What is the best size prop for a 1989 Sport 20 with 200hp blackmax merc?

C

CharlestonSC

Member
I have a 14.25x19 SS now and it takes off quick and tops out at 43mph on GPS at 59K rpm.
Think of a 14.25x21 to give me a better gas mileage and more top speed to get me to my fishing holes faster.
 
CharlestonSC said:
I have a 14.25x19 SS now and it takes off quick and tops out at 43mph on GPS at 59K rpm.
Think of a 14.25x21 to give me a better gas mileage and more top speed to get me to my fishing holes faster.
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A 21p (that's what I run on my 200 merc) might give you faster top end, depending on what your WOT revs read and what your engines factory recommendations are, but it will not give you better gas mileage. Just like a car, the faster you go the more gas you have to burn to achieve and keep that speed. Friction rules, and you cannot change the laws of physics.
 
WOT is 5900 RPM at 43mph normaly I cruise about 4000 rpm at around 28mph so I thought the 21 pitch might bump my speed up a bit at the same rpm so that would improve milage.?
 
Double check your tach as a 1.86:1 lower unit with a 19 pitch prop should have you MOVING right out at 5900 RPM's. 17 or 19 pitch would be as big as I would think one would need.
 
What is the best way to see if the tach is correct? I suspect that is off a bit because when I am WOT 5900 rpm just don't sound like 5900 but that it sounds like a slower rpm could just be me.
 
CharlestonSC said:
WOT is 5900 RPM at 43mph normaly I cruise about 4000 rpm at around 28mph so I thought the 21 pitch might bump my speed up a bit at the same rpm so that would improve milage.?
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There is a lot to be gained from prop testing and setup. You need a tach a reliable method to calculate speed (GPS)and take some notes. Time to plane, minimum planing speed, how does it react to throttle inputs, does it feel sluggish?
I agree with Ferm something doesn't sound right with the numbers. If the tach is right you may already be close to optimal for your setup. A 21 Pitch prop might bump your speed up at the same RPM but there will almost certainly be a trade off. You will be asking the motor to work harder to turn more prop so you may lose some holeshot and it will burn more fuel. I like my outboards propped to run around the maximum factory recommended RPM wide open with a light load around 5800 RPM in your case. Outboards do not like to be lugged around. The timing curve is manual based on your throttle input, meaning if you go to WOT, the timing does too. Maximum timing on a outboard without achieving the proper RPM can lead to pre-detonation. Pre-detonation is difficult to hear in a noisy 2-stroke and it eats away at the pistons and destroys the powerhead. I have a old 2.4L 200 Merc carb motor that I tried on the 20-V. I saw 48 on GPS turning a smaller diameter 19p to 6100 RPM. RPM was on the high side so I tried a 17P large diameter Mirage Plus and it worked better, getting the RPM's down to 5800 with nearly the same speed, holeshot was a tick better. My .02 - Let us know what you find out.
 
My V had a 21 P quicksilver stainless on it with a 2.4 200 when I bought it. I don't know how it ran, I have yet to put my V in the water(wait for next week)
 
It takes off quick if I do WOT it slips a bit so I have to back off a little, but it still gets on plane I'd say in 5 sec or so it has been about 4 months since I'v had it on the water in a fast section, mostly I have been fishing in the creek near the boat ramp so it has been just idle movement.
I will probley just stick with the 19 for now vs. sinking 400.00 for a new prop and do some more testing.
It runs great I was just thinking of moving up and I thought I'd get a bit more out of my gas guzzler 200hp with a bit more pitch
Thanks for all the input, hopefully I will have my boat back together in a few weeks and get back for an update.
 
A 200HP either a 2.4 or 2.5L should be a 1.86:1 lower unit ratio. I know my flats boat with a 21 TROPHY would bump 60 at 5600, so a 19 pitch at 5900 should be running low to mid 50's.
 
I will take you up on that, right now I have taken the lid of of my sport 20 to redo the floor. But I am ready to start finding those Reds.
 
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