finding the right pitch and dia for my alpha 1 260 v8

rlagrow · Jul 13, 2008

Here is the problem. The prop I have has cracked blade.... numbers on current prop are 48 73982 A4 21p othe side of prop 18255.

I know its a 21 pitch, dia my best mesurement is 13.5 and my WOT rpms trimmed up are 5400-5600 I was told thats way to high!! I have great top end, I think! And around 50mph fully loaded but its takes forever to plain out. its my first I/O. ( 1987 190 horizon four winns. 350 v8 260 hp alpha one )

I'm looking for better plaining and the best fuel mileage I can get.

If any info is needed letme me know not sure i covered what i could!!

Thanx Ray

steelespike · Jul 13, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

Not an expert but I believe your max rpm should be around 5,000.
There may be a label on the motor.Diameter seems a little undersized as most of those props are about 14".That said diameter is a product of the design and not directly considered.You may want to confirm the tach is accurate.Poor hole shot and and over reving don't usually go together.
Perhaps the cracked prop is at fault.Are the statistics with the cracked prop?Perhaps you can get the prop repaired and get new statisics.
Is the prop aluminum?They are usually painted.At 260 hp it seems more pitch
would be the norm but that would make the hole shot worse.

rlagrow · Jul 13, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

NO the stats are b4 the crack. I hit a rock on the 4th pulling up to a beach for the night! OK so you think i might look into a 14 or 14.5 dia prop and it I'm at 5400- 5600 i should go to what a 22 pitch. i want economy the hole shot not worried about that much just wanted to take some strain off the motor but i dont want it to over rev and blow it has less then 300 hours on a full rebuild thats why i boought it. any and all insight is wanted and welcome

thank you!!

Bondo · Jul 13, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

Ayuh,.....

Just a Crazy Wild Ess Guess,..........

You've got a prop built for a Mercrury mid-sized gearcase Outboard....

Is the round part of the Prop, slightly Smaller diameter than the gearcase of your Outdrive,..??..??

And,....
You need to verify the accuracy of your Tach,....
Those kind of RPMs will kill that motor in No Time....

steelespike · Jul 13, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

rlagrow said:
NO the stats are b4 the crack. I hit a rock on the 4th pulling up to a beach for the night! OK so you think i might look into a 14 or 14.5 dia prop and it I'm at 5400- 5600 i should go to what a 22 pitch. i want economy the hole shot not worried about that much just wanted to take some strain off the motor but i dont want it to over rev and blow it has less then 300 hours on a full rebuild thats why i boought it. any and all insight is wanted and welcome

thank you!!

While it would appear 14 is the norm You should get the prop designed for your setup/gearcase etc.260hp alpha one. the diameter will follow.Probably the diameter will be around 14.The increased diameter may tend to lower the rpm but generally diameter is not the way to lower rpm.The one inch change is pitch
will get about 200 rpm drop.Rule of thumb is about 200 rpm change for each inch change in pitch. Almost surely to further agrivate your slow hole shot.

codertimt · Jul 13, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

As theses guys have said, verify the tach before you go any further. Your numbers don't calculate... 5600rpm + 21" prop on that motor would net ~65 mph....methinks you're not going quite that fast...

Working back from the 50mph number(which also could be suspect unless it's via gps) and an average slip it would put you at ~4200 rpm. That is more in line with the numbers the boat should be turning and fits the hole shot description more.

So, start with the tach and work from there...

wire2 · Jul 13, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

I have a very similar combination, mine is a 19 ft Glastron SX 190 bowrider with a 5.7

I've upped the power to ~300 hp with a cam, 1.6 roller rockers, aluminum intake and stainless headers.

It's capable of coming up on plane in less than 1 second (1 person in boat) with a cupped 21" ss prop. Top end is 58-59 but at 5500. I now have a 23" ss for it to get rpm's down a bit.

tmh · Jul 14, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

rla, as was said, your numbers don't compute. IF you're hitting over 5000 rpm, you should be flying out of the hole! The problem is, to improve hole shot you need to go DOWN in prop pitch - which increases WOT RPM! Not good. Maybe you tach is set for a 6 cyl engine (I've seen that problem here several times - there's a switch on the tach or something. I'm not sure if a V6 setting would raise or lower the tach rom reading though).

Maybe the prop wasn't in good shape to begine with (for test #'s)? Is it an aluminum prop? With that engine, a SS 4-blade prop may be what you want. Also, sounds like Smart Tabs would help.

Good luck.

rlagrow · Jul 14, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

Ok Here is where I'm at now took a look the prop is a 13.5 alittle smaller then the gearcase local shop loked up numbers it is a mercurser prop for a alpha 1. Looked at switch on tach it is set to for which four winns manual says 4 is the setting for the 8cyl. so now what do i need to test this tach.?

steelespike · Jul 14, 2008

Re: finding the right pitch and dia for my alpha 1 260 v8

See if you can borrow a shop tach to check rpm.

rlagrow · Jul 14, 2008

prop dia

prop dia

A Friend gave me a 14.25 Dia x 21pitch prop to use. the old prop that was on the boat was a 13.5 x 21 pitch will this new prop work or hurt anything?

wire2 · Jul 16, 2008

Re: prop dia

Re: prop dia

rlagrow said:
A Friend gave me a 14.25 Dia x 21pitch prop to use. the old prop that was on the boat was a 13.5 x 21 pitch will this new prop work or hurt anything?

Go for it. That's only 3/8" more radius, you'll hardly notice it.

finding the right pitch and dia for my alpha 1 260 v8

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