Can someone help me out.

[ryanr623]

Can someone tell me what the optimal WOT RPM's for a mid 80's mercruiser 120 is?

At WOT my boat is at 4000 rpm's. Its has been totally rebuilt from the ground up 3 years ago (with paperwork). Have never had any problems with it. Purrs like a kitten and accelerates QUICKLY. But the guy who had it before me has an 18 pitch prop on there for tubing and skiing. Im thinking since i will rarely be doing that out on lake erie, i should get the optimal prop for it. So i need to know what rpms i should be running at WOT.

Or if anyone can chime in on what prop they are running with their 16-18 foot mercruiser 120 boat, i'd appreciate it!

 

[Philster]

Re: Can someone help me out.

4,000 seems low for any motor. I believe its max range is 4400 to 4800.

If she is completely healthy engine-wise, geared properly and the hull is fit and not waterlogged, then she might need a smaller prop. You have to know for sure that the throttle is really fully open and that all things are working from the fuel and electrical system to the meat of the engine. If so, drop down in prop pitch one step.

Most people don't go up in size to pull skiers/tubes, etc. So, don't over-guess the issue.

.

 

[ryanr623]

Re: Can someone help me out.

4,000 seems low for any motor. I believe its max range is 4400 to 4800.

If she is completely healthy engine-wise, geared properly and the hull is fit and not waterlogged, then she might need a smaller prop. You have to know for sure that the throttle is really fully open and that all things are working from the fuel and electrical system to the meat of the engine. If so, drop down in prop pitch one step.

Most people don't go up in size to pull skiers/tubes, etc. So, don't over-guess the issue.

.

Will i lose top end speed dropping down a pitch? Seems like if i raise RPM's i should get more top end, even though it is a lower piptch, since my current prop only tops out at 4000 rpms.

 

[Philster]

Re: Can someone help me out.

The engine RPM should climb, and if it climbs enough you should see a gain in top speed. 1-3 MPH.

The important thing is that you get into the max operating RPM range. Currently, the engine is lugging... and that is bad. You seem far enough from the max RPM range that the engine is under great stress to spin more freely. Not good on the internals.

This assumes all my points from earlier about mechanical and hull health AND assumes you are trimming up/out when building speed. If the drive is all the way down/in, she won't run up to full RPM at WOT.

 

[ryanr623]

Re: Can someone help me out.

The engine RPM should climb, and if it climbs enough you should see a gain in top speed. 1-3 MPH.

The important thing is that you get into the max operating RPM range. Currently, the engine is lugging... and that is bad. You seem far enough from the max RPM range that the engine is under great stress to spin more freely. Not good on the internals.

This assumes all my points from earlier about mechanical and hull health AND assumes you are trimming up/out when building speed. If the drive is all the way down/in, she won't run up to full RPM at WOT.

Thanks for all your help. I am probably not trimming it right. It doesn't have an in/out button.

Sorry to ask, but any tips on how to trim it out properly? i usually just hop in the boat, put the trim down to where it looks even..... and go. Never touch it again till back at the dock when i put it up.

 

[45Auto]

Re: Can someone help me out.

The 120's don't spin as fast as most boat motors, recommended WOT is 4200-4600 RPM. I wouldn't trust what anybody says on the internet, Factory Manual #10 has all the info for your motor, you can download it here and it'll tell you everything you need to know:

http://forums.iboats.com/showthread.php?p=1693398#post 1693398

Sorry to ask, but any tips on how to trim it out properly? i usually just hop in the boat, put the trim down to where it looks even..... and go. Never touch it again till back at the dock when i put it up.

You really need a trim button to optimize your boat's performance.

Put the trim all the way DOWN, then give it one quick bump of UP so it's not bottomed out against the stops (makes it a little smoother). Hit the gas and as it comes on plane start bumping the trim UP. When you're at the speed you want to be, bump the trim UP until the nose of the boat starts going up and down, which is called "porpoising". Then bump it DOWN once or twice until it stops porpoising. That is the most efficient trim for your boat at that speed. Every different speed will require a slightly different trim for optimum performance. Just bump it UP till it starts porpoising, then bump it DOWN till it stops.

I had a 1982 15' Mark Twain with a 120. Tried every prop between a 15" and 23". Didn't make any difference at all in speed. It went 34 MPH at 4600 RPM with the 15" pitch and 34 MPH at 3400 RPM with the 23". It planed about twice as quick with the 15" as it did with the 23".

 

[ryanr623]

Re: Can someone help me out.

The 120's don't spin as fast as most boat motors, recommended WOT is 4200-4600 RPM. I wouldn't trust what anybody says on the internet, Factory Manual #10 has all the info for your motor, you can download it here and it'll tell you everything you need to know:

http://forums.iboats.com/showthread.php?p=1693398#post 1693398



Put the trim all the way DOWN, then give it one quick bump of UP so it's not bottomed out against the stops (makes it a little smoother). Hit the gas and as it comes on plane start bumping the trim UP. When you're at the speed you want to be, bump the trim UP until the nose of the boat starts going up and down, which is called "porpoising". Then bump it DOWN once or twice until it stops porpoising. That is the most efficient trim for your boat at that speed. Every different speed will require a slightly different trim for optimum performance. Just bump it UP till it starts porpoising, then bump it DOWN till it stops.

Awesome, thanks soooo much!!!!. I boat in lake erie so theres always waves. So i will never know if i am porpoising or not. Any advice there? And do you always trim even while cruising, most of the time i am going about 20 due to 1-3 foot waves...

 

[Philster]

Re: Can someone help me out.

Trim it up a little when cruising... and trim up a little further it until it feels right. All the way down is for getting on plane (more or less).

When you trim up, you'll actually feel/see revs climb, so then you back down the throttle and are cruising with less stress on the engine.

Porpoising is different bumping over waves. You'll know it. Takes practice to figure our where the trim should be at various speeds/conditions.