ScarabMike

In another thread, I posted about a possible manifold problem that I was having. It turns out that water was not the problem at all! Apparently, I spun a lower bearing. The good news is, Merc warranty is going to cover it.

My question now is regarding proper RPM at WOT.

I have a 1997 31' Scarab with twin 502 MPI Mags. I just bought the boat and the bearing blew under a test run. The boat came with 24" Bravo I props (NOT Labbed). I suspected all along that they were undersized and planned to replace them with a set of Labbed 26's. In any case, I reported that at WOT I was turning 5000 RPM's. I was under the impression that this was an acceptable operating range for this motor. The manual states an operating range of 4600-5000 RPM's with the rev-limiter set to 5100 RPM's.

Am I wrong? I'm wondering if I cause the bearing failure running at that rate. If I change to 26" props, I should bring down the RPM's, but since I am going to be switching to labbed 26" props, wouldn't that bring my RPM's back up to wher it was with 24" Non-Labbed? Should I consider 28" Labbed Props? The 2 size change should drop me to about 4400-4500 RPM's, but the blueprinting should bring that up to about 4600-4700.

If I'm totally off base her, somebody please point me in the right direction. I certainly don't want to cause another engine failure. as it is, I'm now worried about the second motor that wasn't giving me trouble. It sounds fine, but I wonder if I could have done damage there....

Iggy

For all that I've learned on this board you should not exceed max operating rpm (5000) while lite on fuel and load. Then no less than the minimum operating rpm (4600) with a full load and fuel.
IMHO, I'd say you're propped right.
If you increase pitch to drop rpm (to 4400/4500) you'll be putting too much load on the engine.
I'm sure someone will say different.

Brad Perry

Others may disagree, but I don't think your rpm range had anything to do with the failure. It was going to happen, it was just a matter of time. You just made it happen sooner than if you stayed in a no wake zone forever.

If I had that motor, I would run it at 5000+rpm. No sense in propping the boat below the HP peak of the motor for fear of blowing it up. That motor will take it all day long if it is healthy. The labbed 26 might be a good choice. I would tell the shop the operating conditions now and what motor and solicit their advice. I don't necessarily think that you are doomed to 200 rpm more and that's it. I would think that your existing prop could be blueprinted and at the same time tweaked to give a little more pitch so that you will enjoy the benifits of a more efficient prop while having the right pitch for the job.

Whiteknuckle

I responded to ScarabMikes other post. You have to understand that what I said is purely an opinion. I think your boat is propped correctly. That engine should turn the RPM that you are getting at WOT. I just think that because the manual says WOT is 5000RPM does not mean you should do it all day long. I know others have done it and some do it as SOP, but if you do, expect shorter life on engine and drive. There is no free lunch. Again, its your comfort level. Expect to pay for your habits.

ScarabMike

I thought that my RPM's looked pretty good at 5000. Especially considering that I was running 24 pitch props. You could certainly fell that the props were undersized. There was way too much drag. Even fully trimmed and K-Planes all the way up, I was stuck hard to the water. It was a pretty windy day and there was at least a 2-3 foot chop.

I was running with a full tank of gas and 2 people on board. My gut feeling is that the increased 2" of pitch and the blueprinted props will almost offset one another in RPM's. I will probably drop to about 4900, but who can ever tell without a water test. It's pure guesswork at this point.

I'm actually getting NEW BRAVO I 26" labbed props from Mercury marine. It was part of the deal when I bought the boat. I plan to keep the set of 24's as a spare set. I'm not sure if they are as good as props from shops such as Bronson-Hill, etc, but I don't have a choice here. Since the deal is done already and we were simply waiting for the water test to order the props, I will take the ones that they are giving me. I can always send these out next season if they are not the quality I expect.

BTW. My shop seems to think that the failure may have been caused by a bad oil pump, but we woun't have confirmation until Mercury gives the OK to take the engine apart.

ScarabMike

Whiteknuckle,

Maybe I misunderstood your reply. I only ran at 5000 RPM's for about 5 or 10 minutes. This was my first test runin a boat that 1 purchased during the off-season. It's only early April and I paid good money for the extended warranty (thank goodness). If the motors can't handle running at WOT and a RPM range within spec, I want to know about it now rather than in July.

I actually ran no wake for about 20+ minutes and then at a cruising speed (3000 RPM's or so) for about 20-30 minutes befor I even attempted to open it up. I'm not saying that I bought a performace boat to run at 45mph all day, but I also can't see myself running 75+ all day either.... WHO COULD AFFORD THE GAS? But, I need to feel that it is reasonable to be able to open it up for 15 minutes or so at a time without fear of blowing up my motors each time. If this type of operating is potentially harmful to my motors, then I need to adjust my props, etc to accomodate my style of boating.

Mbam

Mike, unlikley that your failure is related to the RPM. 5000 is right on the money. Also, your math on the props should sounds good.

Tinkerer

I run my boat with 26 pitch bravo 1's at 5400 rpm. Like everyone said you are very safe at 5000 rpm. You just should not make a habit of doing it for 15 min. very often or you will drasticly shorten the life span. Watch your oil pressure when running WOT for an extended period as your oil pressure will tend to drop as the oil gets hot. When your boat is proped right you should be able to bump the rev limiter with just you , 1/4 tank of fuel and not much else on board.

Blackhawkdn

You want your motors to run about 5000 rpm + this time of the year, because as the air temperature gets warmer your Hp will drop and you will lose RPM. I assume this is a new boat, if you ran the motor hard for an extended period of time during break-in
this could have caused the failure. Good word of advice for motor longevity don't run more than five minutes at WOT at a time, back off and let the oil cool down.