Replacement Prop Selection

[candiaoh]

Hello All.

I want to replace the prop on my boat- a 1989 Mercruiser Alpha One, Gen One with a 5.7-liter engine. The boat itself is a 4Winns Santara 210. There appear to be a gazillion options and I am trying to figure out where to even begin. I am not even sure the prop that is on it now is correct. I purchased the boat off of a PO that kept it moored as a fishing boat in Lake Erie.

How and where do I determine the correct/optimal prop? For our usage it runs on an inland lake, mostly cruising and fishing but occasionally is used for tubing and skiing. I'd say it runs about 10 hours in total each summer.

Any guidance would be appreciated!

 

[alldodge]

Need some numbers to determine props
What type and pitch is on it now?
Current WOT speed using GPS?
Current WOT RPM?

 

[candiaoh]

What type and pitch is on it now? I am not sure. Would this be marked on the prop itself? It has three blades.
Current WOT speed using GPS? With 2 people 28 to 32 mph
Current WOT RPM? 4000 RPM

 

[alldodge]

That helps
Type 3 blade

Pitch can normally be found on the hub either actual pitch or part number which will provide the pitch

Speed 32 MPH
RPM 4000

Your motor can be one of 2 types being a 1989 model. Most would turn 4200-4600 RPM. If your motor has a roller cam (noted with block code CD, then it should turn 4400-4800.

If your hull is real slimy then this will create a drag on the hull and slow it down. If compression is not up to spec this means motor has less power. Spec for new is 150 psi

Every 2 inches of pitch will increase or decrease RPM about 400 RPM at WOT. So if your prop is a 21P you could go to a 19P. Again this is if your motor is healthy

 

[brodmann]

The pitch of the prop is the theoretical distance it should travel in one revolution. If it's a 21 pitch prop and you change to a 19 pitch prop, it will give you higher RPM's, less strain on the motor, and will help you plane quicker.

 

[candiaoh]

The pitch of the prop is the theoretical distance it should travel in one revolution. If it's a 21 pitch prop and you change to a 19 pitch prop, it will give you higher RPM's, less strain on the motor, and will help you plane quicker.

Thanks, all. I am going to check out the stamp on the prop this weekend and report back.

 

[QBhoy]

Get the drive ratio stamped on it and the prop pitch. Go from there. Your figures suggest your prop is way over pitched.
A typical example of a boat like this with that set up might have a drive ratio of about 1.47-1 and say a 17-19” prop. 19” would be on the tall side of pitch with that ratio and boat. More likely a 17”
Should the ratio be a lower figure of around 1.6. A 19”-21” would be more in the ball park. Given the model of boat..if she’s a carb and the assumed age..likely the range of rpm is 4409-4800 we are looking at.

 

[candiaoh]

I was finally able to get down to the boat and got the info off of the barrel of the prop.
Its is difficult to see as the prop is in bad shape. It has a small chunk missing off one of the blades. It is aluminum, and the barrel number is: 48 78120 A40 19P.

 

[candiaoh]

Is the drive ratio stamped somewhere on the outdrive and if so where would I look for that?

 

[alldodge]

The ratio can be on the drive but it's normally over time it wears off. The ratio is good for determining slip, but that's not needed to determine what changes should happen

Going from a 19 to a 17 will increase your hole shot and also WOT RPM to within range

Normal a boat would be pitched for a 19 to 21, and with your motor being low it could be it's just getting tired (lower compression)

 

[candiaoh]

So it sounds like moving from a 19 to 17 might be warranted. What about blade #? I have a 3 blade prop now, would 4 blades provide any advantages or create disadvantages?

 

[alldodge]

So it sounds like moving from a 19 to 17 might be warranted. What about blade #? I have a 3 blade prop now, would 4 blades provide any advantages or create disadvantages?

IMO, stick with 3 blade. 4 blade take more HP to turn and increase stern lift which may or may not cause more issues

 

[Chris51280]

My 86 L6 115 has a 21 pitch prop jumps out of hole well but wot is 6000 would a 24 pitch be to much 16.5 bass boat

you need to start your own thread.

 

[alldodge]

I moved the guy

 

[jimmbo]

The boat was kept in the water, moored all season? How crusty/contaminated is the hull bottom?

 

[candiaoh]

So, it has been a while since I started this thread, but I was able to get out on the water this past weekend and gathered some updated data. My rpm maxed at 3300 WOT (not the 4000 I originally reported) with a top speed of 32 mph. It currently is pitched at 19, and as stated above, the prop is not it great condition. Up until this season, it was moored in the water. This season it will live on a trailer between outings. Another note is that it does have a fin on the outdrive I assume the PO used to keep the bow down at slower speeds. Should I replace it with a 17?

 

[alldodge]

With RPM that low I would do a compression test, check timing and see if your getting full advance

 

[Scott06]

With RPM that low I would do a compression test, check timing and see if your getting full advance

Agreed something is not right. Would also verify firing order is correct. I thought on the SBC there are two cylinders that can get swapped wires and still run ok ish.

Has tach been verified for accuracy ?

 

[candiaoh]

That is the thing. It "sounds" like it runs like a top. It idles and accelerates smoothly, all the way to WOT. Maybe the tach is off? How would I check that? Sorry Scott. I sent my reply before I saw your question about the tach.

 

[alldodge]

Get a hand held tach (rent/buy) and connect to negative side of coil