Low top speed?

[DrEvil]

Hey all, I have a 1986 16.5' Peterborough Monza with a 3.0 Merc/A1 drive.

With average weight I run about 36-37MPH @ 43-4400 RPM. To me this just doesen't seem fast for this small of a boat with a 140HP motor. The prop is a 21P and I do have a hydrofoil on the back. GR is 1.98:1.

The hull is clean, the floors are new and there is no water under them. Also, the boat is running a tad light because all of the framework for the rear seats, and the back to back seats are not currently in the boat.

I am going to pick up a 19P to see what that will get for results, but what else should I look at in the mean time?

Thanks for the input and please advise if more info is needed.

 

[hwsiii]

Re: Low top speed?

Evil, your current prop slip is right at 16%, if that is with one person you might want to try a 4 blade prop for your boat.


H

 

[DrEvil]

Re: Low top speed?

Check. Will look into a 4 Blade. Should I stick with a 21P for cruising?

 

[hwsiii]

Re: Low top speed?

If you will fill this out I will be able to do a thorough analysis of your boat and motor and have better answers.

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 forgoing 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

 

[DrEvil]

Re: Low top speed?

Tried the 19P last night same top speed same rpm (36 MPH 4400 RPM)

Seemed to be a better holeshot, planed out at about 3000 RPM.

1:1986 Peterborugh Monza
2:16.5Ft X6 Ft Unknown weight as it is missing the tag
2a:140HP (Assumed)
3:2 with about 15 gallons of fuel
4:Cruising
5:Standard bow rider, Unsure what deadrise is
6:1989 Mercruiser 3.0
7:140 HP 1.98:1 GR
8:4200-4600
11: Has a hydrofoil
12: Mercury Aluminum 14x19P
13. 4400RPM @ 36MPH
14. Normal Elevation
15. New Plugs, Carb is clean, fresh fuel, new oil, Will check cap rotor and wires. They look newer though.
16. Just put it on yesterday from the 21P I don't think it was wrong at all.
17. No
18. Looking for a bit more top end
19. Will do.

 

[hwsiii]

Re: Low top speed?

Evil, pull that hydrofoil, it is slowing you down and stopping you from reaching your maximum RPM and speed. You should have gained about 400 to 500 RPM by changing from a 21" pitch to a 19" pitch in the same make and model of prop. I need to know exactly what make and model of props you are using for theses tests. If you feel you need something to raise the stern later you can buy a set of Smart Tabs SX for about $ 120, but wait until we get you a better prop and you may find you don't want anything.
I can tell you that you reduced your prop slip to 10% with the prop change.
As soon as you can I need some new numbers with the hydrafoil off of the boat, because they should be different than the old ones unless there is a problem I don't know about. But please check the butterfly valve in the carburetor and be sure when you have the throttle pushed all the way forward on the control that it is perpendicular to the throat of the carburetor. Don't just try it by hand at the carburetor, because that doesn't matter.
While you are getting this information I will be running some numbers for you.


H

 

[DrEvil]

Re: Low top speed?

The 19P was a Mercury prop I don't have the number handy.

I'll get in the water tonight and run some more numbers.

Thanks

-V

 

[hwsiii]

Re: Low top speed?

That sounds great.


H

 

[DrEvil]

Re: Low top speed?

Results:

19P ran 4500 @ 38 MPH with just me and Hydrofoil removed

21P ran 4400 @ 35 MPH with just me and Hydrofoil removed

Seems the 19P is better than the 21. Do you think there are any other options to gain a little bit more top end? I plan on smart tabs eventually.

Thanks for taking the time hwsiii, I appreciate it.

-V

 

[DrEvil]

Re: Low top speed?

I just looked at the choke plate... seems the elec. choke is not returning back to where it should when the motor is warm. I have another one and will try that as well next time I go out.

 

[hwsiii]

Re: Low top speed?

In my opinion something is holding your RPM down and that could definitely be it. Lets wait until you change that and make one more run. I expect to see more speed than you are getting. And you are very welcome.


H