Help w/ prop Merc Alpha1 II

[Mercury13]

Just a fairly new owner looking for some prop advice. I have a ' 94 Mercruiser Alpha 1 Gen II od, powered by 5.0L GM. The current prop is a 3 blade 19p aluminum and stamped # is 48 78120 19P. I figured that 19 was the pitch but other than that not sure.

It is in fairly good condition with a bunch of very small nicks. I skimmed the edges w/ a sander to make them uniform, was this an acceptable move ?

I need to get a spare and need help w/ selection. The numbers on the boat are 37mph at 4100 rpms with current setup. Boat is 20 ft. Searay Overnighter.
Any advice on options and features would be greatly appreciated.
Thanks.

Steve
Mercury13

 

[hwsiii]

Re: Help w/ prop Merc Alpha1 II

Steve, the information I found on your boat was that it has a base weight of 2,450 pounds, the motor is 180 HP with a 1.65 to 1 gear ratio and your WOT recommended RPM should be between 4,200 and 4,600 RPM, if this information is wrong then the numbers I am about to give you will be wrong, so please check this information.
The following is my guess of your allup weight.

This is the RPM you ought to be turning by changing to a 17" pitch prop in the same manufacturer and model you have now, and you should be turning right at 4,500 RPM. The prop you have now is showing a Prop Slip of 17% and that is higher than it should be, it ought to be around 11%, my assumption is that the nicks are messing up the water flow on the prop.

H

 

[Mercury13]

Re: Help w/ prop Merc Alpha1 II

Thanks for all the detail. Really appreciated. Sounds like you are suggesting to get the 17p and make the 19 the spare ?

I would like to do SS, but from reading it looks like a good danger of damaging the shaft if a considerable object is hit. The boat is used fairly often in the Delaware River. ( Logs, driftwood etc.. )

Steve

 

[hwsiii]

Re: Help w/ prop Merc Alpha1 II

Steve, that is my recommendation, as it will get your RPM into the upper band of your motor and that is a good thing. But be sure and get the same manufacturer and model of prop, because a change in geometry can mess all the formulas up.

H

 

[hwsiii]

Re: Help w/ prop Merc Alpha1 II

Steve can you take a picture of the prop and post it.

H

 

[Mercury13]

Re: Help w/ prop Merc Alpha1 II

I'll have it up around 3:30 this afternoon.

Steve

 

[Tail_Gunner]

Re: Help w/ prop Merc Alpha1 II

Here's a thought and real world application

22p 4 blade ss 4400 rpm @ 54mph
20p 4blade ss @4950 @ 56 mph

Which one is more effecient..the one running at the peak rpm band....Out of the hole midrange and top end..You will not hurt a motor running at its peak...If you were to replace the valve springs and cam the very same internal's could turn 6000 rpm..Now that does not mean i am suggesting to run @ wot all the time. Personally i have never ran more than 10 minutes watching the heat and oil pressure all the way.

But i do run at 34mph @ 3000 rpm a lot..its very effeicent for a O/B and if its a hot day with a full load..no worry's about detonation or the knock sensor picking up ping and futher retarding the timing..aka dumbing the whole thing down and futher loss of performance.

PS: yes that's 4% slip...add for the cupping and 4th blade more like *8-10

 

[Mercury13]

Re: Help w/ prop Merc Alpha1 II

hwsiii,
Here are the prop pics.

 

[hwsiii]

Re: Help w/ prop Merc Alpha1 II

Steve, I can't see any problems with the prop at all. Get the 17" prop and you will be in great shape. And please repost your numbers on the new prop, as I use these numbers in my database to keep my program updated. Thanks those are excellent numbers tailgunner.

H

 

[Mercury13]

Re: Help w/ prop Merc Alpha1 II

H,

I just took another test run last night w/ the existing prop and the numbers are :
38 mph @3800 rpm's out
34 mph @ 3500 rmp's back.

Needless to say I'm disappointed at the max output of the boat considering the prop is in decent shape.

Meantime I'll look for the next prop.

Steve

 

[Tail_Gunner]

Re: Help w/ prop Merc Alpha1 II

You need to check a few things. First your tach, on the back of the tach there should be a very small switch with 3 setting's...4cyl..6cyl and 8cyl. Make sure that the switch is set to 8 or you will get a bad read..If that is setup right you could put on your muffs place the drive in neutral and run here up slowly to 3500 rpm and 2000 .....have a second tach to verify the true rpm. If those two correspond if you have access to a timing light make sure your timing is set correctly..

I am quite sure something is a miss here, A 305 with 200hp should push a 17p prop on a 20' boat to it top rpm band. The only only other factor's i know of is a hull that's full of barnacles or a boat that has a lot of water in the foam thats saturated..have you checked the weight lately..

 

[Mercury13]

Re: Help w/ prop Merc Alpha1 II

I'll check both the switch and timing in the morning. The hull is clean and freshly bottom painted. New oil change, fuel filter, plugs, cleaned carb, water housing ,thermostat/gaskets...
Remember the current prop is the 19p, alum. I am going to change to the 17p on H's advice.

Steve

 

[hwsiii]

Re: Help w/ prop Merc Alpha1 II

Steve, from the loss in RPM after using the sander I don't think that worked very well. You appeared to have lost about 300 RPM, for a backup prop it will probably do fine. But anytime you change the geometry of a prop it normally changes the RPM. By sanding the I think you probably increased the thickness on the leading edge, thus the loss in RPM now.

H

 

[Mercury13]

Re: Help w/ prop Merc Alpha1 II

H,

The first run with the new prop is :
Mercury aluminum, 3 blade 15 x 17p

39 mph @ 4400 rpm wot

I'll post another test next week.
Thanks again.

Steve

 

[hwsiii]

Re: Help w/ prop Merc Alpha1 II

You are very welcome Steve, I am glad the RPM increased to the upper power band of your motor, that is good for it and your prop slip is now 9% and that is excellent for an aluminum prop.

H