RPM never exceeds 5k

[MAW31]

This may seem like a moot point, but my service manual says operate the motor within the 4500-5800 rpm range for my 88SPL.

Now I know the motor is not new but I'm baffled that i cannot get more than 5k out of the motor no matter what.

compression is 125 all cylinders, idle and WOT jets replaced, SS prop std 13.75x15, pushing a 17' CC myself onboard, linkage observed for WOT.

This all got started when i was at our local "watering hole" and was chatting up a mercury marine mechanic who was surprised thats all I could get...given the boat, prop and HP.

So i got to thinking...how come...

First thing I did was take off the carbs, follow the instructions here, soak em overnight, clean em up, put em back on. On my test run, it really did not change anything...and I didnt expect it to since i 'saw' no real blockage.

i dont mind testing all the details, but where would I go from there? or am I kidding myself, and the operating range does not get you past 5k or is it more like the MPG estimates on cars...u never get whats theoretically possible.

 

[JB]

Re: RPM never exceeds 5k

Do you have PT&T? How do you use it?

Correct trim at WOT can make as much as 1,000rpm difference.

What is the level of the AV plate compared to the bottom of the transom?

 

[MAW31]

Re: RPM never exceeds 5k

good questions.

yep i've got power trim/tilt. theres no gauge to follow i drop it all the way down.

The AV plate is damn close to level with the bottom of the boat.
If i make a hard turn under power i get alot of prop wash.

 

[JB]

Re: RPM never exceeds 5k

Get on plane and then trim the engine out a bit at a time until she just starts to porpoise. Then back off until she stabilizes. You should be able to reach 5500-5800 easily.

 

[MAW31]

Re: RPM never exceeds 5k

what exactly is porpoising?

When the bow bobs Up n down on its own? (not due to waves)

 

[crxess]

Re: RPM never exceeds 5k

Correct.

Also, is CC - center consol or cutty cabin? Is the hull clean?
Do you leave the boat in water all summer?
Gunk accumilating on a hull can rob 25% of performance in just 30 days.

Just a note, Standard prop does not mean correct prop for application. Lower pitch will raise RPM and sacrifice a little speed.

I agree 100% with learning to properly trim at speed. It makes a big difference.

 

[MAW31]

Re: RPM never exceeds 5k

ya, cc = center console. boat sits on my trailer, so clean bottom.

verrrry interesting. i toook it out tonight for a sunset run.

if i didnt see it with my own eyes, i'd of not believed it, but ya...
i gained 300 rpm using the tilt/trim until the bow was bouncing off the still water...then backed if off...no kidding i got 300 additional rpm ...

speaking of props...u know the setup...are you recommending a prop change?

 

[JB]

Re: RPM never exceeds 5k

You could experiment with a shorter prop but that might take her up too high.

You are in the upper half of the recommended rpm range now. Theoretically dropping 1" of pitch should add about 400rpm, but 15" just seems like plenty short for a 17' CC unless it is a pretty heavy boat.

I used 17' to 19' SS props (depending on load and balance) on a DF70 Suzi on my 17' Boston Whaler Montauk and easily got to 5800rpm.

But, hey; I am no propspert. I suggest that you take the details from here and ask the guys over on the prop forum. Give them some details about your boat, too.

One other thing you might try when you have the time. Try raising the engine about an inch (usually one hole in the transom mount). The reduction in drag just might get you the other 500rpm you are seeking, so long as it doesn't produce prop ventilation.

 

[kenmyfam]

Re: RPM never exceeds 5k

Set up is huge for achieving WOT RPM. Motor height, trim, loading of gear, weight distribution can make a huge difference. So can having waterlogged flotation foam in your boat. Once that is optimised you can then consider a prop change. You say you trim it all the way down ??? That can be good for your hole shot but it should be way up once you are planing.