enough rpm 79 inline six

Georgesalmon

Lieutenant Commander
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Apr 14, 2012
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1,793
I have a 19791/2 115hp merc inline six. Running on a heavy glass 20' boat. I'm getting 4800rpm at WOT. That is the minimum engine recommended rpm. Range is 4800-5300. Is this OK? Got a 15p 4 blade prop on it. The new merc AL one. I am happy with the performance overall. Top speed is 31mph GPS. Hole shot good. Only thing worrying me is the rpm at WOT. Would it be better for the motor if I was closer to 5300? Gonna buy a spare prop and thinking about a 13p. Theory says that would get me to 5200ish rpm at WOT. Another thing is that I don't run WOT much, Lake Michigan won't let me. So is it also better if the engine runs faster at 1/2 or 1/3 throttle? Last Sat got caught in a thunderstorm and ran 5 miles at just 10-15 mph using throttle to keep me on top (most of the time) of some pretty big following waves.
 

Maclin

Admiral
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May 27, 2007
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6,761
Re: enough rpm 79 inline six

A factor in this is the engine condition, will it like revving more?!?! Your 15 pitch is very close to ideal.

But for a spare, and perhaps experiments, the smaller 13 pitch prop will make it easier on the engine at all RPM's and let the engine give quicker response with throttle changes. Your top speed may drop 1-2mph but you will get to the top speed quicker if need be. And if you tow skiers it will be the ticket. The 13 will not be much out of line RPM wise, just watch the throttle and RPM's.
 

Texasmark

Supreme Mariner
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Dec 20, 2005
Messages
14,677
Re: enough rpm 79 inline six

I have mercs and ran my tower at 6k every chance I got to to open it up.......it loved it.

I think I share the feelings of a lot of old salts on here that your engine will be happier if you drop your prop size as you said. Your top end mph may fall off, but if you made the new 13P prop a SS with cupped blades, it may increase for several reasons, SS related. Your boat handling in rough water will be greatly improved and you will be able to better hold your speed and boat attitude under rough water conditions and lighten some of the additional load the weather condition poses. BTDT.

HTH,
Mark
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: enough rpm 79 inline six

My info says the gear ratio is 1.7 Calculator indicates you have about 22% slip seems really high even allowing for a heavy boat.
Also would seem the new Merc prop would do better.
I think before any changes you need to confirm your 33 year old tach is accurate. Its possible its only turning about 4000.
Maybe you can borrow a shop tach for a quick run.
 

Georgesalmon

Lieutenant Commander
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Apr 14, 2012
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1,793
Re: enough rpm 79 inline six

SS is not in the budget. I can get the new merc spitfire for about $65 less hub. Pretty sure my tach is correct. Brand new set of gauges from a factory rep. Tach was compared to my RC model airplane engine tach. My boat is over 3000 lbs and deep V. Lotta hull in the water even up on plane. I think engine condition is good. Compresson 120 +- 3 across the board. Starts and runs well after new stator and packs(2007 PO), carb and fuel pump rebuild, new hoses, etc. I have to date only had two people, full fuel and gear in the boat. Expect to have 3 or 4 for normal use (fishing, maybe some grandkid tubing someday when get older.). Based on the above comments I feel good about getting the 13p.

Thanks to all for your comments.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: enough rpm 79 inline six

Makes sense, maybe that 1.7 ratio needs a little less pitch.Let us know your results.
Should make good reference material.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,677
Re: enough rpm 79 inline six

If you are going to go with a cheap alum prop sir, may I suggest a Turning Point Hustler alum which can be purchased from this site for about $100. It has most all the amenities the higher priced SS have except the thin blades and blade strength...two things you may not need/notice. May work well for you.....I have one.

Mark
 

Georgesalmon

Lieutenant Commander
Joined
Apr 14, 2012
Messages
1,793
Re: enough rpm 79 inline six

Thank you, I'll look into that. Thing is, I have an "in" for most marine things at OEM or better pricing. I was in the boat building business for over 30 years from coast to coast.
 
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