Question: What size prop should i use for my boat?

(Posted by: pernagirl00 on 2010-05-31 16:30:55)

I've got an 18' bay liner with a 120 hp outboard engine. I usually use it to travel down steam to fish and also to wake board. Right now it has a 12.75 diameter with a 21 pitch prop on it with 3 blades. It has plenty of power getting it up to plane and will go about 45 mph. The downsize is that my gas mileage is absolutely horrible. I'm using about 3x the amount of gas of everyone else. My prop recently got damaged and I'd like to get one that will increase my efficiency on gas while remaining useful for what I do. What would you recommend? I could probably get a slightly larger diameter prop as well. Thanks!

Posted by: 45 auto on 2010-05-31, 16:37:16

If the engine turns up the correct RPM than replace it with the same if the RPM is low than go to a 20 pitch if to high than go to a 21 with a larger cup.The 120 is the older engine and will use more gas.

Posted by: Cliff on 2010-05-31, 17:44:47

Repair your prop if possible. a dinged and damaged prop is harder to turn through the water, plus it can and will eventually come apart. and you'll be looking for a tow to the docks. buy a spare either way. keep it on the boat or in the tow vehicle. its possible to change a prop in the water, so long as you dont drop anything..... no easy, but possible. and even if you land it 1st, you can get back in the water and continue your day. keep to the same diameter. more pitch will lower RPMs and top end speed, but it will get moving more quickly, which is helpful to getting wakeboarder up on the water. lower pitch wont push you up and out as fast, but you will get more RPMs out of it. it can give you more top speed, but it can result in red lining the motor too. i've put the alpha 4 (4 blade aluminum) prop on those, it works very well on the smaller bayliners with the 3.0, get same/ same if you go that way. definetly a bit more get up and go, not much top speed difference and it seems to help balance out the thrust steer those boats sometimes have. ive even seen the 5 blade stainless steel prop on them, and heard it performed well. but i think its too much prop for that personally.

Posted by: Havoc Squad on 2010-06-01, 17:13:38

21 pitch sounds a little high for a 120hp outboard. Also, never operate a prop with any significant amount of denting or damage. This drastically can affect performance and possibly damage your lower unit gearbox due to unbalanced prop. Get a new prop or get the current one repaired before using the boat again. As for pitch preferences, you might want to check and see if you're outboard is slightly overpitched, most outboards around 85 to 120 hp are in the 17 to 19 inch pitch range. A boat overpitched will struggle or won't reach WOT rpms at a steady rate, whereas a under-pitched prop will reach max WOT too early and too fast. As for gas mileage, you can have it checked out to see if there is something causing excessive fuel burn. However, since this is a two stroke 120hp outboard made well before 2000 where fuel economy began to significantly improve, expect very crappy gas mileage with the outboard you currently have. Hence, if you want something that doesn't eat gas at near 12 to 15 gallons an hour at cruising speed without buying a newer boat or new outboard, buy a Mecrusier or the newer Volvo Penta I/ O boat with a 4 cylinder or 6 cylinder engine. I recommend you look for something in the 1990 to 1998 year range. With a typical 4 cylinder I/ O, they burn around 5 to 6 gallons of gas an hour at around 3400 rpm