BAD BEHAVIOR wrote:Proptech has sent me a new four blade ss 20" prop. I adjusted my motor from being mounted in the top holes in the bracket to the third holes. That gives me ten inches of vertical lift with the jack plate all the way up. This allows me to idle in 1.5 ft of water without trimming my motor up(which causes the back of the boat to dig). I have not run the new prop as it just got here today. (finally) Here's my dilemna. The adjustments I have made thus far gave me great shallow water capabilities (especially for a boat as big as mine) . However , Ive lost about 8 mph off the top end. Now my top end is only around 44-46mph. I really dont care about the number but I ve got a lot of time and money in my rig and I want to be sure I m getting top performance out of her. Should the new prop give me the missing speed ( thats what proptech said)? Will the new prop increase my hole shot? Ive lost a little of my response time(throttle to acceleration) because of the adjustments. What is it I can do to get the extra speed I lost? I am currently running a 3 blade merc factory prop,19". With the adjustments, I can only throttle about 3/4 without over revving the motor. I know first response that its blowin out because of the height but thats not it, the foot is still completely submerged. I miss my speed when its slick calm!!!!! Help will be appreciated!!!
ps Any first hand 3 blade 4 blade knowledge is what Im after!!
If the only adjustment you've made is the height of the motor, Your prop is slipping.
Several questions:
1-Do you have a water pressure gage on the boat?
2-Do you have a low water pickup?
3-What jack plate do you have?
4- What is your jack plate offset? (4"-6"-8"-10")
5-Which Mercury factory 3 blade are you running?
5- are you looking at a speedometer or a GPS to get your speed numbers?
6-Is the loss of top end and higher RPM with the jack plate all the way down?
The first step in setting a boat up is to know your starting point. You've already made an adjustment, but you can still get the information you need. With the boat on the trailer, lower the jack plate all the way down. Adjust your trim until the cavitation plate is parralell with the bottom of the boat. I do this by using a 4 foot level. Use your tongue jack to level the keel. After the keel is level, use your level, while adjusting the trim, to level the cavitation plate. With this done, hold the level on the keel and slide it back to the front of the lower unit. Mark the lower unit where the top of the level intersects the leading edge of the lower unit. measure from the mark to the center of the lower unit gear housing, or the center of the prop shaft. Make note of that measurement. Nothing is set in stone when setting up a boat. But, depending on what setback your jack plate is, that measurement will give you a starting point. If you're running 6" of offset, that measurement should be some where around 5". If it's less than that, you're going to lose water pressure when you raise the jack plate under power. It's gonna run hot.
You' re running a 19 pitch 3 blade prop. If you were turning the proper RPM before you raised the motor, the 20 pitch 4 blade is probably going to be too much prop. You might want to look at a 17 pitch 4 blade. The rule of thumb when changing from 3 blade to 4 blade is to drop 2" in pitch. With the 20" pitch 4 blade, my guess is the hole shot will suffer, and you're not going to be able to turn the RPM you're supposed to at WOT.
