Speed thru water or GPS speed over ground

I understand that, but in the example you gave of your boat you were dropping your rpm by a third. With our engines, dropping our rpm by the same ratio would give us just over 1,000 rpm which is too slow to achieve proper operating temperature. So the point is that adding or decreasing power while staying in the proper operating rpm band for our engines isn't going to change the fuel consumption by enough for us to care about.

To stay in the best rpm range for the FL120, from our cruising rpm of 1700 we can back off a whopping 200 rpm or add an even more staggering 100 rpm.

What I guess that says is that you have much more flexible and versatile engines than we do.

"There's a speed where your boat operates efficiently and the amount of fuel required to exceed that speed increases much more than the actual speed gain. That's my point."

If you want to see a striving for efficiency , look no further than a VLCC or 10,000 TEU box carrier.

The boat and engine operate at ONE speed .

Due to the hull design slowing does not save much , to slow down the bulbous bow needs to be rebuilt for the slower speed.

Fuel burn? Tons per day.

Marin said:

1,000 rpm which is too slow to achieve proper operating temperature.

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Your thermostat should be able to maintain the engine at its proper operating temperature regardless of load.

It's still just .... The slower you go the lower the fuel consumption. Planing boats that operate at higher speeds are another story but most all the boats on this forum have no "sweet spot" other than where the boat/engine runs w the least vibration. There is no economy sweet spot for us. For a FD hull the fuel burn/speed curve is steeper than for semi-disp hulls but ther'e is no magic boat speed that will deviate noticeably from the basic curve.
BUT .... If you didn't have a boat ... Then you could do a lot to maximize fuel economy. A light boat, narrow, skinny at both ends (diamond shaped) and powered by an engine that has it's best fuel efficiency at the rpm the boat will be most often used at (probably about 80% load) or less depending on the engine will produce a boat that will be very fuel efficient. Windhorse is kind of in this classification if you remember that boat. Prop dia, reduction gear can all be optimized for low fuel burn. Just buying a sailboat will go a very long way to supreme economy. One can repower w smaller, lighter engines that are more efficient w different props and gears but you won't come very close to the sailboat.
But if you're not going to sell your boat all you can basically do is slow down.
No Marin. You cannot wear out your Lehmans at 2000 rpm unless your maint goes south or you forget oil or .. or ..

RickB said:

Your thermostat should be able to maintain the engine at its proper operating temperature regardless of load.

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Rick I'd say the oil temperature is the issue that Marin refers to. My engine goes to 190 degrees F in only several (3 min. about) minutes but it takes about an hour at 60 % load to get the oil pressure down to it's continous level. And ov course it takes load not a thermostat to heat soak the oil.

manyboats said:

It's still just .... The slower you go the lower the fuel consumption. Planing boats that operate at higher speeds are another story but most all the boats on this forum have no "sweet spot" other than where the boat/engine runs w the least vibration. There is no economy sweet spot for us. For a FD hull the fuel burn/speed curve is steeper than for semi-disp hulls but ther'e is no magic boat speed that will deviate noticeably from the basic curve.
BUT .... If you didn't have a boat ... Then you could do a lot to maximize fuel economy. A light boat, narrow, skinny at both ends (diamond shaped) and powered by an engine that has it's best fuel efficiency at the rpm the boat will be most often used at (probably about 80% load) or less depending on the engine will produce a boat that will be very fuel efficient. Windhorse is kind of in this classification if you remember that boat. Prop dia, reduction gear can all be optimized for low fuel burn. Just buying a sailboat will go a very long way to supreme economy. One can repower w smaller, lighter engines that are more efficient w different props and gears but you won't come very close to the sailboat.
But if you're not going to sell your boat all you can basically do is slow down.
No Marin. You cannot wear out your Lehmans at 2000 rpm unless your maint goes south or you forget oil or .. or ..

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There's no magic spot but there is certainly a range where a slight addition of power gives a significant amount of speed...where you don't want to be is in the area of the curve where it is taking higher and higher increments of power to produce smaller and smaller increments of speed.

For my Albin 40/135 Lehman, I'm looking at around 1600-1800 RPM to give me economies around the 4 NMPG/ range at 6.5-7 knots.... as I approach 2000-2200 RPM...I'm dropping down into the 2 NMPG range and only gaining a knot up to 8 knots or so. If I drop back all the way to 1400 rpm...I'm not gaining that much economy but am dropping my speed off at a fairly fast rate and going that much slower (4-5 knots) would be painful to only gain 1/4-1/2 NMPG.

RickB said:

Your thermostat should be able to maintain the engine at its proper operating temperature regardless of load.

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You'd think but the coolant temp gauge for each engine doesn't reach the proper temp until the motor's doing about 1300 rpm under load. Don't know how that relates to what's actually happening in the combustion chambers, though.

manyboats said:

Rick I'd say the oil temperature is the issue that Marin refers to.

And ov course it takes load not a thermostat to heat soak the oil.

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Oil is mostly heated by cooling the underside of the pistons and lubricating the valve gear.

The cylinders and head are cooled by the fresh water loop and that is maintained by the thermostat. As long as the cylinders and head are within limits and the oil is warm enough to circulate but not above its upper limit, all is good.

The combustion temperature in the cylinder is the same at idle as it is a full load so as long as the coolant temperature is within limits the engine is happy.

manyboats said:

No Marin. You cannot wear out your Lehmans at 2000 rpm unless your maint goes south or you forget oil or .. or ..

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I'm just going by what people, and one person in particular, in the UK who've had a career's worth of experience with the Ford Dorset diesel which is what the FL 120 is, have told me, and what I've read about the engine's history. The engine was an absolute failure as a truck engine, which is what it was designed as, precisely because of its inabiility to sustain high loads and higher rpm.

RickB said:

The combustion temperature in the cylinder is the same at idle as it is a full load .

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Um, you may want to hook up a pyrometer. You have to have increased energy to increase power output. This is accomplished via more fuel/air. On diesels more fuel increases temp (gassers work the opposite), turbos are used to stuff more air in to 1. add air for combustion and 2. to cool the reaction some. Even with a turbo combustion temps rise as fuel is added to increase power. I know this is oversimplified, so (engineers among us)please don't feel the need to do a dissertation on the exothermic process of internal combustion engines.

twiisted71 said:

Um, you may want to hook up a pyrometer. You have to have increased energy to increase power output. This is accomplished via more fuel/air. On diesels more fuel increases temp (gassers work the opposite), turbos are used to stuff more air in to 1. add air for combustion and 2. to cool the reaction some. Even with a turbo combustion temps rise as fuel is added to increase power. I know this is oversimplified, so (engineers among us)please don't feel the need to do a dissertation on the exothermic process of internal combustion engines.

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There is a difference between EGT and combustion temperature.

most certainly there is. As I said it was an oversimplification, BUT you really don't care about the temp of your exhaust gases. Its used to give a reference to combustion temps. Its much easier to rig an EGT probe in an exhaust than drilling a hole in the head and try to get a direct combustion temp.

I tell yeah fellows. So many great comments. I think finding the sweet spot as Eric put it is what I am trying to do.
If I have to travel 3 or 4 hours to get to a favorite spot how much difference would it really make if it took a half hour longer to get there.
If I can find the best speed for my boat that is best for the boat and engine. Thats a plus
Every boat is different. If you have a boat made to specs in a boat yard with a name brand. Navel architects have already figured that stuff out.
MY boat is a one off. I have no idea how the hull was made, as far as design goes.
The abstract of title says built by me Donald Jones.
MY point is I am starting from scratch Attempting to use all the information at my disposal to determine that sweet spot.

GPS, compass ,tide charts, Charts, weather, batyhscopic readings.
What else is available?
Ahh the INTERNET and good old TF.
I think it prompted a little thought to the tune of 72 reply
I have some great suggestions to consider in my search.

SD

twiisted71 said:

most certainly there is. As I said it was an oversimplification, BUT you really don't care about the temp of your exhaust gases. Its used to give a reference to combustion temps. Its much easier to rig an EGT probe in an exhaust than drilling a hole in the head and try to get a direct combustion temp.

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No, you should care a great deal about EGT, it is directly related to load. It is an important measure because it is an indicator of valve temperature and engine load or overload.

It is not a reference to combustion temperature, it is the temperature of the probe. That probe is alternately heated by exhaust gas and cooled by charge air during valve overlap on a turbocharged engine and by cooling during the other 3 strokes when there are no gases passing over it.

The temperature of combustion is the same at idle as it is a full load. The weight of the gases and heat content of the gas changes with load but not combustion temperature. Drilling a hole in the head isn't going to give you any more information on combustion temperature than the EGT probe.

EGT at idle is lower because the amount of fuel burned is smaller and the combustion gases are cooled more by expansion during the power stroke than would a greater weight of fuel in the same weight of air.

An engine with a properly functioning thermostatic temperature control will maintain the same jacket water temperature across the range from idle to full load. If it runs cool at low loads it is because there is excess water bypassing the thermostatic control or the heat exchanger is oversized. A cooling system intended to perform in warm water might overcool in cold water if the temperature control method is a "one size fits all" version. It is a poor but economical compromise for whoever marinized the engine.

If your engine does not reach the lower temperature of the range specified by the manufacturer at low loads and does not reach maximum temperature during extended periods of high loads in warm water, it is being overcooled.

p.s. Turbos are not installed to reduce combustion temperatures. They increase the amount of air available so more fuel can be burned in the same time period and thus increase power output. Valve overlap in turbocharged engines provides valve cooling ... and also cools the EGT probe.

You're right rickB. Once again I was giving an over simplification. I was loosely using combustion temp (roughly 250-280*) for combustion chamber temp (yes more fuel-more mass-more heat transfer to surrounding structures-higher retained temp in chamber). The actual flame of diesel can reach over 3000* Luckily the engine's cooling system keeps the metal from being heat sinked to anywhere near this. The EGT probe doesn't give split second readouts, it is an average temp of the air coming out of the combustion chamber , NOTHING more.

And I don't care about the temp of the air going out the exhaust unless by some chance it is so hot that it is going to damage the exhaust system. As I said ,and you corroborated, I care about what it tells me about the working of the engine before it gets to the exhaust!

PS On diesels Turbos also cool the combustion chamber temps. They are what gets the air needed in your valve overlap dissertation through there. On diesels leaner + cooler burn. Don't believe me turn up the fuel on a NA diesel and watch your pyro at the same rpms as before you turned it up!

twiisted71 said:

Don't believe me turn up the fuel on a NA diesel and watch your pyro at the same rpms as before you turned it up!

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If you "turn up the fuel" the engine will accelerate, adding more fuel is what makes a diesel run faster. All the governor does is add fuel until the engine turns at the setpoint rpm. If you increase the load, the governor will add fuel to keep it at the set rpm and the EGT will go up.

You can run an unloaded diesel at max rpm and still have a "cool" EGT if there is no load on the engine.

The combustion temperature in the cylinder is the same no matter how much fuel is delivered ... until there is too much to burn.

skipperdude said:

............ The abstract of title says built by me Donald Jones.
MY point is I am starting from scratch Attempting to use all the information at my disposal to determine that sweet spot.

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Take the waterline length, find the formula for "hull speed" on the Internet (yea, I could look it up and post it myself if I was trying to impress people), do the math to find the "hull speed" for your boat.

Wait for slack current in your area and bring the boat to that speed using your GPS. Note the engine RPM and you've found your sweet spot.

That's all you have to do.

Golly gee shucks. Here I've been working on my motors wrong for years. I didn't know I was supposed to build them to operate under no load. Thanks for setting me straight!

To the OP. sorry this went off the rails.

MY point is I am starting from scratch Attempting to use all the information at my disposal to determine that sweet spot.

A very interesting drill is to take an hour some nice day and plot a graph of the rpm vs GPS boat speed.

AS most folks do not (sadly) have a flowmaster , this exercise will at least give a good view of at what speed the hull becomes hard to push.

Hull speed is as usefull to know as the top speed of a Honds.

Nice to know but unless you are a sailor (free breeze) is is certainly not an efficient cruise speed. The Sq rt of the LWL is close to efficient for most.

The simplest way to "improve efficiency" is to use statute miles rather than K in the calculation . 6K becomes 7 statute mph , so the numbers will look better.

YRMV

FF

Re your last post skipperdude the best speed for your boat is probably about 10 knots. The best speed for maximum fuel economy is probably about 3 knots and only you can tell us what the speed is that is the best compromise between speed and vibration .. Commonly expressed as the "sweet spot". But the most economical speed for fuel economy is still as slow as you can stand it. That would be 6.15 knots for me but I've bucked the tide for hours at less than 4 knots. "Sweet spot" has nothing to do w hull speed. It's just a user-friendly thing. Bottom line is still go as slow as you can stand it and speed up a tad.

I just figured that the boat moves thru the water at a given speed per RPM regardless of what the water is doing. If the current is doing 2 knts with me across me or against me I am still moving thru the water at the same speed. Not Speed over ground.
Looking for that hull speed thru the water with enough load to keep the Cat happy
and the wallet if it matters.
SD

skipperdude said:

I just figured that the boat moves thru the water at a given speed per RPM regardless of what the water is doing.

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That's what I've been trying to tell you.

skipperdude said:

I just figured that the boat moves thru the water at a given speed per RPM regardless of what the water is doing. If the current is doing 2 knts with me across me or against me I am still moving thru the water at the same speed. Not Speed over ground.
Looking for that hull speed thru the water with enough load to keep the Cat happy
and the wallet if it matters.
SD

Click to expand...

There are a few other factors such as momentum and chop but for the most past you have it now....just as a lot of people have suggested.

The real question for you is how fast? I hate the thought of going to Florida/Bahamas every year at 6-6.5 knots even though that's a great economy speed. Will going at 8+ knots (a 25% or so increase in speed) kill me if I spend another 50-100% in fuel? Not really as should another $3000 per year of boating when all of boating costs nearly $20,000 per year determine how fast I get someplace (assuming I steam 3200 miles per year)???

Just depends on whether I have those extra days to get where I want to be.

Call me Homer Simpson.
doouh!!
I just wanted to know what thet RPM would be.

SD

Some real data for discussion

I thought I'd through some hard data into the conversation. What you'll see from the data is that there is no optimum speed. Going faster means less economy.

The forum doesn't support tables so it's going to probably look poorly formatted. MPH and MPG are nautical miles.

34' Displacement Hull
LWL 30'2"
1.4 * sqrt(LWL) = 7.7
1.1 * sqrt(LWL) = 6
Engine peak torque 1800 RPM

RPM MPH - GPH - MPG
1000 4.5 - 0.5 - 10
1200 5.1 - 0.7 - 7.8
1400 6.0 - 1.1 - 5.6
1600 6.8 - 1.6 - 4.4
1800 7.4 - 2.2 - 3.5
2000 7.8 - 3.3 - 2.4
2200 8.1 - 4.5 - 1.8
2400 8.6 - 6.1 - 1.4

That is a great chart, what's it mean? I have a 34' mainship with the 200 (or 240 according to the engine manuals) Perkins and at 1400 I'm popping along at a solid 7+ kts.

twiisted71 said:

That is a great chart, what's it mean? I have a 34' mainship with the 200 (or 240 according to the engine manuals) Perkins and at 1400 I'm popping along at a solid 7+ kts.

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it's an example of his boat's performance (I think)

it has nothing to do with your boat (I think)

it just shows that the basic trawler hull design (either full or semidisplacement) run at displacement speeds has a nice gradual speed/fuel consumption till you hit that number somewhere's
around the square root of the waterline length where fuel consumption starts to climb more rapidly than speed as power is applied. (I know)

sorta what some posters have been saying all along....

psneeld said:

The real question for you is how fast? I hate the thought of going to Florida/Bahamas every year at 6-6.5 knots even though that's a great economy speed.

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Me too! My avatar picture is Moonstruck cruising in a "sweet spot" slightly above hull speed.

psneeld said:

it just shows that the basic trawler hull design (either full or semidisplacement) run at displacement speeds has a nice gradual speed/fuel consumption till you hit that number somewhere's
around the square root of the waterline length where fuel consumption starts to climb more rapidly than speed as power is applied. (I know)

Click to expand...

Gotcha, I thought it was some kinda one size fits all this is what your boat should be doing with "x" parameters. And mine was no where near those numbers.

I've been boating my whole life and still can't understand why people like to run big boats with the bow pointed to the sky! On my particular boat I lose around .5 kt when I hit that speed where the stern drops and it takes roughly 100 more rpm (that requires quite a bit more throttle to achieve) to get the speed to begin to climb again. Having a full keel my boat never truly seems to "break free" where you can pull the throttle back and cruise on plane (my Shamrocks were the same way), too much wetted surface area. Our shrimping skiffs essentially the same hull as my Mainship without a keel and a foot more beam would break over and cruise even with large prop tunnels.

Corrected numbers

I copied the wrong numbers for the speed. Here are the correct numbers with the average speed, rather than the speed of a run in one direction.

34' Displacement Hull
LWL 30'2"
1.4 * sqrt(LWL) = 7.7
1.1 * sqrt(LWL) = 6
Engine peak torque 1800 RPM

RPM MPH - GPH - MPG
1000 5.0 - 0.5 - 10
1200 5.5 - 0.7 - 7.8
1400 6.2 - 1.1 - 5.6
1600 7.1 - 1.6 - 4.4
1800 7.6 - 2.2 - 3.5
2000 8.0 - 3.3 - 2.4
2200 8.3 - 4.5 - 1.8
2400 8.7 - 6.1 - 1.4