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Old 01-30-2016, 12:49 PM   #41
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There is nothing about diesel engines in the OP's question.

bay,
Hp = the energy that moves things. HP is the force that will lift 3300lbs so high in a given amount of time. Look it up. Hp dosn't measure .. it is power. One hp is a specific amount of work done.

Your last sentence may be true. But torque does not do any work.
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Old 01-30-2016, 12:52 PM   #42
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Originally Posted by manyboats View Post
The difference is that the boat weighing twice as much will burm twice as much fuel.

That a real factoid, all by itself?

Or does that need a boatload of qualifying assumptions to be true?

-Chris
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Old 01-30-2016, 01:06 PM   #43
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Originally Posted by ranger42c View Post
That a real factoid, all by itself?

Or does that need a boatload of qualifying assumptions to be true?

-Chris
At least a few assumptions ....such as speed...that's why we HAVE a trawler forum.......for us slowpokes.

My trawler is 2X as heavy as my sportfish but burns 1/10 as much to go 1/3 the speed..... so that rule is snapped right there.

But all things being equal except 2 times the weight (speed, naval architectural specifics, etc)....I'd have to let a physics wizard come up with the correct answer. Finding 2 vessels that meet that criterion is...well I would think impractical except in computer modeling.
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Old 01-30-2016, 01:06 PM   #44
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Not much difference in a FD boat.

If it takes 40hp to push a given FD hull of 5,000lbs at a certian speed a hull of 10,000lbs will require about exactly 80hp to push it at the same speed. Quite similar for SD too but a few more variables.

So Chris asking "That a real factoid, all by itself?" Yes as a rule of thumb. But by doubling the weight things like PC, wetted surface and many other things may not increase/decrease at exactly the same rate. so it won't be exactly 1-1 but quite close.
And if you compare different hulls and engines it can be very different.

One of the most common expressions of power required to drive a slow boat is X amount of power per ton of displacement. In that expression it's basically 1-1.
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Old 01-30-2016, 01:31 PM   #45
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david: the basic reason for the difference between gassers and diesel is the energy content of the fuels. despite engine improvements diesel fuel will always have greater energy content.
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Old 01-30-2016, 05:31 PM   #46
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Hp is not force. HP is work done per unit time done by the force called torque
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Old 01-30-2016, 06:33 PM   #47
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david: the bBothasic reason for the difference between gassers and diesel is the energy content of the fuels. despite engine improvements diesel fuel will always have greater energy content.
BTU content is part of it, but is not the biggest difference. Diesel is about 7 lbs per gallon and gasoline is about 6. Both have about the same BTU per lb, so that gives diesel about 7/6 more BTU per gallon or about 17% more.

The bigger difference is that diesels are never throttled. They always suck in the maximum amount of air that the valves and intake ports can deliver. When that air is heated by injecting fuel it expands and produces more power from that fuel than a gasoline engine would because the gasoline engine is almost always throttled so the air in the cylinder is much less, so less power is produced for the fuel burned.

As a corollary, a gasoline engine has to operate near the stioichiometric ratio which is the ideal air to fuel ratio where all air is used in the combustion and little is left over. A diesel always operates at excess, and at light loads a lot of excess air, sometimes ten times more than is needed to combust the fuel injected. The high compression ratio makes it possible to ignite the diesel as it is injected whereas a gasoline engine needs to operate near stoichiometric for it to be ignited with a spark. It won't ignite if it has too much air.

All of which is to say that the excess air nature of a diesel engine is responsible for about 50% better specific fuel efficiency over gasoline and the BTU content another 17%.

FWIW in the laboratory you can run a diesel on gasoline. You need a fuel injection system that won't lock up due to lack of lubrication and high pressure injection system to keep the gasoline from vapor locking.

And I guess I am now just spouting off and should shut up ;-).

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Old 01-30-2016, 07:22 PM   #48
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Hp is not force. HP is work done per unit time done by the force called torque
Yes you're right. Except for the torque part. You can't do work w force. Movement is necessary to do work.

I think what you're saying is that torque causes the piston to go down, the force felt on the con rod and then to the crankshaft that causes rotation. And the torque is applied to the output shaft and produces rotational movement that does work. So hp is created from torque. Hmmmm But the movement only happens if the resistance allows it. But torque didn't cause the piston to go down. Torque is rotational is it not? ... a twisting force. Not a pushing force. The crankshaft turns because the force from the explosion forces the piston down and that force is turned into rotational effort by the mechanical components of the engine. So torque is created by the expanding force and then turned into rotational force (torque) by the engine. In a motor electrical force (voltage and current) is turned into mechanical rotational force that can do work. But the electrical force creates torque w or w/o shaft movement.

1. So hp is not created by torque.
2. And torque is not created by hp.
3. Torque is created by forces in the engine turned into rotational forces that is applied to a shaft.

I think I'm chasing my tail.

Pressure is turned into mechanical movement that converts the pressure into rotational movement that can do mechanical work. The mechanical twisting force in the shaft is torque. And the hp is the work done by movement caused by torque.

What do you think is true?

bayview you say "HP is work done per unit time done by the force called torque". By that you're saying torque can do work. But you can have lots of torque w/o doing any work.
Perhaps you're saying you can apply torque to a machine (gear and a propeller or gear and a pump) and do work. But the work done is not hp. Is hp only a measurement of work? That must be it. But it can't. Because we decide how fast a boat can go by how much hp we apply to appropriate machinery .. like a propeller. We can't propel our boat w torque.
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Old 01-30-2016, 08:46 PM   #49
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"I think I'm chasing my tail.

What do you think is true?"

That part.
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Old 01-30-2016, 08:55 PM   #50
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Well I got someth'in right.
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Old 01-30-2016, 09:13 PM   #51
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So here are the actual definitions of torque and horsepower. Seems fairly obvious to me that an engine produces both of them and both of them are necessary for an engine to do anything worthwhile.

Torque
Noun
Full Definition
1 : a force that produces or tends to produce rotation or torsion <an automobile engine delivers torque to the drive shaft>; also : a measure of the effectiveness of such a force that consists of the product of the force and the perpendicular distance from the line of action of the force to the axis of rotation
2 : a turning or twisting force
Origin: Latin torquēre to twist.
First use: circa 1884

horsepower
noun
: a unit used to measure the power of engines
Full Definition
1 : the power that a horse exerts in pulling
2 : a unit of power equal in the United States to 746 watts and nearly equivalent to the English gravitational unit of the same name that equals 550 foot-pounds of work per second
3 : effective power <intellectual horsepower> <computing horsepower>
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Old 01-30-2016, 09:31 PM   #52
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Well I got someth'in else right .. the expression/word twist.

And I got someth'in else right. You need the rods, crankshaft and piston to convert the force/pressure into rotational effort that excerts torque to the shaft.

So can you power a boat w torque?
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Old 01-31-2016, 10:11 AM   #53
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David:
don't in cylinder fuel injected gassers run un throttled?? Far less pumping losses then.


Way back in the last century the military had diesel multi fuel engines that would burn diesel, kerosene or gasoline.
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Old 01-31-2016, 10:39 AM   #54
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I remember a dragline in a mine in western Alaska that started on gasoline and switched to diesel when it warmed up. Haven't thought about that for a very long time.
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Old 01-31-2016, 10:47 AM   #55
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Some old Diesel engines used a 'pony engine' which was often a gasoline engine that you started first, and then use it to start the diesel engine. Once the diesel is running, you shut down the pony engine. I've only seen one and it was an OLD engine, and I was 10 or so, so it's ancient by todays standards.

One of the problems with direct injecting gasoline is if you ever got air in the lines, you can cause it to ignite in the lines and blow it apart. The direct injection pressures were much lower on gasoline due to it's lower flash point. Also it's viscosity is thinner and more easily atomized by the injector. If I recall the direct injection vettes in late 50's had 200psi injection pressure. Compared to 1400-1500 on Cat, JD, perkins, etc. Deutz had a pop pressure of about 3200 psi which atomized the fuel better, resulting in more power for the same fuel. At those pressures, when testing an injector, you had to be careful since it could inject the diesel fuel through your skin.

fast forward to today, the Common Rail Diesels use a rail pressure of 30,000-40,000 psi and piezo injectors. At those pressures, the materials have to be tool steel hardened so they don't erode.
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Old 01-31-2016, 12:16 PM   #56
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All the old D-8 Cats in the mines I was in had pony motors (or pony engines) but I'm quite sure the dragline (Wakasha sp?) was a dual-fuel engine. May have been just a pony motor starter. They said it started on gas and then switched to diesel but those words could have been applied to either.

The starting engines on the D-8's were 2cyl flat head 4 stroke engines that used the coolant of the mother engine to pre-heat same and the exhaust ran through the main engine too for pre-heat. The pony's had a multi-speed gear box and clutch to turn the flywheel in starting and the engine (main) had a half compression setting. In cold weather it could take 10 minutes to start up. On a 60 degree day only a minute or two was required. When it was 20 or 30 below wood fires were built under the oil pan.


Marin your post #51 was helpful.
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Old 01-31-2016, 12:36 PM   #57
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I have seen an International diesel engine on a generator that started on
gasoline and was switched to diesel when warmed up.

Have used a pup (pony?) engine to start dad's D-2 Cat.
Also seen one on a 80 hp Cat engine in a troller, no fast start with that one if
you stalled in a tight spot.
I think most of the older Cats were started this way.

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Old 01-31-2016, 01:19 PM   #58
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Quote:
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David:
don't in cylinder fuel injected gassers run un throttled?? Far less pumping losses then.

Way back in the last century the military had diesel multi fuel engines that would burn diesel, kerosene or gasoline.
Nope, all gasoline spark ignition engines are throttled to maintain the proper air to fuel ratio.

Gasoline has to be mixed in a fairly tight air to fuel ratio for it to spark ignite, which is an entirely different mechanism from compression ignition. Modern gasoline engines operate with just a few percent of excess air (from stoichiometric) at all power loads. Diesels operate at as much as ten times excess air at light loads.

Stratified charge or lean burn engines can have a higher air to fuel ratio. The gasoline is injected into a pocket of the piston crown. I think that the pocket is near stoichiometric to get it to ignite, but since it is surrounded by mostly air, the overall ratio is high. This reduces combustion temperature and reduces NOx.

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Old 01-31-2016, 01:37 PM   #59
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Exactly right

Quote:
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I'd suggest the advantages of diesel are not limited to larger boats.

Our boat has been built with 315hp 454 gas, and with 260hp Volvo diesel. It gets about 40% more miles per gallon with diesel. A buddy with a 2859 Bayliner repowered from 454 gas to Yanmar 6LP diesel, and saw about the same advantage in fuel economy and thus range.

Even though we are only 26 feet, with limited fuel capacity (ours is 110 gal) the efficiency of diesel vs gas can be a huge advantage. Cruising the distances of the Inside Passage, diesel gives us far greater range on a tank, and thus much more flexibility in the routes we choose to wander. For us, the $7,500 higher initial cost for the diesel has been well worth it.
One of the problems with planing hulls is weight. Added fuel adds weight and the decrease in fuel efficiency due the added weight of fuel. So in your case even if you doubled your fuel capacity you would probably only increase your range by 50%. This was the problem faced by me with my 28, carrying an additional 42 gallons in jerry cans never gave me the full measure in increased range. Another added value of the diesel is torque in big waves, especially with a planning hull. If I had kept the 28 it would have ended up with a Yanmar in it for the very same reasons.
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Old 01-31-2016, 07:06 PM   #60
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This is kinda outside the ops question, but nobody mentioned the safety factor. Gas explodes, diesel fuel does not.
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