I have a very heavy 16.5" welded hull aluminum full windshield fishing boat. I have a j40tlenm short shaft 40 hp Johnson power trim motor. I can spin a 17p prop about 4100 rpm @ about 26 mph. I can spin a 15p prop at about 4400 rpm @ about 27mph. The 17p is an older design without any cupping. The newer 15p is a quicksilver with some cupping. I believe it's closer to 16p. These speeds and rpm are with just me in the boat, full tank of gas, and all fishing gear. If I add 1-2 people it's slower to plane and speed is down 2-3 mph. So I bought a set of 48hp big bore carbs and a set of boyesen reeds. I completely rebuilt the carbs and installed them along with the reeds. I have only ran my 15p (probably closer to 16p cupped) prop. I have gained about 3 mph and about 400 rpm on top. I now blow out the 15p prop out of the hole and run about 30 mph. If I add 1 person speed is down about 1 mph not 2-3. I thought that I would gain more rpm on top but I believe the 50hp exhaust tuner is different. I'm sure port timing is higher on the 50hp block to rev higher as well. I now have copious torque out of the hole and in the mid range. Before If I was cruzing say 3700 rpm and hit the throttle it would slowly run up to max. Now at 3700 rpm it runs up to max rpm almost instantly. I still have to play with the idle air jet as the 48hp carbs have 29/31 as I believe the carbs were from a long shaft. I will install the 33/35 idle air jets from my 40 hp carbs next trip out. Idle is rich but if I try to go even 1/8 turn lower on the screw it gets lean fast. Over all I'm very happy with the upgrade. I believe I probably have about 45-48 hp with loads of torque due to the low port timing. I have less then 200.00 into the project with ebay used carbs, ebay omc brand carb rebuild kits and the boyesen reeds. Here are some pictures of the build.
