What does a “test” propeller do?

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Seafarer
The service manual for my ‘94 Johnson (225, TXERK) calls for use of a test prop for various procedures (such as setting max advance). These are not easy to find and even used ones on eBay are pricey. Can someone explain what a test prop does? Are there alternatives?
 
My guess is that it somehow recreates/simulates "load".
 
JJF is right. They are used to create load on the motor in a test tank without throwing water all over the place.

I don't know what size you would need for your motor, but I found a few on eBay that seem reasonably priced...



 
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I appreciate the responses but I guess I'm more interested in trying to figure out what to do if I can't get the test propeller the service manual calls for. The OMC P/N is 436080 and none of those listed on ebay are that P/N. That part number, or an alternate, is not listed for sale anywhere as far as I can tell. I hate to just skip the steps where the test prop is called for. Right now, that step is setting the maximum spark advance, where the manual calls for running the motor at 4700 rpm. It also specifically indicates the motor should not be run with a regular propeller.
 
Find a calm day and do it out on the water. Start by setting it your "best guess" and then fine tune on the water. You could always stop, make an adjustment, then run it again. Repeat.