Increased prop pitch and the zinc anode on the outdrive is in the way? Remedy?

[auscarter12]

I have a 19' foot Aristocraft with a Mercruiser 165 and a pre-alpha outdrive on it. The outdrive serial number dates it from around 1974-75. I had a 17 inch pitch on it and it would only allow it to run about 27 mph at WOT. I believe the previous owner used the boat for skiing. I, on the other hand, am not interested in skiing - mainly tubing and cruising. I increased the pitch from 17' to 21'. After attaching the new prop, I realized that it hits the zinc anode on the drive. In order to properly detach the anode I would have to split the drive and unscrew it from the inside of the lower unit. I'm thinking about just taking my saws-all and cutting the anode. In essence I would be turning the pointed anode into a flat anode to give the prop enough clearance to run. the prop clears the bottom of the drive perfectly, but the anode is too big. I have never done this before and was hoping for any input or advice. Thank you!

 

[robert graham]

Grind some off for clearance....can't hurt much...

 

[jbcurt00]

Pointed anode?

Changing pitch or DIAMETER causes the prop to contact the anode?

Are you sure you're not talking about the trim tab?

That's a trim tab AND an anode ^^^ You DO NOT want to grind it flat, it's intended to help counter act prop steer. Possibly grind a small flat on the end of it instead of a sharp point?

What rpm are you running WOT w/ the 17pitch?

17 to 21 sounds like a pretty big jump, but I'm not a I/O owner. How'd you decide to jump up to a 21?

 

[auscarter12]

Yes the anode is also a trim tab and looks just like that. jbcurt00. I did not know that tab was to counter act prop steer - it's so small so I thought it was just an anode. Unfortunately to remove or turn the trim tab I have to separate the lower unit from the upper unit and I don't want to go through all of that trouble. The diameter of the new prop is the same as the old one, maybe 1/4 more. The jump in pitch is to increase my top end speed and lower my RPMs. With the 17' pitch, I was only able to hit 27 mph at WOT. For a deep-v boat with 165 horsepower, 27 is pretty slow. My main interest is cruising and tubing with the boat, so I don't need a low pitched prop to get the boat out of the hole for skiers.

 

[auscarter12]

What if I grind/cut a notch into the trim tab instead of grinding it flat? The boat is a 1971 model and does not have power steering, so it already pulls hard to the right anyways since the steering is so heavy.

 

[gm280]

auscarter12, How much is the new prop contacting the trim tab/anode? If it is merely just touching, certainly grind it a little, but if it is really in the way, you are going to have to compromise on something. And I also agree going from a 17 pitch to a 21 pitch is a really big jump. When you were running 27 MPH with the 17 pitch at WOT, what were your RPMs? Because if you were already in the max RPM rage with a 17 pitch, you will be certainly lugging the engine with a 21 pitch and that isn't a good thing to do. So don't prop your boat via more speed syndrome method until you know what your WOT RPMs are... JMHO!

 

[auscarter12]

gm280 Well technically I was not at WOT when running 27 mph on the 17' prop. I haven't opened the motor up to WOT yet because of its age and pending maintenance that needed to be done (carburetor needed to be rebuilt at the time). The carburetor has been rebuilt and the boat is ready to go. Personally, I thought 17' was too small of a pitch for the boat. I was running about 3700 RPM at 27 mph. I understand the top end for those motors is around 4300 RPM. Going from 17' to 21' pitch should drop my RPMs by around 800 RPM since 1' pitch increase is relevant to a drop of around 200 RPM. All I am looking for is a little more speed.

 

[jbcurt00]

Yes the anode is also a trim tab and looks just like that. jbcurt00. I did not know that tab was to counter act prop steer - it's so small so I thought it was just an anode. Unfortunately to remove or turn the trim tab I have to separate the lower unit from the upper unit and I don't want to go through all of that trouble. The diameter of the new prop is the same as the old one, maybe 1/4 more. The jump in pitch is to increase my top end speed and lower my RPMs. With the 17' pitch, I was only able to hit 27 mph at WOT. For a deep-v boat with 165 horsepower, 27 is pretty slow. My main interest is cruising and tubing with the boat, so I don't need a low pitched prop to get the boat out of the hole for skiers.

It's either the same or it isn't.

If you jumped up in pitch AND diameter, again, how did you decide on the 21P prop?

What diameter was the 17P and what's the diameter of the 21P?

Again: What RPMs are you running w/ the 17P?

27mph on an actual gps device or the boat's speedo? W/ an older Mercruiser 165 in a vintage boat, 27mph doesn't sound too bad to me.

How long have you owned the boat?

Have you ever weighed the boat to know if it's still basically the same weight it was when it was brand new? If the motor is 1974, I'd bet the boat is about the same age: 40+yrs old.

For I/O, like other boats, there is lots of variables that go into having the optimal prop for generally 'most' purposes, uses and loads.

This was posted elsewhere on iboats about a Merc 165 and picking a prop:

Re: mercruiser 165 prop suggestions?

Jk, welcome to Iboats prop forum and we will try to help you as much as we can but without more information no one can help you, and I will tell you I need more information than anyone else. But if you would like my help fill this form out and I will help you as much as I can, if you would prefer to not go to this much trouble there are other people who will help you.

Iboats Boat and Motor Info
1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don’t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer’s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

H

 

[jbcurt00]

You posted the 3700RPMs while I was typing my reply.

But again, you've said you were at WOT w/ 3700rpms then said you weren't technically at WOT at 3700rpms.

The 17P and 21P either ARE or ARE NOT the same diameter and you WERE or WERE NOT at WOT at 3700rpms. Neither spec can be both true and false.....

See the list I posted about what all could be variables effecting rpms and speed.

 

[auscarter12]

hwsiii

1971 Aristocraft 19 ft.
19 ft., 91' beam
Mercruiser 165 HP
Outdrive: Pre-alpha 1.62 gear ratio
Dry weight when new: 1650 lbs.
Fully loaded ~ 2300 lbs.
Old Prop: 3 blade aluminum- 14 1/2 in diameter, 17' pitch
New Prop: 3 blade aluminum - 14 1/4 in diameter, 21' pitch.
WOT range: 3900-4300 RPM
Deep V boat
No hydrofoil, but does have self regulating trim tabs


27 mph on a GPS device at ~ 3800 RPM (NOT wide open throttle) with old 17' prop. 4 people in boat, 1/4 tank of gas + supplies for trip (added maybe 600 pounds)

Motor has new distributor cap, new plugs, new wires, rebuilt carb, recently adjusted timing, new ignition coil, 2 new fuel filters.

Old prop (17') is bent in a few places for normal wear and tear. Not sure how long the prop has been on the boat.

I am trying to increase my top speed.

3,000 RPM - 24 mph
3,500 RPM - 26 mph
4,000 RPM - 28 mph

need my social as well?

jbcurt00

 

[jbcurt00]

No

But now someone can determine if the 14.25 X 21P is the right prop for how your boat gets used.

 

[Frank Acampora]

Are you certain you need to remove the lower unit to change the trim tab? Every Mercury lower unit I have ever seen has a plastic plug above the trim tab bolt. Remove the plug and reach in with a deep socket and/or extension.

 

[Bondo]

Ayuh,.... Even though the diameters are the same, the 21" prop, obviously has more rake, puttin' the blades into the anode,....

You can trim the anode so's it clears the prop by 'bout 1/4",...
Or,...
As Frank says, you can remove it, 'n replace it with one that sits further aft, to clear the prop,...
Or,...
You can just remove it, 'n put a flat one there,....

You only need to drop the lower to change the retainin' bolt,.... not the tab itself,...

If ya take it off, or go with a flat plate, my guess is, it'll become a real bear to steer at speeds,....

 

[robert graham]

What would be wrong with adding a small stainless steel plate to the trailing edge of the skeg....stainless plate with small stainless screws....bend the plate sufficient to give your steering trim the relief it needs at speed.....Heck, some of the newer outboards have curved skegs to provide trim. All this very similar to the trim tab on the rear of the vertical stabilizer of a small plan(Cessnas have them)....

 

[auscarter12]

robert graham Bondo Frank Acampora My mistake - upon further inspection I realized that I did not need to separate the units to remove the trim tab. I was unaware of the plastic plug until I looked it up in one of my Mercruiser books. I purchased a smaller trim tab with more of an angle and the prop is still hitting it but not near as bad. I may grind some of it off for clearance until I can get one that fits properly.

Another question - I understand that the trim tab helps with prop steer and the boat pulling. I have installed "Smart Tabs" self actuating trim tabs on the boat. I have yet to test these out and see the effects, but I've only read good things about them. These trim tabs should help offset the effect of replacing the zinc trim tab with a flat zinc anode, correct?

 

[ondarvr]

robert graham Bondo Frank Acampora

Another question - I understand that the trim tab helps with prop steer and the boat pulling. I have installed "Smart Tabs" self actuating trim tabs on the boat. I have yet to test these out and see the effects, but I've only read good things about them. These trim tabs should help offset the effect of replacing the zinc trim tab with a flat zinc anode, correct?

No, they do totally different things.

Also, until you make a run at full throttle and record the RPM's there is no way to tell what prop you may need.

 

[Bondo]

Another question - I understand that the trim tab helps with prop steer and the boat pulling. I have installed "Smart Tabs" self actuating trim tabs on the boat. I have yet to test these out and see the effects, but I've only read good things about them. These trim tabs should help offset the effect of replacing the zinc trim tab with a flat zinc anode, correct?

Ayuh,..... As ondarvr says, two completely different things, unfortunately commonly called the same name,....

I prefer to call the anode thingy We're talkin' 'bout as the Torque Tab, as that Is what it is,.... it's there to help with prop torque,....
but then I've always been alittle different,....

The Smart Tabs, are a Great product, by Great folks,....
Smart tabs are Trim Tabs, as they help control the Trim of the hull,...

 

[Watermann]

I went on the hunt for a new prop and it took me a couple tries to get it right. After reading about your set up, I think you're going to have too much prop for the old 165. I have a lighter boat with more HP and only run a 14.25x23 QS black diamond prop on it. Newer props with more cupping and aggressive rake will really take it's toll on your gear case and the motor will lug not to mention make it a dog to wallow out of the hole.

There's just so many factors to consider to know for sure without trying but if you haven't yet put that prop in the water you can return it for a replacement prop say in a 19p. The 19p would be closer to what the boat would need on it when it was new.

 

[JoLin]

There's just so many factors to consider to know for sure without trying but if you haven't yet put that prop in the water you can return it for a replacement prop say in a 19p. The 19p would be closer to what the boat would need on it when it was new.

I think he'd be better off just returning it at this point. Don't buy another until he's done a WOT test to give him a good feel for what he really needs. My impression is that the OP is spending a lot of money (smart tabs, new prop), fixing issues he may or may not even have.

My .02