Prop Diameter ?

[Sparkscw]

I have an 89 17' four winn with a 2.3L OMC about 2050#.
I am running about 5200-5600 RPM at max and getting about 30mph (verified with gps), lite cruising at 4800-5000 rpm and getting about 25mph.
The prop that is on it is alluminum and a 13x19, the specs say I can use up to a 14x(something).
So I guess my question is: How does the diameter directly effect the top end speed and overall performance of the boat. Would I gain anything by changing to say a 14x19 prop?

Thanks
David

 

[junior1113]

Re: Prop Diameter ?

5200-5600 is rather high 4400-4600 is optimal. id put a 4 blade 20 pitch. diameter is not what you need to change.

 

[hwsiii]

Re: Prop Diameter ?

Spark, the information I found on your motor says it should be turning between 5,200 to 5,600 RPM with a 2:1 gear ratio(verify this please), but when I run the numbers I find that you ought to be able to get about 37 MPH. But when I run the numbers you posted you are showing an extremely high prop slip of 36% so there is something wrong with your setup somewhere unless you had a lot of people on the boat with you when you got these numbers. If you didn't we need to find where the problem is and it very well could be you need to increase the diameter or go to a 4 blade prop and drop the pitch a little bit in order to get more blade surface area to decrease your prop slip.

Prop Slip

If you would take the boat out again with just you in the boat and give me a wide open throttle RPM and speed we can figure out what prop might be best suited for your setup.


H

 

[Sparkscw]

Re: Prop Diameter ?

Spark, the information I found on your motor says it should be turning between 5,200 to 5,600 RPM with a 2:1 gear ratio(verify this please), but when I run the numbers I find that you ought to be able to get about 37 MPH. But when I run the numbers you posted you are showing an extremely high prop slip of 36% so there is something wrong with your setup somewhere unless you had a lot of people on the boat with you when you got these numbers. If you didn't we need to find where the problem is and it very well could be you need to increase the diameter or go to a 4 blade prop and drop the pitch a little bit in order to get more blade surface area to decrease your prop slip.

Prop Slip

If you would take the boat out again with just you in the boat and give me a wide open throttle RPM and speed we can figure out what prop might be best suited for your setup.


H

hwsiiii,
Yes as far as I can determine the ratio is 2.0:1 ( of course this is off the specs), there were only two people in the boat when I got the info posted ( approx 400# of people) 3/4 tank gas (12-15gal) and no other gear. All the specs I can find for this motor put WOT at 5200-5600rpm and because its a 20yr/old motor I am careful not to push it too hard.
I will be going out next week and can retest the #'s. Although out of two trips with all the same info these #'s have been the same.
I don't know what magic you are useing to come up with that niffty chart but, if it will recomend a prop I will give it a try. However, I really don't want to spend several hundred dollars trying different props just to gain a few mph.

Thanks David

 

[hwsiii]

Re: Prop Diameter ?

David, I regularly help people get a better prop than the one they are running currently. It gets better speed or it gets their RPM to where they ought to be instead of way too low or high. I am not a magician and so I am not perfect, but I do get it better the first time. If you will go through the posts on here you will see that.
If somebody is trying to get the absolute maximum speed out of their boat it is a trial and error process, and it takes trying many props to get there. But your problem is simple, you have wayyy too much prop slip, and you only need to decrease your prop slip to raise your speed.
Having said all that, you are quoting a range of RPM for a certain speed, I need good numbers to work with and the ones you have are not good numbers. A boat at any point in time and circumstances has a specific speed at a specific RPM, and with the numbers you have that is not the case. Here is an analysis if I use your ranges for RPM and the speeds you say and you can see why I need exact numbers.

Prop Slip

Here is the information I need if you want 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 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 and model of motor
7. HP and gear ratio of motor IMPORTANT
7A If you have a mercruiser and don?t know the ratio get your serial off the outdrive and go here http://www.sterndrives.com/mercruisersterndrives.html
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
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. 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 forgoing 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

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

 

[Sparkscw]

Re: Prop Diameter ?

hwsiiii,

Don't take my dry sense of humor as derogatory.

I certainly appreciate the help and info.
I will get you the info you have asked for and probably ask about 37 more ???, I am not really looking for a super fast boat ( it is only 128hp after all), I am looking to get this little boat running as efficient as I can.
I will gather the info and post back soon.

Thanks Much
David

 

[hwsiii]

Re: Prop Diameter ?

David, I will be waiting for the new numbers, and please just one person in the boat for that, it makes my calculations easier. LOL
If you will also get the rest of the information as well we will get you a much better prop that you will be happy with. It requires time and effort to get it right and I try to help a lot of people, and sometimes I can take things wrong. I just have no reason to help people who won't help themselves by trying their hardest to get the correct information, but I see you are not that way and it will be a pleasure to work with you.


H

 

[Sparkscw]

Re: Prop Diameter ?

H,
I need to go out next week and get some more accurate running numbers for you to use. I have all the other info gathered and in an excell spreadsheet. I will post all of it by mid-week or I can e-mail to you. Let me know.
For sure I will get you new numbers after we get through with all this and you have helped me to get a little more out of my boat.
Thanks, David

 

[hwsiii]

Re: Prop Diameter ?

David, my email address is hwsiii@yahoo.com if you want to email what you have. But I will tell you I get so much email I sometimes miss things. :redface:


H

 

[Sparkscw]

Re: Prop Diameter ?

H,
I have some more acurate info for you:

# of people: normally only 2 adults in boat # 400
gal of fuel : at time of test 13.5gal

Boat use: fishing and minimal tubing
V and dead rise: I would say a moderate "V" w/chines about 19 degrees
about 11" rise and 32" run measured from the transom center line

yr,make,model engine: 1989 Ford 2.3L

motor Hp: 128 hp
gear ratio: 2:01 ratio
WOT range: 5200-5600 WOT
hydofoil: yes has a hydrofoil installed

Prop info current: MI wheel vortex 992115 13x19
info rpm,mph,gear,gps: 4000 rpm@29.5(gps), 45lbs gear
elavation: 700 above sea level
motor condition: fresh tune everything checks out
new prop: brand new less than 10 hrs no damage
problems to cure: looking for efficiency, mph increase to about 35-37mph
never got over 4000 rpm w/external Tac hooked up
Ok so I hope this all makes sense to you, let me know what you think
Thanks, David

 

[hwsiii]

Re: Prop Diameter ?

David, you mean it couldn't turn more than 4,000 RPM with just you in the boat. The other readings showed 5,200 to 5,600 RPM?


H

 

[Sparkscw]

Re: Prop Diameter ?

David, you mean it couldn't turn more than 4,000 RPM with just you in the boat. The other readings showed 5,200 to 5,600 RPM?


H

H,
That is correct (two people in the boat 400# of people), couldn't shake the wife for a test run like you asked.
The 4000rpm was read from an external Tacometer that I hooked up just for the test run, not the dash Tac.
Now I should add that I installed an engine mounted electric trolling motor 55em such as this:
http://www.minnkotamotors.com/products/trolling_motors/engine_mount/saltwater.aspx

 

[hwsiii]

Re: Prop Diameter ?

What did the dash tach show for RPM, and did you install this since you had readings of 5,400 RPM.

H

 

[Sparkscw]

Re: Prop Diameter ?

The dash tac was reading about 5400 and yes I installed it after I started this post.

 

[hwsiii]

Re: Prop Diameter ?

David, you need to take the hydrofoil off, all it is doing is stealing MPH at top speed. Here are your new Prop Slip Numbers. And if you want to change props you would need to go to at least a 15" pitch prop to raise your RPM to about 5,100 RPM and to get to 5,400 RPM you would need to go to a 14" pitch.

Prop Slip

Prop Change

But in my opinion you need to look at the motor, because you should not need to change to that low a pitch to reach your recommended RPM. I like the trolling motor.


H

 

[Sparkscw]

Re: Prop Diameter ?

The hydrofoil..... so are they not all their cracked up to be? What I have read on line they advertise as these magic wings that will get you up on plane faster.
So I say take off the foil and test run again. Then if that doesn't get me there try to move to a 15 pitch. Sound about right.

I will continue to tinker with the motor, whoever I just did a compression test, and a vacuum test today prior to the test run. All my readings are saying the motor is good to go. I do have a little issue with my carb base plate (worn bushings in the shaft) and I will be swapping that out later this week.
Thanks for all the help and patience.
David

 

[hwsiii]

Re: Prop Diameter ?

No problems at all David I have enjoyed it, but I do believe you are missing some horsepower. It would be good to check everything on the motor before buying another prop though. And yes jerk that foil off, if you decide you need something after you have the right prop then buy Smart Tabs they will help with speed, NOT hinder it. LOL


H

 

[GiMLit]

Re: Prop Diameter ?

I am no prop expert and will not wade into an area I am not well versed at. The foil I will tell you TAKE IT OFF NOW!
I did some upgrading over the winter and on test day I was trying things out and was horribly disappointed with the top speeds I was getting. The drive I had acquired from a boat I salvaged had a foil on it and I left it on just to see what it was like. When disappointed I removed it to pop up with an additional 4 mph top speed. Believe it or not it is 100% the truth. I was simply amazed at the diff and had no real loss on hole shot at all.