More prop advice

[bones774]

I know there is gentleman here willing to give expert advice but at this time I don't have all the info he needs(gear ratio will have to wait for fall haul out).
But in general I have a very heavy cobalt (avg. wt 3800lbs) with a 350 v-8 270HP and a 14.5x19 prop, this prop is not factory installed. The boat accelerates fine to 3000 rpm and about 25mph from there the rpm's flatten out to around 3400-3500rpm and maybe 27mph and no further acceleration.
Manufacturer states WOT RPM at 4200-4500, the engine just doesn't wanna go there, the standard prop is 14x19 could that be such a big difference.

 

[hwsiii]

Re: More prop advice

Bones,that is not unusual at all, normally with the wrong prop blade geometry you either see a high degree of prop slip or the motor just is not apable of reahing the manufaturers WOT RPM. If you will all obalt I am sure they an tell you the gear ratio.


H

 

[Silvertip]

Re: More prop advice

HWSIII - you have a private message.

 

[bones774]

Re: More prop advice

Bones,that is not unusual at all, normally with the wrong prop blade geometry you either see a high degree of prop slip or the motor just is not apable of reahing the manufaturers WOT RPM. If you will all obalt I am sure they an tell you the gear ratio.


H

It sounds like the motor can't reach WOT RPM

I will call Cobalt tmrw. That little bit of difference in prop will affect that much, huh?

 

[hwsiii]

Re: More prop advice

Yes it an Bones.

Thank you Silvertip.


H

 

[bones774]

Re: More prop advice

I called Cobalt first thing this AM, spoke with tech, he does not have specs for 1988 but believes it is "standard gearing at 1.78=1"(Merc 260HP), does that sound correct?
Standard prop from factory was a 3 blade aluminum 14x19.
I'll start collecting more info as possible.

 

[hwsiii]

Re: More prop advice

BonesIboats 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 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

 

[bones774]

Re: More prop advice

OK, this weekend I started to winterize the boat and with the engine off and throttle in WOT I found that the butterfly in the carb did not open, I guess that has been the problem all along, wish I woulda checked when in the water.
Anyway, what do I have to do now to fix the carb problem?
Thanks

I would like to add a mistake I may have made was that i had engine at WOT in neutral and assumed butterfly was open w/o visually checking it, and apparently that was not the case.