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  Outboard revs are limited...

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Author Topic:   Outboard revs are limited...
whalerron posted 09-08-2003 11:06 PM ET (US)   Profile for whalerron   Send Email to whalerron  
I was cruising at about 40 mph on the Delaware Bay yesterday in my 16' 7" Whaler with a 90 hp Johnson on the back. About 12 miles from shore, the motor dropped down to 4000 rpms and would not accelerate beyond that. It almost felt like I had run through a batch of seaweed. We stopped the boat and looked back at the prop. There was nothing on the prop and nothing wrapped around the prop. Weird! I started off again and the motor behaved the same. We stopped again and still couldn't see anything on the prop. So, I raised the motor to have a look at things. Lo-an-behold, there was an old mossy milk jug folded over the front of the gearcase on the lower unit. The jug had been split from top to bottom and it was folded evenly over the lower unit. After removing that, the motor ran normal again.

I had to laugh. What are the chances of hitting a plastic milk jug in the middle of the Delaware Bay at 40 mph in just such a way that the bottle becomes wrapped around the leading edge of the lower unit? Maybe I should start playing the lottery?

I am still wondering why the bottle caused the rpms to be limited. I wonder if it was an issue with drag?

- ron

Peter posted 09-09-2003 07:46 AM ET (US)     Profile for Peter  Send Email to Peter     
That is something. It is most likely drag that limited the engine speed. It doesn't take much.

Reminds me of an event many years ago as a kid. I was out with a friend on his 13 Sport powered by an Evinrude 35 and we were moving along at WOT in open water when all of a sudden the engine kicked up as though it struck an underwater object, thorougly spraying us with water until he was able back off the throttle. Turns out we ran over an aluminum can that bent itself perfectly around the lower unit between the gear housing and the anti-cavitation plate thereby causing enough drag to pop the motor up.

Tom2697 posted 09-09-2003 10:25 AM ET (US)     Profile for Tom2697  Send Email to Tom2697     
You don't state what year motor you have...but, many newer motors have rev limiters that will limit the rpms to 3000 or 4000 rpm if the engine gets too hot. Sounds like that milk jug was preventing good water flow to your intake. Some motors will shut down one bank of cylinders, or alternating cylinders, and then switch once these get too hot. This is to allow cool water to continue to circulate through the motor and to allow you to still get home. The first time I experienced this was on a '90 Yamaha Super Jet (jetski) so the technology is not really new.

As for playing the lottery with the luck you had, doesn't sound to me as though your luck was good!

Peter posted 09-09-2003 10:34 AM ET (US)     Profile for Peter  Send Email to Peter     
Most overheat engine speed governors knock the engine speed back to 2000 rpm or below.
brisboats posted 09-11-2003 05:47 PM ET (US)     Profile for brisboats  Send Email to brisboats     
.....and would be accompanied by the overheat alarm. There is a reason racers put a nosecone on the bullet its all about drag. I think that milk jug would prevent thre prop from getting clean water also.

Brian

whalerron posted 09-11-2003 11:15 PM ET (US)     Profile for whalerron  Send Email to whalerron     
I thought about the jug and how it could be interrupting the waterflow. But, if the jug was disturbing the flow and aerating the water, wouldn't the motor have a tendency to overrev instead of underrev?

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