Whats the right rpm's

[cdail28590]

I have a 28' Bertram and my motors only turn up to 3700 rpms at full throttle. The props appear to be the correct ones from the factory that are 17x 17. The boat gets on plane great and tops out at 33mph. The rpm seems low, does anybody know what should be the max for 310hp 350 vortec motors? If the rpms are to low any suggestions on the correct size props?

[34 silverton]

i have 454's and turn 3800 i'm happy wiyh that 350's should run around 4400 wide open but that is abusing them it sound like you are getting your speed and plane ok so why run them harder

[cdail28590]

I don't want to run them harder I just want to make sure the boat is propped right. The engines might not be running as efficient as they can even though I don't it turn it hard.

[controlled chaos]

It depends what brand 350 you are running as the PCM, Mercruiser, Crusader motors are slightly different in their own way but generally a 350 should rev to 4800 safely at WOT. Most guys settle for anything between 4400-4600 which is fine. I have a 350 PCM in my boat and it revs to 4650 at WOT. If you have ruled out a dirty bottom, damaged props, or any other bottom issues then you definately need to look at timing and then I would think about taking the props into a shop to get worked on after that. If you are seeing 3700 rpms at WOT then your engines are being grossly overloaded and this is not good for longevity. I am sure that your fuel mileage is suffering due to this as well.

34 SILVERTON...Don't take this the wrong way but your 454's may be overloaded as well. I don't know your situation but 3800 is pretty low for a 454. They make their power in generally the same range as a 350. WOT should be between 4600 and 4800 rpms. Cruise may be a little higher in a 454 due to the torque curve.

My old boat with 454s cruised best at 3300rpms and WOT was 4400 b/c I was running bigger props with 1.91:1 trannys. This was a commercial application and believe it or not, we got over 3000 hrs with these motors and we took them out running. JMHO of course.

Cheers,

Dave

[cdail28590]

The boat is kept on a trailer so theres no growth even though it is bottom painted. Props were reconditioned last year and there are no signs of any damage and to my knowledge I haven't hit anything since they were repaired. Both motors will only go to 3700 so I think its definately the props, I just wanted to make sure.

[controlled chaos]

I am no professional marine mechanic but I have done a few inboard boats with GM gassers. Did the motors ever spin up past 3700??? If they did not then props is a good place to start but you already determined that. I would still make sure your timing is all good to go. I am not trying to insult your intelligence but here is exactly how I would do my timing when I am trying to dial in an engine. Again, just my technique and I hope you get you motors running right really soon. I Start out at the dock and in the water so I can do multiple sea trials until it is as good as it is gonna get with a warm engine idling at your recommended idle and if I remember correctly anywhere from 9-12* is where a 350 runs best at idle. Then I tweak it further and as long as you do not exceed 28-30* BTDC @ 4000rpms under no load then you should be in the safe zone. 32* or more and you get into a tricky area. I would also check to make sure your carbs are looking good. Sometimes carbs can be a menace when trying to problem solve on an inboard. Make sure you secondaries open freely and fuel/air mixture is right. Chances are... You have prop issues and you already know that, I am just bringing up other things just in case it may help. I have had my share of issues with inboard boats and sometimes it is a combination of a few different things all at the same time. I'm sure you know what I mean. Inboards are a labor of love!!! Good luck and keep us posted with the results.

Cheers,

Dave

[nautiduck]

Have you done a compression test on the engines? Timing is the other area I would check. If those are the props that came with that boat and engine combo then I would suspect something else. Are you carrying extra weight, hull water logged, lots of extra gear, extra filled fuel or water tanks? Find out what the boat should weigh and then get it weighed (dry as possible tanks emptied no gear) at the local truck scales on the trailer then weigh truck and trailer again without the boat on the trailer. Also you may have a distributor issue I had one on a 350 that wasnt mechanically advancing correctly. Would doubt two of them doing this but its possible, and one could be lagging and lugging the other.

[fishfighter]

Have you done a compression test on the engines? Timing is the other area I would check. If those are the props that came with that boat and engine combo then I would suspect something else. Are you carrying extra weight, hull water logged, lots of extra gear, extra filled fuel or water tanks? Find out what the boat should weigh and then get it weighed (dry as possible tanks emptied no gear) at the local truck scales on the trailer then weigh truck and trailer again without the boat on the trailer. Also you may have a distributor issue I had one on a 350 that wasnt mechanically advancing correctly. Would doubt two of them doing this but its possible, and one could be lagging and lugging the other.

Weight is a big killer. Ever look to see what junk we put on board? 3700 rpms will in fact kill those engines. Vortec 350's rpm WOT should be at 4400-4500. Timing could be a problem.Do you still have glass fuel tanks? E-10 gas could be eating the fiberglass which will travel to the fuel filters,starving the engines of fuel. That in itself will keep the engines from turnning full rpms. Another question is,what are your engine temps running? Did you check your exhaust lately? Block risers/exhaust will drop your rpms. Anyway,what was your rpms before having them tuned last year?

[cdail28590]

Every since I had the boat it would only run 3700 rpms. I have replaced the glass tank with a new aluminum tank. The motors only have about 50 hours on them as they were new crate motor drop ins. Running temps are around 170 degrees and there doesn't seem to be any smoke coming from the exhaust with the exception of little bit of white smoke while running but that doesn't appear to be anything out of the ordinary. I have checked the timing and its at 12 base and advanced to 28, so that appears ok. With both motors turning to 3700 it appears to have to be the props. The boat weighs around 11,500 lbs and its not waterlogged as I had it apart over the winter replacing the tank. I normally run 60 gallons of fuel (as I don't need much playing on the river) no water tanks and maybe 300 lbs of food, ice, drinks, and coolers with 2 people on board.

[T380Fisher]

I have 454 Crusaders in my boat and my book says 4200-4400 WOT. Mine turn in at 4350 in the spring and 4200 by haulout in the fall due to growth and amount of "stuff" that always seems to makes its way onboard during the season.

Silverton34....Yikes....just because my engines turn up to 4350 doesn't mean I run them there. If you are WOT at 3800 RPM you are severly overloading your engines even if you run them at a much lower RPM.


cdail28590.....If your props are standard (ie not cupped) figure about 200 rpm per inch. You need to lessen your pitch by about 3". (if it is just in the props). My previous boat was a 31 SeaRay Sedan with Mercuiser 350's. My props were 17 x 15. It weighed 11,000 lbs.

If you call a good prop shop, they should be able to figure out the correct diameter and pitch for your ride based on power, weight, waterline length and hull type.