Horrible Performance 1987 Proline 21 w/OMC Seadrive

[seachest]

Hi folks - looking for some advice here on propeller sizing. I recently replaced the powerhead on my 1987 OMC Sea Drive (140 HP) . The shop which supplied the head and lower unit sent me a 15 X 15 prop. I have to run it at 4400 RPM to get any " decent " 20 MPH out of it. Fuel burn is about 12 - 14 GPH and I can't imagine this being any where close to normal. Please help if you can with what I should do regarding the proper pitch. There's not a whole lot of room regarding the diameter as I can barely get my index finger between the blades and the cavitation plate . Any and all advice is welcome

Thank you all in advance

 

[steelespike]

I don't know much about your Sea Drive Does it use a 2 stroke OMC power head?
What is the displacement?Do you know the rated rpm. It appears it might be a 1.6, 1.8, 2.5, 2.6 Do you know the gear ratio?
I suspect both the tach and speedometer.Assuming 1.86 ratio ; 20 mph at 4400 is 40% slip.Typical slip is in the low double digit numbers.
​ 13% slip calculates to 29 mph. 5500 at 13% nets 36 mph.
You need to verify the tach and use a gps for your speed.
You could use a induction tach.
As far as diameter goes usually it follows the pitch. as pitch goes down usually diameter goes up.
Pitch goes up diameter goes down.

 

[seachest]

Thank you for your response Steelespike - Unfortunately it's a bit over me head but here is some information gleaned from the shop manual : W.O.T. Range 5000 - 6000 RPM, POWER RATED: 140 HP @ 5500 RPM. Displacement 110 CI (1800 cm3) Gear Ratio : 13:26 (.500) . The posted speed was based on GPS readings and yes it is a 2 stroke oil injected. Perhaps this information will help and you may be able to shed some further light on the subject. I greatly appreciate your help and look forward to your response.

 

[Texasmark]

I helped a guy sometime ago with a seadrive and fixed his problem. You say you have to run it at 4400 to get a 20 mph speed. What throttle setting is this? Is this all the way forward, aka wide open throttle? If not my answer below is incorrect. Outboards are not like cars. They are built to run at wide open throttle all day. As a matter of fact my experience indicates that 2 strokers like it that way. If you are trying to "soft pedal" the throttle you are on the wrong road.

Also as spike said dia changes with pitch change but for a given size gearbox...the contents of the casting in front of the prop there is a given set of props that fit it. Since you have clearance with the 15, your Seadrive must have the large diameter gearbox that higher hp engines use allowing for 14-15" dia props.

I will assume that you have the throttle firewalled and only get 4400 rpm. The engine is lugging and you need to change the "gear" in the "transmission" you are using. Since the ratio between the drive shaft and prop shaft are fixed by the engine gearing at 2:1 you are going to have to use your other gear......the prop, or have a look at your procedures.

Reducing the load on the engine goes a LONG way in helping in that situation. This is where proper engine trim comes into play. Proper usage of that function is for best performance of the boat at the desired speed. What works at half throttle may not be what you want at WOT. 20 mph and you are barely on plane (getting the hull on top, not in, the water). If you tucked the trim in tight to get on plane now is the time to push the prop away from the boat to get more of the boat out of the water which will increase rpms and speed. Do you currently use your trim to your advantage, experimenting on what works best under a certain set of conditions...planing out, running at WOT kind of things? If you do and can't get the rpms up any faster than that....and your tach is correct...... you have too much pitch in your prop and have to reduce it.

From what you said you need a significant pitch change if running at WOT and proper trim angle, but I think I'll stop here and let you do your home work and get back on here with answers to my questions and we'll go from there. Were YOU running this boat with this engine before the powerhead change? Were you running this same prop? If so, how were things?

Mark

 

[steelespike]

If wot produces only 4400 @ 20 mph then either the tach is wrong (high) Or you have a serious motor/driveline issue.
If we use your 20mph (gps) 2.00 ratio,13% slip,15" prop = about 3200 rpm.
If the tach is accurate and wot is actually 4400 you can't make up the rpm with a prop change. Your 1600 rpm short of 6,000.
4400 at 20 just doesn't add up.
Theoretically your 15" prop moves forward 15" in one revolution .But water is a liquid so the prop slips a little. This is calculated as slip
and typically is ABOUT the low double digits. Presently your numbers and a 2.00 ratio indicate about 36% slip possible but highly unlikely
thats why the tach is suspect.It's pretty common for a tach to be set wrong or just plain not accurate.Especially as they age.

 

[seachest]

Sorry for any misunderstanding but 4400 is not WOT. There is more room to go and I am fairly certain I got it up in the 5500 + range. But there the fuel consumption on the flowmaster all but pins at 20 GPH. I have compare the flowmaster totalizer and added fuel accordingly and it is very accurate. The tach is only one year old and correlates to the mechanics instrument at least in the lower RPM ranges. I have played with both the motor and trim tabs to get to the most optimum settings without cavitation. Unfortunately, I did not record the speed when I throttled up into the near WOT range but there was certainly a noticeable increase - But the fuel consumption was insane. Does this help any ?

 

[seachest]

Also, Is there a way to measure the gear ratio without tearing things apart ? E.G. Pull the plugs and put the motor in gear, turn it by hand, and count the number of revolutions of the prop vs. one complete revolution of the motor ? Sounds logical to me but I'm not sure if that would even be possible .......

 

[steelespike]

Exactly pull the plugs note the location and condition. Mark the flywheel and prop so you can measure accurately.
2.00 to 1 is indicated by the numbers you quoted. Motor 2 ,prop 1.
A 140 could use about 17 gph at wot. Todays very efficient Evinrude Etec 150 uses 15.15 to 16.45 gph A carbed 2 stroke would be significantly higher.
Sometimes we use 10% of the hp to figure in your head but its much closer to 12 or 14% on a carbed 2 stroke.
So if we are insisting the speed is accurate and the tach is right your at 36% slip.A typical pontoon boat has slip around 20,25%
Excessive application of the tabs might increase slip.
Does the boat respond to up trim? If it doesn't and lays down really flat that would slow it and may increase slip.
Is the bottom clean and smooth?

 

[seachest]

Thank you once again. Will work on determining the ratio tomorrow. Cold and dark here in the North East The power head is a V 4 two stroke with individual carbs for each cylinder so I can imagine high consumption but 20 GPH + @ wot seems really excessive. The bottom is smooth but painted which I realize adds to drag. I will have to zero (fully up) the tabs and see what happens with trim/tilt of the motor. As I recall it responds properly, unfortunately I haven't been able to get out much with the early onset of sub freezing weather. The gear ratio should be an easy determination as she is on a trailer in the driveway presently. Will keep you posted on what I find. Also, in reading Texasmarks response I should be running at or near WOT. Did I interpret that correctly ?

 

[steelespike]

At wot the rpm needs to be near the top rating. 6000. You said you reached 5500 thats a good start.
Once the numbers are right you can of course operate at what ever rpm you prefer.

 

[Texasmark]

Well it looks like we are moving forward. Seems your priority is fuel consumption and everything else just falls as it may. You really don't have to check your gear ratio however it takes a little time but is no way complicated as Spike pointed out. Your 5500 at WOT is a fine number and your engine should be very happy with that. I guess your hole shot (getting up on plane) is ok also or you would have said something about it. So I see nothing wrong with your current prop....looks like a good choice.

So this leaves me with............what needs fixing other than fuel consumption? You can go to boat www sites and get performance numbers for new boat-engine combinations. There are fuel consumption vs boat speed charts posted and you will notice that the best fuel efficiency is just above planing speed which is easily detected just below the 20 mph mark and an abrupt decrease in the upward slope of the consumption curve.... and worst at WOT.

Don't have any info on your engine and if I did I don't know what I'd do with it but years ago I do know OMC made some cross-flow and loop charged combustion engines of the same HP and I think 140 was in that pileup. Back in 1968 or 69....brain's getting fuzzy, OMC came out with a revolution in 2 stroke engines and it was Loop Charging. The process worked best with 3 cylinder engines as the timing of the exhaust pulses were just right to make the system work. They added other amenities in that design that could be used on 4 cylinder engines, not in full loop charging initially, but fuel saving enhancements. Fuzzy on this but I do think they did loop a 4 banger but I don't know what that consisted of. The V6 engines were a walk in the park as they were just 2 ea 3 cyl. engines with 2 tuning chambers.

Where all this is heading is determining whether or not your WOT fuel flow is excessive or to be expected for what you have.

So sir, I just think you are wishing for what you have not and you will either have to live with it or get a boat that is more fuel efficient. I can't add anything to what you told us with you there and we here in the way of fixing the problem you mention.

Good Luck,
Mark

 

[dazk14]

If you're running an aluminum prop, a switch to a well fitted Stainless prop will help improve fuel mileage. 3 or 4 blades? A Sea Drive is HEAVY.

Do you experience ventilation when trimmed out.

How do the plugs look? light brown, dark brown.

Is the motor warming up to proper temps. Stats not functioning will suck more fuel.

Let's start with the above....

 

[seachest]

Well folks, as they say "feed me garlic and call me stinky" - I discovered part of the problem - My GPS was set to "Nautical" rather than "Statute" readings. Hence the top end of only 24. How stupid do I feel. So I reset it and was getting 28+ MPH @ 5200. I have since switched from the 15 x 15 prop to a 17 P x 14.25 D Turning Point 3 Blade aluminum and I'm seeing 32 + MPH @ 5300 RPM which is WOT in this case. Still playing with the trim and considering a Hydrofoil for cornering. But all in all I'm fairly happy with the performance. I wish to thank you one and all for your advice and contributions to solving my problem. It was all greatly appreciated and graciously considered. Now can someone explain how to close out the thread ? Do I do that or does a moderator ?

 

[steelespike]

Your solution is fine just let it ride.It will gradually fade down the "page" and may pop up and help someone doing a search for help.

 

[Canadian Seadrive]

Seachest, your speed and rpm sound about right. For comparison, i have the 1990 20ft proline with 1.6L seadrive(115hp). At WOT i get 28 mph at about 5300rpm. Ive got the standard aluminum prop for that engine.

 

[GA_Boater]

Seachest, your speed and rpm sound about right. For comparison, i have the 1990 20ft proline with 1.6L seadrive(115hp). At WOT i get 28 mph at about 5300rpm. Ive got the standard aluminum prop for that engine.

It has been well over a year since Seachest visited. Please don't reply to inactive topics, which by our definition is 3 months of deadness - This one has had no activity since shortly after Seachest started the thread.

Closed.