Under propped?

manyboats said:

Jeff,

If you burn 4gph at WOT and 3gph at cruise you're at 75% load in cruise.

Check the manufacturers specs for WOT fuel burn.

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That's kind of misleading as you wouldn't be able to generate the horse power at cruise that you potentially can at maximum rpm. It all depends on the maximum available HP at the cruise rpm.

Ted

O C Diver,
That's true but it's close enough for what we're doing here I'd say.

Frequently information is useable and beneficial when you throw out the details that make most people say "what are you talking about?".

The problem is the propeller that accepts 100% power at 100% rated RPM, looses its HP absorbing ability VERY rapidly .

Dropping down a few hundred RPM will lower the load a HUGE amount, which is why engine builders attempt to get you to prop for the pin, and then operate at lower RPM.

For engines that service lots of applications the builders RPM for generators for emergency or prime power production make the most sense to use.

Emergency power is short term, days or so, Prime is 24/7 forever.

Most engines come with 4 sets of power graphs , from 24/7 to sport fish , climbing up on the plane.

Each RPM will have a power rating .

The 24/7 rating for LRC makes the longest life setup.

But operating at a very small percentage of the 24/7 rating is not recommended at Cruise RPM.

Additionally, the chart that TT provided us with is not a really good example because that engine produces about the same hp from 2100 on up. A more typical NA engine would not have the flat area at the upper end of the curve. If TT were to overprop to where his new red line is, and limit his RPM to that point (usually reseting the stop on the pump) he would, in his case be giving up very little HP because the curve is so flat at that point.

Brooksie said:

Additionally, the chart that TT provided us with is not a really good example because that engine produces about the same hp from 2100 on up. A more typical NA engine would not have the flat area at the upper end of the curve. If TT were to overprop to where his new red line is, and limit his RPM to that point (usually reseting the stop on the pump) he would, in his case be giving up very little HP because the curve is so flat at that point.

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I think that power curve is pretty typical. Attached is another one from a Deere 6090 rated for near continuous operation. The same engine rated for continuous duty has the same curve shape.

But your point is taken about the HP being nearly flat after the engine reaches peak torque. There is an argument that says you should prop to intersect the line at peak torque (and not go over). What I don't know is what happens to engine life expectancy when you run at the Crankshaft power curve rather than the Propeller power curve. I image it must go down or we would all be propping that way.

>I agree. But why is that different from an M2 running 24x7 at 65% load factor? To get a 65% load factor on an M2, you need to drop the RPMs to 1850. Shouldn't it be 2100 just like M1?<

To change the LOAD the usual method is to select a load , gen head , water pump size or Propeller that fits the desired load at the desired RPM..

Its not just throttle position.

Thats why matching the engine loading , RPM , cruise speed and prop is more work than simply propping like a ski boat for full RPM at full throttle.

A 2100RPM engine would usually be run at 80% load at 90% of Rated Rpm.

The compromise of 1800 for cruise at 80% power (for 1800RPM) will usually result in very long engine life with high efficiency.

Low RPM operation is sometimes limited by the eng mfg , some fall off the chart at 1500 , so that would be the designed LR cruise, with the chart HP available at 1500 as the guide for loading.

Contemplating this concept shows how so many boats have far bigger engines than required , and mostly get by quite well as the HP rating is auto, tractor , or yard implement , flash peak BS,, not industrial.

I've gone through two or three props to find my diesel's sweet spot--best thing I did was reduce my prop's pitch and bring everything within specs. WOT for my Volvo MD2030 is 3600 per Volvo specs (3200 - 3600). I cruise at 2900 RPM which is 80% WOT or 52.6% load according to Volvo's load/RPM graph. Volvo recommends 300-500 below WOT for cruise but this is simply too noisy and uncomfortable.

Our CAT 3116 has a recreational "E" rating, max RPM 2800, cruise 2400. However the 2400 spec is way to high for how we use the boat. At 2400 she is plowing along at 10 knots trying to get up on plane, burning 9.7 GPH. We have found our sweet spot a 1600-1800, which gets us 7 or 8 knots burning a bit over 3gph. I think most of the turbo charged diesels are cruise rated for planing boats, not FD or SD trawlers. Contrary to popular legend, we have found no issues with running our CAT at the slower speeds. Almost 2k hours on the clock now with no problems.

FF says;
"To change the LOAD the usual method is to select a load , gen head , water pump size or Propeller that fits the desired load at the desired RPM.."

Fred there's no such thing as "desired rpm". Unless you have a variable pitch prop the load is fixed. The rpm to load to is exact. The rpm where your engine develops it's maximum power. 3000rpm in my case and I need to prop to that engine speed as closely as possible. Preferably any variation should be in the underpropped direction as in 3050rpm being preferable to 2950rpm.

Your cruise prop notions are like trying to drive a diesel truck around in high gear. Not for normal people and normal boats.

I turn my engine on....pull out of the marina and 5-12 hrs later having run at the same rpm and generally the same load all that time, then pull it to idle when I dock and shut it off.

Trucks are in high gear when rolling down the highway at cruise speed (50-80).....if they pulled away from the loading dock got pushed to speed by a helper and cruised flat lands for the whole trip with no stops and coasted into the next loading dock...I bet they would have one gear.

What's the diff?

psneeld,
Most trucks at 80 would be very heavily loaded compared to your boat at slow speed. 50 too depending on the truck and size of load. But the real world difference would be that all the hills, cities and towns would dictate that the truck would get 100% loaded very often and your boat almost never or possibly never. I think some here are afraid to run WOT even for a very short time.

But relative to your example there could be no difference depending on the truck. A truck at 50 with nothing in the box could be equivalent to you
re running 6 knots.

manyboats said:

psneeld,
Most trucks at 80 would be very heavily loaded compared to your boat at slow speed. 50 too depending on the truck and size of load. But the real world difference would be that all the hills, cities and towns would dictate that the truck would get 100% loaded very often and your boat almost never or possibly never. I think some here are afraid to run WOT even for a very short time.

But relative to your example there could be no difference depending on the truck. A truck at 50 with nothing in the box could be equivalent to you
re running 6 knots.

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Totally disagree......but would revise my speeds to lower 45-55 if talking not very aerodynamic vehicles.

OK, for those of us too stupid to just know this stuff and still trying to figure out the lingo and the relevant information.... and how to apply said relevant information...

OK, so on my 1978 Perkins 6.354 engines, my manual says a max rpm of 2800 and a max continuous rpm of 2400, what one am i using to base my cruising speed...

It does not provide any curves or graphs or recomendations for cruising speed (mostly i assume because they have no idea what the engine will be placed in). And I dont understand the inforamtion well enough to find this information when i Google it.

I would like to know what my cruising rpm should be and what I should base that on. I have them in my 1978 34' Californian.

The closest I have come to figuring it out, is that somewhere someone told me (or I read) the easiest way to figure out the most efficient cruising speed was to figure out what rpm I cruise at that produces three wakes, one at the bow, one in the middle of the length of boat and of course the stern wake.

This may be totally BOGUS, but thats what I have done. It turns out this is right at 1800 rpm and at that i burn just under 2 gph per engine and my speed is 7-7.5 knots.

Am i WAY off base? Where can I find this information or what terms do I search under?

Diver,
Here's my opinion but you should talk to an engine manufacturer. Or an engineer that knows this kind of stuff.

I think 50% load is high enough. The technical editor of Passage Maker Magazine says 75%. Others may say minimum regular load whereas the lube oil temp is at or above a certain point.

For a 50% load your engine will burn somewhere north of 3 gph. 3.25 to 3.5 I'd guess. Probably closer to 3.25.

If that consumption slams your pocketbook or ears slowing down will put you in the underloaded category (by my 50% std) and your engines may live long and happy lives. They already may have several thousands of hours at a 30% load.

But ideally I think one should run at 60 - 65% load. If you talk to an engine manufacturer I'd like to hear what he says. And I'd like to know that the numbers came from an engineer. Not a salesman or a mechanic.

Seems it's been a long time since we've heard from you. I've been wondering. I'm no longer in Thorne Bay. Live in NW WA up the Skagit River.

Percentage of load isn't equivalent to percentage of rated horsepower or percentage of full rpm.

Load is the actual fuel burned over a period of time divided by the rated full-power fuel consumption for the same period of time.

Diver, Perkins publish curves for most of their models here (second page).
Perkins Diesel Engines & Powerpart Parts: Products

If not listed, provide model number etc and I will see what I can find out.

dimer2 said:

Diver, Perkins publish curves for most of their models here (second page).
Perkins Diesel Engines & Powerpart Parts: Products

If not listed, provide model number etc and I will see what I can find out.

Click to expand...

Yeah, i found that page, but cant figure out which one of those is my old 6.354 engine... I read the instructions on what the numbers mean, but it doesnt make sense to me... How do I apply that to my engine?

On re reading your post, is the 6.354 NOT the model number? it sounds as though its not.. how do I find the model number?

never mind, i found a site that explains where to find the model number... will check that out this weekend.. and check back ...

manyboats said:

Diver,

For a 50% load your engine will burn somewhere north of 3 gph. 3.25 to 3.5 I'd guess. Probably closer to 3.25.

If that consumption slams your pocketbook or ears slowing down will put you in the underloaded category (by my 50% std) and your engines may live long and happy lives. They already may have several thousands of hours at a 30% load.

But ideally I think one should run at 60 - 65% load. If you talk to an engine manufacturer I'd like to hear what he says. And I'd like to know that the numbers came from an engineer. Not a salesman or a mechanic.

Seems it's been a long time since we've heard from you. I've been wondering. I'm no longer in Thorne Bay. Live in NW WA up the Skagit River.

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I have been out and about... still in K Town...

Part of my problem is I guess I dont truly understand the terminology....

Load is defined as what? And From what I have read isnt load based on a curve? I guess I am not sure of what number to apply the 50% or 60-65% of load to... Does that make sense?

Almost every boat has a sweet spot that feels right, sounds right and rides best. Whether that is 1800 or 2400 rpm or something else is not pertinent other than as a topic for endless debate. Run her where she feels the best and enjoy her for many many years.

Diver,
As I understand it load is a percentage of the total amount of power available at WOT. And the percentage of the amount burned at WOT is supposed to represent very closely the percentage of load. Percentage of rpm isn't worth talking about in this regard as the load curve is so different from the rpm.

It is said that if you burn 4gph at cruise and 6gph at WOT your'e at 66% load. 3gph = 50% load ect.

Being a heat engine making half as much heat equates to making half as much power. And making half as much heat will take half as much fuel.

The Ford Lehman makes about half it's power (60hp) at half it's fuel consumption but what rpm that occurs on a specific boat is dependent on many things like prop loading, hull drag and engine efficiency. Most trawlers have low engine efficiency, low prop loading and high hull drag.


Bob Coffer wrote;
"Almost every boat has a sweet spot that feels right, sounds right and rides best"
Yes but it has nothing to do w engines except that some engine speeds are smoother than others. And engines could care less if they vibrate or not. If there is a sweet spot for engines it should or would be between two places.
1. A high enough load and engine speed so the heat developed is sufficient to allow the lube oil system to perform well by insuring the rings, pistons and cylinders will perform as designed. And w enough load to insure the bores do not become glazed.
2. A low enough load and running temp so the engine does not suffer abnormal wear.
Diesel engines have a tendency to be very noisy at low loads and speeds. These conditions can be "sweet" for the boat operator but very bad for the engine. Generally speaking a diesel engine can be more "happy" at relatively high speeds and low loads but those are the conditions where the operator senses the engine is about ready to self destruct from what he considers to be excessive noise. Many operators may think the engine is overstressed under such conditions and the opposite is usually true.

>The closest I have come to figuring it out, is that somewhere someone told me (or I read) the easiest way to figure out the most efficient cruising speed was to figure out what rpm I cruise at that produces three wakes, one at the bow, one in the middle of the length of boat and of course the stern wake.<

OR,

Measure the waterline (LWL) as the boat floats , figure out the square root of the LWL and cruise there in Knots.

Convert to statute miles if inshore using statute miles on chart.

5nm per hour = 6 statute miles.

Light, sweet hulls can cruise at the sq rt X 1.15 and still be cheap to operate.

Diver,
You've not been sold down the river. You wrote; "The closest I have come to figuring it out, is that somewhere someone told me (or I read) the easiest way to figure out the most efficient cruising speed was to figure out what rpm I cruise at that produces three wakes, one at the bow, one in the middle of the length of boat and of course the stern wake."

That is true. That's half of hull speed. Most ships run like that as cruising near hull speed would take a ridiculous amount of power. The bow wave returning under the stern is running on the first returning wave. If you have the second wave under the stern and the first amidships you're going half as fast. The first wave returning under the stern is the strongest by far and is said to push the stern fwd. This is one of the reasons a FD hull is so efficient. Most trawlers befit little from the returning wave or waves. The stern of a FD hull is shaped much like the face of the wave under it. The steeper the stern angle (or QBBL) the more the hull is driven fwd by the stern wave. With the flat sterns of the SD hulls the stern is thrust deeper in the water causing more drag.

Large ships at sea (and the inland lakes) may go a specific speed to benefit from the 4th or 5th returning wave. I once read of a great lakes ore boat running on the 7th returning wave. An aircraft carrier may go at hull speed to launch aircraft into the wind when there isn't any and cruise the rest of the day on the second returning wave at only 1/3 of hull speed. Each time the following wave returns it gets weaker so the first is best and at the only speed that is practical for trawlers.

Well, on the survey if my boat, a 34' Californian 1978, it said I had a semi displacement hull.

Does that change the "three wave" theory? Since it's not a FD hull?

Diver,
With that hull (that is SD for sure (even close to planing)) waves don't mean enough to be considered. However Fred is absolutely right "can cruise at the sq rt X 1.15" (about one knot slower than hull speed (or 1.34) for best economy at a reasonable speed. THAT IS THE BEST SPEED TO RUN. However slowing further will deliver even greater economy. Just any speed slower than (sq rt 1.15). Any speed over 1.15 (probably about 7 knots for you) will result in large increases in fuel burn.

But no the "3 wave" statement still applies but you'd be running very close to 3.25 knots. If you want low fuel consumption that bad you should consider getting a FD boat. And beyond that there's FD catamarans.

Three-wave wake?

Nice try Mark but if it was we'd see almost no wake at all as you'd be going 3 knots. What looks like a 2nd wake is a different wave. That's the 2nd bow wave. The second wave in this discussion is a 2nd following wave.

The second following wave comes clear in to the midships, pushes against the hull, bounces back out and then returns at the stern.

In your pic it almost looks like it but it isn't.

Nice try though.

Regardless, this is how it looks at normal cruise (six-plus knots, one knot below hull speed).

manyboats said:

Nice try Mark but if it was we'd see almost no wake at all as you'd be going 3 knots.

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This is four knots.

Mark are you sure that's 4 knots? Looks like 2. At any rate a dramatic display of how speed consumes power. Practically no wake but still going over half speed.

Willy from behind at 7 knots. You can see the two bow waves well off to the left. The following wave, much larger, is at the stern. Of interest to this "3 wave" discussion is the fact that the mid section of the hull has no wave anywhere near it. In fact the water line is well below the static WL. The sea is being pushed aside by the fwd end of the boat and the inertia of that water pulls the water away from the hull amidships. This photo shows that clearly. Then when the inertia is spent the wave repels itself off the sea and and heaps up under the stern. I'm probably a tad over HS here. The following wave is probably a tad aft of the stern where that big hump of water is. It's too far aft to be pushing the hull fwd w much force. Slow down a knot (like FF says) and the following wave moves up under the stern pushing fwd as best it can. This is how our FD hulls move so efficiently at a little below HS. Like the BART system. The subway train uses it's motors to go up the hills and turns the motors into generators going down the hills re-cooping a great deal of the power lost going up the hills. Same w the FD hull. But I think the larger advantage of the FD hull is that it's stern is shaped like the wave it's riding on and causes little drag or fuss. Hence the water flows out from under the stern gracefully w little fuss.

OK, so (trying to simplify here, cuz I find that if I cant say it simply i really dont understand it) if I take the length of the water line on my boat (32 at water line', boat over all length is 34') and the square orrt of that is 5.6ish.

my combination of efficient and still reasonable speed is 5.6 knots?