Mercruiser 5.7 maxing 4k rpm Prop incorrect?

[nazz2005]

Hi, just pulled my prop i have a 15x17p stamped on it. this prop has been on my boat since i got it 6 yrs.(sitting a while) 2000 24' BR Mercruiser I/O 5.7 250hp boat weight 3750. gear and people fuel i do about 4200. current top speed by gps is 35.2 mph at 4000-4050 rpm. believe engine is up to spec...new plugs,cap,rotor,oil,fuel,risers,exhaust manifold water pump,fresh bottom paint..
Does come out of hole nice,plans perfect, then is maxed instantly..Just would like to get into safe operating range 44-4800 rpm..
Sound like a prop problem?

 

[hwsiii]

Re: Mercruiser 5.7 maxing 4k rpm Prop incorrect?

Nazz, 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 gladly 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 maximum 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. manufacturer 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 a c ross 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, use a board under the keel and stiking out to the anti ventilation plate for a referene
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. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for $ 50
RPM ___________ Speed ___________ No. of people ____________ Gas ________
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

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H

 

[johno71]

Re: Mercruiser 5.7 maxing 4k rpm Prop incorrect?

I am having the same issue. Here is the completed form as best as i can do it right now.

1. Year, make and model of boat
1991 Wellcraft 233 Eclipse

2. Length, width and base weight of boat, look for boat decal on back of boat
23.5', 8'6" beam and I am guessing around 5000lbs with me & 1 person on board as normally used

2a.What is the maximum recommended HP for your boat
Don't know for sure, but it is a 330hp 1996 350ci mercruiser engine from eagle engine sales.

3. Number of people and gallons of gas normally on boat
2 people and about 35 gallons of gas

4. What do you use the boat for
Cruising and anchoring out

5. Is it a Deep Vee and if so how many degrees of deadrise
I am not sure

6. Year, make. manufacturer and model of motor
1996 mercruiser 5.7l 350ci

7. HP and gear ratio of motor IMPORTANT
330hp (advertised) probably more like 300hp. 1.5 gear ratio in alpha one replacement drive from sterndrive engineering.

If you don?t know the ratio, you need to pull the plugs out and put a piece of tape a c ross 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
4400-4800

9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a board under the keel and stiking out to the anti ventilation plate for a referene
Not sure

10. Is it a bass boat or does it have a pad bottom
it is not a bass boat, I am not sure what a pad bottom is

11. Does it have a hydrafoil, dolefin or trim tabs
It has a dolefin, SE300

12. Make, model, diameter, pitch and whether SS or aluminum prop
QL 14.8 x 17 3 blade 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. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for $ 50
RPM - ~3900 Speed 42mph No. of people 2 Gas 20gallons

14. Are you at sea level or a higher elevation, give us the elevation in feet
Sea level (florida)

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
New engine with about 6 hours on it

16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
Prop has been on there for a while, Installed new engine and new outdrive. I think it is the wrong prop because i am not getting above 3900 rpms, and I can feel some vibration when it is in gear. No vibration when it is in neutral.

17. Does the prop show any damage that you can see
Prop shows light damage. Some knicks and dings.

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
Trying to make sure I am not overloading the new engine i just installed. I am pretty ok with the speed, Just want to make sure i treat the new engine well.

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.
I can do this, but I don't really want to run the boat with the damaged prop. The outdrive is brand new and I don't want to damage it by putting undue stress on it. I am just tryin to figure out a good baseline prop to begin testing. I am not sure what the factory diameted prop should be and I am a little concerned the the diameter is too large.

Thanks for the Help,
John

 

[steelespike]

Re: Mercruiser 5.7 maxing 4k rpm Prop incorrect?

Haven't seen much of hwsiii since the oil spill.
Someone with more knowledge than me should be along.
Speaking very generally my info indicate a 3,900lb dry weight.
and originally 260 hp. If your boat is a deepV it will have a pronounced V at the stern. At 330 hp I don't think a 17" pitch is excessive. You are lacking about 900 rpm to make 4,800.This would require about a 4" pitch change.(200rpm per inch rule of thumb)Putting you at about 13" seems really small to me.There are only 2 props
smaller on the chart.You might varify the throttle opens all the way don't assume because its new that its right.Also me sure it is timed right and advancing as it should. The Doelfin could slow it some.Your better off with trim tabs and or a prop with stern lift.Is the bottom clean and smooth?I would try it without the foil.It will likely plane slower but may gain some speed and rpm.You may want to check that the tach is accurate.

 

[johno71]

Re: Mercruiser 5.7 maxing 4k rpm Prop incorrect?

I think you are correct about the doelfin. I had it out once without it on there and it was a little faster but slower to plain. I know the boat lists at 3900 dry, but I have added a lot of weight in the interior. I also keep a small generator on board along with a hatch top a/c unit. The bottom is clean with fresh bottom paint. I will double check the carb, but the last time i looked it was opening all the way. Mabey something is hanging up in the linkage or cable. I was thinkin about going to a 4 blade prop, but I am concerned about the outdrive lasting. I know 330hp is pushing it on an alpha one with a boat as heavy as mine. I think the "extra grip" on the holeshot may be a bit too much for the drive.
Thanks,
John

 

[steelespike]

Re: Mercruiser 5.7 maxing 4k rpm Prop incorrect?

I think your probably right about the weight.
Two ways to think about the 4 blade;it does offer stronger bite but also you should be able to plane quicker possibly at less power.Over all less strain on the drive.You also should consider trim tabs either active tabs or passive like the smart tabs.When considering strain on your alpha1 imagine the strain from a fin that is controlling your 5,000lb boat.

 

[raysprops]

Re: Mercruiser 5.7 maxing 4k rpm Prop incorrect?

If your R.P.M. is below specs by this much and everything else is good your most likely over proped. Before changing props the one other thing you can check for is engine room venting, get the boat wound right up and then watch the tach while you have some one crack the engine cover to check that there is no restriction in the venting. Also if you have had to replace an impeller (water pump) you may want to check that there are no pieces caught up in the exhaust, you would be suprised what I find in the thru hubs of props I repair. If all else is good try a 16X 16 3blade or a 15X16 4 blade. try to test them out before you buy as aprop that doesn't work is worthless to you.