3.0LX Mercruiser - Alpha One - Prop

[BigOtis]

Hey Guys,
I am wanting to change the prop out on my 1997 Regal to something with a little more top end speed. The specs on the boat is - mercruiser 3.0LX with a Alpha one outdrive and currently has a 17 pitch prop on it. My top end on the boat is 28mph at 3500rpm. Not much water sports behind this boat, mostly just cruising. So I dont really need the pulling power.

Any thought on which prop to go to? Can you direct me to it here, so i can purchase.

As well as what top end speed can I expect out of this craft with say a 21 or 23 pitch prop?

As always, Thanks guys,
BigOtis

 

[eavega]

Hi BigOtis. So, when you say your top end is 28 MPH at 3500 RPM, are you indicating what your speed is at 3500 RPM, or are you saying that your WOT RPM is 3500, and at that RPM your top speed is 28 MPH? Your WOT RPM for that motor is 4200-4600 RPM, so if you are only getting 3500 RPM at WOT, you are lugging your motor and you are over-propped.

If you are referring to a cruising RPM, then you need to determine what your engine speed is at WOT RPM, and adjust accordingly from there; increasing pitch will decrease your RPM and vice versa.

For comparison, I have a Mercruiser 3.0 L on a 17.5' Stingray, and run two props; I have a 20P SST 3-blade for speed, and that will get me about 40 MPH, at WOT (4300 RPM), cruising speed of around 30 MPH @ 3500 RPM. I also run a 17P 4-blade Aluminum for water sports, and I get a top speed of about 38 MPH @ 4500 RPM WOT (although I rarely run WOT when I'm using the 4-blade) cruising around 26 MPH @ 3000 RPM. Both of these speed measurements are with GPS, and with the vessel loaded with full 20 gallons of fuel, three people and gear.

Hope that gives you a starting point...

Rgds

Eric

 

[fishrdan]

My top end on the boat is 28mph at 3500rpm.

Is that the boat's Wide Open Throttle (WOT) RPM and speed? If it isn't, then we would need to know the WOT RPM and speed to help select a prop. If is it, then you most likely have other problems that need to be addressed, before buying another prop.

The engine should be able to obtain it's WOT RPM (4400-4800 RPM) and at 3500 RPM it's too far off to resolve with a prop change.

Things to check;
- Engine health, compression, timing, when was the latest tune-up?
- Hull clean and current prop in good condition?
- Water logged foam, are you dragging around an extra 700# of water soaked foam?
- Altitude, what elevation are you boating at?
- Drive ratio should be 2.0, has someone swapped it to another gear ratio that won't work with your engine?

 

[HT32BSX115]

currently has a 17 pitch prop on it. My top end on the boat is 28mph at 3500rpm. Not much water sports behind this boat, mostly just cruising. So I dont really need the pulling power.

Howdy,


Before you change anything, get a KNOWN Tach and speed measuring device (your Smart Phone will do!)

Tachometers are FREQUENTLY OFF! You cannot make any decisions at all if you cannot determine your speed and RPM!

Once you do that, According to just about ALL engine manufacturers (I/O, Outboard or Inboard) , propeller pitch is chosen to achieve max rated RPM (in the recommended range) at "Wide Open Throttle" (WOT) with the boat loaded at approx the weight you'll operate it.


Cheers,


Rick

 

[BigOtis]

Hey guys, sorry for the delay had to go out of town but back now.... So, WOT will yeild me around 4300RPM and 35mph with 17 pitch prop. Engine is strong. Everything seems to be in great running order engine wise. I cruise for nice ride at 3500rpm and get 28MPH out of the vessel. I guess my propper question to ask is what could I get speed wise in going to a 21-23 pitch prop? I will keep 17 pitch prop for water sports etc. I just want something to get more speed out of this boat.

 

[Chris1956]

That 17P prop is about right for your engine/boat combo. Installing a larger prop pitch will cause your WOT RPMs to drop, lugging the engine. So a 19P prop would give about 3900RPM, a 21P prop would give about 3500RPM and a 23P prop would give about 3100RPM at WOT. Also, your may get less RPM, due to dropping the motor out of it's proper powerband.

 

[dazk14]

97 is when Regal came out with their Fas-trac LSR hull, which certainly should be a 40+ mph boat.

Is that what you have? If so, a SS prop is somewhat of a must and possibly a 4 blade as they put so much air under the boat, many props simply ventilate before you can get much positive trim.

Are you trimming out to max just prior to ventilation? Does your prop allow much positive trim? Do you use it for watersports, or strictly looking for top end speed?

 

[steelespike]

If that is a 18 footer typically a 3.0 would be propped with a 21 or 23" prop for max speed around 40. Would probably struggle with a big load and water sports.
Heavy loads and water sports would require about a 17" prop and empty you would have to watch the rpm or it would over rev.
As suggested you have issues with the motor or boat or both. A quick compression check should tell how the motor is.
When you pull the plugs note the cylinder and condition. At 3500 and 28 mph I wonder if your on 3 cylinders.
If there is a foil on the outdrive it may help hole shot but slow then top end but probably only account for about a couple of mph.

 

[jestor68]

Both of his rpm/speed numbers work out to theoretical speed with no slip factor. Impossible.

If going actual 35 mph using a 10% slip, he'd be reading about 4800 rpm with a 17 pitch prop and his gear ratio.

Need to figure out what number(s) are off before spending money.

 

[Moondog57]

New to this forum but dealing with the same outdrive. I’ve got an aluminum prop which looks like it’s suffered a shark attack but we don’t run in shallow water. But kids have used the boat too, so there’s no telling where it’s been. But I’ve also got the old SS prop off this same drive with a spun hub. Short of sending it somewhere for a new hub to be pressed into it, are there kits available where I can do this myself? I’ve seen some kits online but uncertain whether they’d fit this or what’s involved in installing a new hub. I’m pretty handy and work on my own vehicles so unless it requires some unusual equipment I’m willing to attempt this as the SS prop is in otherwise good condition. Thanks

 

[porscheguy]

New to this forum but dealing with the same outdrive. I’ve got an aluminum prop which looks like it’s suffered a shark attack but we don’t run in shallow water. But kids have used the boat too, so there’s no telling where it’s been. But I’ve also got the old SS prop off this same drive with a spun hub. Short of sending it somewhere for a new hub to be pressed into it, are there kits available where I can do this myself? I’ve seen some kits online but uncertain whether they’d fit this or what’s involved in installing a new hub. I’m pretty handy and work on my own vehicles so unless it requires some unusual equipment I’m willing to attempt this as the SS prop is in otherwise good condition. Thanks

In the future, it’s best to start new threads rather than bumping ancient ones where the original posters may have moved on.

as for your props. On the aluminum, throw it away if it looks that bad. New aluminum props start at $90-$100. Repairing old beat up aluminum just isn’t cost effective. As for the SS prop. You’ll need to have a new rubber hub pressed in. It’s not a big deal to have done. I just had one changed last week for $85. The kits like the mercury flo torq are used on newer mercury props that don’t accept rubber hubs at all. But you’ll need a shop to do it. I’m not sure where you’re located, but you can do a google search for prop shops near you. You may find a place locally that’s buried in an industrial park.