Prop theory: Not my strong suit

KeepitReal · 07-29-2011, 01:53 AM

By suggestion from some members, Im reposting this from my original location in MerCruiser repair section. Feedback so far is that a 15x16 4-blade stainless prop for my Diablo 180 4.3 Mercruiser is not the right prop by any means. I'm looking for a good all-around prop with a good hole shot and decent water sports ability. Top speed isn't critical but right now I'm only getting 38 mph at too high an RPM range-5400.

First of all let me thank everybody for the very useful information I have found throughout the forum so far. I just bought my first boat last week, a 1993 Diablo 180 with the 4.3 Mercruiser Alpha One Outdrive Thunderbolt IV ignition. I didnt know a thing about boats when I bought it, but all the information here is helping me learn quickly.

When I first bought the boat, I got a WOT RPM of 3900-4000 max and 28 mph on GPS speed. It sputtered and hesitated on acceleration and dieseled when you tried to shut it off too quick. After reading all the info on how to diagnose low WOT RPM's, I discovered the ignition timing was WAY off. It was set at 2 degrees ATDC when it should be 8 degrees BDTC. Fixing that and replacing the plugs, wires, cap and rotor did magical things to the performance of the boat. No more sputtering, hesitating, or dieseling. I got about another 10 miles per hour out of it, 38 mph GPS speed. My carburetor idle speed and throttle cable had to be adjusted to accomodate for the change in timing, but all is in order now and she runs like a champ.

My concern now is that now my boat may be over-performing. The spec for my boat on WOT RPM's is 4400-4800. I get a WOT RPM of 5200-5400 at a top speed of about 38 mph with 2 people in the boat on smooth water, trimmed all the way down. I have a 15x16 4-blade stainless prop. With this prop, I get a great hole shot and quick plane.

I know the prop plays a role in the WOT RPMs. But I have also read about 3.0 Mercruisers going 45 mph on the water, so it seems like 38 max in my 4.3 may still be low. Prop theory is still new to me and I'm trying to wrap my head around it all.

Questions: Is my prop the prime suspect of my high WOT RPM's? Other than the prop, are there other things that will make my WOT RPM's too high? Does my particular prop (that gives a great hole shot and quick plane) cause me to suffer on the top speed?

Any guidance will be greatly appreciated

***mikeyt*** · 07-29-2011, 02:17 AM

I answered all of this in your other thread.... you didn't trust my advice? I am 'the prop guru'.

KeepitReal · 07-29-2011, 02:23 AM

Oh, thanks Prop Guru

I just re-posted because the other response to my thread said I might get more advice in this section. One clarification........I'm a newbie so bear with me. When you said I should be running a 19" 3-blade or 20" 4-blade, that refers to the pitch correct? I should stay with my 15" diameter? I know, this is probably a dumb question but I just want to make sure-

***mikeyt*** · 07-29-2011, 02:46 AM

Yes, the 19" or 20" is the pitch. The diameter for a prop with that pitch will be between 14" and 14.5" with 14.25" being the most common. Running either of those props should have your WOT rpms very close to 4800 & your boat should run in the high 40's but still have a reasonably good hole shot. Not comparable to the 15" but still reasonably good.

You might also hear a suggestion here about running a Mercury High 5 prop. I dont have a lot of experience in dealing with that particular prop. Some members here do run it and like it, some have had mixed results with it. It's a bit on the finicky side with getting with the right application but when it's right it is a nice performer. Pricey though.