Over Propped?

[krisnowicki]

lost about 40 hp in my motor switch from the vortec 350. I was running a 14.5 x 21 inch prop. Today when I went out i was using the same prop. When I go to WOT off from idle she revs up to about 2300 then sits there for about 20 secs until it gets on plane. Then it will go up to about 3300 and thats it. When it is not under load it will rev and hold up to 5500 but i never do that. Also since my when it is not under load I can see the tach show the 650 idle to 5500 with no jumping the tach has to be right. Also, I was doing about 30 mph (29.2 gps)at 2300 with a 21p prop. This computes with the prop calc. So my questions are, 1. Is this typical behavior of a boat that is way over proped? 2.if so is the 21 to 19 jump going to be enough or should I go to a 17? I do tubing and cruising. 3. If not what would cause this to happen.?

 

[TilliamWe]

Re: Over Propped?

You are overpropped. Borrow a 17 and see how it does.

 

[krisnowicki]

Re: Over Propped?

I dont know any one with a 17 .... my boat wieghs 3400 I am going to have to buy a 17" what diamater should i look for

 

[Bondo]

Re: Over Propped?

I am going to have to buy a 17" what diamater should i look for

Ayuh,......

I just posted this on another thread,.....

Ayuh,.....

Diameter is determined by Blade Design,.....

Buy your prop by Pitch,.... The Engineers that designed it took care of the Diameter......

 

[TilliamWe]

Re: Over Propped?

The reason I said to borrow a 17" is because just by the numbers, it still won't be small enough. So if you buy one and it only raises your WOT ROMS to 4200, you are gonna be upset.
It really sounds to me like you need to find your "local" Mercruiser dealer that participates in their propeller demo program. Go to MercuryMarine.com scroll down to "Find a Dealer" click on it, and then you'll see a place to put in your location and then at the bottom of the page you'll see a place to check prop demo.

The reason I say all this is because at only 3300rpm WOT with a 21, a 17 "by the book" would only raise your RPMs by 800. That's not enough. But since you are so severely overpropped now and your motor can't get into its "power band", a 17 might allow it to spin so much more freely and it might be the best pitch. But I'd hate to see you spend real money on a 17 and then have to go buy a 15. With the demo program, you buy a prop and as long as you don't damage it, you return it for the correct one.
Now you know why Mercruiser matches outdrive ratios to specific engine/drive/boat combinations.

 

[krisnowicki]

Re: Over Propped?

I was thinking the same thing. When I say that 3300 was wot, i would open it up and it would scream but move up slowly. It would not stop right at 3300 that is as far as i was willing to punish my engine. Sorry for not saying this earlier. I would bet it would get up to maybe 3600 or 3700