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Running on the muffs >1500rpm

troym

troym

Advanced Contributor
Was curious about the basis behind the precaution of not running the engine at >1500rpm when using the garden hose and muffs for cooling.

The only thing I've found in the books is that the suction of the outdrive water pump (impeller) at higher rpms could collapse the garden hose, restricting the water supply.

I usually use my well pump when I run the engine in the driveway. The well delivers 50-60psi and 25gpm at the discharge. I've never seen any indication of the hose collapsing. I don't make it a habit of running the engine at more than idle speed, but have revved it a bit from time to time.
Even at higher rpms, there has been sufficient flow to force water out around the muffs and supply the engine, as indicated by water issuing from the idle ports and out through the prop hub, and normal running temps.

Just wondering if there are any other concerns.
 
No other concerns from me. I too have around 60-70 psi from the hose.

But what purpose would it serve to run above 1500 rpms on the trailer ? And the drive you have it all the way down?

I read many times people reving above 5000 rpms they are just plain crazy.
 
No real reason other than to make sure everything is as it should be after working on it. I'll go to maybe 2500 to 3000 tops... with the drive all the way down... always. I suppose the same level of assurance can be attained without revving it, but it sounds soooo good!
 
As long as the garage door is open and there's cold beer in my garage fridge, I don't get any complaints.
 
I'm thinking Merc may be skirting an issue of the plastic guide tube for the water pump copper line melting. using suction collapse as an excuse
I've seen so many of those melted into a ball... has to happen on muffs... can't really melt plastic under water.
 
I have run it for a second at 2000-2500 just to clear the carbon out. ( wink )

But i got a stringer no tube another good reason to have a stringer HAH.
 
on plane it`s not under water ,hy.The reason is if the pump sucks more than the hose can supply it will flatten the hose cutting off the water untill the pump burns up and water pressure not the pump is cooling the motor.If you use the "splash test" your safe. Turn on the water and look at what is splashing out with the motor off.
Start the motor and 90% should disappear. Raise the idle untill it all stops splashing and thats your max rpm limit.
 
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