Prop and top speed evaluation

[lekmedm]

Greetings to the gurus!

So, like many, I am wondering if my boat is properly set up and tuned. As per the "Read this first!" post I will try to be as complete as possible in my description.


1) Performance issue you are trying to correct.
Is my current top speed the max I should expect for my boat with the current prop?

2) Current prop manufacturer, model, aluminum or stainless and as a minimum.
Michigan Wheel, #011007, aluminum, 3 blade prop. (<---updated)

3) Current prop diameter and pitch (required).
15x17 (<---corrected)

4) Wide open throttle RPM and speed with an average load (very helpful)
4500rpm @ WOT; 28mph as per GPS.

5) Engine/drive make, model, year, and HP
OMC 460 King Cobra, 1988, ~350hp.

6) Boat make model, year, length and weight
Cruisers Inc. VeeSport 2660, 1988, 26', ~6700lbs. (dry)

So my engine is supposed to run between 4400 - 4800 rpms @ WOT as per the factory service manual. I seem to get up on plane easily enough. I was wondering if 28 - 29mph is the most I should expect from her given the current parameters. Mind you, I don't need to go faster. I just want to make sure everything is dialed in the way it should.

 

[cribber]

Re: Prop and top speed evaluation

You didn't say what gear ratio you're running which is needed in the calculations that determine optimum performance. This link is to a decent prop calculator that provide theoretical speed and slip percentage http://www.go-fast.com/Prop_Slip_Calculator.htm

Using a gear ratio of 2.0 and plugging what you provided puts you at 13% slippage. Not too bad considering the weight your pushing if this is correct. Please post your geat ratio for the experts to assist.

I'm not an expert on props so for recommendations and I'd defer to the experts on here for advice what pitch to select.

 

[QC]

Re: Prop and top speed evaluation

I think it is a 1:43 ratio which makes the numbers ugly . . . 35% slip. All info has to be perfect for any of this to work.

 

[cribber]

Re: Prop and top speed evaluation

QC that seriously skews the slip percentage and thanks for clarifying!!! Hopefully lekmedm will post the gear ratios.

 

[TilliamWe]

Re: Prop and top speed evaluation

4) Wide open throttle RPM and speed with an average load (very helpful)
4500rpm @ WOT; 28mph as per GPS.

That top speed stinks, and I am thinking QC is right, there is WAY TOO MUCH propslip.

 

[lekmedm]

Re: Prop and top speed evaluation

QC that seriously skews the slip percentage and thanks for clarifying!!! Hopefully lekmedm will post the gear ratios.

Thanks for replying, guys. As requested, straight from the service manual...:

Upper Gear Ratio: 21:17
Lower Gear Ratio: 17:30
Total Ratio: One revolution of U-joint shaft (input shaft) results in 0.70 revolutions of propeller shaft (output shaft)
Overall Ratio: 1.43 revolutions of U-joint shaft results in one revolution of propeller shaft


Sounds like QC has bad news for me... : What to do to improve...??? Is this a propeller issue, or is there an engine tune necessary?

 

[lekmedm]

Re: Prop and top speed evaluation

Whoops.... I made a mistake...

I just went out and checked the prop I used to get those speed results.

It's a Michigan Wheel aluminum 3 blade prop (#011007) that's 15 x 17

Ideas...?

 

[QC]

Re: Prop and top speed evaluation

Well I have good news and bad news . . . Bad news first. 17" pitch = 44% slip.

Here's the good news. I am not confident at all in evaluating a somewhat heavy, single screw cruiser as far as slip is concerned. I actually think you need more diameter, but I really have no clue . . . Hopefully hwsi will happen by.

 

[lekmedm]

Re: Prop and top speed evaluation

Hmmm... I was just studying my factory service manual...

There's a chart listing boat sizes, categories, speed, and prop sizes...

So, I guess I fit in the 22-28ft. Heavy Cruiser group that should go 30-37mph with a prop that's 15.5" diameter and 15" pitch. That sounds like I'm on the low end.

What will happen to my speed if I go down in pitch from 17 to 15 and up in diameter by 1/2"?

Also, the surface of the blades of my 15x17 aluminum prop are a bit pitted. Can they be resurfaced with something and repainted?

 

[hwsiii]

Re: Prop and top speed evaluation

Lek, QC is definitely right about your prop slip, if that gear ratio is right. My initial calculations show you should be getting a good bit more speed out of your setup, and your prop slip is 45%. Using your actual upper and lower gears I do not come up with a 1.43 gear ratio, please verify this through OMC.
If you will fill out this form I will be glad to do a comprehensive analysis and see where you stand.
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

 

[Dhadley]

Re: Prop and top speed evaluation

A few things to keep in mind as you go thru this process - the big Ford makes a ton of torque. The King Cobras don't usually work well with aluminum props as evidenced by the cavatation burns you describe on the prop. The power of that packaged can rarely be harnessed by an aluminum prop.

Somehow I get the feeling the motor isn't quite stock as produced by OMC. Have you made any changes - exhaust, intake etc? Is the exhaust still thru the drive or thru the transom?

 

[QC]

Re: Prop and top speed evaluation

Using your actual upper and lower gears I do not come up with a 1.43 gear ratio

21:17 and 17:30

17/21 = .81, 30/17 = 1.76, .81 x 1.76 = 1.43 . . .

 

[hwsiii]

Re: Prop and top speed evaluation

Thank you for the save QC, I meant to say I had gone and looked up the gear set and found the gears to be different, so I used that instead, and came up with a 1.41 gear ratio.

http://www.ebasicpower.com/pc/GLM22520OMC3FGear%20Set,%20Upper%20OMC%20Cobra%2086-93%205.7L,%205.8L,%2021:16.html

In my opinion this boat is a prime example of how changing to a better 4 blade prop from a 3 blade can increase the top speed dramatically. Usually an easy way to tell if you will gain speed with a 4 blade prop versus a 3 blade is if your prop slip is above 16% to 20%, and it does not matter whether your boat is large or small and whether you have a small outboard or a large Inboard or inboard/outboard, prop slip is the only thing that matters in these circumstances.

My preliminary analysis shows you should gain at least 7 MPH with the change. I would appreciate it if you will fill out the form so I can do a more thorough analysis.

Prop Slip

I also would like you to verify the maximum RPM with a mechanics tach, as 45% prop slip is as high as I have ever seen on a boat. Also look at the bottom of the boat and ascertain if there is any growth or buildup that is causing this problem.

H

 

[QC]

Re: Prop and top speed evaluation

Here's where I got the 1.43 to begin with . . . OMC place

 

[hwsiii]

Re: Prop and top speed evaluation

Thank you very much QC, I will use that site in the future for my numbers, it is apparently better than the one I use. LOL


H

 

[lekmedm]

Re: Prop and top speed evaluation

I am almost speechless with the degree of effort you guys are willing to put in to helping me analyze how to optimize my boating setup. A heartfelt thanks to all of you!

Well, hwsiii, the list of questions you have posed are quite comprehensive, and I hope I'll be able to answer all of them satisfactorily. I'll do my best. A little bit of backround on the boat: I bought it in July 2007, and, until recently, had nothing but problems with it. First, I ended up having to replace the carb, then adjust the shift linkage (which I had to learn), and finally I struggled with cooling issues. Right now all of those issues have been resolved. The engine has had a tune up including new plugs and filters and oil in the outdrive. So, now on to the questions and answers...


1. Year, make and model of boat
1988 Cruisers Inc. VeeSport 2660

2. Length, width and base weight of boat, look for boat decal on back of boat
26 feet long, 10 foot beam, 6700lbs dry weight

2a.What is the maximum recommended HP for your boat
It's hard to say. The same hull came with several engine options including a dual setup. My Ford 460 is the original engine on the boat (rebuit ~6 years ago; 213hrs on it now) offered by Cruisers Inc. and rated in the area of about 340hp.

3. Number of people and gallons of gas normally on boat
To date, just 2 adults and ~60gal gas (tank holds ~110gal).

4. What do you use the boat for
Cruising.

5. Is it a Deep Vee and if so how many degrees of deadrise
I think that's how it would be classified. I don't know what the degrees of deadrise would be. I can tell you that I have the same boat as this one for sale on iboats.

6. Year, make. manufacturer and model of motor
1988 OMC 460 King Cobra. Rebuilt ~6 years ago (2 owners previous). The model # off the cylinder head was completely faded.

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
The HP is supposed to be around 340hp. I have no idea what the gear ratio is, and I have no way of checking it.

8. Manufacturer?s recommended Wide Open Throttle (WOT) range
4400 - 4800 rpms

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
Drivetrain is I/O.

10. Is it a bass boat or does it have a pad bottom
Not a bass boat. I don't know what a pad bottom is.

11. Does it have a hydrafoil, dolefin or trim tabs
Hydrofoil on the outdrive + trim tabs.

12. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop
Michigan Wheel; 011007; 15" dia.; 17" pitch; 3 blades; aluminum.

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 ________
WOT RPM = 4500; Speed = 29mph; # people = 2; Gal. gas = ~ 50 gal. I'm not sure what you mean by "trim the prop" or what "ventilating" is (newbie here ). The outdrive was lowered completely during my test.

14. Are you at sea level or a higher elevation, give us the elevation in feet
Sea level.

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
Carburetor is brand new and has been on engine for ~10hrs of use. Compression unknown. Spark present all around (system previously converted to Pertronix).

16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
This is a new used prop with a bit of pitting on the blade surfaces. Previously I had a SS prop on there, but when I could only gt 9mph @ 4300rpms with that one, I have concluded that the SS prop has a spun hub. I am not convinced the current prop is the wrong prop, other than my OMC FSM suggests a smaller pitch (15") for a boat my size. I am just looking for optimization.

17. Does the prop show any damage that you can see
The blade edges are smooth. The surface of the blades has light pitting.

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
I am just looking for the boat to cruise within its optimal speed range.

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.
I wish I could, but the boat is out of the water and winterized.

 

[Dhadley]

Re: Prop and top speed evaluation

If your drive is mounted as low as the one in the pic the plate (you call it a hydrafoil) is, most likely, slowing you down. Have you tried it without the plate?

 

[lekmedm]

Re: Prop and top speed evaluation

If your drive is mounted as low as the one in the pic the plate (you call it a hydrafoil) is, most likely, slowing you down. Have you tried it without the plate?

OK, maybe it's called a "plate". I'm a newbie and here to learn.

Here's a pic of my boat's outdrive. You can't see the plate really well from this angle, though. I haven't tried running without it. I thought it was supposed to help you get on plane faster.

 

[lekmedm]

Re: Prop and top speed evaluation

For clarity, that plate, hydrofoil, or whatever, that I have mounted on my outdrive would be mounted in this place circled in the picture:

 

[cribber]

Re: Prop and top speed evaluation

Dump the hydrofoil... you really don't need it with the trim tabs your running. I/O drives really don't need hydrofoils with the correct prop and likely is hurting your performance. Also, drop the trim all the way down on your I/O to reduce exposure to the elements over the winter and help in reducing moisture from collecting in your lower unit.

You've got all the experts helping you out here with hwsiii, QC, Dhadley and I've relied on their experience and knowledge to help me out.