What Pitch?

[greatoutdoors]

Hi Guys. I just bought a Mercury 40hp motor. The gear ratio is 2.0 It has a 11" diameter and 12" pitch prop. My boat is a 14' semi v that when all told with passengers and gear, motor etc, is probably at 850 lbs. It is rated for a 30hp tiller and 45hp console motor. The boat rockets out of the hole and gets on plane very quickly, rides on about the last two feet of the hull, and I'm near 40mph, if not, then I'm at 40. I don't have a tach. The WOT operating range on the motor is 4500-5500. I would imagine the engine is near the top of that range with the small pitch prop. I would like to gain top speed while dropping the rpms a little. So, I would imagine I would need a higher pitch prop, correct? What size would you guys suggest? The information I gave is as accurate as I get get at this point. Thanks.

 

[Silvertip]

Re: What Pitch?

Your numbers are incomprehensible because using 4500 RPM and the other data you supplied yields negative slip numbers which are not theoretically possible. Using 5500 rpm and the same data slip is still negative. You don't have a tach yet you want to drop rpm and gain speed. That doesn't happen unless you are extremely underpropped (engine is over reving). If the engine is over propped (not reving in range) then adding pitch will likely slow you down and lug the engine to its early death. You need a tach and accurate speed data (gps) to know what the engine is doing before spending money on props.

 

[hwsiii]

Re: What Pitch?

Great, I have been reading your past posts and I have to tell you that you are working in the dark when you don't know how many RPM and how many miles per hour you are going. If you will look at my number 13 you will see you can buy a tach and hour meter for $ 35 for your motor, just remember it has a short wire and has to be mounted close to the motor and you cannot add to the wire. And I am sure you know someone who has hand held GPS or even some cell phones have GPS built in so you will have a true GPS speed.
I also looked up the information for your 14' Aqualine boat but ouldn't find anything about it. When I looked up your motor I found that a 1970 Mercury 40 HP should turn 5,300 RPM at WOT and has a gear ratio of 1.7:1, please verify this. If you will fill out my form it will show you how to KNOW what the real gear ratio is for sure.
I also ran prop sliip numbers using the gear ratio of 1.7:1 and a true 12" pitch prop, if you are turning 6,300 RPM your maximum speed would be 38.5 MPH, but you would definitely be running past maximum RPM too much. In my opinion you are probably running about 34 MPH GPS speed.

Boat and Motor

Prop Picker

Prop Slip

Without real numbers to work with, no one can help you, so please fill out this form and I will help you as much as I can.

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 across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, 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, number of blades 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 digital tachometer and hour meter here: http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories $ 35
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. 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 as well as timing and advance, 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

 

[greatoutdoors]

Re: What Pitch?

Great, I have been reading your past posts and I have to tell you that you are working in the dark when you don't know how many RPM and how many miles per hour you are going. If you will look at my number 13 you will see you can buy a tach and hour meter for $ 35 for your motor, just remember it has a short wire and has to be mounted close to the motor and you cannot add to the wire. And I am sure you know someone who has hand held GPS or even some cell phones have GPS built in so you will have a true GPS speed.
I also looked up the information for your 14' Aqualine boat but ouldn't find anything about it. When I looked up your motor I found that a 1970 Mercury 40 HP should turn 5,300 RPM at WOT and has a gear ratio of 1.7:1, please verify this. If you will fill out my form it will show you how to KNOW what the real gear ratio is for sure.
I also ran prop sliip numbers using the gear ratio of 1.7:1 and a true 12" pitch prop, if you are turning 6,300 RPM your maximum speed would be 38.5 MPH, but you would definitely be running past maximum RPM too much. In my opinion you are probably running about 34 MPH GPS speed.

Boat and Motor

Prop Picker

Prop Slip

Without real numbers to work with, no one can help you, so please fill out this form and I will help you as much as I can.

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 across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, 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, number of blades 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 digital tachometer and hour meter here: http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories $ 35
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. 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 as well as timing and advance, 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

Thank you. I borrowed a GPS from a friend today, so I will be using that on the next trip out. Tach wont be here for about two weeks. As for the gear ratio and WOT, I got the info here under 40hp classic. http://www.marineengine.com/parts/prop/wot_prop.html
I also found some specs which match yours, but my motor is a model 400 and not a 402. So, I can't be totally sure until I do the test. Found those here. http://www.piranha.com/motor_specs.php?id=3816
I can tell you that your numbers seem to be accutate except for beam at waterline which is 49". I will fill this form out as soon as I am able to install the tach and provide you some more accurate numbers. I'll PM you when I do as it may not be for a few weeks. Thanks again.

 

[greatoutdoors]

Re: What Pitch?

I got the boat out today with the gps. I hit 28mph, but I had 3 guys on the boat, 225lbs, 200lbs, and 180 lbs and a bunch of gear. I was also running up river against the current of the Susquehanna river. It was a 3 mph current, as we were drifting down according to gps. Couldn't get to the lake to try it with one guy and no current. I was pretty pleased, I didn't think it would go any faster than around 22-23 with all that load. Tach should be here by next weekend, so I should be able to fill out the form then.

 

[hwsiii]

Re: What Pitch?

That sounds wonderful, I will be here.


H