Max RPM advise

[DWRAT]

My brand new Mercury 150 4 stroke with a 15X15 Enertia prop is getting 5,300-5,350 max RPM.
It has a great hole shot, mid range power & hitting 40 mph.
Any reason running 5,300 rpm could be problem?
Just trying to save buying a 15.3X14 prop for another $500.00
I know the range is 5,000-5,800 and I have always heard to try to be at the top of the range, but why?
Thanks
Dan

[Rick McC.]

[quote="DWRAT"]My brand new Mercury 150 4 stroke with a 15X15 Enertia prop is getting 5,300-5,350 max RPM.
It has a great hole shot, mid range power & hitting 40 mph.
Any reason running 5,300 rpm could be problem?
Just trying to save buying a 15.3X14 prop for another $500.00
I know the range is 5,000-5,800 and I have always heard to try to be at the top of the range, but why?
Thanks
Dan[/quote]

I'd be happy with what you have now.

[mpilot]

[quote="DWRAT"]My brand new Mercury 150 4 stroke with a 15X15 Enertia prop is getting 5,300-5,350 max RPM.
It has a great hole shot, mid range power & hitting 40 mph.
Any reason running 5,300 rpm could be problem?
Just trying to save buying a 15.3X14 prop for another $500.00
I know the range is 5,000-5,800 and I have always heard to try to be at the top of the range, but why?
Thanks
Dan[/quote]

That 15x15 is what all of the Brunswick brands ship with. You'll pick up a little rpm with the drop in pitch but you'll lose some back with the diameter and no real speed gains. I'd stick with what toy have.

[Bamaman]

40 mph is the max you should expect out of that engine and boat.

You're good to go. The 150 FourStroke I think is a 5800 rpm max motor anyway. Close enough.

[r&s20]

The reason to try to get to the top of RPM range, is so when you load boat heavier than normal (or pulling tubes) you will still be within the WOT RPM range. maybe lower rpm's but still range.
If you are hitting 5300 with normal load i would run what you have

[Ozark Bob]

I have the same engine on my South Bay 522RS turning a 15.3 X 14 Q3 prop, same as the Enertia. The best I can get at WOT is 5400 rpm. I was also concerned about not getting into the upper rpm range so I contacted Mercury for their opinion. They said "Your rpms are exactly where we recommend. This should cause no issues with your engine. Issues can arise when the wot rpms are much lower ie. 4000 rpms."

[DWRAT]

[quote="Ozark Bob"]I have the same engine on my South Bay 522RS turning a 15.3 X 14 Q3 prop, same as the Enertia. The best I can get at WOT is 5400 rpm. I was also concerned about not getting into the upper rpm range so I contacted Mercury for their opinion. They said "Your rpms are exactly where we recommend. This should cause no issues with your engine. Issues can arise when the wot rpms are much lower ie. 4000 rpms."[/quote]

Bob,
What top speed are you running?

[DWRAT]

Thanks everyone for your replies.

Dan

[Ozark Bob]

I get 35 mph at 5400 rpm with just my wife and I on board and about half fuel. When I bought the boat it had a 15.25 X 14 PowerTech prop and I could only get 32 mph and 5300 rpm. I got a pretty good improvement with the Q3 prop but I am still searching for a few more rpms. I am going to try raising my engine 1 hole to hole #3 and see what happens. Not sure I will see any gains but it's worth a try.

Your numbers sound really good. Do you have the full center tube? Mine is the 3/4. You could pick up a couple hundred rpms if you went to a 14" prop but I don't think you would gain any speed. I looked at your numbers on the BAM Prop Slip Calculator and with you 15" prop turning 5350 rpms it showed a Theoretical Speed of 40 mph. Assuming a 200 rpm gain with the 14" prop it showed a Theoretical speed of 41 at 5550 rpms. Both those numbers indicate 0% slip which is pretty much impossible. How are you measuring your speed? Boat speedo or GPS?

http://www.go-fast.com/Prop_Slip_Calculator.htm

If you do decide to try the 14" prop and like it you wouldn't have any problem selling the 15 X 15 Enertia.

Bob

[DWRAT]

GPS speedo, 3/4 toon, full under skin, motor one hole down from the top hole, 2 people and a dog with full camping gear running in the Colorado river but in a wide section. (see picture)
Maybe the flow helps outs a bit but from many past testing, only 1 mph in the narrow section.
GPS reading in the center bottom of crappy picture.

http://i1129.photobucket.com/albums/m51 ... G_2047.jpg

[Ozark Bob]

Those are great numbers. Wish mine would go that fast. I think we have very similar boats. Mine also has the 3/4 center tube with full under skinning. Overall length 23'9". Makes me wonder why I can't get a couple of more MPHs. I think if I had your boat I would leave just as it is and enjoy!

Bob

[Scott1]

If you are comparing boats and speed you need to know how each motor is mounted. If mounted too low in the water you will not get the top speed.