4 blade prop??

[Gfunk]

I have a 150 Optimax, 25" shaft with a 19" pitch stainless prop on my 20" transom V. I bought the TH Marine jack plate to get me 5" of lift and 4" of offset from the transom, just haven't had the time to install it yet..... currently, I'm getting between 5000 and 5100 rpms, with a speed of about 34-36 mph with 2 adults and 1 11 year old on board.
My question has to do with this, I'm pretty sure I'll have to lower the pitch of my prop even after I move the engine back, was thinking to a 17", was going to get an aluminum 3-blade, but I found a 14 1/2"x17" diameter 4-blade stainless prop I can get for the price of a new aluminum 3-blade. Does anyone have any feedback on these types of props on our boats, or any feedback on the pitch/diameter once I move the engine back/up?

THanks in advance!
Gfunk

[tsubaki]

Before you spend money on the prop, make sure you get the engine height about right.
I've got a hydraulic jack plate on mine and found the best performance with the cavitation plate 1 to 2 inches above the bottom of the boat. The setback and sharp V bottom seems to allow this without water intake problems.
I'm running a 14x19 and getting 44mph.

[THEFERMANATOR]

You are losing ALOT of RPM's if you have a 25" shaft on a 20" transom. I have seen as much as 400-500RPM increase in raising an engine just 2 inches, let alone 5 inches. Get your engine to where the cavitation plate is level or just above the bottom of the hull before you change props as you could see almost 1,000 RPM's by raising the engine up 5 inches(I used to get a good 1,000-1,200 when I would jack up my jack plate on my flats boat).

[Gfunk]

This is what I have so far, hard to tell, but we've got the motor about as high as we can get it and still feel comfortable, although the cavitation plate is still below the boat by 2-3". Thanks for the advice though, I'll just stick with my current prop till I can get this thing raised and see what it does.

[Destroyer]

Once you get your engine height dialed in here are some things to consider:

4 blade props will give you a better hole shot and the least vibration, but at the cost of top end. (The more blades in the water, the more drag/friction on your engine.) A 2 blade prop gives the best high end, but at the cost of more vibration and poorer low end, which is why most people run 3 blade props. They're a good compromise between the 2 blade and the 4 blade props.

Right now you're running a 19P stainless prop. The equivlant prop in Alum would be a 21 pitch. As a general rule of thumb you drop 2 inches of pitch when you switch from Alum to SS...so if you're going from SS to Alum you add 2" to your thinking. So if you're thinking about dropping to a 4 blade 17P, the same prop in Alum would be a 19P. To my way of thinking, that's too much prop for a 150 engine. I think that the first prop I'd try would be a 17P Alum for a baseline, and then work from there once i saw what my RPMs were.

[THEFERMANATOR]

Originally Posted by Destroyer

Once you get your engine height dialed in here are some things to consider:

4 blade props will give you a better hole shot and the least vibration, but at the cost of top end. (The more blades in the water, the more drag/friction on your engine.) A 2 blade prop gives the best high end, but at the cost of more vibration and poorer low end, which is why most people run 3 blade props. They're a good compromise between the 2 blade and the 4 blade props.

Right now you're running a 19P stainless prop. The equivlant prop in Alum would be a 21 pitch. As a general rule of thumb you drop 2 inches of pitch when you switch from Alum to SS...so if you're going from SS to Alum you add 2" to your thinking. So if you're thinking about dropping to a 4 blade 17P, the same prop in Alum would be a 19P. To my way of thinking, that's too much prop for a 150 engine. I think that the first prop I'd try would be a 17P Alum for a baseline, and then work from there once i saw what my RPMs were.

Careful with changing pitches when going from stainless to aluminum. New aluminum props are now being built slightly different to account for this. I've done a few swaps recently, and actually found teh WOT RPM's went down some with an aluminum of equal pitch because it wasn't as fast on the top end due to the extra blade thickness. Get the engine height correct first, but also keep in mind engine height may need to be changed according to which prop you run as some like to surface whereas others need to be buried. And find a prop shop that will work with you. I know my local prop shop will swap out props for a small fee if it isn't the right one so long as it is still in new condition(no scratches or chips in it, or cavitation burns).

[Destroyer]

Originally Posted by THEFERMANATOR

Careful with changing pitches when going from stainless to aluminum. New aluminum props are now being built slightly different to account for this. I've done a few swaps recently, and actually found teh WOT RPM's went down some with an aluminum of equal pitch because it wasn't as fast on the top end due to the extra blade thickness. Get the engine height correct first, but also keep in mind engine height may need to be changed according to which prop you run as some like to surface whereas others need to be buried. And find a prop shop that will work with you. I know my local prop shop will swap out props for a small fee if it isn't the right one so long as it is still in new condition(no scratches or chips in it, or cavitation burns).

There's a local marina at a lake (Budd Lake) by me that has a wall full of used but good props. He'll let you take any one of them and try it out for free so you can find the right prop for your particular engine/boat combination. I wasn't aware of the new Aluminum props, but in retrospect it doesn't surprise me. Manufacturers are constantly changing products to make them better/cheaper/more fuel efficient, etc. Regardless of what kind of prop, if you have a dealer that will allow you to swap out props, I still think that a 17P is a good place to start to establish a baseline for performance for a 150 hp engine on a V20, and then adjust from there.

[bgreene]

Originally Posted by Destroyer

There's a local marina at a lake (Budd Lake) by me that has a wall full of used but good props. He'll let you take any one of them and try it out for free so you can find the right prop for your particular engine/boat combination. I wasn't aware of the new Aluminum props, but in retrospect it doesn't surprise me. Manufacturers are constantly changing products to make them better/cheaper/more fuel efficient, etc. Regardless of what kind of prop, if you have a dealer that will allow you to swap out props, I still think that a 17P is a good place to start to establish a baseline for performance for a 150 hp engine on a V20, and then adjust from there.

What about the 10 bladed prop ?

[Destroyer]

Originally Posted by bgreene

What about the 10 bladed prop ?

Too expensive...I can't afford one.