Correct prop size?

dev6315

dev6315

Member
Jul 11, 2009
371
Myerstown, PA
Boat Info
2005 SeaRay Sundeck 240
Engines
5.7mag Mercruiser with bravo 3
After one of the first runs on my 250 Sundancer this past weekend I was able to get a bit of running info while underway. One of my biggest questions is the prop.
I ran her gentle to break in the rebuilt engine and drive. It has a 7.4 454 running a Bravo 1 with a Mirage SS 15.5" dia x 17 pitch prop. 3 blade. If I wanted to, I could have spun the engine easily close to 6000rpm. It seemed that 4750-5500 was almost cruising. This seems high. Am I under propped??
I know the hub is fine and the wheel is true, I had it serviced before re-installing, it came with the boat from the PO. Any insight is appreciated before looking for another prop.
 
That seems high for the RPM range - my '89 250 DA with 7.4/BravoI combo cruises on plane comfortably in the 3000-3300 range (at 19-24knots) and WOT should be somewhere in the 4200-4800 range. I run a Mirage 17p (sounds like the same prop you're running) and my BravoI is a 1.5:1 gear ratio. Something sounds amiss with your setup....
 
Boykster, thank you very much for the information, it is very helpful in diagnosing a baseline. When I bought the boat the outdrive casing was shot. I got a great deal on a complete rebuilt Bravo 1 unit. I am thinking I am running the higher gear ratio. Thankfully I still have the old drive and all the internals are perfect with low hours. I am going to switch out the upper gear set with the 32 teeth gears, that way I know I am running a 1:5:1 set up, then theoretically I should get your numbers.
If anyone out there can shed any other insight that would be appreciated. I am going to most likely tackle this job tomorrow, thank god I saved the old drive for parts.
 
I have to admit, I'm not quite sure how to do something like that. To find the tach is lying to me would be a much easier solution to this problem.
 
Borrow a timing gun. Clip it around spark plug wire and it displays RPM, compare it to your tach.
 
Yep, a timing gun will do the job. good to have in your toolbox anyway come tune-up time. Pretty inexpensive for one with a digital tach.
 
Thanks guys. I will pick a digital unit up tomorrow. I opened the drive up today (needed to replace a shift cable and bellow anyway) and confirmed that I have 32 teeth on my upper gears and 27 teeth on the drive shaft coming from the engine. I am definitely turning a 1:5:1. At this point it would have to be the tach. I also double checked and I am turning a 15.5" dia 17 pitch 3 blade prop.
 
Yeah - sounds like it's the tach. You're essentially running identical gear to mine; cruising at 4000+ rpms is just too high. My sweet spot is right around 3000-3100 for speed vs comfort vs fuel economy (yeah right). 3300-3500 is more 'fun' though ;-)
 
I doubt it's your tach, if they go bad they usually bounce around or just quit.
You can always put on more prop to get the rpm band you need.
I definately wouldn't push that Chevy over 4600 RPM's unless it was well insured.
 
If I were to re-prop. What would be a good jumping off point to start 20P-22P?
I will hopefully get out today for the timing gun, but the tach is pretty sure, it doesn't jump around.
 
What is your GPS speed when cruising? With a 7.4, 1.5 ratio, and 17" pitch, your 250DA may be a bit underpropped, but I don't think as much as the tach numbers you provided. Like others said, check RPM's with another tach. Also, on the back of the tach, there is a switch for 4/6/8 cyl. Check to see where it is set at. Also, move that switch back & forth a bit, and then set it back to 8 cyl. The contacts on that switch can get corroded, and cause erratic and increased RPM readings.


I would stick with a Mirage Plus -- its a great prop. I think your 250DA with a 7.4 and 1.5 ratio should work well with a Mirage 17" or 19" pitch prop. Your boat should hit well above 40 mph and WOT RPMs should be able get into the 4400-4800 range. Since you confirmed you have the 1.5 ratio, I don't think you want anything higher than 19" pitch.
 
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