Prop Size, RPMs, & overheating

jongolds · #1 06-05-2006, 12:32 AM

I've finally got some time in my new to me 73 Tsunami with a 96 Merc 225. It's got a 15 1/4" x 15" SS prop. RPMs top out at 5k and prolonged running at 4k causes it to overheat.

I'm still working on getting a manual so I'm not certain but I think I should be topping out at a higher RPM. What do all think? Also, could running with to much pitch cause you to overheat? I've never heard of that before but intuitively it kind of makes sense to me.

I havne't changed the impeller yet. I know I should but I was told the previous owner had done it before selling it last fall so I was hoping to be able to cut that corner this seasons. I know that's not the smartest thing but there is only so much time in the day and I'd rather spend it on the boat than working on it.

I'll need top get a new prop to have a spare anyway so I'm thinking about what to try. I'm open to the idea of a 4 blade too.

gss036 · #2 06-05-2006, 01:33 AM

5000 would be at the low end of the WOT rpms. Top end should be somewhere around 5400/5500. I run a 14 1/2 X 17 stainless Merc prop (some cup) and get around 5400 top end on my 89 200 Merc. I don't think you would be lugging at 5000. You say you are overheating at 4000, what temperature are you considering overheating (degrees)? I cruise at 42/4300 for 15-18 miles w/o any rise in temp. Idleing you might get up to 180, but once you are over 2300/2500, the poppet valve should open and cool back down and stay there.
The 1st thing I would check are the thermostats, maybe even change them as they are only about $10 each. They should open at 143 degrees, if not you could be restricting the water flow. The 2nd thing would be the poppet valve assy. Do you have a water pressure guage? If so what is your idle/WOT pressure? That would be a good indicator of the impeller. My Merc factory manual says mine should 12 psi, but engine models differ.
Are EFI or carbed?

jongolds · #3 06-05-2006, 10:21 AM

I'm surprised. I was guessing WOT rpms should have been closer to 6k. My old boat's I/O had a WOT range in the area of 5,500 and I thought outboards were higher.

Unfortunately, I have no water pressure guage and no numbers on the temp guage. The pee hole seems to flow alright at idle. The temp guage just goes from C to H. No numbers. It's normally (in my limited use) run about 1/4 of the way up off of C. When it was over heating, it ran all the way over to H. Bringing the throttle back down dropped the temps right away.

I was really hoping that being overpropped might be the cause of the overheating. It doesn't sound like my pitch is too far off though. I'm not thrilled with the low 30s top speed either. I was expecting more. Another benefit of it being the prop would have been that I could run it carefully as is for another couple of weeks with out worrying about damaging anything and then just changed the prop in shallow water once it warms up. Doing the thermostat or pump will all require pulling the boat out. That means waiting for high tide to pull it out and to put it back in.

CHANCE1234 · #4 06-05-2006, 05:19 PM

I would guess, thermostat as well, sometimes they get clogged with sand and seaweed. The way to best check to make sure they are opening at the right temp is to remove them from the engine, and put them in a pot of water at about medium temp. With a cooking thermometer monitor the temp and around 140 or so check them to see if they open. you will have to take them out of the water momentarily to check the gap. Hopefully this helps.

eggsuckindog · #5 06-09-2006, 12:07 AM

I would also be concerned that you only get 5K out of a 225 with a 15" prop, that thing should swing a 17 easy.

Trayder · #6 06-09-2006, 03:46 PM

do you check the engine out after you purchased it?

new t-stats, water pump, basics?

start from scratch

jongolds · #7 06-12-2006, 12:10 PM

The prop will get about 5800 rpm. I was hitting a wall at 5k because that's when the computer would start dropping RPMS due to the over heating problem. From a cold start I was able to get them up higher.

I've changed the thermostats & poppet valve (still not sure what this does). Haven't done the pump/impeller yet but it's looking like that's gonna be next. There's not much more I can do while it's in the water and I can't get it out until high tide late Sunday afternoon.

Right now I'm selling the bunk trailer that I have for it to get something with brakes on both axles. Considering that I don't trailer much, and that everyone says the bunks are better for the hulls, I was planning to stick with the bunks but I'm not sure now. In the event something happens and I need to get it out of the water on short notice, I don't want to be worrying about the tide.

Bigshrimpin · #8 06-12-2006, 02:34 PM

Poppet - is a spring loaded water pressure relief valve.
This is a nice little diagram of merc's cooling system. I don't think the cooling on the 225 is much different.

jongolds · #9 06-13-2006, 02:00 PM

Thanks Bigshrimpin, very educational.

gss036 · #10 06-13-2006, 10:20 PM

With the new poppet assy and stats not fixing the problem, I would suspect the impeller also, maybe not reassembled correct, or bad base gasket,pilot tube and gasket not sealing or blockage (from old impeller coming apart?) in water tube going into the block. If you are getting hot enough for it to slow down, you are asking for trouble.Take a look at this article w/pictures. A little dish soap makes the impeller go in easier.
http://www.in-depthangling.com/forum...5/o/93/fpart/1