Changing prop on 3.0 Merc w/ Alpha 1

[z0lt3c]

Hey Guys-
Thanks to all this great information I finally got my boat running well. Now that I have all the upkeep items straightened out, I decided it was time to upgrade my prop.

My old prop was an aluminum 14.5x19P Michigan Wheel unit which has been beat to all hell with multiple bends and gashes in all three blades. I was reaching ~34mph ~4200rpm, 1 person, minimum weight, based on my dash instruments. My new prop is a stainless steel 14x19P quicksilver unit in like new condition.

I've done alot of searching but I still have a few questions;

1. The new prop already has a hub kit installed, can I simply re-use this hub kit with all the hardware from the old prop?
   
2. What is the best way to brace the prop to keep it from turning while torquing the nut? I've read suggestions which state to use a 2x4 to pin the prop against the outdrive? Why can't I just use a impact wrench like I would on a axle nut on a wheel?
   
3. Any concerns with the size of my old and new props? This is going on a 1990 18' Larson bowrider with a Mercruiser 3.0 engine. Looking to improve my hole shot for water sports.

Thanks,
Z

 

[Dave.Mishchenko]

Re: Changing prop on 3.0 Merc w/ Alpha 1

> 1. The new prop already has a hub kit installed, can I simply re-use this hub kit with all the hardware from the old prop?

Do you mean the thrust hub, tab washer and nut as shown in the image? If so, then you can re-use those with the new prop.


http://i47.tinypic.com/2wei1bm.jpg

> 2. What is the best way to brace the prop to keep it from turning while torquing the nut? I've read suggestions which state to use a 2x4 to pin the prop against the outdrive? Why can't I just use a impact wrench like I would on a axle nut on a wheel?

The nut should be torqued to 55 ft-lbs. The 2x4 method is what the operation / maintenance manual suggests and it works well. I use a torque wrench on mine up to 55 ft-lbs and then past it to be able to align 3 tabs on the tab washer. Don't forget to lubricate the prop shaft.

> 3. Any concerns with the size of my old and new props? This is going on a 1990 18' Larson bowrider with a Mercruiser 3.0 engine. Looking to improve my hole shot for water sports.

You should see some improvement with SS, but given that your WOT RPM is 4200 RPM you could drop the pitch or perhaps go with a 4 blade prop. But that you should be retested with a more typical load (i.e. more people / pulling tube rather than one person) and ideally with a GPS.

Given the damage to the prop I would also suggest checking the prop shaft to ensure it is still in spec. If it's been bent too far you could get a rear seal leak which would lead to damage in the lower unit.

 

[Cptkid570]

Re: Changing prop on 3.0 Merc w/ Alpha 1

Can you post back here and let us know the difference between your 14.5x19 alumunim vs the 14x19 stainless?

I have a 19' boat with 3.0 liter and find the 19 pitch is the best prop for me for just cruising around.. for hole shot and water sports, I switch to a 17 pitch. But, my 17 pitch is 14.5.. you dropped diameter to 14 over 14.5, so i'm interested if that .5 makes a difference or not... and if the stainless is a lot better than aluminum.

I never knew to tighted to 55lbs.. i've always just put a towel on the prop blade and held it by hand and tightened.. never had one come off . I'd just be careful not to overtighten it.

 

[steelespike]

Re: Changing prop on 3.0 Merc w/ Alpha 1

Can you post back here and let us know the difference between your 14.5x19 alumunim vs the 14x19 stainless?

I have a 19' boat with 3.0 liter and find the 19 pitch is the best prop for me for just cruising around.. for hole shot and water sports, I switch to a 17 pitch. But, my 17 pitch is 14.5.. you dropped diameter to 14 over 14.5, so i'm interested if that .5 makes a difference or not... and if the stainless is a lot better than aluminum.

I never knew to tighted to 55lbs.. i've always just put a towel on the prop blade and held it by hand and tightened.. never had one come off . I'd just be careful not to overtighten it.

Diameter is a secondary consideration usually just follows the design of the prop.
With the rough condition of the old prop I don't think any erformance comparison will mean much. Will be nice to see how the stainless does though.

 

[z0lt3c]

Re: Changing prop on 3.0 Merc w/ Alpha 1

Thanks for the info guys, was able to get the new prop installed. Snapped a quick pic of the old and new prop, I'll post some comments when I get the boat back in the water this weekend. The old prop looks worse in person but you can see all the folded edges on each blade;

Only one problem, this stupid block of wood thing, I'm still not sure how to "pin it" -- I wound up using my impact wrench instead which made quick work of the whole job. However, when I was futzing around with the wood, I wound up damaged the little fin on the outdrive above the propeller. Can anybody point me in the right direction to replace this piece?

Thanks,
Z

 

[Dave.Mishchenko]

Re: Changing prop on 3.0 Merc w/ Alpha 1

That's the trim tab. If you're looking down on the outdrive there will be a rubber cap directly above it. Pull that and you'll get access to the bolt that holds it in place.

You'll want to mark the angle at which it is mounted and set the replacement to match.
http://www.iboats.com/Anode-Trim-Ta...62201213--**********.818981946--view_id.45064

Above are some samples of what you can get. The last one is an offset trim tab and you might want to go with it if you plan to switch to a lower pitched prop in the future. You could also run it as is if you can't get a replacement right away and you'll just find that the boat pulls a bit to the right. Besides the anode function it's there to offset the rotation of the prop which causes the boat to steer to the right.

 

[z0lt3c]

Re: Changing prop on 3.0 Merc w/ Alpha 1

That's the trim tab. [snip] The last one is an offset trim tab and you might want to go with it if you plan to switch to a lower pitched prop in the future.

Can you help me understand the benefits of the offset trim tab and why it would be preferred for a lower pitch prop?

You really got me thinking of picking up another lower pitch prop now, especially after seeing how easy it is to swap. All of my boating is done up in Keuka Lake, NY -- not alot of need to "cruise," I would like to maximize the boat for tubing.

Thanks for the quick response.

 

[Dave.Mishchenko]

Re: Changing prop on 3.0 Merc w/ Alpha 1

When you're changing props the angle and shape of blades will change will will affect the clearance between the blades and the trim tab. On my 3.0 L / Alpha 1 Gen 2 the stock prop had a pitch of 21 inches. I swapped in a 19 inch pitch prop and the blades would no longer clear the trib tab. In my case I just used a grinder to remove enough of the tab to have sufficient clearance.

With a change in pitch you have to keep engine speed in mind. A 1 inch drop in pitch will generally raise the WOT engine speed by 200 RPM. In your case if you want to drop the pitch you'll get better hole shot and low end performance, but you have to watch that the WOT speed of the engine doesn't take the motor out of it's recommended range (4400 - 4800 - that's what my 07 - 3.0L is listed at). Likewise running the engine at WOT below the range may mean that it is laboring with the current prop and a drop in pitch would be helpful.

Once you get out on the water you'll want to record your WOT engine speed and boat speed again and then post something in the prop forum.

 

[rs1lollie]

Re: Changing prop on 3.0 Merc w/ Alpha 1

Hey Guys-
Thanks to all this great information I finally got my boat running well. Now that I have all the upkeep items straightened out, I decided it was time to upgrade my prop.

My old prop was an aluminum 14.5x19P Michigan Wheel unit which has been beat to all hell with multiple bends and gashes in all three blades. I was reaching ~34mph ~4200rpm, 1 person, minimum weight, based on my dash instruments. My new prop is a stainless steel 14x19P quicksilver unit in like new condition.

I've done alot of searching but I still have a few questions;

1. The new prop already has a hub kit installed, can I simply re-use this hub kit with all the hardware from the old prop?
   
2. What is the best way to brace the prop to keep it from turning while torquing the nut? I've read suggestions which state to use a 2x4 to pin the prop against the outdrive? Why can't I just use a impact wrench like I would on a axle nut on a wheel?
   
3. Any concerns with the size of my old and new props? This is going on a 1990 18' Larson bowrider with a Mercruiser 3.0 engine. Looking to improve my hole shot for water sports.

Thanks,
Z

I have a 1990 maxum 3.0l alpha 1 out drive and I'm using a Mercury Alpha4 prop 48-834854A45 it is a 4 blade 14x20 and I'm running 42mph @ 4800 rmps, I get a great hole shot, plains quicker even at lower speeds, I use this prop for everything from water sports to just cruising and only lost a mph or 2. I do not have a problem with clearance with the trim tab or any other part of the drive with this prop

 

[rs1lollie]

Re: Changing prop on 3.0 Merc w/ Alpha 1

When you're changing props the angle and shape of blades will change will will affect the clearance between the blades and the trim tab. On my 3.0 L / Alpha 1 Gen 2 the stock prop had a pitch of 21 inches. I swapped in a 19 inch pitch prop and the blades would no longer clear the trib tab. In my case I just used a grinder to remove enough of the tab to have sufficient clearance.

With a change in pitch you have to keep engine speed in mind. A 1 inch drop in pitch will generally raise the WOT engine speed by 200 RPM. In your case if you want to drop the pitch you'll get better hole shot and low end performance, but you have to watch that the WOT speed of the engine doesn't take the motor out of it's recommended range (4400 - 4800 - that's what my 07 - 3.0L is listed at). Likewise running the engine at WOT below the range may mean that it is laboring with the current prop and a drop in pitch would be helpful.

Once you get out on the water you'll want to record your WOT engine speed and boat speed again and then post something in the prop forum.

Just Quoting on that you dropped a prop size and now no clearance with trim tab?? you must of went from a 14.25 dia to a bigger dia prop, I went from a 19p 14.25dia to a 20p 14 dia and have plenty of clearance .

 

[oldjeep]

Re: Changing prop on 3.0 Merc w/ Alpha 1

Just Quoting on that you dropped a prop size and now no clearance with trim tab?? you must of went from a 14.25 dia to a bigger dia prop, I went from a 19p 14.25dia to a 20p 14 dia and have plenty of clearance .

Depends on the prop. I've got a 21 pitch hustler prop that requires a performance trim tab to clear. Same diameter and pitch as the quicksilver prop it replaced, just a different design.

 

[z0lt3c]

Re: Changing prop on 3.0 Merc w/ Alpha 1

Replaced the trim tab and got back out in the water today with the new prop.

Still don't have any means of GPS measurement, but took some additional readings at WOT based on dashboard instrumentation. The new prop seems to reach the same 4200-4300 RPM range at WOT as the old prop. However, new top speed is almost 37 mph, a good 3 mph increase given similar conditions and load. There might have been a little more left, I was more shocked that I broke 35 mph, which I never could before.

Some positive additional observations; The boat seems much quicker and more responsive at 3000 rpm and above then with the old prop. Seems to be more speed available in this rpm range then versus the old prop. Of course, the vibrations at higher speed is also greatly reduced, sans damaged prop. I haven't gone for an extended cruise yet, but this prop really seems to optimize cruising type operation.

Not so positive ... I am still struggling with my hole shot at low speeds and I just think this is too much prop for my boat (still). The boat will plane out nice around 10mph, but won't plane again until 21-22mph -- running in that 10 to 20 mph range, very difficult to get on plane. This is similar to before and maybe even slightly worse?? Could just be my memory.

I have a 1990 maxum 3.0l alpha 1 out drive and I'm using a Mercury Alpha4 prop 48-834854A45 it is a 4 blade 14x20 and I'm running 42mph @ 4800 rmps, I get a great hole shot, plains quicker even at lower speeds,

This is a great data point, thanks for the info and pictures. 42mph out of a 90 3.0L w/ Alpha 1 Gen 1?? I'm impressed and with a 20P no less. Makes we wonder if I'm running down a cylinder

Your pictures raised some more questions for me. You seem to also have a RH prop, like mine... however your trim tab is pointed in the opposite direction. I installed my new one identical to my old one, but maybe it was never installed right in the first place. Which way should the trim tab go with a RH prop?

 

[rs1lollie]

Re: Changing prop on 3.0 Merc w/ Alpha 1

z0lt3c check out this post that I posted on which way my trim tab should be. http://forums.iboats.com/showthread.php?t=351391

Yes I would check your compression and or make sure your running on all cylinders. BTW your trimming the out drive to the sweet spot to get the most from your boat, Right???

 

[z0lt3c]

Re: Changing prop on 3.0 Merc w/ Alpha 1

z0lt3c check out this post that I posted on which way my trim tab should be. http://forums.iboats.com/showthread.php?t=351391

Thanks for the link. Sounds like my trim tab is not pointed correctly. I am assuming your picture above is the final location you determined? I installed the tab on the trailer, did you do the adjustments while the boat was still in the water?

Yes I would check your compression and or make sure your running on all cylinders. BTW your trimming the out drive to the sweet spot to get the most from your boat, Right???

Compression is good. However, I don't know the last time the carb was rebuilt or the throttle cables were replaced.

Yes I am using power trim adjustments, however, if you have any suggested reading on this subject it would be helpful, none the less. I am still very much a novice (2nd season).

Thanks.

 

[rs1lollie]

Re: Changing prop on 3.0 Merc w/ Alpha 1

Thanks for the link. Sounds like my trim tab is not pointed correctly. I am assuming your picture above is the final location you determined? I installed the tab on the trailer, did you do the adjustments while the boat was still in the water?



Compression is good. However, I don't know the last time the carb was rebuilt or the throttle cables were replaced.

Yes I am using power trim adjustments, however, if you have any suggested reading on this subject it would be helpful, none the less. I am still very much a novice (2nd season).

Thanks.

Yes the pic is of final adjustment location. What I did was I brought a alen wrench with me and did it in the water, tilted up out drive made adjustment ran boat stopped raised out drive made adjustment and so on till I got it were I was satisfied with it. a lot of messing around but worth it.
This might be a good read onto what not to do and how to trim :http://forums.iboats.com/showthread.php?t=409196

 

[steelespike]

Re: Changing prop on 3.0 Merc w/ Alpha 1

While the ss is a little more sofisticated it is also more aggressive.Thus you still struggle a little with hole shot and are short on rpm.I really think any new prop would have improved your speed some based on the old prop.You may be able to have a good prop shop remove a little pitch or some cup.Done carefully I think you
can gain some hole shot and maintain your speed.