Prop Issue/Questions?

[Tupelo]

Newbie to the forum, and to owning a boat. Just purchased a used 2008 Smokercraft Pro Mag 182, with a 115 Optimax (I think it has a "stock" prop on it..basically what came with it when the boat was new).

I am thinking I might need a new prop, as I been a little disappointed with both the hole shot, and speed.

At about 5600 RPM, I am only getting about 24/25 mph (according to GPS). I have hit 27/28 a couple of times, but I am thinking that the boat should be a little faster (engine will rev higher..I can get the higher speed at WOT, and about 58/59 hundred RPM).

I am clueless as to what kind of Prop I should look at (3 blade, 4 blade, 5, etc.), so if I am going to spend the $$, want to make sure I get the right one.

Stainless appears to be the way to go, however I am a little worried about hitting rocks...since I love to fish. Heard that it can bend the drive shaft, etc.

Anyway...help would be appreciated. If someone else has a similar size boat, that would be great too. (18.5 feet, 96 inch beam).

[115 optimax]

What is the pitch of the prop?I have a 1725 Proguide/115 Opti with a 13x19p AL prop and i can hit 45mph @ 5700 RPM.You should be in the 40mph range.Sounds like you have too small of a pitch.

[yarcraft91]

Max rpm for the Optimax 115 is 5750. Don't let it turn any faster.

If the tach is right, you have excess drag somewhere. That could be a motor mounted way too low on the transom or a motor is trimmed way too far in. How do you adjust the trim once the boat is on plane?

You may be turning less rpm than you think. It's possible the tach is set up wrong or not working correctly. What kind of tach do you have- standard analog or something else? If analog, there's a rotary switch on the back- where is it set?

The motor did not come with a standard prop. The prop is added at the point of sale. The right prop for that boat is probably a 17 or 19 pitch. What is yours?

[Tupelo]

Is there a way I can tell the pitch (I don't have any info as to what was installed). I bought a spare prop which I think is smaller...19 I believe.

[Tupelo]

Originally Posted by yarcraft91

Max rpm for the Optimax 115 is 5750. Don't let it turn any faster.

If the tach is right, you have excess drag somewhere. That could be a motor mounted way too low on the transom or a motor is trimmed way too far in. How do you adjust the trim once the boat is on plane?

You may be turning less rpm than you think. It's possible the tach is set up wrong or not working correctly. What kind of tach do you have- standard analog or something else? If analog, there's a rotary switch on the back- where is it set?

The motor did not come with a standard prop. The prop is added at the point of sale. The right prop for that boat is probably a 17 or 19 pitch. What is yours?

I adjust the trim by tilting the motor up. Have it down at its lowest setting on take off, and once it starts getting up to speed I tilt the motor up. Basically until it cavitates a bit and then drop it down.

The Tach will be what came with the smokercraft. I will take a look at it tonight to see what I can find.

Not sure if it helps, but I did notice it will porpoise with 3 people in the boat, when I start to tilt the motor up. Does not do it with 2 people.

[yarcraft91]

Originally Posted by Tupelo

Is there a way I can tell the pitch (I don't have any info as to what was installed). I bought a spare prop which I think is smaller...19 I believe.

Look inside the hub of the prop. You should find some number in there, roughly this format: 13.5 x 17, which would mean the prop is 13.5" diameter, 17 pitch.

[yarcraft91]

Originally Posted by Tupelo

I adjust the trim by tilting the motor up. Have it down at its lowest setting on take off, and once it starts getting up to speed I tilt the motor up. Basically until it cavitates a bit and then drop it down.

That sounds right. With boat on trailer, adjust the trim to the normal running position after you trim out. Is the anti-cavitation plate over the propeller higher or lower than the bottom of the hull?

The Tach will be what came with the smokercraft. I will take a look at it tonight to see what I can find.

The tach has either a moving needle to point at rpm, or a digital readout.

Not sure if it helps, but I did notice it will porpoise with 3 people in the boat, when I start to tilt the motor up. Does not do it with 2 people.

That may depend on where the people sit. If all in the stern, that's why you had porpoising.

[Tupelo]

Originally Posted by yarcraft91

Look inside the hub of the prop. You should find some number in there, roughly this format: 13.5 x 17, which would mean the prop is 13.5" diameter, 17 pitch.

Ok..I couldn't see the number clearly, but it had this serial number on the side: 48 77350 A45 23P. I will have to go back with some cleaner...the number is in the hub, but it was pretty dirty, and I didn't have the right stuff with me. However when I google that number it comes back with 3 Blade Alum Merc 13.25 x 23 RH #4877350 A40 Powerboat Prop


So...that would be a 23 Pitch?

My Spare Prop is a brand new Vortex - 12 3/4 21P. At least that is what is on the side of it.

[Tupelo]

Originally Posted by yarcraft91

That sounds right. With boat on trailer, adjust the trim to the normal running position after you trim out. Is the anti-cavitation plate over the propeller higher or lower than the bottom of the hull?


The tach has either a moving needle to point at rpm, or a digital readout.

That may depend on where the people sit. If all in the stern, that's why you had porpoising.

Ya...there was too much weight at that back. That is what I figured, but thought I would mention it in case. The RPM is analog (needle). I will check the cavitation plate next time I go out.

[yarcraft91]

Originally Posted by Tupelo

Ok..I couldn't see the number clearly, but it had this serial number on the side: 48 77350 A45 23P. I will have to go back with some cleaner...the number is in the hub, but it was pretty dirty, and I didn't have the right stuff with me. However when I google that number it comes back with 3 Blade Alum Merc 13.25 x 23 RH #4877350 A40 Powerboat Prop


So...that would be a 23 Pitch?

My Spare Prop is a brand new Vortex - 12 3/4 21P. At least that is what is on the side of it.

Yep- looks like that's a 23 pitch prop. For your motor/boat combo, I'd think a 17-19 pitch prop would be the correct choice, but let's check a few things that you will find on labels on the motor.

Lower unit gear ratio- 2.07? Max rpm- 5750? If so, that 23 pitch prop, running at 5800 rpm, should be pushing your boat at a speed in the mid-50's- and that's 10+ mph faster than you should see with a 115 on an 18' boat (i.e., not gonna happen). More likely, your tach isn't reporting rpm correctly and you're not turning as many rpm as the tach says. Check the rotary switch on that analog tach. You wouldn't be the first person on this board who bought a used boat where that switch was set wrong and the original owner never figured it out. A local Mercury dealer will know the correct setting, which, IIRC, is 6P, but don't take my word for it- ask a dealer.

You can probably trade that brand new Vortex 21 pitch for a Vortex with different pitch at a cooperative dealership.

BTW, my experience is that too much prop pitch makes porpoising worse.