03cobraa
Cadet
- Joined
- Aug 6, 2009
- Messages
- 13
Hey Guys I cant achieve max RPMS! At WOT it will only go to 3300 RPMS at 41 MPH GPS. The recomended max rpm is 4200-4600. I'm about 1000 RPM low. Boat starts and runs great! Here are the specs:
Iboats Boat and Motor Info
1. Year, make and model of boat:1987 Thundercraft Malibu 190
2. Length, width and base weight of boat, look for boat decal on back of boat:19ft and 6' 6" wide Open Bow. Sorry no specs from NADA on weight
2a.What is the maximum recommended HP for your boat: for 1987 125HP
3. Number of people and gallons of gas normally on boat:4 adults 16 Gal
4. What do you use the boat for : Tubing/ski/pleasure
5. Is it a Deep Vee and if so how many degrees of deadrise: yes deep V
6. Year, make. manufacturer and model of motor:factory 1987 4.3 Chevy
7. HP and gear ratio of motor IMPORTANT Manual shows 125HP for 1987 2bbl
gear ratio of 1.68:1
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:4200-4600RPM
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:NA OMC Cobra
10. Is it a bass boat or does it have a pad bottom:NA open bow
11. Does it have a hydrafoil, dolefin or trim tabs:NA
12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop: factory OMC Cobra 14 1/2 X 19 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. 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 tavhometer and hour meter here:
http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories
RPM __3300_________ Speed ___41 GPS________ No. of people _____4____ Gal. Gas ___16 GAL._____
14. Are you at sea level or a higher elevation, give us the elevation in feet:750ft
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: checked butterfly yes WOT
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop:FActory not reaching WOT
17. Does the prop show any damage that you can see: A few nicks on the edges.
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: Should reach Max RPM's maybe more MPH
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.
Iboats Boat and Motor Info
1. Year, make and model of boat:1987 Thundercraft Malibu 190
2. Length, width and base weight of boat, look for boat decal on back of boat:19ft and 6' 6" wide Open Bow. Sorry no specs from NADA on weight
2a.What is the maximum recommended HP for your boat: for 1987 125HP
3. Number of people and gallons of gas normally on boat:4 adults 16 Gal
4. What do you use the boat for : Tubing/ski/pleasure
5. Is it a Deep Vee and if so how many degrees of deadrise: yes deep V
6. Year, make. manufacturer and model of motor:factory 1987 4.3 Chevy
7. HP and gear ratio of motor IMPORTANT Manual shows 125HP for 1987 2bbl
gear ratio of 1.68:1
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:4200-4600RPM
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:NA OMC Cobra
10. Is it a bass boat or does it have a pad bottom:NA open bow
11. Does it have a hydrafoil, dolefin or trim tabs:NA
12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop: factory OMC Cobra 14 1/2 X 19 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. 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 tavhometer and hour meter here:
http://cgi.ebay.com/ebaymotors/Tacho...Q5fAccessories
RPM __3300_________ Speed ___41 GPS________ No. of people _____4____ Gal. Gas ___16 GAL._____
14. Are you at sea level or a higher elevation, give us the elevation in feet:750ft
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: checked butterfly yes WOT
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop:FActory not reaching WOT
17. Does the prop show any damage that you can see: A few nicks on the edges.
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: Should reach Max RPM's maybe more MPH
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.
If I use a reasonable 13% slip I get 4400, and 4250ish at 10%, That tells me you are propped about right for cruising. If your hole shot is decent I think she is reasonable as is. You are also misstating the horsepower if she is indeed a 4.3. Lowest rating was around 175 for those.