Overpropped...But Why?

[Sidclark]

My MT36 has the Ford Lehman 135 with a WOT of 2500 rpm. I can only get 2300 rpm. Boat has a clean bottom and a four blade prop. At 2000 rpm I'm at 7 knots. This sounds a little slow to me. I'm trying to figure why the PO would set the boat up like this. Is there some advantage somewhere? Should I worry about 200 rpm? There is a three blade spare onboard, but not sure the diameter or pitch. I assume it's a stock prop. Does anyone know the stock diameter and pitch? I may just put the spare back on if it's called for...

 

[rslifkin]

If it's easy enough for you to switch props, I'd check for any markings on the spare to indicate size, then throw it on and see if the boat behaves better with it.

Of course, I'd make sure there's nothing preventing you from making full engine power or something else going on first.

 

[firehoser75]

Check out sbmar.com looking for info on properly propping naturally aspirated engines.

Unless there is some other issue (such as throttle, stop, some adjustment,etc.) you are probably overpropped and are therefore potentially overloading your engine which can be damaging in the long run. You also could consult a prop shop. Search online for the info the shop would require to give you the right specs for your prop. First step I would suggest is to read Tony's tips and info on this, ensure no other cause, then consult prop shop if needed.

 

[mahal]

At 2000 rpm I'm at 7 knots. This sounds a little slow to me.

This might be the reason why the previous owner did it. If it were propped correctly your boat would even be slower. Older Grand Banks (mostly with Lehmans) were overpropped from the factory. There were plenty of discussions on this on the GB owners forum.

 

[DavidM]

If he propped the boat correctly, I agree it would go a bit slower at 2,000 rpm than before. So run at 2,100 ro 2,200 rpm to make 7 kts and the engine will be happier.

If there is nothing wrong with the engine then 7 kts is about all you are going to get out of that engine.

David

 

[mahal]

Isn't he limited to 2000 rpm for the 80% load rule?

 

[PacificBlue]

Why? The boat got heavy over time

Take some pitch out of it and let the engine breath easier.

 

[Comodave]

Take the spare prop to a shop that does prop scanning to find out the size, if the prop isn’t already marked.

 

[Nomad Willy]

Pacific Blue wrote;
"Take some pitch out of it and let the engine breath easier."

Indeed yes. 2550 to 2600rpm IMO.

 

[DavidM]

If the speed is the same 7 kts before and after repropping then the load will be the same even though the rpm will be higher.

David

 

[Nomad Willy]

But David the load on any given stroke will be higher on the lower rpm engine. Less sidewards load on the soft aluminum piston and the cylinder wall. When engines get worn out the pistons and cylinders are the point of maximum wear. The weak link .. so to speak. I'm about to put on my other prop back on after it's been re-pitched one inch down. 14 to 13". I'm hoping to get 3100rpm that will be 100 over rated rpm of 3000. I could call it my sweet spot.

This is the prop. Michigan MP. A rather old design thats a tad bit less efficient but smoother and w lots more reverse thrust. Bought this prop about 08.

 

[STB]

Controls are normally set up to stop 100rpm short of max-WOT-under-load.

With the boat off fully advance the throttle at the control then go down and disconnect the cable at the throttle arm and fully advance the throttle arm by hand. Does it have a little or a lot more movement?

Or, if you can leave someone else with the helm underway and are comfortable, you can do the same thing with the boat running. Cruise with full throttle at the controls, then disconnect the cable at the arm and fully advance the arm. How many more RPM do you get?

If the nominal number is 2500 and you are seeing 2300 from the controls, you are only 100 RPM off -- and it could be at the controls.

Maybe you are off an inch of pitch. Maybe. But, I'd check simple stuff first.

My last boat (1977 Californian 42') had way too much pitch. It should have been getting tob2400 or 2500 WOT-Load andbonly hit 2000. My idle in a trawler was about 7kn!

2" of pitch later, I was at 2400 WOT-Load and cruised at 2100. Idle was nixe and slow. Cruise was almost 10kn

The mechanic said it was crazy, but folks did that to cruise at lower RPMs and get a quieter ride.

 

[Lepke]

Hull speed for a 36' boat (if that's the waterline length and not overall) is 8 knots. You're not going to get noticeably more speed if that's the goal.

 

[rslifkin]

But David the load on any given stroke will be higher on the lower rpm engine. Less sidewards load on the soft aluminum piston and the cylinder wall. When engines get worn out the pistons and cylinders are the point of maximum wear. The weak link .. so to speak.

Agreed. Engines are all a balance. High RPM can cause more wear, but making more power per stroke also causes more wear (especially on piston rings, etc.). And making more power per stroke tends to lead to higher EGTs which can shorted valve live and things like that. So it's all about balance. RPMs at a reasonable level relative to the engine design, then power output relative to RPM at a reasonable level for the design. And that'll give you max continuous power.

 

[jleonard]

The first thing I would do is make sure the tachometer is accurate by using a hand held tachometer on the engine.
Then you have accurate data.

 

[FF]

"If the speed is the same 7 kts before and after repropping then the load will be the same even though the rpm will be higher."

Just so, the problem is the term "over propped" is not used correctly or well understood.

A boat at cruise speed , whatever it is , requires a certain amount of HP regardless of the engine speed to produce it..

Many cruisers pride them selves on a minor fuel burn, 2 or 3 GPH cruise is prized .

At about 15-16 hp per gallon per hour many boats cruise on 30-40 HP .There is no need to operate a 100HP or 200HP + engine at high RPM to make 40HP.

Looking at the Mfg HP curves may show the engine is capable of creating 55HP at say 1600 RPM.

That RPM would be the goal for a cruising prop . NO,, the engine will never be tasked to run 2500 RPM , which would be overloaded.

The advantage to selecting an efficient operating speed that better loads the engine at modest RPM is NOT over propping or overloading .It is simply better matching the engine to the boat.

This concept is for displacement boats , not sport fish or water skiers.

 

[rslifkin]

Some manufacturers will void warranty if the engine is over-propped, even if it's not pushed hard. And some also say that the engine can produce 100hp at this RPM, but as-propped, in service it shouldn't be above this curve, say 60 hp or 3.2 gph.

Depending on the engine design, asking for, say, 50 hp cruise power at 1500 RPM instead of 2000 may or may not reduce lifespan. It will generally save a little fuel, but depending on the engine it may not be much.

 

[Cigatoo]

Good advise from JLeonard.
1. Get a photo tachometer. Measure RPM at the front crank pulley at 500 RPM intervals all the way up to wot (wide open throttle.) (Out of gear) This will assure that you have the correct information. Compare to your ships tachometer.
2. Can you reach rated WOT when out of gear? If so you may be over propped or dirty bottom or excess drag. You get the idea. If you cannot reach wot in neutral look for a restriction at the throttle cable or linkage. Does someone have the high idle screw turned back to restrict RPM? Disconnect the throttle cable at the pump and try wot by hand.

Don’t touch the prop until you have verified the above.

 

[sunchaser]

Sid C

I'm with Jay Leonard and Cigatoo.

One more oft missed variable - through the water boat speed. I assume the OP is measuring it with GPS. Find a straight safe section of water and measure boat speed in one direction and then reverse over the same track at about 4-5 different verifiable RPM including wide open throttle. Write down results to the nearest 1/10th of a knot.

Good luck and good question.

 

[Sidclark]

I've calculated hull speed at 7.5 knots. I'm perfectly happy to cruise at 7 kts. Let's assume rpm is correct and throttle stop is correct. Am I better off cruising those 7kts at 2000rpm overpropped or at 2200rpm correctly propped? Not worried about fuel rate, just want to be nice to the engine, but not kill it with kindness....

 

[DavidM]

I've calculated hull speed at 7.5 knots. I'm perfectly happy to cruise at 7 kts. Let's assume rpm is correct and throttle stop is correct. Am I better off cruising those 7kts at 2000rpm overpropped or at 2200rpm correctly propped? Not worried about fuel rate, just want to be nice to the engine, but not kill it with kindness....

2,200 for sure. Your engine will love you and not because you are being unreasonably kind. It will love you because running 200 rpm faster at the same speed and hp output will reduce the stress on engine components.

Stress kills engines. A little more rpm is much less stressful to the engine than higher piston forces and EGTs.

David

 

[Nomad Willy]

2200 re your question but I’d prop this boat for 2300rpm.

Another element that everybody will need to face is that as scum and growth covers your hull you’ll loose both boat speed and engine speed. So whatever you prop for engine speed (rpm) will come down over time. So even if you prop to rated rpm you’ll be overpropped in time. You can only avoid that by underpropping the same amount of rpm you loose from hull growth.
It’s easy to consider rated engine speed as some kind of redline. It probably isn’t. The Perkins originally in the small Willards saw decades of service in London taxi cabs and for that service they were rated at 50hp at 4000rpm. But in marine service like the Willards rating was 36hp at 3000rpm. So I suspect that most marine engines will live longer at a higher rpm and less load. Like truck drivers downshifting on a hill. And if it still required heavy throttle - downshift again. With the 4-107-8 Perkins engine one could probably underprop several hundred rpm and w a light’ish load suffer less wear than w the engine running at a lower rpm and enough load to develop the same power.

I had a Nissan Maxima w a 3ltr. Inline 6 cyl. Diesel. The noise was awful. But w a heavy load like a hill to climb it suddenly became far more quiet. So much so that I was very glad come to a hill for some relief from the diesel racket. On that car w that engine noise was far greater w light loads. It was noisy w light loads.

Most people overprop to reduce engine noise but that very likely could be maximizing engine load and engine wear. But re this load v/s wear and noise may be different w different engine designs. I almost always cruise my boat at 2300rpm and don’t think of it as noisy but it’s a pre-chamber upper cylinder design. So engine design may have a lot to do w load v/s noise.

 

[SteveK]

I've calculated hull speed at 7.5 knots. I'm perfectly happy to cruise at 7 kts. Let's assume rpm is correct and throttle stop is correct. Am I better off cruising those 7kts at 2000rpm overpropped or at 2200rpm correctly propped? Not worried about fuel rate, just want to be nice to the engine, but not kill it with kindness....

IMHO, what sounds off is you are only getting 7KN @ 2000 RPM with a more efficient 4 blade.
I can only compare to my GB and defer to others with same boat as yours.
With one engine down I was getting 7 knots @ 1600 with 2500 max under load.
Overpropped usually lowers the RPM to attain same speed.

 

[BandB]

I'm always concerned if not getting max RPM. On a lightly loaded boat you should get slightly more, so perhaps 2550 but I'd never settle for less than 2500. Now, step one is to be sure the 2300 is accurate. From there, if confident engine is tuned properly, then play around with props until you are right. With the right prop, 80% load will be a higher RPM and you'll achieve the same speed as now, if not more. I consider max RPM at WOT a key indicator of health.

 

[sunchaser]

I've calculated hull speed at 7.5 knots. I'm perfectly happy to cruise at 7 kts. Let's assume rpm is correct and throttle stop is correct. Am I better off cruising those 7kts at 2000rpm overpropped or at 2200rpm correctly propped? Not worried about fuel rate, just want to be nice to the engine, but not kill it with kindness....

As suggested, best you verify the tach and speed are accurate.

On our vessel, we much prefer 1600 to 1800 RPM for long engine life and propped to reach full RPM. If we had smaller engines the optimum RPM would be higher.

Our engines, not unusual for many of our trawlers, were designed for 1500 or 1800 RPM genset work. I am not an advocate, in most cases, of running at 80% load for longest health - for our slow generally overpowered trawlers. Sure, pick a few minutes now and then to run at near red line RPM.

If you read Tony Athen's site you will find him saying he's never seen an engine wear out prematurely due to running it "slow." And rotating things follow closely the rule that wear increases by the square of RPM increase. Think water pumps, alternators, transmissions, struts etc.

But your boat so run at verifiable RPM you receive the most votes on or???

Disclaimer - adequately powered SD and planning vessels have a very different criteria for this topic.

 

[rslifkin]

For a good use of the 80% rule, I'd say that if the manufacturer doesn't publish a max continuous RPM or power output spec, 80% of max rated or achieved RPM (assuming you reach or exceed max rated) is generally safe for continuous output. That'll usually put you around 50 - 55% of max power output, which should be safe on pretty much any engine (although some will be fine with putting out more continuously).

Of course, that doesn't mean you have to run it that hard all the time. Being more gentle is perfectly fine as long as it gets up to temp, etc.

 

[Al]

My MT36 has the Ford Lehman 135 with a WOT of 2500 rpm. I can only get 2300 rpm. Boat has a clean bottom and a four blade prop. At 2000 rpm I'm at 7 knots. This sounds a little slow to me. I'm trying to figure why the PO would set the boat up like this. Is there some advantage somewhere? Should I worry about 200 rpm? There is a three blade spare onboard, but not sure the diameter or pitch. I assume it's a stock prop. Does anyone know the stock diameter and pitch? I may just put the spare back on if it's called for...

You hadn't included your reduction gear ratio. You need to know all the factors :
https://www.vicprop.com/calculator.htm

Work through this and let the forum know how the homework resulted.

 

[markpierce]

I consider my boat/prop/engine combination well-balanced. (NA John Deere 4045)

Dead-slow, 800 RPM, 3.3 knots
No-wake, 1000 RPM, 4.2 knots
Slow, 1400 RPM, 5.7 knots
Standard cruise, 1800 RPM, 6.3 knots, 43% load, 1.7 GPM
Full speed, 2200 RPM, 7.3 knots (hull-speed), 72% load, 2.9 GPM
Wide-open-throttle, 2400 RPM (engine rating), insignificant speed increase, 4 GPM

 

[jleonard]

I've calculated hull speed at 7.5 knots. I'm perfectly happy to cruise at 7 kts. Let's assume rpm is correct and throttle stop is correct. Am I better off cruising those 7kts at 2000rpm overpropped or at 2200rpm correctly propped? Not worried about fuel rate, just want to be nice to the engine, but not kill it with kindness....

You can calculate all day, but you need to know that when your tach says 2000 rpm your engine is actually turning at that rate.
I have a hand held tach and have helped many diesel engine boaters calibrate their tachometers. It is rare that they were correct when first checked.
On fuel use....it takes X horsepower to move your boat at 7 knots. That will take a certain amount of fuel no matter what rpm the engine is turning (any difference will not be significant).The engine will be a little happier at 2200 rpm but in MY opinion that small difference in that range will be insignificant.

The bigger issue is that actual rpm can be greatly different than what your boat tachometer is showing. It's not always linear.
Thus my first sentence above.

 

[Cigatoo]

To get your gear ratio use your photo tachometer to measure engine RPM while in gear at a set RPM. Say 1,000 RPM. Without touching the throttle / changing RPM measure the rotation on the prop shaft using the photo tachometer. Divide the two together to get your gear ratio.

You need to know all this info before going anywhere near the prop.

WOT RPM no load
Max Rpm obtainable under load
Gear ratio

My opinion FWIW, is that as long as you are running under hull speed and under 2,000 RPM continuous (135 Lehman) being a little over propped won’t hurt.