21" to 19" no RPM change?

[whofan]

Buddy has a 19` Lund pro v powered with a Mercury 150 EFI. He is running at 4950 RPMs with a Mercury 21" Vengence Prop.

He wants to get 5500 RPMS.

He buys a Michigan Wheel Balistic 19" Prop.
He`s still running at 4950 RPM with the 19" prop with no noticible change in speed at 45-46 MPH WOT.
Motor is mounted at the first hole

Anybody Know what the problem could be?

 

[hwsiii]

Re: 21" to 19" no RPM change?

Fan, 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
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

 

[This_lil_fishy]

Re: 21" to 19" no RPM change?

If I was a betting man the numbers aren't exactly the same, but close enough to be annoying. My guess would be that the 21" is much less efficient then the 19", also are they both aluminum? As a 19" SS prop may be close to the 21" aluminum.

Ian

 

[jason32038]

Re: 21" to 19" no RPM change?

Changing prop size doesn't really change your engine speed much it's the pitch of the blades. Try a prop with a lower pitch. Go down one notch eg. 20 to 19 or 19 to 18. Stick with a 21" prop.

 

[whofan]

Re: 21" to 19" no RPM change?

Buddy has a 19` Lund pro v powered with a Mercury 150 EFI. He is running at 4950 RPMs with a Mercury 21" Vengence Prop.

He wants to get 5500 RPMS.

He buys a Michigan Wheel Balistic 19" Prop.
He`s still running at 4950 RPM with the 19" prop with no noticible change in speed at 45-46 MPH WOT.
Motor is mounted at the first hole

Anybody Know what the problem could be?

Both props are stainless steal.
Motor is a EFI. Not optimax.

He said motor is running strong and smooth.

His whole rig is in showroom condition.

The 21" is Mercury original. The 19" is what he tried to get the Rs up.

Both props don`t have a scratch on them.

As far as pitch goes doesn`t that change the size of the prop?

Both are 3 blade props. I`m not sure about gear ratio, prop cup or diameter.

Prop was reconmended by dealer/ mechanic.

He went to the mechcanic and told him there was no change . He said somethings not right.

GPS speed is same with both.

The only thing I can think of is the engine could be starving for fuel at WOT?

He said engine runs smooth at WOT not missing or anything.

 

[hwsiii]

Re: 21" to 19" no RPM change?

Theoretially there should have been at least a hange of 200 RPM, but that is not always the ase by any means. I would hek that there is enough fuel getting to the engine beause it very well ould be stalling at that speed and have nothing at all to do with the props. That is why I have the form I use to hek if there is possibly a motor problem or a tah problem. I have diagnose many motor and tach problems on here using my mathematical analysis proedures.


H

 

[whofan]

Re: 21" to 19" no RPM change?

Theoretially there should have been at least a hange of 200 RPM, but that is not always the ase by any means. I would hek that there is enough fuel getting to the engine beause it very well ould be stalling at that speed and have nothing at all to do with the props. That is why I have the form I use to hek if there is possibly a motor problem or a tah problem. I have diagnose many motor and tach problems on here using my mathematical analysis proedures.


H

Thank you for your reply. I will relay your information to him and see if he wants to persue this further useing your form as I don`t have all the info myself.
Thank You! hwsiii

 

[hwsiii]

Re: 21" to 19" no RPM change?

You are very welome.


H

 

[kick bass]

Re: 21" to 19" no RPM change?

Mr H. is correct!
actually I would recommend the 21" prop. It is funny but at that RPM the 19" prop is actually more efficient. I don't want to get in on the formula right now. You have a problem with you engine rev's. How long has he had the boat and did it ever perform better? Also, when he reaches this speed how does the engine sound? Does the sound change when he gets to max RPM's?
Mr H said that it should have increased the RPM's by at least 200, this is correct. actually Mr H was conservative at his estimate. the boat owner has the right idea, bringing the RPM's up. and he chose the right RPM. Evidently he has done some home work. Again ask if the engine sound changes when the RPM's max out. also coud you get him to run the boat at different speeds and tell us what the tach says? like 3000 R's 4000 R's 4500 ? he should have a 1:87 gear ratio and 5600 Max RPM's.

 

[jason32038]

Re: 21" to 19" no RPM change?

No it doesnt change the diameter of the prop. The pitch is the angle of the blades on the prop. Changing the pitch from say 19 to 18 will raise rpms about 500-550 rpm. Going from say 19 to 20 pitch will lower rpm. Changing the diameter of the prop doesnt change rpm much at all. There are many prop size\pitch combinations that will get him the performance he's looking for. Go to a known professional prop shop with a good reputation. The prop diameter and pitch are on the prop listing the diameter first and pitch is the second number I believe. I reccomend that you guys stick with a 21" prop. Look on the prop and find out what the pitch is and go down one number. This will allow the engine to wind up more because the pitch isnt at such an angle that it is pushing more water and putting more stress on the engine that is needed. On the other hand...getting the rpms up to 5,500 doesn't necessarily mean faster acceleration and top speed and could even result in worse fuel mileage.

Both props are stainless steal.
Motor is a EFI. Not optimax.

He said motor is running strong and smooth.

His whole rig is in showroom condition.

The 21" is Mercury original. The 19" is what he tried to get the Rs up.

Both props don`t have a scratch on them.

As far as pitch goes doesn`t that change the size of the prop?

Both are 3 blade props. I`m not sure about gear ratio, prop cup or diameter.

Prop was reconmended by dealer/ mechanic.

He went to the mechcanic and told him there was no change . He said somethings not right.

GPS speed is same with both.

The only thing I can think of is the engine could be starving for fuel at WOT?

He said engine runs smooth at WOT not missing or anything.