Deer chaser... why too much prop? I am maxing out on RPM with the 21 pitch... if I got down in pitch, won't I lose speed as I'll hit my max RPM that much sooner?
Mine is a 2 stroke... what speed are you getting with your 75 Merc 4?
Deer chaser... why too much prop? I am maxing out on RPM with the 21 pitch... if I got down in pitch, won't I lose speed as I'll hit my max RPM that much sooner?
Mine is a 2 stroke... what speed are you getting with your 75 Merc 4?
I have a rt178 with a 75hp four stroke with a 17p prop and top speed is around 37. I run my boat on the heavy side with 4 batteries. I am not sure about your boat or set up but I would thing when you get set up right you would be around 37-40 mph. Also are you sure 5000 is your max RPM.
Yes, WOT is 4800-5250 for my motor. I believe 4 strokes handle higher RPM, in general.what RPM are you runing at that speed.Originally Posted by fishon396
I am not sure how accurate the factory tach is but 5200 and max rpm on mine is 5500 I think.
The 2 stoke does have a lower rpm range. But something is not adding up. Your rpms must be way off. There’s just no way for a 75 hp motor, can turn a 21p. Two things, 1 you have a big foot lower uint on the motor. Or 2 what your rpms are not what your tach is reading. A 75 on that hull with the standard 17p prop will run 40 mph all day. Some reach 42.
But that it also a different gear on the lower unit.
I would stop stop by a shop. And just have a mechanic look at it. To confirm everything lower unit pitch prop, correct RPMs. Then start from there.
Your lower unit should have a 2.33 gear ratio. Which means you could need a 19p prop. But even a 21 is to high for your lower unit.
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What exactly is a big foot lower unit? I will hook up another tach to it and see what it does next time I am out.
21" is way too much prop. As far as SS, you have a lifting surface on your boat. A SS prop will give you much more bite, particularly at trim. Trim tab - your prop turns to the right. Setting the Trim tab to starboard put port pressure on the lower unit, which eliminates wheel torgue. Trimming too much will cause steering torque. Trick is trimming (which ain't gonna be much with a tin prop) until steering is neutral.
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Too much prop: with the 21 pitch, I can hit max RPM of 5, 250 - would that not suggest that I do not have too much prop?
Otherwise, my RPM gauge is wrong, as was suggested in a previous post.
I have a 2 stroke Merc 90 on my 185, and I run a 20p prop and hit about 45 mph. A 21p on a 75 sounds like too much pitch.
I will be checking the RPM accuracy on my tach this weekend.
Do I have it correct that if I have too much prop, I would not be able to hit my WOT RPM?
Curious what whole your motor is mounted to
Second hole
Thanks. That's the same as mine, and the max speed I can get out of my 75HP is 32MPH.
I don't think displacement is more with your 90... as I think it just has bigger carbs on it, same displacement, so there shouldn't be a reason I'm getting 14MPH less than the 90....
So, I will check the RPM's this weekend and see if my built-in guage is wrong... and see if I can't find a 19P prop to test out too. I already have 2 of the 21P ones (one is a backup), so I'd hate to buy a 3rd!
Perhaps you can borrow one from someone in your area, there might be a forum member in your area that can loan you one to try. A dealer in the area might be of some help also. They certainly aren't cheap, no need to buy one until you are sure. My tach is accurate, and my speeds are GPS and not speedometer, and my boat is running with three group 31 batteries and usually a full livewell. You are correct, both motors are the same displacement, fuel delivery is the difference. Good luck!
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