loss of speed with the cowl on

[bigtwin]

I just purchased a 10 1/8" x 15" ballistic prop. for <br />my 98 25hp 2cyl. Evinrude. During testing the motor was turning 5440rpm at 31.5mph (gps) 13% slip. 15' semivee hull alum. fishing boat with full floor, live well, trolling motor, two batties, and 350lbs of fuel, gear and people. I do not have a tach. in my boat so I use a d.v.o.m. (digital volt ohm meter)with an inductive pickup clamped around on of the plug wires and set on 2 cycle mode. The inductive pickup is rather large so I have to test with the cowl removed, however when I put the cowl back on I loose about 1.5-2 mph that calculates to a 200-300 rpm drop. I assume that the engine is not getting enough air with the cowl on. any one know of a way to increase the air flow to my motor without excessive noise, or the worry of water intrusion.

 

[cobra 3.0]

Re: loss of speed with the cowl on

I'm not sure about your "cowl problem" or if this is related to it, but a 13" pitch prop would in my opinion be better for your application. I had a similar set up with a 30hp Johnson 15.5' Alumininum Espadon with floor, etc. and the 15" pitch worked, but not as well as a 13" pitch. The 15" pitch will be very weight sensitive, while the 13" pitch won't.<br /><br />When you tested with the cowl off and cowl on, did you have the same weight in the boat?

 

[bigtwin]

Re: loss of speed with the cowl on

Cowl is another tem for the motor cover, and I have the same amount of weight in the boat.

 

[walleyehed]

Re: loss of speed with the cowl on

You have an exhaust leak causing the drop in RPM with the cowl on. The cowl vents plenty for needed air for normal running conditions, but exhaust contains no O2, so it slows the engine..

 

[tashasdaddy]

Re: loss of speed with the cowl on

agree with walleyehed. give me clean air.