Prop question

Sand Wedge

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Nj
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Canyon 336
I have a 1990 sailfish 252. I just repowered with 2 mercury 200 verados. The dealer put on the mirage plus props 17 in and boat was proposing at his speeds. They changed to a 15 inch mirage solved problem. I'm only getting 42 mph trimmed out which is 5 mph slower than the old motors. I'm reading 6100 Rpms. And burning gas at 1 mile per gallon. Does anyone know where the prop size should be as of know I'm not thrilled with the choice I made. I was anticipating better results. Any info would be great. Thanks for your time.
 
Sand Wedge,

You did not tell us what the WOT of the 200 Verados is. 6100 rpms is screaming pretty high..are you in the recommended range? At what speed/rpm are you only making 1 miles per gallon..that are you running 26 mph burning 13 gallons per hour each engine?? I am spinning 17P wheels on the 150's on my 23 Gulfstream and I am getting 2-2.3 miles per gallon. My neighbor is spinning 14x23 wheels on the 150 Suzuki's on his 25 Seafox and he is averaging 2 miles per gallon...something does not sound right....
 
Chris they say 6100-6300 is the range rpm you should be in wot wich I am. At 6100 trimmed out im getting 42 mph At a cruise speed is 30 mph I am getting the 1 mile per gallon. Wot I'm at .07 gallon per mile. The performance numbers are worse with the 4 strokes than my old strokes. Not sure where to go from here.
 
Agree with the other poster that says something doesnt sound right. Boats usually porpoise if engine is mounted too low. Also, is there a big difference in weight with the vrods compared to your old engines ? Did the dealer look into any of these things before changing your prop? Is it possible that you're now underpropped but cant tell because of the rev limiters ? I would simply tell the dealer that you're not satisfied with the performance and let him figure it out what the problem is..thats what you paid him for. Based on what you say your numbers are, he really cant argue that everything is fine.
 
John I did ask dealership to go over everything and they said they think everything is fine the reason why I'm posting this is a try to find out if there's any other options I have I think I'm getting horrible
fuel economy. And top end speed is not there. The motors are 150 lbs heavier between both motors wich is not a huge difference. I don't know if my expiation is set to high and this is good but I have nothing to judge it against.
 
Here is some test data from Mercury with a pair of 200's on a 262 Mako..granted a different boat, but it is a 4500 pound deep v hull

Performance Summary
Top Speed (MPH@RPM) 53.4 @ 6250
Acceleration 0 - 20 MPH (seconds) 4.66
Acceleration 0 - 30 MPH (seconds) 6.50
RPM Range Speed (MPH) Fuel (GPH) Fuel (MPG)
1000 4.1mph 1.2gph 3.4mpg
2000 8.6mph 3.4gph 2.5 mpg
3000 18.3mph 7.4gph 2.5 mpg
4000 30.4mph 11.6gph 2.6 mpg5000 39.7mph 17.8gph 2.2 mpg
6250 53.4mph 36.4gph 1.5 mpg

A slightly wider hull with shallower deadrise with typically run faster burning less fuel...

Mercury Name Enertia
Diameter/ Pitch 14" x 19"
Number of Blades 3
Material Stainless Steel

There is another one with a 25 Whaler Conquest (4900 pound dry weight)..

Engine Speed Fuel Economy Sound
(RPM) (MPH) (GPH) (MPG)
1000 5.0 1.2 4.2
1500 7.0 2.3 3.0
2000 8.4 3.4 2.5
2500 9.8 6.6 1.5
3000 16.8 7.9 2.1
3500 23.1 10.3 2.2
4000rpms 31.0mph 13.4gph 2.3mpg
4500 35.7 19.8 1.8
5000 39.8 30.2 1.3
5500 43.6 42.0 1.0 140 -
6000 51.9 42.0 1.2 167 -
6200 52.0 42.0 1.2 167
Prop = Propeller Name Enertia
Diameter/ Pitch 13 5/8" x 21"

One thing to remember is that these are ideal condition tests with brand new clean hull boats without all the stuff in them that we put....so take the numbers with a grain of salt..but if you are burning 2x as much fuel, then something is not right. By the way, how are you determining how much fuel you are burning?
 
I going off the merc monitor gauges tells you mph mpg and gph
 
If you're porpoising, you may just be trimmed too high or need some tab. You shouldn't need any less that 17" pitch props, I'm running 14X19's on mine (Yami 200 carbs). If I had to do it over I'd get 17x15.25's. The wider hull & shallower deadrise will run faster if it's calm, if it's rough you'll run slower. Ask to try the 17's again with you driving the boat and see where the cavation plate is. How does the boat sit with them? Once you get the props dialed in, let us know the perf #'s.