I installed a pair of new Mecruiser 5.7 EFI engines (260hp each) in my 26ft Chris Craft Stinger last year with Mirage 14 1/4 X 20 props. Time to plane, mid-range acceleration are both good, and my top-end tac readings both are 4600rpm indicating 54mph on GPS. This is obviously not my idea of a heart stopping top-end, and the boat is prone to porpoise -badly- at all speeds, requiring about 3-4 degrees of trim tab even at top-end with drives tucked in. I have been told that Stingers and Bajas both have a tendency to porpoise by design due to there very high drive heights. Is there any truth to this??? Is there anything (short of trading the boat) that can be done to eliminate or mitigate the problem? I have heard some 4-blade props can tame ugly handling problems like chine walking. Could they help me here? If so, anyone have any recommendations before I start prop swapping?

Thanks.

I've had a porpois problem appear and disappear with different props. I think it has to do with the ability of the prop/boat to carry the bow. Another variable that you can try is drive height. If your drives are really high, you could try a spacer to set them a touch lower. This would help the prop to carry the bow.

What is the max rpm's for the engines? 4600 seems kinda low, so a smaller prop may help the engine to produce more power, which would help the prop/hull do it's work.

I have the exact same problem in my Liberator 241. I'm running 21P Mirage + props. I was out playing last night with the boat. I did some passes with my drives trimmed and my tabs all the way up. Bouncing like crazy. Then I put the tabs down a little so the porpoise was gone. Then I trimmed the drives way up. Barely any porpoise and was actually a little bit faster than without the tabs down.

Thanks for the tips, Cord. I'm running Alpha drives on the boat. Any idea what normal drive heights are with these drives? I can measure mine and see if this could be a real problem or a myth.

The WOT range for the 5.7 EFI is 4600-4800rpm. I agree I'm a little low, but still within range. I may just try different/smaller props and see how they perform. What props have you had the best luck with? It would give me a starting place in looking for better props for my hull.

Rock brown,
My friend has an 86 with twin 260's. I think he was running 60 with cleaver props. He also had some proposing. I'll find out what his combination is and let you know.
Dan

Thanks, Dan. I'll be waiting.

Rock brown,
My friend is running 21 pitch 3 blade merc props. He thinks they are Mirages. One engine turns to 4,200 and the other turns 4,500. He has never verified that his tachs are correct. You should be running the same rpms on both motors. He runs between 60-61 on GPS with a light load. The GPS number is with me running next to him in the river so it is not exact numbers but real close. He has the tabs up and the boat loves to porpoise. He tried cleavers on the boat but they didn't help the porpoise problem and midrange numbers weren’t as good. I wish I had better info for you. If he ever gets his boat out of storage I'll verify the prop type. I wonder if the hull acts that way because both of the props are spinning the same direction.

Good luck,

Dan

Thanks for the info, Dan.

It's beginning to sound like a drive height problem. I know it's not about the prop direction. I installed new counter-rotating drives when I replaced the engines. Wow, what a difference that made around the docks! Really appreciate your responses.

One thing, though..You might what to verify the WOT numbers with your friend. If it's one thing I have learned about engines: They do not like high manifold pressure (high power) with low rpm's. Marine engines are built tough, but sustained high power outputs with low rpm's put an incredible strain on the cylinder walls, cams, piston/piston rods, etc. Mercury warns of possible catastrophic engine failure with prolonged operation below 4600.

Again, Thanks a million for your help.

Next problem...Anyone know of someone who makes spacers for the Alpha One Gen II drive?????? Also, what are good x-demension figures for these drives? I'm measuring mine this week.