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Mercury 40 and 44 cu-in Speed & Performance

Posted: Wed Jan 11, 2012 2:03 pm
by Mike Sea
I have noticed several posts on the performance of the 40 hp Mark 55 on a small boat. Most of the speeds quoted are in the ball park, when I compare them to my numbers. The boat I built is 13 ft long, 5.5 ft wide and weighs about 250 lbs. The design is similar to the Flying Saucer except it has a flat bottom with non-trip chines. The bottom was taken form Hal Kelly’s Jinx and can be seen at http://www.svensons.com/boats/?f=Mechan ... Jinx_3.jpg .
To date I have run 3 different Mercury motors on this boat a 1951 KG9 (25 plus hp), 1960 Merc 300 (35 hp) and a 1964 Merc 500 (50 hp). The KG9 and Merc 300 are 40 cubic inch motors and the Merc 500 is a 44 cubic inch motor. I can find no difference between the Mark 55 and Merc 300 to justify a 5 hp difference.
All the speeds quoted are GPS with a 170 lb driver steering from the rear cockpit.
The KG9 produced a speed of 38.7 mph with a 9.5 x 12 Mercury prop.
The Merc 300 produced a speed of 35.4 mph with a 10.75 x 13 Mercury prop ( I think the boat would have been a little faster with a 10.75 x 12 prop).
The Merc 500 produced a speed of 42.6 mph with a 10.5 x 15 Michigan Wheel small hub cupped prop (designed for OPC racing).

Re: Mercury 40 and 44 cu-in Speed & Performance

Posted: Wed Jan 11, 2012 2:41 pm
by upspirate
Can you post pics of the boat and also how high is the motor mounted?

(Cavitation plate in relationship to the bottom)

Re: Mercury 40 and 44 cu-in Speed & Performance

Posted: Wed Jan 11, 2012 7:58 pm
by Mike Sea
The cavitation plate was about 0.5 inches above bottom of boat for KG9, and level with the bottom of boat for the other two motors.

Re: Mercury 40 and 44 cu-in Speed & Performance

Posted: Thu Jan 12, 2012 7:38 am
by upspirate
Thanks for the pics, I like the anti-trip bottom design.

Randy will like this post when he gets back from vacation! :wink:

Re: Mercury 40 and 44 cu-in Speed & Performance

Posted: Thu Aug 08, 2013 10:46 am
by TUGMYWAY
A good KG9 was more like 30 to 32 HP. The lower 25 marking was to just piss of the Johnrude guys? :x