Propping and overheating- your thoughts?

[ufm82]

I will post this here due to an issue I've been having that isn't making sense.
I have a 1993 Merc 150 on a 19' ProLine center console. I run a 19 Tempest Plus for the best all around performance. I've been having a problem as of late and I wonder if I am cauing it by being overpropped.
When towing a tube, I cannot run in the midrange without having an overheat alarm go off. Idle to 2500 rpm is fine and over 4,000 is fine but you can't tow a tube that fast. So, 3,000 is the ideal rpm for me. However, the temp gauge quickly shoots up and the alarm goes off at that range. I don't have any handling issues, the prop never blows out or slips, the engine runs strong. Just overheats.
I have a 13 pitch Mirage that I have run in the past but have not put it on this year as it will over rev the engine at WOT. But it gives me awesome pulling power for tubes and is likely easier on the engine.
My question- can the 19 be causing the overheat? T-stats are good, poppet valve is good, water pump was done this year, water jackets are clear and it pees fine. All seems fine until I pull a tube. Your thoughts?

UFM82

 

[cshyde]

Re: Propping and overheating- your thoughts?

You may be lugging the engine in that RPM range. I would put the other prop on and see if you still have overheating issues. If you do then water pump and thermostat may need to be looked at.

 

[steelespike]

Re: Propping and overheating- your thoughts?

What is the wot rpm? I would wonder if the 3000 creates a condition that starves the motor for water. Have you played with the trim to see if it changes anything?
Unless the motor is right on the temperature edge I think it wouldn't be likely that
lugging would cause an almost instant over heat.

 

[ufm82]

Re: Propping and overheating- your thoughts?

Trimmed full down- no reason to trim up at that speed. It's always been my understanding that the water pump provides water until water pressure into the inlets overcomes the poppet valve and opens it, allowing full flow through the engine. I'm guessing that at that rpm, the pump isn't getting it done but the poppet isn't open because I'm not going that fast. (Maybe 15-20 mph) Shouldn't the pump keep up there?
I haven't swapped the props yet- it's not had this issue before. I'll be interested to see what happens- if my rpms go up due to the lower pitch, my speed will still be the same and the issue should be worse I would think. I'm puzzled.
I did the poppet last year after some sand clogged it and assume it's working properly but maybe I need to drop the lower cowlings and pull it out to check. More work and I feel like I'm chasing a ghost.

UFM82

 

[steelespike]

Re: Propping and overheating- your thoughts?

I was thinking some wierd didturbance in the water that a slight trim out might
change.If it is the rpm and the pump it would seem you could force it with the smaller prop by playing with the rpm just to see if it will do it.Run the motor at the same driveshaft speed that appears to cause it with the19.