Tohatsu 6HP, hot exhaust

[MattFL]

I've got a 2012 model year Tohatsu 6HP with 54 hours on it. The water pump and thermostat in it now were installed about 1 year ago and have about 10 hours on them. The motor runs great, but the plate on the bottom of the motor that must be where the exhaust runs, gets HOT. We just started using the motor in salt and traveled about 20 miles this weekend at 3/4 to full throttle, and when we got back that plate on the bottom of the motor was white with a thick layer of salt, where the water spray had steamed away leaving only the salt behind. Is it normal for this plate to get so hot on this motor? It has no other signs of overheating, runs great and has good water pressure.

 

[pvanv]

What is your actuall model? Did you use an IR thermometer on the power head to verify that it is running around 150F? The bottom of the block has the oil pump cover. The exhaust cover plate is on the top of the lower casting.

 

[MattFL]

The model number is MFS6CDS. I didn't have a thermometer with me, but the plate on the bottom is too hot to touch when running fast. At idle it's warm but not super hot, and it heats up almost immediately when you open the throttle which is what made me think it's the exhaust. I uploaded a picture below, the arrow points to the part that gets super hot.

 

[Sea Rider]

If there's no internal water passage undergoing a diam reduction and consequently heating up more than it should due to salt layers, crust whichever, that part always gets very hot as deals very near with hot gasses exiting from the exhaust system. My 5 HP horse does about same when running for long periods at say 3/4 throttle, guess it's the nature of the beast. As long the powerhead doesn't overheat and smoke, all will be fine.

If peace of mind is what you're after, run the motor at fast idle on a barrel with Salt Away Flushing Product for some time and check with a new water run if the heating issue has lessened to a lower temp than previous. Do you count with the Tohatsu flushing adapter ?

Happy Boating

 

[pvanv]

That is the exhaust port going into the driveshaft housing. It will get hot at speed. Normal.

 

[MattFL]

Thanks guys for the feedback. Sea Rider in this case it's definitely not salt build up. Just over 50 hours on the motor since new, and it saw salt for the first time about 2 weeks ago, prior to that it was fresh water only.

 

[Sea Rider]

Thanks guys for the feedback. Sea Rider in this case it's definitely not salt build up. Just over 50 hours on the motor since new, and it saw salt for the first time about 2 weeks ago, prior to that it was fresh water only.

With fresh water use there's nothing that can visibly be seen adhered to the lower powerhead, while with salt water yes as experienced. Other issue to account for would be If the motor was running at wot towards the min lugging rpm range, imposible to tell without a tach installed and a wot run....

Probably don't know, but all Tohatsu motors delivered with a prop are way over propped, one less pitch from current one will benefit to achieve a much better water performance...

Happy Boating

 

[MattFL]

I've got an hour meter tach on mine that remembers highest RPM. Typically it comes back around 5600. Last trip out we pick up some weeds and aerated the prop and hit the rev limiter, the highest recorded RPM was 6318 so the rev limiter must come in somewhere around 6300. We use it on a small 13' jon boat, it tops out around 17mph and hits the rev limiter with just me in it, or about 13.5mph with me, my son and our fishing gear. So for my particular use I think it's not too bad, but it would definitely be too much prop for a bigger boat like a sail boat or big inflatable, etc..

 

[Sea Rider]

Conclusion : the exhaust port going into the driveshaft housing will get hotter than any other area around the middle leg, it's the nature of the beast. Just curious, is the motor running with the OEM factory delivered prop or you have changed it for a lower pitch one ? Excellent to count with a tach to know what's going on with the motor at high revs.

Happy Boating

 

[MattFL]

It's the factory OEM prop. The boat probably weighs 100# and is completely flat bottom, not much to it. This is it shortly after I got the motor, not full speed:

https://youtu.be/zI2v81l89mU