1973 Merc 1500

The motor in question is on a 17' Checkmate, but I have not found a Hydrostream here yet in West Michigan. I have done everything to this motor and it seems to run well. A problem has popped up with the stator that could be related but it was working earlier as I tested it when replacing the rectifier. It only runs 50-54mph @5000-5400 with a LaserII 20 or a Merc stock 21@100rpm less and no lift. It depends on the temps outside as to what it runs. The tach is calibrated, the new speedo seems to be close or 2-3mph less than other boats. Prop calculators put the speedo pretty close. I have been told it should do an easy 60mph. Others have told me Mercury rated them at the head so they are over-rated. Mercury says use a 23-25 prop for this boat, and run 4800-5800 rpm. It goes slower with a SS 17 @5700rpm and 48mph. It has 120psi compression +/- 2psi and appears to be in very good condition. The hull is mostly a vee with small trihull sides and no pad. The transom is 20" but the motor has been raised 2" so the cav plate is about 1-2" above the bottom. I have most of the interior removed and the as-new weight was under 1000lbs. I have tried larger and smaller props with less success. What should this motor do? Any help would be great as this is the second summer on this.




The performance you are describing is about average for that boat. It's been 10 years since I've seen one. I'd try to find a Yamaha drag 22 pitch or a 22 pitch 14" diameter inline chopper and see if the boat reacts. This may get you to 60. Also look at the bottom and make sure the hull is flat front to back. You are looking for hooks and valleys mainly in the last 4 feet of the hull. If you find some, fill and sand until the hull is perfectly flat front to rear. There is more speed to be had here than in any prop you'll find.

                              Get yourself the 4 ft straight edge and go for it.



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.

Mark C.


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