Change Prop or Bravo 3 Gear Ratio

[loutax]

I recently changed the motor in my Formula 252SS from a Mercruiser 7.2(454) to a 383 Stroker. Similar horsepower, less torque.
When I went to water test the boat No Pickup at all.

I changed the props from 26 pitch to 22 pitch which improved holeshot significantly.

Problem is that wide open throttle rpm is still only 4650. Spec on engine is 4800-5200RPM.

I am running a BRAVO 3 with a 1.81:1 gear ratio.

Should I change the gear ratio on the Bravo Three to a 2.0:1 or a 2.2:1 or should I go down to the last prop available at 20 pitch. Is there any difference between those two solutions. Props are obviously cheaper to change.
Top speed is not as important to me as holeshot.

 

[hwsiii]

Re: Change Prop or Bravo 3 Gear Ratio

Loutax, you haven't given near enough information for anyone to help you with your problem, other than an educated guess from the RPM you are doing right now to get you back up to 5,200 RPM and that would be a 20" pitch prop set. The part I don't understand at all is why you had a set of 26" props on there.
If the new motor had about the same HP at WOT then the new motor would have to be producing the same torque to produce that same HP, and it isn't. Torque and HP have a direct mathematical relationship to each other.

Illustration

Old Motor 330 HP Producing 600 lb. ft. Torque Theoretically​

330 HP = (Torque 600 lb. ft. X Shaft RPM 2,873) / 5252

Gear change to match Shaft RPM​

H

 

[TilliamWe]

Re: Change Prop or Bravo 3 Gear Ratio

I agree. I understand the holeshot suffering, but the top end should have been very similiar. Are you sure that your tachometer is reading correctly?

 

[HT32BSX115]

Re: Change Prop or Bravo 3 Gear Ratio

I think your tach might be off too. Also, what are you using to determine your speed?

Your boat is not that much larger than my 87 211 Liberator (I'm 21' not counting the swim-step I think yours includes the swim-step)

I also have a 1.81:1 drive and I run 26p props. I get about 57mph(GPS) at 4600 RPM and 8% slip. (and it launches like a rocket by comparison to the original 460 King Cobra )

How fast did it run before (with the 454) at 4600 RPM?

 

[hwsiii]

Re: Change Prop or Bravo 3 Gear Ratio

Lou, 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 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
RPM ___________ Speed ___________ 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, 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