New prop suggestions to increase overall performance?

[Lee J]

Hello,

My 'new to me' boat runs really well, but I think I might be able to get more top speed out of it with a different prop. I have a Mercruiser 233 which is a Ford 351W with Alpha 1 outdrive. The gear ratio is 1.32 (edited this). Another internet source says WOT should be 3800-4200rpm. Right now my boat gets on plane extremely fast for a boat it's size and will top out at 32.5mph on the gps.

The part that makes me think a new prop will help is that it wants to rev higher than 4200...it actually gets over 4300 and starts bogging, I only did this for a second and then dropped it back down to 4200. I sort of think the bogging is because it's got that rochester 2bbl carb and cant provide enough air/fuel, but I could be wrong.

I'll provide the specs below...any recommendations would be greatly appreciated.

1) Performance issue you are trying to correct.
I'd like to get more top speed, better cruise speed (currently 27@3500rpm), and optimum fuel efficiency

2) Current prop manufacturer, model, aluminum or stainless and as a minimum.
It says "48 16440 a40 16p" on it. Which I think is a 16x16 3 blade aluminum prop made by mercury from what I can find out. It looks like a ship prop, wide blades. On another forum someone called it an "elephant ear" prop and I can see why.

3) Current prop diameter and pitch (required).
16x16

4) Wide open throttle RPM and speed with an average load (very helpful)
WOT RPM seems to go until it can't which is over 4300 RPM. Speed was 32.5mph. WOT RPM max should be 3800-4200 from what I've read.

5) Engine/drive make, model, year, and HP
Mercruiser 233 (Ford 351W 233HP) , 1.32:1 gear ratio

6) Boat make model, year, length and weight
1976 Silverline 22 foot hardtop. Weight was listed at around 3900lbs when new.

 

[QC]

Re: New prop suggestions to increase overall performance?

I'm finding 4000 - 4600 WOT RPM. What year model? I found '75, '76 and '77 . . .

Define "bogging". If it is propped correctly it should should reach a max RPM and then stop rising. Technically this is a full lug condition as it has reached the point where horsepower available is equal to horsepower required. That's kind of a good thing

Edit: I find 1.32:1 ratio . . .

 

[Lee J]

Re: New prop suggestions to increase overall performance?

Hmm, It's a '76. Those numbers would change things quite a bit! When I say bog, its gets up over 4300 and then drops off, almost like running out of gas, but runs perfectly with no hesitations all the way up to that.

I had seen 3800-4200 from a few different posts I found on a google search, so it wasn't definitive. This one says it applies to all merc 233's and was from a seloc manual:
http://wiki.answers.com/Q/What_is_rpm_for_Mercruiser_233

The others were just from guys posting within other threads on other forums. Could the 4000-4600rpm figure be for the merc 255 with the 4bbl carb?

I think you are totally right about the ratio, I think I misread this chart:
http://www.seilermarine.com/info/mercruiser-drive-ratio-chart.htm I saw 351 and just assumed it was for the alpha 1.

 

[QC]

Re: New prop suggestions to increase overall performance?

I'd like others to chime in on WOT RPM, as I get mine from a ridiculous source . . . Piranha Propellers I use it for older I/Os as it is hard to get that data. The 1.32 was consistent with some Merc info I have. Funny thing is that shows the 255 as 3800 - 4200.

It should "drop off" when it reaches the max as there ain't no mo if she's running right and propped right.

 

[hwsiii]

Re: New prop suggestions to increase overall performance?

Lee, I find your gear ratio to be 1.7:1 instead of 1.32:1, please verify this.

And when I run a prop analysis on your prop I show 15 % prop slip with a 1.7 gear ratio and 34% with a 1.32 gear ratio, so I don't know what to believe, if my numbers are right or yours. In my opinion, I also believe that even 15% prop slip is more than it should be, especially with an elephant ear prop.

Prop Slip

The part that makes me think a new prop will help is that it wants to rev higher than 4200...it actually gets over 4300 and starts bogging, I only did this for a second and then dropped it back down to 4200. I sort of think the bogging is because it's got that rochester 2bbl carb and cant provide enough air/fuel, but I could be wrong.

We have to find out exactly what the maximum recommended RPM should be for your motor and you will need a serial number or model number for the outdrive as well so they can tell us what the gear ratio is, the easy way to know that is to call a mercruiser dealer and ask them to look it up.
We need to know this so we will know if your motor is running right or you need to rebuild the carburetor to get your motor running right.

When we have these answers then we can go forward and make better informed decisions.

QC, that is about when the RPM started hanging from 4,200 to 4,600 and I have it listed at 4,200 and 4,600, so I don't have the RIGHT answer. When Lee calls the dealer he will find that answer as well.



H

 

[Lee J]

Re: New prop suggestions to increase overall performance?

I think the engine is running well. When I got it it had a slight hesitation and I changed the plugs, fuel filter, and rebuilt the carb and it seemed to make it run great. It has a pertronix ignitor ignition, so no points. I am still trying to find out the WOT throttle figures, 4600 seems really high after hearing the engine run at 4200.

The serial number on the outdrive is 4202134, and after doing some more research I think I was wrong to call it an alpha one. I think a Mercruiser 233 is a seperate deal and I really think the ratio is 1.32:1. I have to check a couple more things though.

Thanks again everyone for your responses!

 

[Lee J]

Re: New prop suggestions to increase overall performance?

Ok, just got off the phone with Mercury and they confirmed that the WOT rpm's should be 3800-4200 and the ratio is 1.32:1.

So it looks like I'm getting an insane amount of slip? HWSIII, I was reading some of your other posts and I'm wondering if maybe a 4 blade prop is what I need, or are my numbers so out of whack do you think it could be something else?

Also, not sure if it's relevant, but the prop was repaired a year or so ago( he kept all receipts). It had fresh paint covering some pitting. If a pic is needed, let me know and I'll upload one tonight.

 

[hwsiii]

Re: New prop suggestions to increase overall performance?

Lee, I have never seen an elephant ear produce that kind of prop slip on a boat like yours. There are possibilities that we just need to go through until we find the real problem, and I ran a new Prop Slip calculation so we can go through them one by one.

Prop Slip 2

The first thing is we need to know that you have a CLEAN bottom on the boat with no slime, barnacles, protuberances, a ROUGH bottom or barnacles on the prop.

Then we need to be sure that there is nothing in front of the outdrive by the transom that is disrupting the water flow to the prop.

1. Prop slip is 34%, and that is just outrageous.

2. Prop slip at cruising is about the same as at WOT, that is expected.

3. Maybe the tach is off and the engine is only turning 3,300 RPM and not 4,300 RPM. That will be easy to check with a mechanics tach, but I do not believe that is the problem.

4. Maybe they actually repitched the prop when he had it at the prop shop, you can check that by taking it to the prop shop and have them check the pitch. But it would have had to have been repitched to about a 12" pitch to get the prop slip to 12%, which is a reasonable slip % with that prop on your boat, and I question this. You say it gets on plane very quick, that make this a possibility, but you ought to be able to run a higher pitch than this.

There is another possibility that is highly questionable, and I don't believe it is viable, but the hub could be slipping.

And you are very right, I don't believe going to a 4 blade at this time will cure your problem, we first need to find the problem.

I tried to list ALL of the potential items that could be causing this, we just need to check each one of them until we find the problem.


I would like for you to take me about 8 pictures of the bottom of the boat and the transom and a couple of the prop, so I can see if I see anything unusual.



H

 

[Lee J]

Re: New prop suggestions to increase overall performance?

I really do appreciate all the help! Here are some pics I just took. There is some stuff on the hull, about 80% wipes off with just a finger. I'm not even sure what it is, but it's feels like fine sand once it's on your fingers.

 

[Lee J]

Re: New prop suggestions to increase overall performance?

and a couple more:

 

[hwsiii]

Re: New prop suggestions to increase overall performance?

Lee, from looking at the prop pictures it appears to me that you have a major cavitation problem with that prop, and in my opinion, that is where at least one of the problems is with your prop slip.

I want to think about this some more today, and study all of the pictures again, to be sure I don't miss anything with the boat as well.

You did an excellent job with the pictures, I just wanted to let you know.



H

 

[QC]

Re: New prop suggestions to increase overall performance?

I agree ^^^^^^ and for those following this thread, those pits in the prop are from Cavitation which is literally a boiling of the water surrounding the propeller due to pressure drop and then a subsequent collapsing of the bubbles that eventually knocks little holes in the prop. It is actually somewhat rare and is usually confused with Ventilation which is the prop grabbing air, either from the surface or due to some turbulence in the vicinity of the prop. This is why the plate above your propeller on I/Os and OBs is actually called an Anti-Ventilation (AV) plate, and not an Anti-Cavitation plate as it is often referred to.

 

[Lee J]

Re: New prop suggestions to increase overall performance?

Wow, that is very interesting stuff! I've always thought cavitation and ventalation were basically the same thing, so this is great info.

Thanks so much QC and hwsiii for spending the time and energy to help troubleshoot this!

 

[hwsiii]

Re: New prop suggestions to increase overall performance?

Lee,

There is some stuff on the hull, about 80% wipes off with just a finger. I'm not even sure what it is, but it's feels like fine sand once it's on your fingers.

It is harder doing this long distance, than actually being there, but your pictures sure help a lot.

I can't tell how large those particles on the bottom of the boat are, but in theory, if they are large enough and they are really stuck to the bottom of the boat instead of just laying there, then they could be disrupting the clean water flow to the prop and creating the cavitation you are having.

I would appreciate it if you would take a washcloth and try to clean those particles off, and tell me how hard they are to get off. Also, if you would just look back at the last 5' of bottom before the transom and just tell me how much of those particles are there, that will answer my questions.

I find it very unusual that an elephant ear prop, of that size on your boat, has cavitation burns all over it.

There is a possibility that the particles are causing it OR it could be that the prop shop just ruined the prop when they tried to redo it for the previous owner, but if those particles don't come off easily that will help us make a better decision of which way to go.



H

 

[QC]

Re: New prop suggestions to increase overall performance?

I noticed CF numbers Lee. Where are you?

 

[Lee J]

Re: New prop suggestions to increase overall performance?

I'm by the Delta area in CA, near Antioch. The previous owner lived near Chico.

The particles are like small grains of sand and come off completely with a buffer. With my finger alone most of them rub right off. I'll definitely spend the time to remove all of them that I can get to with the trailer bunks in the way.

So my path forward: it sounds like I should clean the hull and then retest speeds/rpms. Once I eliminate that as a variable, then we work on prop pitch, etc?

 

[Lee J]

Re: New prop suggestions to increase overall performance?

I spent a couple hours cleaning the bottom of the hull and finally took it out today. Conditions were not the best for top speed tests, very windy.

I took a couple passes down a smooth section in both directions and the numbers were a little different, but no real improvement.

Basically, the GPS readings were around 31-33mph. About the same as the 32.5 I clocked last time. The thing that was slightly different this time is that the engine would max out at 4150rpm, not the 4300 it did last time.

Could my prop be that bad, or do you still think there is something else going on here?

 

[90sunbird]

Re: New prop suggestions to increase overall performance?

you boiled the prop, cavatation must be bad lots of slip. the 4 brl is not gonna do what u think if the motor is bogging your prolbem isnt a 2 brl carb believe it or not some times the boat will run better with a smaller 2 brl then a big 4 brl the factory put them on for a reason the smaller venturi will get better flow at low rpm!!and rochester is a nice carb for fuel and low end performance. i would adress the engine bogging issue before getting nuts about a prop cuse when your motor is running at peak you will need to get a new prop

 

[hwsiii]

Re: New prop suggestions to increase overall performance?

Lee, buy a 15" pitch Solas Rubex 4 blade aluminum prop from here at Iboats for less than $ 135. I have to assume at this time that the prop was ruined at the prop shop.



H

 

[QC]

Re: New prop suggestions to increase overall performance?

i would adress the engine bogging issue before

Please read the whole thread . . . there is no indication that there is an engine issue.