How Fast am I Going?

Randy, 

I have a 1987 Vegas XT with a Johnson Venom 225 - low water pickup engine turning a 28 pitch prop. Seems that the speedometer stops working around 60. The pickup is mounted in the center just right of the drain plug. I know that some engines have the pickup in the lower unit of the motor but this one does not.  Is there something else folks are doing to get somewhat of a reading on speed?  

Thanks

Jerry

 

Jerry, 

Get the pickup and move it at least 10 inches from the centerline. It needs to be away from the propwash. You could use this formula for speed. Take engine rpm X gear ratio (in decimal ) X the prop pitch ( unless you have a pitch gauge this is a guess based on what it should be or is stamped) and divide by 1056. This is your theoretical speed with no slip. Multiply by .9 for 10% slip and your pretty close. Remember you need slip or you have no thrust so 10% is pretty close. I've heard of the low slip numbers but they don't seem to know the real pitch of the prop. I don't take too much stock in peoples numbers like that. I say slip is good if it is not too excessive. For your boat let's use the engine at 5500 X .54 the gear ratio X 28 for the prop and divide by 1056. The answer is : 78.75mph. Now take that times .9 and we get: 70.875mph. That is about right for that rig turning 5500. I suspect you are turning 6000ish but you can see how the figures work. 

                                                              Randy

 

 

Please submit whatever questions you have to IHRTechTalk@yahoo.com.  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Thanks,
Mark C.

 

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