prop pitch

[Ducker2]

Can anyone tell me what they think. I have a 16 foot legend sc with a 2006 mercury 4 stroke efi with a 12 pitch prop. Im running about 5100 rpm at roughly 22 mph. I think i should really get closer to 29 or 30 mph so i was thinking of dropping down to a 10 pitch prop that will increase my rpm. The rpm rating on the motor is between 5500 to 6000 wot. I have read for every pitch of prop you have an increase of 200 rpm. So if i dropped down 2 sizes to a 10 pitch i would increase my rpm by roughly 400 rpm. Is this manageable and has anyone done any testing with different pitch and diameter on there fishing boats??/

Thanks Rick

[NMG]

What size engine and is that with a light load or full load? Also, are you trimmed up to get that 5,100? Do you know what the max HP rating for the boat is?

[Ducker2]

That’s with a load it’s 40 hp trimming it up to high it just bounces and that is the max up rating on that boat

[67ZL1]

Assuming that the engine is running properly then i’d Look at your motor height. Running on plane you should be able to see your cavitation plate ontop of the water. A quick way to set it is to have the cavitation plate sitting about 1/2” to 1” above bottom of hull.

[lape0019]

My personal opinion is I don’t think you’ll ever see 30mph out of that boat. You may gain a mile or two but I’ve got a buddy who has a Rebel 1455 with a 40HP fourstroke (I know not the same thing but it’s lighter) and his top speed is about 36kilimoters an hour which is about what you’re getting.

Either way, your RPM’s are too low. You are lugging the motor down making it work harder than it needs to.

The advice above is pretty good. Start by looking at what hole your motor is mounted to the transom is. If it’s to low, it’ll cause drag which results in lower RPM’s. To high and you won't have enough water pressure. My old boat was factory rigged in the 3rd (top) hole. It was later moved to the second hole where I guess the previous owner liked how it handled because that’s where it stayed. So if you are in the first hole, I’d look at raising the motor a bit before spending money on a prop. That alone can gain you 100-200 rpm’s.

After you’ve confirmed this, then look at props. 200 rpms per pitch is an average but seems to be quite accurate and is what I ended up with on my old boat.

[NMG]

Great advice here. Definitely look at the mounting height first. If that looks okay, then I'd suggest a lower pitch prop. You'll probably also find that you'll get higher revs in the spring and fall with cooler weather, and that in the hottest days of summer, your performance will be down a bit.

Also, try getting a sense of how the boat performs with a light load, a "normal" load and a heavy load. Most people would normally prop for a "normal" load, but if you are running heavy loads allot, then you may find that you have to go down a couple of pitches to get better performance. It's entirely possible that the impact on your speed will be negligible, since you'll be getting better RPM's. Overall performance, getting on plane, cruising speeds, acceleration, etc., should be much better though.

[Ducker2]

this is where my engine is sitting now

[Ducker2]

If i place a piece of wood from the cavitation plate to the bottom of the hull it looks like i could raise it up a good 3 inches to get the cavitation plate level with the bottom of the hull I think the vent on the top is just exhaust does this have to be under water.

[lape0019]

Without seeing where the bolts are that attach the motor to the transom, it is hard to say for sure.

If your cavitation plate is 3" under pad, I would say you could raise it a bit but look at where the bolts are coming through the motor at the top of the transom and seem if you are able to raise it a bit. Doing this would require minimal tools or a few buddies to support the weight. A cherry picker, sold sockets and a pull ring that attaches to the flywheel bolt is all you really need. If you aren't comfortable doing it, any marina that deals in Merc will have everything needed. I assume a good marina could tell you if it is setup properly too.

[Ducker2]

I raised it up 2 holes tried it tonight faster hole shot top speed 29 mph and runs at 5900 rpm

[Ducker2]

Thank you everyone for the helpful advice that was great

[lape0019]

That Is seriously awesome.

On the plus side, you don't need to spend money on a new prop!

[NMG]

[FishToLive]

You might not notice but you will probably save $$$ on gas....easier/quicker to plane, lower RPM to cruise,...