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Author Topic:   new OMC ss prop with vent holes
simonmeridew posted 07-13-2001 10:27 PM ET (US)   Profile for simonmeridew   Send Email to simonmeridew  
I just got and installed a new ss prop for my Johnson 70 because I found a new part of the bottom with my aluminum prop 2 weeks ago. The directions were poor from Michigan Wheel regarding the vent holes on the hub: there are 3 of them, and they supplied 3 plastic cones which I assume I'm supposed to plug the holes to enhance or alter performance. What gives? The directions only say that the holes may not work in every application. Any suggestions? 1984 Montauk, fishing and cruising, salt and fresh. Thanks
Dick posted 07-13-2001 10:47 PM ET (US)     Profile for Dick  Send Email to Dick     
In my experience the best thing you can do is experiment and find out what works for you. Start with the holes plugged and try all options untill you get the best performance.
lhg posted 07-13-2001 10:52 PM ET (US)     Profile for lhg    
You must have a Michigan Wheel "Rapture" model. The vents allow exhaust gases to escape while accelerating, introducing air bubbles in front of the blades, giving them some slippage to more quickly bring up the RPM range of the engine, resulting in quicker hole shot. Once the boat is running 20MPH or so, the flow of water over the holes seals them and the prop operates in non-aerated water.

A boat will plane at a slower speed with the vents plugged, or at least a lower RPM range, but will not accelerate as quite as fast. Vents are only used with aggressive high rake props, because they initially take a bigger "bite" on the water at slower speeds.

My recommendation would be to run the prop with the vents open, and see how you like it.
Remember with a prop like a Rapture, your engine should be up at least one bolt hole above dead low. If you have too much slippage, then insert the vent plugs.

The "Rapture" should be a good prop for your engine. You will notice an increase in overall performance and top speed, with increased bow lift providing a smoother ride.

vdbgroup posted 07-15-2001 12:06 AM ET (US)     Profile for vdbgroup  Send Email to vdbgroup     
I switched to a Rapture last fall on my Montauk w/ 90 v4 Johnson 13x17. Noticed much better gas mileage, but probably due to the original prop losing about an 1" of diameter after several reworks. I use a manual Jack Plate to fine tune engine height, I highly recommend you get the engine up as high as possible without slipping the prop. Vent holes wide open.

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