[adnet99]

2000 Mercury 115 overheats when pulling tubes or running over 3500rpm. The impeller, Popet Valve, Thermostat have been changed as well as cleaning out crud in the water lines. It runs great with lots of water pissing but the overheat alarm goes off when under load or high RPMs. Slow down to idle and the alarm shuts off fairly quickly. Any suggestions?

[j_martin]

Is it actually overheating, or is the alarm sensor too sensitive. If you can comfortably hold your hand on the head for a few seconds, it's pretty cool. If the alarm goes off, you should just about stick to it. (not recommended)

If it is overheating, you might have small passages in the head, exhaust chest area, and base plate plugged up with splinters or pieces of water pump. About all you can do is pull parts off till you find it.

A water pressure gauge would give some usefull information.

hope it helps
John

[adnet99]

Thanks. I had a local mechanic check for blockage as well as do all the other work at a tune of $800+ and it still has the same problem. He said that the pipe extension (water tube that is used to make it a long shaft) is plastic instead of copper and that may be it. But after spending so much and them keeping it for a month I just brought it home.

There seems to be a connection with the additional load of pulling a tube float or higher speeds. Pulled a skier for over 5 miles at 3300RPM with no alarm. But pulling a tube (much more drag) at 3000RPM will cause the alarm to go off.

[j_martin]

Is it possible that you're overpropped? What is your WOT rpm with a normal load. If your boat is fairly big, it might be worth while to use a lower pitch prop for tubing and skiing.

I have a 150 hp on a Tracker bass boat that I use for family play sometimes. I use a different prop for skiing and fishing. I go down 2 inches for skiing.

Mercury's don't like to lug. Not only can it make them overheat, but it greatly increases stress and wear. Yer engine will live a lot longer and happier overrevving than lugging. Right on, of course, is the target.

[adnet99]

The boat (19’ Seapro) and motor are 2000 and I just bought them a couple of months ago. When I took the boat on a test run it ran great and didn’t overheat. However, the water was very cold (winter time). When I started using the boat (warmer weather) it started overheating or at least that is when the alarm started going off. The water coming out of the piss hole was weak and at higher RPMs would dwindle to nothing. That is when I had all that work done. The impellor (old one) seemed OK but I had it changed anyway. The pissing was better but still had the higher RPM problem.

From the previous owner’s paperwork the motor and boat were bought together. As for revved RPMs I have only gotten it up to 4800 before the alarm went off and then I had to back it down.

It also seems more prevalent when I fill the 57 gallon fuel tank and/or have a boat load of people.

[CharlieB]

4 cyl 115?

WOT 5250, any less and it begins to heat.

I have a 4 blade 22P that lugs mine right at 5250 full tank and running alone, anyone else or pulling tubes I switch to my 3 blade 23P, loaded it still turns 5400, alone 6100, I lose almost 7 MPH with the 3 blade but man it WILL come out of the hole.

[j_martin]

There are many things that can cause a motor to overheat. Running lean, lugging, water pressure problems, water delivery problems, water exit problems, timing.

It looks like you might be lugging it. I run an XR4, which is one screaming v6 on an 18 foot bass bote, and I change props for skiing and family stuff where top end isn't important, but it'll be pulling quite a bit. I go down from 24 to 22 pitch for the pulling. Running empty, you should be up to 6000, and see 5000 fairly quickly at full throttle.

If that isn't it, the next thing I'd do is install a water pressure gauge to get some more information. I believe there's a pipe plug near the thermostat where you could hook one up, and they aren't very expensive.

hope it helps
John

[adnet99]

I have a three-blade prop but the boat is in the water so I can’t tell the size or pitch. I plan on getting a water pressure gauge. Thanks much for you rhelp.

[adnet99]

Any other thoughts or suggestions?
Thanks in advance

[CharlieB]

V6's will turn higher rpm than the I-4 however, ideally you should turn 5250 to 5500 WOT, any less and you are over-propped, which will cause excessive engine loading even at 3500 and create more heating.

Original Merc prop should have ID on the outside of the hub, it may be awkward but trimmed all up the prop should be out of the water and readable, careful you don't go swimming unless you want too!

With a 2.07 gearcase you should be right near a 3 blade 22 pitch, maybe less depending on the weight of your boat and how you load it.

[adnet99]

Is there a chart somewhere that can tell me what prop is best for my boat/motor?

I bought this boat used and I suspect that that prop was changed prior to selling. I took it for a short run prior to buying it but didn't have the overheating problem until I got it home, filled full of gas and loaded it with kids.

Also I noticed that when pulling just one kid on a tube that there is a difference if the kid pushes his toes in the water vs keeping them out. One would think that something so little wouldn't have much effect but the kid dragging his feet at 3300rpm caused the engine to overheat again.

So, I looked at my prop and it appears to be rather old and painted over so I couldn't see the size. It is a three blade but definitely could be the wrong size.

So how do I determine the right size?

Use
Tubing / skiing = 50% of time
Running / fishing = 50% of time

I would rather be slow and able to pull tubes than go fast and overheat with tubes.

[j_martin]

That rig's heavy, and the engine fairly small for it. Figuring 40 mph, 2.00:1 gear ratio in the lower, 5500 rpm, 15% slip, it comes out about 18 pitch prop.

That might be on the high side.

Here's the calculator, go figure. <grin>

http://www.rbbi.com/folders/prop/propcalc.htm

hope it helps
John

[adnet99]

You guys are awsome...changed props and everything is super.

old prop = 13x19p
new = 13.7x15p

Boat runs like a young stud. Mega thanks for all your help.