prop help

[SPOONMAN1]

I have 2001 21' Miariah with a 300 hp 5.7 Merc.- Alpha 1 gen 2. Bought the boat in the spring it had a 17 pitch 3 blade prop (new prop had probably never been run????) I replaced it with an old vari pitch prop I had. At slower speeds ( 20 to 30 mph) it wasn't sure it would stay up on plane pulling knee boarders, so I put the 17 back on. Now top speed seems to be around 40 mph at aruond 4200 rpm. all this info is approxamate due to fact that it is not a big deal that I get the most speed or power out of the prop. I feel like 21 pitch or 23 pitch would be better, but I don't know. any advice? Thanks

 

[Bondo]

Re: prop help

so I put the 17 back on. Now top speed seems to be around 40 mph at aruond 4200 rpm.

Ayuh,.... If yer only pullin' 4200 rpms at Wot now, yer Outa horsepower...

It'll never pull a Bigger prop...

I'd do a compression test, 'n tune up... it sounds Waayyy short of 300 hp...

 

[jestor68]

Re: prop help

Increasing pitch will only pull your rpm lower.

Your range for that 350 Mag is about 4600-5000 rpm. Which means you aren't even reaching the lower range(4600).

Your slip numbers (13%) are on the high side, indicating a heavy boat.

After checking on the health of your 11 year old motor, you need to think about a larger diameter prop with a reduced pitch; like a 15 X 15 to get your rpm up to at least the minimum recommended.

What type of boat is this; a BR or a Cuddy?

 

[SPOONMAN1]

Re: prop help

This is a bow rider. Like I said the numbers were approximate. My 18 yr old son said he thought it was closer to 45 mph at 4500 rpm's. I don't know if I have ever ran it WOT at top end just not wanting to press my luck. This is a brand new motor with less than 50 hours on it and it runs like a top. It is the reason I bought the boat because the engine was just replaced by a marina, but they were the ones to put the 17 pitch blade on. I don't understand why anyone would put such a prop on that set up. Anyway I would like to get the mph's in the upper 50's and still pull a kneeboarder. The old vari pitch prop may be fine but I am not sure. I don't know how to check it out to see what I should be running.
Thanks

 

[90stingray]

Re: prop help

You need to make a wot pass... trimmed out... and see what the engine will turn and what the gps speed is. Keep in mind if it wants to rev thru the roof, you need to throttle back. Running wot is not pressing your luck. They are made to be run flat out.

 

[jestor68]

Re: prop help

You go by the motor's WOT rpm range to prop a boat. These ranges vary from motor to motor.

Your range happens to be 4600-5000 rpm. Typically a motor's horse power peak is somewhere in the middle of that range; in this case we'll call it 4800 rpm. So, if you let it fall below 4600 at full throttle, you're losing power throughout the rpm band, and it won't perform to it's potential.

This is an area where there can be no guess work. You must run the boat and record the WOT rpm and speed(speed by GPS is best). You only have a 400 rpm window to play with.

BTW: The Mercury prop selector comes up with 18" pitch for your combination and weight. So the 17" is not that far off. For example, a 15.5 X 17 would run about the same rpm/speed as a 14.5 X 18.

One other thought comes to mind on a re-power. Did the dealer replace it with an actual 350 Mag, or a crate 5.7? You may actually have a 240/250 hp motor in there(?).

The plot thickens, as I thought we were dealing with a 2001 350 Mag by Mercuiser.

 

[Bondo]

Re: prop help

This is a bow rider. Like I said the numbers were approximate. My 18 yr old son said he thought it was closer to 45 mph at 4500 rpm's. I don't know if I have ever ran it WOT at top end just not wanting to press my luck. This is a brand new motor with less than 50 hours on it and it runs like a top. It is the reason I bought the boat because the engine was just replaced by a marina, but they were the ones to put the 17 pitch blade on. I don't understand why anyone would put such a prop on that set up. Anyway I would like to get the mph's in the upper 50's and still pull a kneeboarder. The old vari pitch prop may be fine but I am not sure. I don't know how to check it out to see what I should be running.
Thanks

Ayuh,.... Buy another boat,... That ain't Never gonna happen...

 

[jestor68]

Re: prop help

The Mercury site indicates that with that set up, you should see between 48 and 53 mph with an 18" pitch prop at around 4500-4600 rpm.

That is in line with what other like size/powered recreational boats are doing. I doubt that you will achieve "high 50's".

 

[Tail_Gunner]

Re: prop help

This is a bow rider. Like I said the numbers were approximate. My 18 yr old son said he thought it was closer to 45 mph at 4500 rpm's. I don't know if I have ever ran it WOT at top end just not wanting to press my luck. This is a brand new motor with less than 50 hours on it and it runs like a top. It is the reason I bought the boat because the engine was just replaced by a marina, but they were the ones to put the 17 pitch blade on. I don't understand why anyone would put such a prop on that set up. Anyway I would like to get the mph's in the upper 50's and still pull a kneeboarder. The old vari pitch prop may be fine but I am not sure. I don't know how to check it out to see what I should be running.
Thanks

Very nice boat: Is this a mpi engine or carbed. Since the rebuild have you lost power or increased power

 

[Tail_Gunner]

Re: prop help

You go by the motor's WOT rpm range to prop a boat. These ranges vary from motor to motor.

Your range happens to be 4600-5000 rpm. Typically a motor's horse power peak is somewhere in the middle of that range; in this case we'll call it 4800 rpm. So, if you let it fall below 4600 at full throttle, you're losing power throughout the rpm band, and it won't perform to it's potential.

This is an area where there can be no guess work. You must run the boat and record the WOT rpm and speed(speed by GPS is best). You only have a 400 rpm window to play with.

BTW: The Mercury prop selector comes up with 18" pitch for your combination and weight. So the 17" is not that far off. For example, a 15.5 X 17 would run about the same rpm/speed as a 14.5 X 18.

One other thought comes to mind on a re-power. Did the dealer replace it with an actual 350 Mag, or a crate 5.7? You may actually have a 240/250 hp motor in there(?).

The plot thickens, as I thought we were dealing with a 2001 350 Mag by Mercuiser.

Jestor is going in the right direction. IF if its a mpi engine Just what type or engine is in there....I have seen on this site people who have replaced current roller blocks and vortec heads with older (but new) solid cam and non vortec heads design. Now this is just a opinion but a 350mag with mpi in that hull should hit the mid 50's with a 19p prop...Mariah has a fast hull

 

[SPOONMAN1]

Re: prop help

The P.O. let the orig. engine freeze. The boat originally had 5.7. The marina ordered him a Mercruiser 350 mag with vortec heads and it has throttle body inj. My company issued me a I phone yesterday, it may have GPS on it somewhere, other than that the speedo is all I have. I don't have the initiative to reseach all the advantages and disadvantages of SS, or 3 blade vs 4 blade. I had hoped to see mid to upper 50's from maybe a 21 pitch and still get up on plane to pull a kneeboard. If it is not going to happen I will probably go wiyh a 19 and limit the kneeboarding. My last boat was a 20' crownline, that would hit 63 and pull anything at any speed with a 23 pitch but had a fat block 454. (I miss it)

 

[IamCanadian]

Re: prop help

I may be wrong, but if it is TBI it is a 260hp Version.

 

[Tail_Gunner]

Re: prop help

...

 

[Tail_Gunner]

Re: prop help

Even a 260 hp engine should push that boat to 50. However looking at the TBI version merc still used 454 injectors and throttle body so it can build 300hp..cross referencing part numbers shows the old motor used vortec heads and you say it has new vortec head's the cam would be the only hold up here.

The engine has a mefi 1,2, or 3 and it dosnt matter which one..just added feature's. I would ask the marina if they had the ecu flashed to the new engine. If not its time for a tune up.Call these guys and tell them exactly what you have.. http://www.obd2allinone.com/sc/details.asp?item=mefitune. You will be quite happy with your new power and more than probably better econ.

Merc's orignal programming and Volvo's is differnt (strange) and there are big performance gain's to be had.

Same boat smaller engine... http://www.boatpoint.com.au/reviews/2006/bowrider/mariah/mariah-sx20-8766

Does this boat have a Bravo drive???