need help
 

I have a 1990 42 cig revolution twin 750hps with #6s. 1.5 ratios. Prop shafts are in line with the bottom of the boat. I am trunning 16x27 merc. 4 blades. At 5100rpms I run 75 on gps. Bud at cig use to run the boat at 5900 rpms at 83 on gps. The props were in bad shape so Merc redone them. Is the speed lost in the props. Nothing else on the boat has changed. Does merc do a good job? Should I try to have these props reworked or invest in some new 4/5/6 blade merc/herring/hydromotives ? Thanks Heath

Re: need help
 

Well somehow you have lost a considerable amount of RPM. So I would think that something might be going on with the motors. When was the last time they have been freshened? Merc does an OK job not steller but I cannot imagine hacking the props that bad.I am happy to take a look at them for you if you would like.

Julie

Re: need help
 

Wow,

Mercury invented Long Hub cleavers and the Lab Finishing process. Before most people on this board, including the respondents on this thread knew what Long Hub props were let alone Lab Finishing, Mercury was doing the process. I'm sure they are not the cause for that type of performance loss.

I spent 10 years there and I can tell you the boys in the Lab would be hurt by some of these comments. Now, that being said, the quality control in the Lab has suffered over the last 2-3 years. I know, I've sent new stuff back over the last couple years when needed if the standard wasn't what it used to be. Dennis C. may roll in his grave occasionally too. However, As far as bringing your 16x27 back to within spec. as to not lose 8mph and 800 rpm, I'm sure they handled that just fine.

My partner in this business has been doing those big cleavers propellers since 1986. He pitched 90% of those props for almost 20 years. Time and experience are key and Mercury and BBLADES have it.

I guess my blood is still a little black.

Check the engines and or bottom, etc.

Re: need help
 

Are you sure about the 5900 rpm and the 83 mph? How many sourses?

Re: need help
 

Thank you Ben,

I may have changed or missed the point at hand (or not). The props are actually performing better since the rework if we look at efficiantcy.

There may be a number confusion here.

Or,,,, one example of what we tend to overlook in this day and age...............slip can be good????????!!!!!!!!!

Darn they brought them baack to true spec.

Re: need help
 

Heath,
Are the props you are running now the same ones that ran the boat at 5900@83?

If so I doubt you have a prop problem. There must be something else going on.
What changes have been made to the boat since it ran at that speed and rpm?
Are your motors supercharged or naturally aspirated?
Do you have dyno sheets on the motors?
Wet sump or dry sump #6's?
How does the boat get on plane?
Is there a rooster tail at WOT when trimmed for max speed? If so, how high.

Matt

Re: need help
 

The motors have just been totally gone through. They are natural. Bud at cig was the only one who told me the old numbers. Also I was there when they dynoed the motors and there is only 6hp difference. The drives are wet sump. Also one motor will some times run 200 to 400 rpms higher. And that really throwes me off. The boat sat 6 years before we got it and I have not looked at the bottom. That might be part of the problem. Should I be happy with the speed I am getting out of it? I know it is a big old heavy boat. It is a Kevlar hall. Merc did tell me that 2 of the of the blades were bent on one of the props. The boat likes to porpoise a lot so I have to run a little trim. That might be where I am losing my rpms. Would spacers help with the porpoising? I am just looking for the most speed out of it with out going up in power. Thanks for all the input Heath

Re: need help
 

If your motors are making 750 hp you will have to turn more than 5100 rpm to get that kind of hp.
Do you recall at what RPM max torque and HP were produced?
Did you see the rpm difference between motors before or after the props were worked? Is the rpm difference at wide open throttle or at cruise?
Your prop slip is really high, almost 25%.
I think you should be able to see more than 71 from your boat considering the power you have.

Re: need help
 

I went down and looked at the boat to day. The prop shafts are 2inches above the bottom of the boat. That's probably getting the porpoising and why I have to run with the tabs down. The motors turned 6000rpms on the dyno. We never run the boat before we redid everything. I think the rpm difference my be from the timing on the motors, going to time them on Monday. I think if I put some spacers on the drives, pull up the tabs, and get the motors a little more dialed in I should see a little more speed. Maybe not 83 but upper 70s would make me happy for the size and age of the boat. Thanks for all the help Heath