Why Isn't Thrust "everything"?

Discussion in 'Hybrid' started by SURV69, Apr 1, 2010.

1. Joined: Apr 2010
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SURV69Junior Member

Comparisons between internal combustion engines and electric always comes down to comparing equivalent horsepower . . . why?

Why isn't the one and only "real" factor involved in comparisons of various options . . . THRUST?

Isn't it THRUST that determines if something moves and at what speed or momentum that something moves at?

Even on land isn't it a form of THRUST that determines how something moves, given any other set of parameters(such as friction and such)?

Does voltage, ampreage, wattage, shaft horsepower, brake horsepower, prop horsepower matter, when it comes to whether something will move?

Sure, a 20,000 lb displacement hull will take more power than a 5000lb displacement hull, but isn't that eventual need for power based on the available THRUST that is pushing the boat along it's way?

When I see a Torqeedo engine claiming 96 lbs of static thrust, is there a good reason to believe that another "trolling" motor of 101 lbs of static thrust is not equal to the challenge for pushing the boat similarly?

Also, how does someone compare an IC motor, with it's HP and such all determined at the high-end of it's speed to an electric motor which is being operated at a much lower speed . . . the much lower speed that most IC motors run at(especially on sailboats)?

Maybe I'm missing the whole picture, but this is how I see it.

If a person is pushing a car, he's going to have a tough time and might get it moving slowly(on level ground).

If he has two buddies help him, they will get the car moving more easily individually and to a certain point be able to maintain a faster speed, cause three people will apply more thrust(force) on the car.

Now, I know there's a point where they can only go so fast, but our marine comparison is our displacement hull sailboat and in general we're looking at the more efficient less than hull-speed motion.

Kipawa propellers has shown that a better designed propeller on a MinnKota trolling motor will increase thrust.

A cowling around the propellers of any motor(marine), will also increase thrust(again the lower speeds of displacement hull sailboats).

So, shouldn't the ultimate conversion be to THRUST and not to HP? I mean, don't we actually have the more useful conversion backwards?:?:

2. Joined: Oct 2002
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lewisboatsObsessed Member

Increasing thrust by a significant amount usually requires an increase in torque and power to provide the increased thrust. You tripled the "people power" to move the car easier and to maintain the faster speed. The propellers increase thrust but I bet they drain the battery faster to maintain that increased thrust. Cowlings allow the blade of propeller to interact and transfer power to the water more efficiently by concentrating the power applied to the water. Those two trolling motors may use the same amount of power but one has a more efficient transmission system or a slightly better propeller and thus rates the increased thrust rating...or it might just be pure marketing BS. Older 10 hp 2 stroke outboards often ran to 12 or 13 actual hp. The amount of thrust of a true 10 hp might be more than the amount of a true 12 hp though...depending on its configuration, speed, propeller type, condition and condition of propeller, Seems to me you have to compare the package in it's entirety rather than a single aspect.

3. Paul KotzebuePrevious Member

Thrust = Power divided by Speed.

That's why thrust isn't everything. Naval architects look at thrust or power, depending on what type of analysis or calculation is required. For example, the axial force on a propeller shaft is thrust. Power required is drag (which is equal to thrust) times speed (in consistent units).

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SURV69Junior Member

Generally speaking, for sailboats we're talking about one speed . . . slow.

Fast speed isn't easily attainable by propulsion. Even the Hull speed, a speed which can often be surpassed under sail, is hard to achieve under power.

SO, we're either looking at about 3 knots or pretty much a waste of power.

I wish to stick to 3 knots or less.

As far as everything else, all the other data seems to be more a function/formulation of wattage and voltage. Issues that I would think more affects what I use for power, like the number of batteries and whether or not they're parallel or series. If I need more batteries, I'll add them

Obviously, I want to make the most efficient use of wattage, voltage and proper pitch of the propeller, to get the most usable thrust from the propeller to move me at about 3 knots. I may need more speed from time to time. Maybe 4+ knots for short periods(a few minutes).

More speed may be more inefficient, less speed might be more inefficient, but 3 knots is my target and is what is probably about twice the "natural" speed of my boat(I figure about 1+ knots as the natural speed . . . the speed that requires very little thrust to move).

SO, given this small window of parameters, I don't think I need to worry about speed approaching the thrust output(offsetting the thrust).

After all, we are talking about a sailboat. Many sailors choose no engine at all and are very well satisfied.

For this, I think I need to know how various engines move the boat at less than hull-speed and still feel that thrust is the best indicator of what it takes to move a boat.

BTW, I've never gone more than 1/4 throttle on my 9.9 HP IC motor, and even at 1/4 throttle, felt the increase in speed was not worth the faster engine speed.

I'm not in a hurry . . . I just want to get there.

5. Paul KotzebuePrevious Member

That's fine, as long as you understand maximum thrust is dependant on the engine and propeller combination. Actual thrust required at any given speed is equal to drag and varies from boat to boat. Engine power is constant, which is why engines (and electric motors) are more likely to have power ratings than thrust ratings.

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jonrSenior Member

Give me a large enough crow bar (or gears) and I can create any amount of thrust that you want. But when it comes to moving boats at any practical speed, I'm terrible at it.

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