Boat Performance

This is an interesting discussion. I just went back and read it from the start.

Jeff F said:

This is an interesting discussion. I just went back and read it from the start.

Click to expand...

And then posted by mistake

I completely get the over propping argument. But implicit in doing that is an acknowledgement that the motor is too powerful for the intended purpose. You really do forfeit the ability to get maximum power from the motor. I'd have trouble making that trade-off.

Motors are always too powerful for the prop, at some rpm. That is because the engine torque curve is always convex, and the prop torque curve always concave. They are not a good match for each other - equal only at (at most) 2 points. This is very unlike the same engine mounted in a truck, with a choice of 6 gears to match available torque to load.

A variable pitch prop would allow much better matching of load to available torque. Most airplanes have variable pitch props for exactly this reason.

"A variable pitch prop would allow much better matching of load to available torque. Most airplanes have variable pitch props for exactly this reason."

This is true but the savings from perhaps $10,000 to $15,000 investment in a VPP would not pay financially.

It would be great for engine life , fuel burn and noise while cruising , but its an expensive luxury.

"You really do forfeit the ability to get maximum power from the motor. I'd have trouble making that trade-off."

Most displacement boats have a "hull speed" where added power results in bigger and bigger waves , and fuel flow but only a minor speed increase.

Sinking the stern as the bow attempts to climb its own wave is the usual result of the oversized engines stuck in many trawlers.

2-3 GPH is 30-50HP , propping to dump 120 or 135 or more HP into the water may only result in a 1 or 2K speed increase , bit BIG stern waves.

markpierce said:

Installing a propulsion system that can result in overheating the engine at normal/capable engine speeds is a harmful waste of time. If one wants optimal fuel-consumption/speed efficiency, move at a knot below hull speed.

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EXACTLY!

Thanks Mark... for crossing the t's and dotting the i's in a simple true, factual answer to this subject. Your statement is KISS to the utmost!!

I've been reading this thread's "what if this" and "why not that" for some time.

Takes a mellow guy like Mark to make a short statement that in this case pins the tail on the donkey...!

As with most overpropping threads, what's missed by the members with planning hulls is that the hull (hull speed) is the limiting factor. Consider my boat. With 135 HP, my boat can reach a speed of 8.6 knots. To reach 10 knots requires 450 HP (original engine in my boat) and a huge set of trim tabs to go above 10 knots.

My John Deere develops 135 HP at 2,600 RPM. If I prop my boat to reach 2,600 RPM, it means my 8 knot cruise goes from 1,700 RPM to about 2,200 to 2,300 RPM. My 7 knot cruise goes from 1,450 RPM to 1,800 to 1,900 RPM. First, there's no reason to endure the additional noise and secondly, does anybody really believe running an engine 500 RPM faster all the time improves life expectancy (all other factors being equal).

Really fail to understand why people can't understand that it's ok to to have a self imposed maximum RPM other than the manufacturer's. Finally, why is it ok to use a variable pitch propeller to maximize load / speed at reduced RPM but not a fixed pitch propeller at the same pitch?

Ted

jespchee said:

1100-6.8 Kts
1400-8 kts
1800-9.2 kts
2100-13.4 kts (however I saw 14.5 on my smartphone app)

Any advice is welcomed as I am new to big boat performance.

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There is something wrong with these numbers. You increased rpms by 300 (1100-1400) revolutions and gain 1.2 kts.

Then you increased your rpms by 400 revolutions (1400-1800) and gained 1.2kts.

Ok, that sounds normal.

Then you increased your rpms by 300 revolutions (1800-2100) and gained 4.2 kts. This isn’t possible! There is something wrong with the data.

I don’t know if the tach stuck at 2100 or if something is wrong with the speed data.

O C Diver said:

Finally, why is it ok to use a variable pitch propeller to maximize load / speed at reduced RPM but not a fixed pitch propeller at the same pitch?

Ted

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Of course it is OK. An engine is only overloaded if it is overloaded. The pitch of the prop is one factor of many. I'm sure there are owners who should not have any setup that ever allows engine mismanagement, as they will mismanage it. Other owners will never have a problem with the same setup.

My boat seems to be overpropped by about 200 rpm (2800 max vs. 3000 spec). I chose to leave it that way, as full throttle operation accounts for less than 1% of operating time. The engine is loaded more at 1400 - 1600 with this prop, where 90% of the running is done, and this load is welcome as it is still only around 35% of available torque. Operating slightly overloaded 1% of the time will have negligible effect on engine life, and less than the effect of reduced piston travel at normal cruising speed.

I did not suggest that variable pitch props were economical when their cost is considered. Just that without them, the physics of props virtually guarantees lower load operation at part throttle.

I find nothing wrong with overpropping on a trawler that has excess HP. Ted's posts explain it well. Running hull speed with a larger than needed engine I would rather have it at lower rpm at higher load to keep firing temps and pressures up. To a point...

But it does leave a caveat: If operator was heavy on the throttle with it overpropped, it can harm the engine. Some engines more susceptible to that than others.

Sharne Lisa

I think you would not be wise to fit 4 blade propellers, as you will overload the engines at higher RPM. My 38 year old 38ft 16.5 ton DW semi displacement trawler yacht has identical twin Ford Sabre 212BHP engines, swinging 26 inch 3 blade propellers. I make similar but slightly higher speeds up to 2100 RPM, and can make 18Kn at 2400RPM without smoking, and holding 90c temperature. (The manufacturers claimed 19KN).
By running at 7kn the fuel consumption is increased threefold, from running at 12kn on the plane.
Mike

Ted wrote;
“does anybody really believe running an engine 500 RPM faster all the time improves life expectancy (all other factors being equal).”

Yes. Becuase there will be far less load. Load is what wears out engines. Not rpm.

But to get that result using 2-3 or 400rpm faster would be more likely. The rpm range that would be more efficient would be quite narrow and the degree of efficiency change would diminish as rpm went up and vary all over the map w all the other variables.

Marin Faure made a post about 10 yrs ago about taking his props to the prop man (Kroger & Sons in Ballard Sea) to get them pitched to rated rpm .. FL’s and 2500rpm.
He achieved his goal (2500rpm) and was supprised to see his fuel consumption drop. It’s mostly about load. It’s possible (even probable) that his noise level decreased too.

Run two engines without load, one at 1500 rpm and one at 2000. Which will wear out first? Almost certainly, the higher speed one.

What wears an engine out is far more complicated than "load". In addition, if we call "load" the power required of the engine, with a given boat at a given speed, the load is constant and does not change as you mess with rpm and prop.

Will an engine last longer running at greater rpm with lower torque, putting out the same power, as one running at lower rpm but more torque? The answer is not obvious unless taken to extreme, you'd need to do a lot of empirical testing to prove one way or another for the moderate cases we are talking about.

I'd like to see a scholarly study on this. Absent that we are just speculating.

Lets jump over to the discussion of genset longevity at light loading.....

Never mind....

mike r smith said:

...
By running at 7kn the fuel consumption is increased threefold, from running at 12kn on the plane.
Mike

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Trawlers aren't capable of running at plane.

The problem w little generators is that when you increase the load dramatically you decrease the rpm where the engine can’t match the load so the rpm and the load tank. So wherever the noise is comming from both rpm and load drop. And as the rpm drops the capacity for the engine to address load decreases.

I worked in a powerhouse (1400kw) in Alaska where the engine and it’s 90” dia flywheel absorbed almost all the shock load when the load was instantly doubled. When the load was doubled the increase in noise was deafening. But the rpm hardly increased at all. Working rpm was 327 and the rpm drop was in the vicinity of 10 or less. 98% of the increased noise was from load.

So when the skipper increases the rpm on his engine the power/load increases as does the rpm. It sounds like the noise increase is a function of rpm but I think it’s comming mostly from load. But when you decrease the pitch of a prop on a boat and don’t don’t change the load the rpm increases. That’s what happened on Marin’s boat. He didn’t say there was less noise or more but he did say there was less fuel burned that shows there was less power being developed and applied. It produced more torque on the shaft and moved the boat better.

But a good place to experience this rpm/load v/s noise function is in a diesel-electric powerhouse where the engine is capable of maintaining rpm keeping rpm constant.
On a boat having and using an instrument to measure sound before and after a prop change could reveal where the sound is comming from. It’s comming from both of course but IMO primarily the engine load.
And perhaps it depends what kind if injection the engine employs. I had a Nissan car (diesel) that definitely got almost quiet w a big increase in load. I usta look for hills. But I don’t know for sure the injection system caused the engine to be quieter under heavy load. At a steady 30-40mph that car was loud. Stick your food in it and over half the noise disappears.

Now that I think of it I could do an experiment in my boat slip. I’d need a insturment to measure the sound and to make the same recording wthe boat tied to the dock/float and then underway at the same rpm .... obviously under much less load. So we’d know if a big difference in load made a big difference in noise. Or w a truck on and off the throttle on a grade. Or a boat towing another big boat and not .. at the same rpm.

I guess I’m fishing for input that would be relative.

Let's see now:


If I read it correctly... NW intimates the following.


If boat is tied up still, and engine is turning a prop at let's say 1000 rpm, then the engine is experiencing more load than if the same boat is traveling through water while same engine is turning same prop at same 1000 rpm???


I'm no mathematical specialist on hull-water friction of a tied-up stand-still hull while prop pulls-in/pushes-out water. Likewise, I'm no mathematical specialist on hull-water friction of a moving hull while prop pulls-in/pushes-out water.


But - It seems to me that their may be considerably more load on engine that is created while hull-water friction [inc bow and stern waves built-up] happens to a moving boat than the load created on engine of tied-up stand-still boat.


Clarity... Please! Would like to learn mathematical/physic reasoning... and ... resulting answer.

Art,
The resistance is the prop load. Underway the load is reduced by fwd motion. If the prop reaches it’s pitch speed the resistance will be 100% water friction over the blades. Minimal load for whatever speed that is.

I can’t remember what rpm my boat is capable of tied to the float but it’s far less than while underway. So underway the engine load is much less but the rpm is much higher.

O C Diver said:

As with most overpropping threads, what's missed by the members with planning hulls is that the hull (hull speed) is the limiting factor. Consider my boat. With 135 HP, my boat can reach a speed of 8.6 knots. To reach 10 knots requires 450 HP (original engine in my boat) and a huge set of trim tabs to go above 10 knots.


Ted

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Interesting...

When I repowered I seriously thought of replacing the original 330 Cummins engines with 220 hp cummins engines, to make things more efficient, and also save a few bucks durring the repower.

Then I realized that for the most part with the Cummins B series that it’s just that the lowwer HP versions run at a lower RPM.

So... I decided to use 330’s and just generally run the boat at hull speed, but preserving the ability to run at 15 knots should the need arise.

"Yes. Becuase there will be far less load. Load is what wears out engines. Not rpm."

If it takes 50 HP to move the boat at the desired speed , that is the "load".

Weather the engine is in an efficient operating range (RPM) to produce that 50 hp will change fuel burn.

"Load is what wears out engines. Not rpm.""

Most folks use piston miles , as a wear guide , which is RPM , not load dependent.

This is why many older tachs count total RPM, not hours.

ksanders said:

Interesting...

When I repowered I seriously thought of replacing the original 330 Cummins engines with 220 hp cummins engines, to make things more efficient, and also save a few bucks durring the repower.

Then I realized that for the most part with the Cummins B series that it’s just that the lowwer HP versions run at a lower RPM.

So... I decided to use 330’s and just generally run the boat at hull speed, but preserving the ability to run at 15 knots should the need arise.

Click to expand...

My boat was available from the factory with twin "B" series engines, either 330 or 370 HP. Mine fully loaded with fuel and water, wouldn't climb over the bow wave (plane) with 450 HP. 660 to 740 HP would probably have made a big difference, or the trim tabs.

Ted

Nomad Willy said:

Art,
The resistance is the prop load. Underway the load is reduced by fwd motion. If the prop reaches it’s pitch speed the resistance will be 100% water friction over the blades. Minimal load for whatever speed that is.

I can’t remember what rpm my boat is capable of tied to the float but it’s far less than while underway. So underway the engine load is much less but the rpm is much higher.

Click to expand...

Hi Eric - My mind is still twisting [pun intended] regarding load basis on a twisting prop in water.

Following outcropped quotes from your post are also highlighted above in your post.

"... resistance is... prop load. Underway the load is reduced by fwd motion. "... pitch speed... resistance will be 100% water friction over the blades."

It still seems to me that the prop being tied up at a dock has less resistance for cutting through water and pushing that water to the rear than same prop needing to cut through water and, simultaneously needing to push through the friction-resistance factors established by pushing a hull through water as well as the boat's bow and stern wave.

Then - These additional "friction" factors also come to forefront... sea growth on hull, raspy texture to old bottom paint, size of keel, appendages sticking through hull or attached to hull. Also, unless it is a planing hull that has stepped up onto the water surface and is on full-plane [reducing sq. ft. hull contact to water - thus less friction resistance] then additional speed increases the friction load of hull surface through water. As well for a planing hull... once it's on plane... added speed increases its friction factors by ever increasing multiples. That's why little speed boats have thousand + HP to go 125 mph. As boat speed increases in linear mph, friction load of hull contact to water ever increases higher and higher in "per speed" multiples.

Seems that if prop was in Olympic sized pool [i.e. a location large enough for no backwash limitations and the prop was turning at what ever specific rpm] that the load on that prop would be less than if same prop, at same rpm were pushing a [friction surrounded hull surface] boat through water. That scenario seems logical to my mind. Therefore, if boat is tied to dock [although the prop would experience some water flow restriction by the water it moves being dragged past some hull parts] the tied-up boat with prop turning would be a fairly close representation of my mention of a prop with no boat hull friction at all that is turning in an Olympic pool.

Art,
Think of my little boat w a 37hp engine or a 20hp OB on a 16’ skiff.
Tied to the float neither can develop much rpm unless the boats were equipped w ultra low pitched towing props. I think my boat makes about 2200. The OB would run fine but wouldn’t make much rpm.

Now if you cut them loose on their own my boat makes 3000rpm and the OB probably about 5000.
Now put your imagiation hat on and imagine a big helicopter towing them fwd. My boat is a bad example because it’s FD but if it were a planing hull it would go much faster than usual and if it wern’t for the governor my little diesel would rev up to 4000rpm or more. And the 20hp OB would probably self-destruct. So at stattic the load is max and as speed increases the engine load goes down.

Now think of an overpropped (op) boat. The engine at WOT can’t make rated rpm because the prop load is too great. Reduce the rpm and throttle to the point where the engine makes all the power and rpm it can. This is the maximum speed the op boat can do. Reduce the rpm further untill the engine can easily turn the prop and push the boat. This is below the normal cruising rpm but the engine makes normal cruising power at less rpm and and more throttle but is a bit more efficient. Burns less fuel at cruising speed. One can increase throttle and speed here but very little as the prop load quickly loads the engine to excess at far less speed as the normally propped boat. But at that one point, actually a very small range of rpm the op boat is more efficient.

Boat hull friction is not part of the example above. I see your reluctance to exclude it but the boat in the swimming pool or tied to the float has no hull friction. The water is just being used to brake the engine. You’ve heard of “brake horspower”? Of course and they are highly related. In this case the “brake” being the water to load the engine. But as the boat hull advances (by what ever means) the load on the engine becomes less and less untill 100% of the load/drag seen by the engine is just the friction of the water over the blades. There is no thrust .. in either direction.

Well there’s so many variables I may have gone astray or run aground so to speak. Most are probably becoming more and more confused and at this point that may be myself as well.

Art said:

Hi Eric - My mind is still twisting [pun intended] regarding load basis on a twisting prop in water.

Following outcropped quotes from your post are also highlighted above in your post.

"... resistance is... prop load. Underway the load is reduced by fwd motion. "... pitch speed... resistance will be 100% water friction over the blades."

It still seems to me that the prop being tied up at a dock has less resistance for cutting through water and pushing that water to the rear than same prop needing to cut through water and, simultaneously needing to push through the friction-resistance factors established by pushing a hull through water as well as the boat's bow and stern wave.

Then - These additional "friction" factors also come to forefront... sea growth on hull, raspy texture to old bottom paint, size of keel, appendages sticking through hull or attached to hull. Also, unless it is a planing hull that has stepped up onto the water surface and is on full-plane [reducing sq. ft. hull contact to water - thus less friction resistance] then additional speed increases the friction load of hull surface through water. As well for a planing hull... once it's on plane... added speed increases its friction factors by ever increasing multiples. That's why little speed boats have thousand + HP to go 125 mph. As boat speed increases in linear mph, friction load of hull contact to water ever increases higher and higher in "per speed" multiples.

Seems that if prop was in Olympic sized pool [i.e. a location large enough for no backwash limitations and the prop was turning at what ever specific rpm] that the load on that prop would be less than if same prop, at same rpm were pushing a [friction surrounded hull surface] boat through water. That scenario seems logical to my mind. Therefore, if boat is tied to dock [although the prop would experience some water flow restriction by the water it moves being dragged past some hull parts] the tied-up boat with prop turning would be a fairly close representation of my mention of a prop with no boat hull friction at all that is turning in an Olympic pool.

Click to expand...

"It still seems to me that the prop being tied up at a dock has less resistance for cutting through water and pushing that water to the rear than same prop needing to cut through water and, simultaneously needing to push through the friction-resistance factors established by pushing a hull through water as well as the boat's bow and stern wave."

If you want to prove it to yourself just take your twin engine Carver and only run one engine - how many rpm max can you achieve with one engine vs 2?
I have run this test with diesels and the resultant loads from a slower moving hull (or non moving) on one engine are quite large.
If you have vacuum gages on your twin gas engines you can also see what your engine pulls with one engine at a specific rpm at say 5 kts vs tied to the dock at same rpm. Fuel flow gages would work as well.

One of the major factors in prop power absorption is slip ratio. This is the difference between the speed the prop is moving through the water, and the pitch advance speed. At 1000 rpm a 20" pitch prop wants to go 20,000" (1000 x 20") each minute, that is the pitch advance speed. If it does so, it absorbs no work from the shaft (other than viscous friction on the blades). It also does not push the boat. So props pushing something have a slip ratio, they only move maybe 60% of the pitch advance speed, perhaps 12,000" in the example. That requires the prop to push water to the rear, and absorbs work. The higher the slip ratio, the more work, until the prop cavitates. Tied to the dock, you have 100% slip ratio*, so the prop is absorbing more power than if the boat was moving at the same rpm.

*To be technically accurate, the slip ratio should be calculated relative to the local water flow, at the dock the prop will create some local current so it will not be slipping 100%, though higher than if the boat were moving.

Yes 100% slip at the dock. But it’s still doing work pushing at the boat. Not as much as it would if the boat was at cruising speed.

“Local water flow” Yes if the boat was moving through the water at the props “advance speed” there would be no thrust or slip. Just parasitic drag from the water rushing across the blade surfaces.

DDW,
Can you apply this to overpropping?

Smitty wrote;
“I have run this test with diesels and the resultant loads from a slower moving hull (or non moving) on one engine are quite large.”
Yes the prop is pushing water aft against huge amounts of water (heavy) already there. Much if this existing water needs to be pushed out of the way to make room for the water the prop is pushing aft. Against all this resistance the prop at the dockside slows dramatically.

But as I asked DDW, can you apply the above theory and trials to overpropping?

Personal experience:


I often "beach" [ground] the forward most nose [about 3' or so] of our boat's bow on island's very edge. Where we boat most islands' have firm, root entwined edges that fall off extremely sharply. In saying that... I mean... from a couple inches water to 7' deep in a matter of just a few feet off shore. Depth at boat transom is 17' to 20'. Therefore with engine cooling scuppers in latter 1/3 portion of hull length there is no chance of sucking up debris. With boat temporally "grounded' at the bow... I then throw a light weight but strong aluminum front anchor ashore and with our runabout I go out to drop a rear anchor. From that point I leave ample front anchor slack-line so I can back off island edge by motor and pull in rear anchor line until boat is well positioned just off island edge with ample slack left in lines for 4' +/- tides.


I mentioned above simply to say the following. I've for many years hit the island edge with both engines' tachs registering 800 rpm. I've not noticed any change in rpm whether it was moving slowly just before grounding or once boat has stopped moving.

I've read all posts in this thread regarding props and their "loads". I just wish my props could talk. I've more questions than I have answers!

Happy Prop-Load Daze! - Art

Let's first define "overpropping". On this thread it seems to be a prop or pitch selected such that in calm water, at full throttle, the engine cannot reach it's rated rpm steady state.

All that means is the prop absorption curve meets the engine output curve at less than max rpm. At anything less than that rpm, the engine will be capable of more torque than the prop can absorb, so it is running at reduced load. It is worthy of note that with a higher displacement (more fuel or water or stores) or in a headwind, or in a seaway, a perfectly "propped" boat by the above definition is unlikely to be able to reach full rpm, and will therefore become "overpropped" by that definition.

If we pick a boat and an rpm and a displacement and a weather condition, the result will be a certain speed, and a certain slip ratio. Increasing the prop pitch but changing nothing else, several things will happen: the prop will absorb more torque. The engine will fuel more (the rack or the electronics) to achieve the same rpm. The boat speed will increase, but not in proportion, as hydrodynamic resistance is non-linear. Therefore the slip ratio will increase.

For a modest increase in pitch, the increase in torque required is modest, and the headroom the engine has in torque is usually large except near top rpm. As others have said, whether this will increase economy depends on the fuel map and the resistance curve of the hull. Very difficult to predict the result, even with those in hand.

Here is a engine and prop curve plot taken from this article on sbmar.com, which is a pretty good discussion of the subject.



Note that you do not need to run at much reduced rpm to unload the engine (relative to its capability) by 50% or more. In this example cutting from 2000 rpm to 1700 reduces its loading to 50%. Changing the prop pitch a little bit moves the prop curve up and to the left a little bit. It only has to move a little bit to change the intersect point (max rpm) a fair amount. But it changes the cruise power demand much less.

I have a typically overpowered SD trawler (AT34). Max rated rpm is 3000, but I cruise it at 1400 or so. Even a pretty substantial change in pitch will not overload the engine at 1400, if anything it might be considered underloaded. If I habitually ran it at full throttle and only got 2700 vs the 3000 spec, then yeah, it's overloaded. But I don't do that.

Another wild card in the discussion is mechanical vs common rail electronic control. The "throttle" on a mechanical diesel just adjusts the governor speed set point. The governor then adds fuel to achieve that rpm. If you overload the engine such that it cannot achieve the set rpm, then it will be constantly overfueled and eventually damaged. Electronic engines are considerably more efficient, the more modern ones will not allow this kind of damage to occur, they will de-rate to prevent it. These controls were developed for truck engines, and in light duty applications (pickups) they run a low rpm and low load for a very long time.

Great discussion that I am struggling to understand.

Load has been mentioned so I’ll ask a silly question. For wear and tear on an electronically controlled common rail engine, what load % typically be efficient and “best” for the engine?

dhays said:

Great discussion that I am struggling to understand.

Load has been mentioned so I’ll ask a silly question. For wear and tear on an electronically controlled common rail engine, what load % typically be efficient and “best” for the engine?

Click to expand...

Hey Dave - Where did that come from??

Now I'll be even more confused! :lol:


Post Scrip/Edit: Having now fully read DDW's post - I see where you are coming from!

dhays said:

Great discussion that I am struggling to understand.

Load has been mentioned so I’ll ask a silly question. For wear and tear on an electronically controlled common rail engine, what load % typically be efficient and “best” for the engine?

Click to expand...

Good question and I'm not sure anyone (except perhaps the engineers at the manufacturer) knows. I suspect the engine will run longest (on the hour meter) that runs the slowest and with the lightest load, provided it is warm enough to avoid coking and drooling and all those kinds of things (some of which common rail eliminates). On the other hand a good argument could be made that IF you have a certain amount of work to do, there is some considerably higher load that will get the greatest amount of that work done per engine lifetime. An engine run at low rpm and load isn't doing much work, and will wear out just because of piston and bearing travel.

In our application (well, mine anyway) I have an oversized engine that I am not going to run any significant percent of the time at or near full power. So a question might be asked, in my 380 hp Cummins that I am going to run at 7 knots requiring about 80 HP @ 1400 rpm, would it prefer I do that at 200 more rpm and 40 ft-lbs less torque*? A little less load on bearings and rings, but the same proportion more surface travel per mile run. I suspect with those kind of minor variations, it makes very little difference. These marine diesels in yachts get killed by corrosion and neglect far quicker than most of them wear out.

On a smaller engine run at 80 - 100% of it's capacity, then I think you'd want to get the prop right on. For the rest of us a bigger consideration is probably, do you like the sound better at 1400 or 1600 rpm?

*They are directly related: HP = rpm x torque / 5252