how I should interpret all of the above?
Thanks!
Hey Dharr… hoping to get your thoughts one more time. Here's my update:
I bought a Solas 4 blade 8.75" 7P prop, and it was not successful, but there was MAJOR change. I went from 4600 RPM to around 7000 RPM WOT (engine is rated for 5400-6100). I felt what I assume was prop slip (does that mean the hub spun?) in the higher RPMs....it felt like it was "chunking" for lack of a better descriptions. My speed stayed the same, around 7.4 mph.
I'm assuming going from the 9" to the 8.75" with lower pitch (with visibly smaller prop blades / lower per blade surface area) increased my RPMs. I had read that I'd lose 4-500 going to 4 blade, but I assume the smaller blade size was the difference.
I'm wondering if I should stay to the 7P 4 blade and go to a 10" Solas instead of the 8.75" to slow the RPM and increase surface area of the prop.
Any thoughts?