Prop for 4.3L Merc I/O

[stephanofd]

Hope a few of you can help me with this. Local prop shop says I have the right prop but after reading some of the great info here, I'm not so sure.

Iboats Boat and Motor Info
1. Year, make and model of boat
1993 Celebrity 180 Status
2. Length, width and base weight of boat, look for boat decal on back of boat
18', 91" beam, 2500 lbs base weight.
2a.What is the recommended HP for your boat
175
3. Number of people and gallons of gas normally on boat
2 adults, 2 children, assorted gear ~ 700lbs + 25 gallons of gas
4. What do you use the boat for
Some cruising, mostly towing and skiing.
5. Is it a Deep Vee and if so how many degrees of deadrise
It's a Vee hull, couldn't find the deadrise, if anyone knows, please share.
6. Year, make and model of motor
1993 4.3L LX Mercruiser
7. HP and gear ratio of motor IMPORTANT
175 HP, 1.81:1
7A If you have a mercruiser and don?t know the ratio get your serial off the outdrive and go here http://www.sterndrives.com/mercruisersterndrives.html
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
4400 - 4800
11. Does it have a hydrafoil, dolefin or trim tabs
Stingray foil, no tabs.
12. Make, model, diameter, pitch and whether SS or aluminum prop
Currently Quicksilver 14 1/4 x 21 SS 3 blade
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.
WOT is 5400 RPM's GPS speed is 41.5 MPH.
14. Are you at sea level or a higher elevation, give us the elevation in feet
297 ft.
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 forgoing could be the reason your prop is not attaining full RPM
Engine was tuned in June 09, compression on all cyl's in 160 - 175 range.
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
Just bought the boat in June, it came with it. The RPM's are exceeding the high range for this motor. I don't have a problem holding back on the throttle, this prop gives me pretty good hole shot, it can be on plane in ~ 10 seconds. I would like to buy a spare prop and would like to know which way to go. Was thinking 4 blade 23 to get my RPM's down and keep my hole shot. Not worried too much about TS.
17. Does the prop show any damage that you can see
It's clean and sharp.
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
See # 16.

Thanks for any help you guys can give me.

 

[steelespike]

Re: Prop for 4.3L Merc I/O

hwsiii should be along with some specific info.
It seems to me you either have a lot of prop slip or your tach is off.
You might confirm it is set right and accurate.
You would have to go with about 24" pitch to get around 4,800
seems extreme to me.Would really kill your hole shot.

 

[sleeper]

Re: Prop for 4.3L Merc I/O

5400 rpm on your boat with a 21p prop should be 50+ mph. You are showing about 30% prop slip currently, which seems extremely high. Your tach could be off, has it been checked?

 

[hwsiii]

Re: Prop for 4.3L Merc I/O

Spike and sleeper are definitely right your tach is definitely wrong. Try turning the switch in the back back and forth to clean the contacts and the get a tach that you can use temporarily the next time you run the boat and give us the new numbers and we will pick a prop for you.


H

 

[hwsiii]

Re: Prop for 4.3L Merc I/O

double post

H

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

I will check the tack next weekend if the weather is clear. I will clean the contacts before we hit the water. Any suggestions on where I can borrow a tack temporarily?

Thanks for the help.

 

[steelespike]

Re: Prop for 4.3L Merc I/O

If you have a marina you use regelarly you may be able to borrow a shop tach long enough to do a test run.

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

If you have a marina you use regelarly you may be able to borrow a shop tach long enough to do a test run.

I do, and I will check with them this week. Rather not have to buy one.

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

Talked to my prop guy today. I am going to take my prop in and let him put some more cup into it to help pull the rpm's down. If that doesn't work then the shop mechanic said that he could take a run out to the water with me to check the tach, but there was no way he was loaning me his $500 shop tach.

 

[hwsiii]

Re: Prop for 4.3L Merc I/O

First lets see what the RPM are, then see if we need anything done to the prop.


H

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

First lets see what the RPM are, then see if we need anything done to the prop.


H

Oops! Sorry, too late, dropped it off Tuesday afternoon to get a new throttle cable put on. That's when I talked to him about the RPM's and he suggested adding cup. He called Wednesday morning to let me know it was done. How's that for service. I didn't think about getting the cable replaced when I originally posted, could that have an effect on where WOT will be? I talked to the shop mech. and he told me that he checked the RPM's in June when it was in for a tune-up and everything looked good, but he never went over 1500 RPM's. He suggested trying with the new cup and see where my RPM's were and if there was still a problem, he would see when he could get to the water with me to check WOT with his shop tach. I might have mentioned before that he wasn't letting that shop tach out of his shop unless it was in his hands. I don't have any other way of checking the RPM's.

Thanks for all the help and advice, I will report by Sunday what I got on the water Saturday.

 

[hwsiii]

Re: Prop for 4.3L Merc I/O

LOL, well we will see when you take it out.



H

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

LOL, well we will see when you take it out.

H

Well, got it out Saturday morning with pretty smooth water. After getting the motor warmed up ~ 170F, I took it out for a WOT straight run across the water. With our normal load (see OP), our newly cupped prop, and a new throttle cable, I hit 44.2 MPH (GPS) at 4900 RPM's. This seems a bit more inline with what I think it should be. Hwsiii, can you plug this into your spreadsheet and tell me if this is normal? and if so, could a different prop give me any better top end or hole shot? I would like to buy a spare prop. I might as well buy one that gives better performance with top end or hole shot, I need to know which one I should be buying for and what I should get. Does that make sense?

 

[hwsiii]

Re: Prop for 4.3L Merc I/O

Stephan, the cup you had added helped bring your prop slip down from 30% to 22%, but that still is not good. You are now running at least a theoretical 22" pitch with the extra cup added to the prop. And in my opinion 22% prop slip is still too high for your boat and motor setup, in my opinion you still have one of two problems, either the stingray is stealing WOT speed or the tach is wrong.
Pull the stingray and run the boat without it and lets see what the new speed is. We need to know what is not right before we pick a new prop, the only way to make an informed decision is to know what everything is doing and what it is helping or hurting.

Prop Slip

The following shows what pitch prop is required to reach the speeds attained with less than 10% prop slip.

Prop Pitch by Speed

H

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

Pull the stingray and run the boat without it and lets see what the new speed is. We need to know what is not right before we pick a new prop, the only way to make an informed decision is to know what everything is doing and what it is helping or hurting.

H

Will do. I have a trip out of town next weekend, so it might be two weeks before I can actually test it out. Getting off early enough on a weekday is usually pretty tough.

Thanks for the insight. I will get you an update on the Stingray in a couple of weeks.

 

[hwsiii]

Re: Prop for 4.3L Merc I/O

That sounds great Stephan, I will be here. I hope I made it clear enough why I said what I said. LOL



H

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

That sounds great Stephan, I will be here. I hope I made it clear enough why I said what I said. LOL



H

I think I followed it, if not, you can straighten me out after my next report.

 

[stephanofd]

Re: Prop for 4.3L Merc I/O

Stephan, the cup you had added helped bring your prop slip down from 30% to 22%, but that still is not good. You are now running at least a theoretical 22" pitch with the extra cup added to the prop. And in my opinion 22% prop slip is still too high for your boat and motor setup, in my opinion you still have one of two problems, either the stingray is stealing WOT speed or the tach is wrong.
Pull the stingray and run the boat without it and lets see what the new speed is. We need to know what is not right before we pick a new prop, the only way to make an informed decision is to know what everything is doing and what it is helping or hurting.
H

Hwsiii,
Looks like the Stingray wasn't doing anything but helping me plane. I will have to talk to my mechanic about taking a cruise next weekend with his shop tach. Without the Stingray on the stern drive, I hit 44 mph at 4800 rpm's and 45.2 at 5000 rpm's. Not much difference with or without the Stingray on WOT, but boy I sure could tell the difference in planing. Instead of planing off in 8 to 10 seconds, it was taking 25 to 30 seconds, and when I eased back on the throttle, it would drop out of plane at about 15-18 mph instead of the usual 10-12 I get with the Stingray.

Any other ideas other than a run with my mechanic and his shop tach?

 

[wca_tim]

Re: Prop for 4.3L Merc I/O

it sure sounds like something isn't right...

running a 21" prop on a ca. 18 foot boat with a 4.3 in it 5000 rpms was 50+ some change and it planed in nothing flat, a 23" prop was upper 50's touching 60 or 61 some days at 4700 or so. that was with almost a thousand pounds less than you carry and 50 horsepower more. surely something is wrong. is something bent (ie skeg)? is your bottom dirty (the BOAT!!)? how about waterlogged foam or something - any chance that you're carrying hundreds of extra pounds of weight? what about trim? is your trim working right and is it trimming out as far as you think it is or trying to plow it's way through the water... or the other way is it trimmed way out or something? that's a LOT of slip for such a small boat and maybe more prop than you should be able to turn...

Do you any friends or a shop that'll loan you a different 21" prop? without the slip you're getting i would guess that boat should run sweet with something like a 23" laser... vented to help bring it out of the hole quicker. I'm interested to see what others think...

how about gear ratio? Is there any chance the boat wound up with a 4 cyl upper ratio at some point? That would put you at 10% slip and make a lot more sense... a 21" prop should be about right or maybe even a little tall for your setup based on what I'm reading...

 

[hwsiii]

Re: Prop for 4.3L Merc I/O

Stephanof, you need a 21" pith Solas Amita 4 blade prop, it will derease your prop slip a lot and improve your hole shot and aeleration very muh. You should be as fast or maybe faster with this prop and you will like the way the boat moves up higher in the water olumn and how smooth it runs.


It is available right here at Iboat and here are the numbers and ost.
Price: $103.99
You Save: $43.01
Compare: $147.00
Discount: 29.3% Off
Add Amita Aluminum Propeller 1513-140-21 To Your Cart

Propeller Specifications:
SKU:#1513-140-21
Manufacturer: Solas
Brand: Amita
Material: Aluminum
Diameter: 14?
Pitch: 21?
Blades: 4
Rotation: Standard (Clockwise)
Usually Ships Within: 1 Business Day


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