I need prop advice

[donzi182003]

Hey everyone i am in need of advice. I fixed my boat and it has no miss and no alarm. It was the neutral position switch. Simple. However it still only gets 43-4400 wot. I know for a fact that it has to be the prop because i took it out right after the mech worked on it and it ran beautifully. It has always gotten those rpm since the day i bought it. On the motor it has a 4500-5000 operating WOT range. I at least want to be at 47-4800. What prop would work out on my boat best, based on some of your experiences. Also I would be interesed in purchasing one if anyone has the ideal prop.

Boat specs:

2003 18 classic
350 mag mpi 300 hp
alpha 1.62 ratio
mirage 25pitch prop.


Thanks,
Ed

[BigGrizzly]

Your prop is too big you need a 23 Turbo 1 or a Vector. The Vector will lift the stern and give a loos er feeling but may enhance the speed abour 1 mph Are you sure about the drive ratio.

[DickB]

I have the same boat/motor/outdrive but a '99. It came with a 23 Mirage prop and runs right at 5000 RPM at WOT.

[donzi182003]

Big Grizzley,



I am 100% sure about the drive ratio. My uncle has a 22 with a 496 ho and he has my prop. That was the first indicator that I was over propped. Where can i find a vector prop? I am assuming I would pick up speed by acheiving the right prop size? I know that props make a huge difference with the o/b's



Ed

[donzi182003]

Dick B,


Whats your top end speed and what is your optimal cruise?


Ed

[DickB]

64.9 MPH best ever; usually 62-63 MPH GPS and 65 indicated. Optimal cruise? As in fuel mileage, or what? Dunno; I like to cruise about 40 MPH a lot but also just loaf around at near idle a lot too.

[donzi182003]

Dick,

I cruise at 45-48 on a good day but 40 on normal conditions. What I mean by optimal is how many rpm's do you run a t 40 and at 50 mph just to have a point of comparison. Also arent i supposed to pick up a little more speed in salt? I assume you run in fresh water.

Ed

[Barry Phillips]

Donzi18, I had a bone stock 98 Classic 18 running a carbureted 350 making 250HP through a Bravo 1 drive spinning a 23” Mecury Vengeance prop. The boat ran really well with this combination, 65 mph confirmed, with one passenger and 30 gallons of fuel at 4,800 rpm. My current boat is a nice turnkey 22 running a 496Mag 375 HP. At 70mph, I am happy with the performance of this boat. One concern I have is I think it is little under propped with the motor turning 5000 rpm. I would like to see the motor run in the 4,800-rpm range. Most 454mags and 502s I have seen run 25 inch props as dose the 496mag HO.

I have ridden in a dealer boat very similar to yours equipped with a 350mag, Alpha drive turning a 23” Mirage Plus, as I recall it was running around 62 mph with 4 of us a board. I though it could easily make 66 to 68 mph with less weight and more break in, since the dealer was using a gentle hand. Last but not least, I saw a test on Power Boat TV on a 97 18 running a 300hp 350mag, with a Bravo 1 drive turning a 25” Mirage Plus. If you believe their numbers the boat ran 75mph at 5,000rpm. I believe the top end on this boat would be closer to 68 mph. Power Boat TV did not explain there testing procedures. Hope I did confuse the issue. Donzi seams to bounce back and forth between 23 and 25” props over different years and drives. Seems my 22 can run more prop and your 18 a little less. Can you barrow a 23” Mirage Plus to find kind of base line?

[donzi182003]

Barry,

I do not know anyone who has a 23p mirage to spare. My prop came with the boat from factory. I know its not right though because 4300 is the best ill turn with 2 passengers on board. Ill hit 4500 if im lucky by myself.

Ed

[boldts]

donzi18,

Give most any major prop shop a call. Most will let you run a prop for testing purposes. Only thing to remember is, if you ding it, you bought it! I'm not talking the local dealer. They are usually all about making money, not necessarily doing the best for the customer. Throttle-Up is a big shop in FL. They'll have something for you to try. There are others. I bought a Turbo1 up in Michigan, tested and needed a still smaller pitch. There was a restocking fee, but I was able to get the right pitch for my boat in a mater of 2 days.


On another thought, are you reading the boat tach at 4500 or a separate tach/dewel meter on the engine. It's not making a lot of sense that other Donzi owners with pretty much the same set-up are turning 5000 and your not. Could be a simple boat tach reading wrong. Especially since they are running 23" Mirage Plus props. Who knows, maybe the props are that much different performance wise.

If the tach is correct, no doubt you need at least 1" maybe even 2" less pitch to get the engine up to 4800. BTW, Grizzly didn't steer me wrong in any of the advise he supplied to me over the years I've known him. He's an asset to the boards.

[tmdog]

Donzi18- - I have a 23 Mirage+ for sale at a reasonable price. Let me know. Prop is like new- -no dings.

[Barry Phillips]

Donzi18 your drive ratio dose not seem right boat sold in Florida, I thought that the 1.64 ratio applied more to boats used in highland areas, like Lake Tahoe. When I first got my 18 the tack was only reading 4,400rpm but the boat seemed to run really well. So I checked it against a shop tack and my VDO was not calibrated right, which was easily remedied with an adjustment screw on the back. Did you verify that the drive ratio is 1.64, you can take the serial number and call Mecury Marine. Your gear ratio with that much prop seems like the problem. A 23” should put you in the 4,700 to 4,800 rpm range. What is your top speed at 4,400 rpm. My 18 never ran below 62 on a hot humid day, again in cool fall NY weather it could run 65mph with little effort. What is your trim setting at WOT? My 18 would air out nicely with the trim just above neutral position on the gauge. Classics seem to fly higher than they really do, set of the pants.

[TBroccoli]

Trim out the drive.

[donzi182003]

Of course I trim the drive. I trim it as high as the trim set will let me. It even shoots a small tail at that setting.

Top speed is 60-62 gps.

Ed

[Barry Phillips]

Hi Ed, to bad your down there and I up here. I could try your 25" and you my 23", 62 mph at 4,400 are not right. My 22 seems to gain a lot of momentum between 4,000 and 5,000 rpm, roughly 55 to 70 mph, so adding another 400 to 600 rpm is a big deal. If I trim to far out my rev limiter alarm begins to sound. Your boat is fairly new, did you call Donzi, you’re not to far from Sarasota, and maybe they will trade props with you as a warranty issue. When I was having the RPM issues with my 18, Donzi sent my dealer a Mirage Plus, and my boat actually lost 200 rpm so I stayed with the Vengeance .