so I have been rebulding a boat (1983 mercedes 19', 120 mercrusier, alpha 1 sterndrive) Compression is 120 psi on all cylinders, rebuilt carb, and put on new to me lower unit off another 120 mercrusier. I took it out for the first time this weekend and I could not get it over 3000 rpm, nor could I seem to get above (estimated) 10 mph. It was very slow. from Rpm ranges of 1000-3000 we might have gone 3 mph faster, but I am not sure (no speedo yet).
Now, the original prop was a 17 pitch 15"diameter prop, but it is beat to $%&. so someone gave me a prop that was off a 120 horse mercury outboard, it is a 19 pitch and seems to have a smaller diameter and a smaller barrel. It fits, but it is smaller then the old one. the other problem is the tilt trim will not stay in any position when it is in drive because it get forced down to it's lowest position, so I know that is not helping at all.
So my questions are,
Does the barrel of the prop effect speed? and exhaust? and is the barrel size directly related to the prop diameter? (bigger diameter prop = bigger barrel)
Will a bigger diameter prop help this issue?
I know pitch will make a difference, but should I expect changing the diameter to a 15.5 and pitch to 17" increase rpm by 1500? or should I go to a 15" pitch?
Is 15.5 a common diameter for a 120 merc alpha 1 sterndrive?
Will fixing the trim help all of this as well?
I am very new to props and boats in general, but i like knowing everything about it so I am comfortable purchasing a new prop, and not worry about trying props, and exchanging them a bunch of times to find the right one...
Please help!!
Now, the original prop was a 17 pitch 15"diameter prop, but it is beat to $%&. so someone gave me a prop that was off a 120 horse mercury outboard, it is a 19 pitch and seems to have a smaller diameter and a smaller barrel. It fits, but it is smaller then the old one. the other problem is the tilt trim will not stay in any position when it is in drive because it get forced down to it's lowest position, so I know that is not helping at all.
So my questions are,
Does the barrel of the prop effect speed? and exhaust? and is the barrel size directly related to the prop diameter? (bigger diameter prop = bigger barrel)
Will a bigger diameter prop help this issue?
I know pitch will make a difference, but should I expect changing the diameter to a 15.5 and pitch to 17" increase rpm by 1500? or should I go to a 15" pitch?
Is 15.5 a common diameter for a 120 merc alpha 1 sterndrive?
Will fixing the trim help all of this as well?
I am very new to props and boats in general, but i like knowing everything about it so I am comfortable purchasing a new prop, and not worry about trying props, and exchanging them a bunch of times to find the right one...
Please help!!