350 mag mercruiser

[blackleytroy]

I have a 350 mag. 300 hp, and alpha 1 outdrive. This is in a 1992 Celebrity 220 cuddy cabin. It currently has a prop 48-78120-19 and from what I can find on the internet, this is a pretty standard aluminum prop? It runs fine when it gets up on plane, but getting there takes some time... and pulling tubers is a real chore. The engine< according to my tach, maxes at about 4000 rpm currently.
Any thoughts on what kind of prop I could change to in order to be able to get onto plane quicker? Not concerned about top end speed as it currently can hit close to 50 mph, and thats plenty.
Thanks

 

[Eric.G]

Re: 350 mag mercruiser

If your not concerned about top end speed and just want to get up on plane quicker and be better suited towards water sports, you'd be looking for a 17 pitch prop according to what you've stated above. And if you can stand the price, a Stainless Steel prop would serve you better than aluminum. SS doesnt deflect as much and since its stronger, the blades are thinner making them more efficient... getting you on plane quicker than the same prop in alum.
All that being said, your numbers above dont seem to make sense. Thats not a lot of prop for 300HP in a 22 ft boat... unless you have 5 or 6 adults and a tube....

Eric

 

[John_S]

Re: 350 mag mercruiser

All that being said, your numbers above dont seem to make sense. Thats not a lot of prop for 300HP in a 22 ft boat... unless you have 5 or 6 adults and a tube....

I'd agree that offhand going to 17" P seems small, but I suspect your boat is a good deal heavier than a typical runabout. Do you have the speed (gps) at wot? I would suspect that your outdrive ratio is 1.47/1.50, but would be good to know for sure. A 300hp 350 mag would have to be a modern MPI replacement engine, or significantly modified 350 mag from '92.

 

[blackleytroy]

Re: 350 mag mercruiser

Took it out today and with a gps, speed at wot was 63 KPH, or about 43 MPH and rpm was about 3600 according to my tach. Not sure about the gear ratio of the out drive, how can I find that out? As we bought this boat second hand, I'm not sure what the original prop size and pitch were.
Thanks

 

[blackleytroy]

Re: 350 mag mercruiser

Took it out today and with a gps, speed at wot was 63 KPH, or about 43 MPH and rpm was about 3600 according to my tach. Not sure about the gear ratio of the out drive, how can I find that out? As we bought this boat second hand, I'm not sure what the original prop size and pitch were.
Thanks

Should also mention that this was at about 5000 ft elevation as I'm sure that makes some difference? The boat was in California when we purchased it, so allot less elevation.

 

[John_S]

Re: 350 mag mercruiser

Look over the drive carefully. There should be the ratio marked on it. You could also rotate the engine by hand. It would take 1.5 engine revolutions to turn the prop 1 revolution.

Your rpms are way low. Is the tach correct? You are getting good speed for the rpms you have. Is this a 300hp mpi, or a 250hp carb?

 

[blackleytroy]

Re: 350 mag mercruiser

I will have to look to see if I can find the gearing for the alpha 1. As for the tach, it seems to be working but I can't be too sure. It is a 350 magnum with 4 bbl carb. Will see what I can find as far as the gearing.
Thanks again.

 

[oops!]

Re: 350 mag mercruiser

that will most likely be a 167 or 147

 

[blackleytroy]

Re: 350 mag mercruiser

Hello again. I found what I believe is the gear ratio for the outdrive. It has "150R" on it. I would assume the 150 is the gearing and the R is the rotation as it is a right hand rotation. With this information, any thoughts on the prop that would help?
Thanks

 

[QC]

Re: 350 mag mercruiser

Should also mention that this was at about 5000 ft elevation.

This is probably the whole story. Annnnnd . . . '92 Magnum's were rated 240 propshaft hp. 1.5 .

I am getting 0% slip with 3600 and 43MPH, so I'd like to see the tach confirmed. 43 MPH in a 23 footer at 5000 ft. with 240 hp is very respectable, so I do think it is performing well. With the proper tach I coudl see a 17 or maybe even a 15 in your future . . .

 

[blackleytroy]

Re: 350 mag mercruiser

Thanks QC. Someone mentioned that the 17 pitch prop is pretty small for the size of boat, does that make much difference? Also, the current prop is a 3 blade aluminum if that makes any difference. Like I said earlier, not concerned with the top end, just would like to get on plane quicker. Is it worth the ss prop or 4 blade, or? Any advice?
Thanks again

 

[blackleytroy]

Re: 350 mag mercruiser

should have also asked... does the diameter of the prop have any bearing (I'm sure it does but not sure what). I think?? my current prop is 16.5".

 

[John_S]

Re: 350 mag mercruiser

Someone mentioned that the 17 pitch prop is pretty small for the size of boat,

That was considering you had 300hp, not 240hp, which then has to be derated for your elevation.

 

[QC]

Re: 350 mag mercruiser

Confirm the tach is right, run WOT one more time trimmed right and with a typical load. Then let's do this one more time. If you do not care about top speed, and the numbers are close, I think you will end up with an aluminum 14.5 x 16 4 blade. That's a guess at this point without confirmation on the tach. Good luck!

 

[Tail_Gunner]

Re: 350 mag mercruiser

Confirm the tach is right, run WOT one more time trimmed right and with a typical load. Then let's do this one more time. If you do not care about top speed, and the numbers are close, I think you will end up with an aluminum 14.5 x 16 4 blade. That's a guess at this point without confirmation on the tach. Good luck!

The above is correct you do have a tach or speed error. A 19 pitch prop on a 5000lb boat is huge at 5000 ft and you have stated as much with a sluggish hole shot yet decent top end.

 

[Eric.G]

Re: 350 mag mercruiser

With the engine at a speed of 2500 or 3000 rpm, (can be on the muffs if desired) give the dash a couple of wacks with the palm of your hand. If the rpm changes up or down, that points to the tach having an issue. If it doesnt change, that still does NOT prove its correct, but its an easy test that might show its bad. I've had to replace the tach on every boat I've owned... though I do prefer older craft.

Eric

 

[blackleytroy]

Re: 350 mag mercruiser

Well it seems that the tach is accurate. Now just a question, I am going to get a new prop, but when I went to purchase one, the gey at the boat shop mentioned that this prop (14.5x17) seems like a small prop for this boat? Should I be looking at a 16x17 or 16x19 instead? How does the diameter of the prop affect the boat?
Thanks

 

[Titanium48]

Re: 350 mag mercruiser

Are the 1500+ m elevation mountain lakes going to be your primary boating destination? Remember that the popular Alberta prairie lakes are around 700-900 m elevation, and the big BC interior lakes are between 350 and 700 m. You might test the boat at lower elevation before deciding on a prop.

 

[Tail_Gunner]

Re: 350 mag mercruiser

Took it out today and with a gps, speed at wot was 63 KPH, or about 43 MPH and rpm was about 3600 according to my tach. Not sure about the gear ratio of the out drive, how can I find that out? As we bought this boat second hand, I'm not sure what the original prop size and pitch were.
Thanks

Then lets use the above numbers...19p....1.5 ratio....43 @3600 that is 0%. Take it to 4900 rpm thats 59mph @ 0% but 10% is generous.... that's53 which does sound right. At this point i would find out if your timing was set correctly....check the wires and plugs to confirm the motor is right. Sorry but something is amiss....How may i ask did you check the tach...and from 1000 to 4000?

 

[Tail_Gunner]

Re: 350 mag mercruiser

I have a 350 mag. 300 hp, and alpha 1 outdrive. This is in a 1992 Celebrity 220 cuddy cabin. It currently has a prop 48-78120-19 and from what I can find on the internet, this is a pretty standard aluminum prop? It runs fine when it gets up on plane, but getting there takes some time... and pulling tubers is a real chore. The engine< according to my tach, maxes at about 4000 rpm currently.
Any thoughts on what kind of prop I could change to in order to be able to get onto plane quicker? Not concerned about top end speed as it currently can hit close to 50 mph, and thats plenty.
Thanks

Here your saying 4000 @ say 48mph..? again that is 0% slip at least its consistent. I take it the boat really struggles to plane and eventually winds out to 4000....is that a empty load or are there passengers at those numbers..

19 @ 4000
18@4200
17@4400
16@4600
15@4800...


Now if you had say 5-6 people that would be within reason..but if not again id have the timing check..