I have a 20' Maxum SR2000 with a Mercruiser 3.0LX and a 19" Mercury High Five SS prop. We don't waterski, so hole shot's not important to me. We do occasionally drag the kids on tubes, but 75% of the boating we do is just cruising the rivers and lakes.
I'd like to get more top-end speed, as well as pick up a spare prop, so I'm thinking about using my high five as a spare most of the time and going with a three blade aluminum prop. I'm changing a lot of parameters, so I'm really unclear as to where to go with the pitch.
WOT for my motor should be about 4400-4800. With a typical load and properly trimmed, I'm running at about 30-32 mph (per gps) on calm lake water at 4800 rpm. Problem is, I can't run full throttle without exceeding my rpms by another 600 or so, so I have to keep it backed off.
I've read that for each 2" of pitch increase, rpms should drop 200-400, but I've also read that increases in the number of blades will also drop rpms. In my case, I want to shave off at least 600 rpms AND I want to drop two blades.
Off the top of my head, I'm thinking a 23" pitch might get me in the ball-park, but I'm relatively new to boats, so I'm hoping to get input from more knowledgeable people (which pretty much means anyone reading this).
I should add that my current gear ratio is 1.98:1 (or 1.94:1?) and I'm well aware that my slip is VERY high, like 20-25%. My High Five is pretty nicked up, so that's probably the main reason.
Last edited by hopslover; August 2nd, 2011 at 06:14 PM.
Reason: More info
I very highly doubt that your 3.0L 20 foot boat will be able to push a 23 pitch prop. I don't know of many that ever have been able to do so....
Maybe a 21, more likely a 19. It's very difficult for me to guess how going from beat up/knicked up High 5 to a nice clean 3 blade will impact things....
Is there any way you can test drive a prop from somewhere? Beg/borrow one from someone else? 21's and 19's are a dime a dozen on Mercruisers...any friends or local marinas that can loan you a used one for a quick RPM & speed test-drive? That would be my strongest suggestion. Anything else would be a guess....and I honestly believe a 23 will be a mistake....
Thanks for the replies. After researching elsewhere, it looks like a three-blade 19 is probably the answer. There's a new-in-box Black Max 19 with Torq II hub kit for $80 locally, so I think I'll pick that up and see how it works.
I also need to look at my high five more closely and see if it has the correct thrust washer and clearance between the prop and housing. I guess if it's not seated in the housing correctly or there's too much clearance, too much gas escapes and increases the slip.
I'd also like to check my tach to see if it's accurate. Anybody know how to do that? Can an automotive tach tester be used?
I have a riding lawnmower with a BS engine and a small digital tach. It is very stable and very repeatable in the readings. The BS runs out to 3600. Don't know the upper limit of the device...probably 9999. It has a wire that has about 6 turns wrapped around a spark plug ignition wire. One would assume that it has two other wires, haven't looked but makes sense, one for 12v and the other ground.
If you have a distributor ign in your Mercruiser 3.0 (4 cycle) and use one of these it will/should read half your real rpm's being the dist only fires the plug every other rev. Being designed for/adapted to a lawnmower which has magneto ignition, the plug gets fired every revolution like a 2 cycle outboard does.