Re: prop woes - at my wits end!
to give you a comparison, i have a 19' bayliner capri with a 130 hp merc i/o. The gear ratio is 2:1 and I run a 14x21" prop, motor turns 5000 rpm and i get around 40-42 mph. my weight with just me is about 2500 lbs, and it's a 18 degree deadrise for what it's worth. (deadrise number doesn't mean that much)
gear ratio of a 2000 johnson 150 is 1.86 I think, so doing some propeller math
Code:
efficiency = 1
Ratio 1.86
rpm/pitch 12 13 15 17 19 20 21
4000 24.4 26.5 30.5 34.6 38.7 40.7 42.8
4500 27.5 29.8 34.4 38.9 43.5 45.8 48.1
5000 30.5 33.1 38.2 43.3 48.4 50.9 53.5
5500 33.6 36.4 42.0 47.6 53.2 56.0 58.8
Ratio 1.98
rpm/pitch 12 13 15 17 19 20 21
4000 23.0 24.9 28.7 32.5 36.3 38.3 40.2
4500 25.8 28.0 32.3 36.6 40.9 43.0 45.2
5000 28.7 31.1 35.9 40.7 45.4 47.8 50.2
5500 31.6 34.2 39.5 44.7 50.0 52.6 55.2
Code:
efficiency = 0.85 (15% loss)
Ratio 1.86
rpm/pitch 12 13 15 17 19 20 21
4000 20.8 22.5 26.0 29.4 32.9 34.6 36.4
4500 23.4 25.3 29.2 33.1 37.0 38.9 40.9
5000 26.0 28.1 32.5 36.8 41.1 43.3 45.4
5500 28.6 30.9 35.7 40.5 45.2 47.6 50.0
Ratio 1.98
rpm/pitch 12 13 15 17 19 20 21
4000 19.5 21.1 24.4 27.6 30.9 32.5 34.1
4500 22.0 23.8 27.4 31.1 34.8 36.6 38.4
5000 24.4 26.4 30.5 34.6 38.6 40.7 42.7
5500 26.8 29.1 33.5 38.0 42.5 44.7 47.0
I threw the 100% efficiency numbers in there so you can always work out numbers on your own for other loss numbers,
given roughly the same size/weight boat maybe a little more power for you but if it's a deeper hull might balance out, so a max speed of 40-45mph, 40mph at least given a 19' boat with a 2000 150hp outboard, a 17" pitch prop at 5500 rpm is what I would buy as a first choice.
if you ran a stiletto 4-blade 15" pitch and only got 5200 rpms with rooster tail, you then either got a bad tach or your motor isn't making full power. Just because a motor has good compression doesn't mean it'll make full power, you might want to check over the motor, clean the carbs if it has them. and if it's a new boat with a bare fiberglass hull and you keep it in the water then check the bottom for growth, that would go along with poor fuel economy.
the cav plate can be slightly below or above the keel of the hull, it all depends on the hull and motor combo. If it's more an 1.5" or 2" then that's probably lower than it needs to be. But up to an inch below the keel is not going to be enough of a drag to drop rpms so i wouldn't worry about it. You could try raising the motor height, but you'll probably find the prop will ventilate more, you seemed to describe that from using the 14.25x15" prop.
for a 19' boat even though they call it a deep-v offshore, it's a small boat. I think you're getting misdirected on a 4-blade. they will have better bite more drag and not turn as many rpms as a 3-blade. looking at props they say a johnson 150 is supposed to turn 4500-5500 and you were pulling 4900 rpm with a 14.25x17" prop. To me that ballparks pretty well at 42mph if your mph reading is accurate. i would guess here that you're losing top end due to the 4 blades. i would stick with a 3 blade first, keep your variables down. get used to the boat and when you feel you have it trimmed right and the motor is making full power and will turn 5500 with a lower pitch prop, then start swapping props. right now you have no good baseline. i always had good luck with props from overton's, are you getting hit with a 20% prop return charge? that would suck.
