prop help please!

[mattyb973]

i have a 19' open bow. its a mercruiser 5.0/alpha1. weighs in at 2500lbs dry or 3000-3500 wet/gear/ppl. on it now is a very beat up aluminum 19p 3 blade. i can't get on plane without ppl moving to the front. i'd like to fix that. what i've tried so far and results.

current prop - aluminum 3-blade 19p
max rpm - 4100
max speed (gps) - 45mph

1st try - stainless 4-blade 19p
max rpm - 3600
max speed (gps) - 40mph
results - still couldn't get on plane without ppl up front and max rpm way too low

2nd try - stainless 3-blade 21p
max rpm - 3600
max speed - 40mph
results - similar to 4-blade stainless. no plane and rpm way too low. really just tried this prop cuz it was laying around the marina so why not.


i think i'd like to go to a 4-blade aluminum prop. i want to be able to plane off without really moving ppl around and the top speed loss isn't critical to me. this boat should do 50mph. it should also be around 4500rpm wot as well. steer me in the right direction guys. i'm thinking maybe an aluminum 4-blade 17p?

 

[Art Bernard]

Re: prop help please!

This is a previous post I did. Might help you on your prop choice



Is 56mph @4800rpm at wot (wide open throttle)? Your engine has an operating rpm range, you want to stay within that range. I dunno what you know about prop pitch and you haven't posted your current prop size so here is an explanation I hope will help you. A prop's size is measured by diameter (first number) and pitch (second number) so a 15 x 15 prop will be 15" in diameter with a 15 deg pitch. with reguards to pitch, the higher the pitch number the more agressive the angle of the blades and the less rpm you will turn but you will get more "power" but less speed. The lower the ptich number the less agressive the angle of the blades and the higher the rpm you will turn. This will give you more top end speed, but sacrifice power and hole shot. Each degree of pitch is worth approx 400-500rpm so lets say your engine has a rpm range of 5000-6500rpm and the prop you have on it now (lets say a 15 x 15) is turning 5500rpm @ wot. If you drop a degree of pitch making your prop a 15 x 14 you should turn around 5000rpm. If you raised your pitch making it a 15 x 17, you should turn aroun 6000rpm. Cup is another way to gain a bit of pitch. Cupping your prop is worth about 250rpm and can be done to fine tune the prop. Lets say you are turning 6000 rpm with a 15 x 17 prop. Adding cup would put you around 6250 rpm. Bear in mind to that your boats hull, the weight of your boat, and how the weight is distributed around the boat will effect rpm. I recomend a stainless prop, you will get better performance out of them and I strongly recomend that you do not exceed the rpm range for your motor, if fact you should leave a bit of cushion. Hope this helps you and good luck.

 

[Robbabob]

Re: prop help please!

MattyB,

Looking at your results is right on target with expectations. You need to drop your pitch 2" when staying with a 3-blade and 3" going to a 4-blade.. Your rpm will increase approximately 200 per inch, improving your hole-shot and increasing engine efficiency.

good luck!