Am I getting the most out of my engine?

[starkyb]

My 1988 75 horse Mariner won't go past 5400 rpm and 35 mph at wot. It redlines at 6000. How do I get the most out of my engine?

 

[jbcurt00]

Re: Am I getting the most out of my engine?

What redlines at 6000rpm? The gauge? Is it original to the boat, is the motor? Are you the original owner? Check the factory manual, it should have a peak/max rpm range. You may or may not be in that range at 5400rpm.

EDIT: According to the condensed owners manual, your RPM range should be 4750-5250 for a post 1986, Mariner 3cyl 75hp OB.

If you know the diameter & pitch of your prop, post it. As well as the boat, and how you use it: towing a couple skiers or 1 of the monster huge new inflatables, fishing etc. and any details about # of people (weight) aboard &/or gear, esp water & fuel (heavy). It may help those that can (not me unfortunately) help. And are you positive all else is operating as it should?

 

[oldman570]

Re: Am I getting the most out of my engine?

As JBCurt has stated 5250 is ideal top RPM for that motor and over that can be bad. If you are turning up to 6000 and pulling skiers you might want to change the prop to one that will get the RPM down to specs.
Oldman570

 

[starkyb]

Re: Am I getting the most out of my engine?

Ok with that being said, my stainless prop has some damage on the leading edge of all three blades. if I have it reconditioned to like new condition, how much and how will it improve the performance of the motor.

 

[jbcurt00]

Re: Am I getting the most out of my engine?

I asked a few questions in my last post, that you haven't answered..... Esp @the prop diameter & pitch. And as stated before the manual for your motor lists 4750-5250 as a max rpm range. Unless those details weren't for the correct motor: 75hp 3cyl post 1986 Mariner. Three blade stainless is helpful, but.....................

It may prove to be the wrong prop. If it's spinning at 5400rpm w some damage (however slight) now, once tuned up, it should have reduced friction & drag both as it bites the water & travels through it. W/out that extra drag & friction, in theory it should be able to turn at a higher rpm, if only moderately. Since you haven't mentioned how you're reading the rpm's, I have to guess at a gauge on the dash. You're already spinning above the recommended range of the motor, so more probably wouldn't be better for the motor......

But a damaged prop does reduce performance & produces vibrations that are not conducive to the long & happy life of any part of your motor & prop. Think of a ceiling fan, it's not spinning nearly as fast, but the slightest unbalance (even just 1 dusty blade) causes it to spin funky, make strange scrapping noises. That's why many fan makers send small sticky backed lead weights w/ their fans. The consumer 'gets' the pleasure of fine tuning the fan & getting it to balance & work right so the motor will last longer..

Boat weight load (& distribution fore~aft, starboard~port), weather & water conditions all contribute to boat performance, so even if you do recon the prop, if conditions aren't exactly the same as the last time you 'tested' it, the results won't necessarily tell you how much of the change was based on the prop.

Spend some time reading thru the Merc/Mariner threads, there's lots of info available @iBoats. It is very useful & helpful. You may find some answers to your questions.

 

[starkyb]

Re: Am I getting the most out of my engine?

The boat is a Tracker Tournament V17. I use it for fishing 90% of the time and go tubing with the kids every once in a while. I wasn't able to locate the pitch or the diameter on the prop. I'm not sure where to look or if it is even on there. I would be satisfied with a gain of 4-6 mph after having the prop reconditioned as all three blades have some damage on them. I would like to but a spare aluminum prop but I'm not sure which are good quality or the best place to shop for one.

 

[CharlieB]

Re: Am I getting the most out of my engine?

Generally, the SS prop will provide better performance than and alum prop of the exact same size/pitch due to the lesser 'flex' of the blades and the 'slicker' surface of the material, at the addition cost, of course, and the increased risk of gear or shaft damage if the prop strikes something other than water.

But you are putting the cart before the horse, so to speak, without a tach to verify WOT RPM you cannot begin to make any evaluation of performance of the engine or prop.

Before even thinking of changing prop size or pitch, you need to absolutely know RPM, from there you need to verify the motor is tuned and adjusted exactly as required in the FACTORY service manual, only then are you ready to evaluate a prop.

 

[starkyb]

Re: Am I getting the most out of my engine?

Sorry I forgot to state that I have a tach on the dash that works well. At wot it runs at 5200 to 5500 rpms depending on how much weight is on the boat. Where do I find the pitch and diameter of the prop and where can I find a factory service manual?

 

[jbcurt00]

Re: Am I getting the most out of my engine?

The boat is a Tracker Tournament V17. I use it for fishing 90% of the time and go tubing with the kids every once in a while. I wasn't able to locate the pitch or the diameter on the prop. I'm not sure where to look or if it is even on there. I would be satisfied with a gain of 4-6 mph after having the prop reconditioned as all three blades have some damage on them. I would like to but a spare aluminum prop but I'm not sure which are good quality or the best place to shop for one.

As CharlieB is saying and as several, me included, have mentioned, you are over RPM on the prop now, based on your posted info & however you are measuring it. So again as Charlie has said, you need to be absolutely sure you know what RPM you're at now before making any changes to the prop. Motor's cost a lot more then either gas (wrong prop makes you run inefficient) or a new prop (if you've got the wrong one).

EDIT: You posted a reply while I was typing that ^^^

Iboats, Ebay & google are all possible sources of manuals. Search thru the motor forums, there are online manual sources posted in a few threads. Use the search function in the Merc/Mariner OB forum. Some are free & easy to download, others are not easily downloaded but easy to read & use. Others are at a cost. Avoid the Seloc manuals, they are often not good resources unless there is no others available, but it's debated as to whether it's better then nothing...

 

[jbcurt00]

Re: Am I getting the most out of my engine?

The pitch & diameter are usually stamped on the hub portion of the prop, engine side, inner face. Small stamping, often very hard to find on prop that has been in service for a bit.

There are prop manufacturer & retail sites that have prop selector functions on their websites, try a few of those. Find more then 1 and definitely try to find 1 that lets you input as much info as possible. Some include boat weight & length to find the right prop. My original 1973 Johnson 115 manual includes prop selections on it's specifications page. There were in excess of 12 choices. So even w/ that info it'd be a little trial & error to find which would work best for me & whatever boat I had this engine mounted to.

 

[starkyb]

Re: Am I getting the most out of my engine?

Thanks. I will check again. Realistically I will probably not try to buy a new prop but is it unrealistic to think I could gain a few mph if my rpms stay the same with a prop that has been reconditioned?