Prop Size, RPMs, & overheating
 

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.

Re: Prop Size, RPMs, & overheating
 

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?

Re: Prop Size, RPMs, & overheating
 

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.

Re: Prop Size, RPMs, & overheating
 

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.

Re: Prop Size, RPMs, & overheating
 

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.

Re: Prop Size, RPMs, & overheating
 

do you check the engine out after you purchased it?

new t-stats, water pump, basics?

start from scratch

Re: Prop Size, RPMs, & overheating
 

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.

Re: Prop Size, RPMs, & overheating
 

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.
http://www.biggerhammer.net/mercury/...5_blizzard.jpg

Re: Prop Size, RPMs, & overheating
 

Thanks Bigshrimpin, very educational.

Re: Prop Size, RPMs, & overheating
 

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

Re: Prop Size, RPMs, & overheating
 

I'm not too worried about damage except for from the first time it happened. Since then I've bene wathcing the temp guage closely and you can watch it rise till the revs are limited and then see the temps fall right away as the revs fall.

That's why I didn't think it was the pump at first. The pisser puts out a very strong stream and it cools down very quickly as the RPMS are dropped. If the pump was the problem it shouldn't cool right? I don't know but the pump is the last thing I can thing to try unless it's just the temp sender. But, I would think it's a seperate sender for the guage and the computer anyway so that shouldn't be it anyway (unless I'm wrong again).

That article is great. I printed it out to use for next time but I've already lined up a mechanic that will come to the house to take care of things once I get the boat out of the water this weekend. I wanted to get on this guys customer list anyway so this works out well in the long run.

For anyone else changing poppet valves after talking to the mechanic, he said I should have used a very tiny amount of sealant even with the cork gaskets and you don't need to torque the cover down too much. I of course kept tightening trying until the gasket bulged out the sides while trying to get rid of the leak. Also, regarding the thermostats, sealant is your choice. I think the sloppy application of too sealant much caused a small leak in my old stats that corroded the spark plugs as it ran down the cylinder heads. On the other hand, on my replacements, I didn't use any sealant and I had to redo one of them to add a little sealant to close up another small leak.

So, that's where I stand at this point. I'll keep you posted and thanks again for all the advice.