Prop Help Please!

[Xesvuli420]

1987 Dixie Bandit
3.0L OMC/ Cobra IO
140hp
WOT Range 4200-4600rpm

My used boat came with a 13.25x3x17p Prop. It was really bad UNDERpropped. I could only get 3/4 thru the throttle before I was at my MAX WOT range.

IF I was to push it to see what RPM the boat would hit with that prop, It would most likely be around 5200-5600

SO, I ordered a 13x3x21p prop to bring d

own the RPM range, and boy did it. Now at WOT I don't even hit 4000rpm unless the water is perfectly still, and I am going STRAIT!

Here are my findings...

13.25x3x17 = >5100rpm, 34mph top GPS speed (Prop from previous owner)
12.8x4x17 = >5100rpm, 32mph top GPS speed (New Ski Prop)
13x3x21 = 4000 max RPM, 38mph GPS speed (Speed Prop)

My question...

I assume that now I need to go to a 19p prop to hit the "desired" WOT range, but if I do will I lose or gain MPH? To me it seems that if now I am at MAX RPM @ 4000, and my "power" is created from 4200-4600rpm, then I should have more top speed if I do go down a size.

I need to decide what to do to get the vessel in PEAK operating conditions, So any help, advice, or testimonials would be greatly appreciated!

Thanks guys

 

[Gary H NC]

Re: Prop Help Please!

16 pitch to a 21 was a huge jump.I would bet a 19 would have been better.
Post this in the prop section...Those guys there will really help.

 

[walleyehed]

Re: Prop Help Please!

Yea, 5" pitch jump is huge.
The best way to find out is run the 16P wide open just long enough to find out where it would go but you got numbers with a 17 anyway so that's ok. 5100 won't hurt it. A good, high-quality alum in 19P 3-bld will put you likely at an optimum range over-all. I'd keep the 4-bld (12.8 sounds like a comprop) for a power prop.

 

[Xesvuli420]

Re: Prop Help Please!

I know it was a huge jump, but i was told to expect around 100-200 rpm loss per inch, so If i was WAY under propped with the 17, I did the math. from a 17 to a 21 is 4 inches. 200 Drop+4 inches=~800 Drop. at best my WOT was 5100, but im sure that it could have hit 5500, either way lets use both.
5100rpm - 800 = 4300
5500rpm - 800 = 4700
My WOT range is 4200 - 4600, so according to the whole "100-200" per inch theory, the 21 would be right.

Thats why I made such a big jump. I was trying not to have to buy prop after prop until I hit the right one. Oh well.

So heres my questions...

Would it hurt the boat if at full throttle, I was only at 3900-4000 rpm when WOT range is 4200-4600?

Would I increase in TOP speed if I got a smaller 19pitch prop.(less prop, but more rpms)..If so by how much would you guess MPH wise

Which one would give the best gas milage?

Thanks guys

 

[Xesvuli420]

Re: Prop Help Please!

Yea, 5" pitch jump is huge.
The best way to find out is run the 16P wide open just long enough to find out where it would go but you got numbers with a 17 anyway so that's ok. 5100 won't hurt it. A good, high-quality alum in 19P 3-bld will put you likely at an optimum range over-all. I'd keep the 4-bld (12.8 sounds like a comprop) for a power prop.

Correct it is a comprop, and I am keeping it as a ski prop. I just want to get one that gives my boat its top speed possible.

 

[steelespike]

Re: Prop Help Please!

The problem with being over propped is the motor is lugging whether its at 1,500 rpm or 4,000, bad for the motor.The 100 to 200 rpm rule of thumb is
a very approximate figure based on same brand,same style,same material
prop.Even then no guarantee.I usually go toward the light side because you can still use the prop by watching the rpm.Its possible you could have a prop shop remove some pitch or some cup if any.I've seen this a number of times on here with excellent results.You do need to be sure the shop knows what its is doing.If you do go for the 19 stay with the same brand/style.

 

[Xesvuli420]

Re: Prop Help Please!

So heres my questions...

Would it hurt the boat if at full throttle, I was only at 3900-4000 rpm when WOT range is 4200-4600?

Would I increase in TOP speed if I got a smaller 19pitch prop.(less prop, but more rpms)..If so by how much would you guess MPH wise

Which one would give the best gas milage?

Thanks guys

Thanks again

 

[Xesvuli420]

Re: Prop Help Please!

Really? no one can tell me if top speed will go up or down by decreasing a 21p prop to a 19p prop?

 

[hwsiii]

Re: Prop Help Please!

Xes, I need to know exactly the make and model of eah prop that you have and I also need this form filled out if you would like my help.

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 Tiny Tach for $ 50
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

 

[walleyehed]

Re: Prop Help Please!

Really? no one can tell me if top speed will go up or down by decreasing a 21p prop to a 19p prop?

It's not that simple and if it were, I know 2 of us here would be dirty rich...

OK, if a 21 is turning 3900-4000 and your engine developes it's power at 4400 (a variable that isn't IN any formula) and a 19P put you "about" 4400 it would be logical that if the correct 19P style was used, yes, your speed would be greater. Not all props are created equal or even close so sometimes just recommending a certain pitch is not the answer.
You say "decrease" are you thinking of having the 21 re-pitched????