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Incredibly stupid question on boat applications: is the prop direct drive?
Not sure if directed at Erat, or a general question.
Most (all?) "direct drive" applications with internal combustion engines (wherein the engine turns the prop through purely mechanical power transfer, rather than through a generator and electric motor) use a reduction gearbox, as the efficient propeller speed range is greatly less than the peak-power engine RPM. The reduction ratio varies with propeller size and blade pitch; some are 2:1, some are 10:1.
Boats with inboard engines also customarily incorporate a reverse gear, housed within the same gearbox.
Ships powered by reciprocating steam engines (eg: WWI Battleships), are generally "direct drive" in the true sense. The prop turns at crankshaft speed, with reverse being accomplished by reversing the rotation of the engine.
Disclaimer: not a marine engineer, just a guy who, as a kid, worked on a few yachts and sailboats in a below-the-waterline capacity.
Not sure if directed at Erat, or a general question.
Yes, mostly directed at Erat...or anyone smarter than me (the list is long). I was curious if it was a 1:1 to crankshaft speed, as I have no idea how the props are designed. Especially with reference the the Devel Sixteen motor. Watching the video it seems very "peaky", with all the power in the top end.
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Originally Posted by czubaka
Incredibly stupid question on boat applications: is the prop direct drive?
depends on the boat. Inboard prop driven boats like my 5 liter is dirrect drive. Flywheel harmonic balancer prop shaft prop. My ski boat has a transmission. Although it is 1:1 it has a neutral and reverse. It is also way slower because of the pitch of the prop. But gets out of the hole way faster.
Edit* there are far smarter people than me who can explain prop slip and pitch as it relates to boat speed and angle.
All I know is bigger pitch means higher top end with a lot of slip at low speeds and smaller pitch means more out of hole but less top end.
Generally on a boat, you want torque to peak at the top end, whereas on a car, you want torque over a broad range of speeds.
Going fast in a car usually entails you accelerating through multiple gears but rarely, if ever, attaining "top speed". Going fast on a boat usually entails you being at wide open throttle, briefly accelerating to and then holding top speed for extended periods of time. Most often, you want the engine to produce the best torque at the higher RPMs (with torque falling off before redline), and once you have your engine doing so, the pilot then selects the prop size and pitch which will allow the boat engine to hold the prop at that speed while going through the water. That is to say that if your boat engine cannot reach peak torque, then you need to reduce your prop's aggressiveness. If your engine reaches peak torque and then continues past it, you need to select a prop suited for higher speeds.
At the same time, selecting a "less aggressive" prop is like selecting a lower gear in your car. You'll be able to accelerate faster with a lower pitch because the prop is more effectively forcing water rearward whereas a high pitched prop tends to push water outward or "centrifugally" for lack of a better term until it reaches the higher speeds. This is why one prop gets a better "holeshot" (time it takes to go from zero to hydroplane) while the other prop can quickly jump to a big advantage once it's on plane.
Source: mostly heresay, but I've been learning since purchasing a 5.7L I/O "fish and ski".
Ok, I watched it, and it still does not make sense.
* More than half of the props are outside the water
* Whatever remaining portion of the prop blades in the water are actually in bad water, meaning, the water is all churned up and there are millions of compressible air bubbles in it.
I would understand running props deep in the water to get the bubble free stuff. (I'm pretty sure there's a name for it - we call it "rotten water" in Turkish)
That whole thing just looks wrong.
Trump should just ban that thing.
I had to google it because it was driving me nuts. Never would I have thought that the reduction in drag outweighs having half of your driving mechanism out of the water. If you had brought that idea to me id if said you were nuts. But I suppose it becomes obvious after reading how it works, that you can make up for that with pitch and blade count.
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Most all high performance or race boats are "surface drive" type, including the 5 liter. In fact, when a hydroplane is running at optimum conditions there are only two points of contact. The skid fin and half the prop(also rudder).
This is why they call it "flying the boat". With a controllable front canard you can get the perfect balance.
It's fun to watch a jersey skiff on full plane trying to balance itself on one point of contact. The prop.
This prop generally does not exit the water, however when it does it is only beneficial.
Commie, why do you think it is "bad water"? It may look washed out and bubbly, but it isn't like you think it is.
I believe what you are referring to is what we call "prop wash" and that is the bubbly mushy stuff. If you've ever ski'd or wakeboarded in or through it you know how soft and mushy it can get. The bottom of the boat does not create this, the prop does.
I still can't figure out the whole "rotten water" thing.
edit:
I posted this message too soon.
You have a point about the difference between prop wash and bubbles created by the bottom of the boat.
But still...
Rowing teams avoid getting behind another boat.
I mean, they have no props, and the oars are not even comparable to a prop in terms of creating bubbles, and they still avoid any bubbles in the water in the name of efficiency.
The fact that I am unable to wrap my head around this at 2 am does not mean it's wrong.
It obviously works.
I will admit that drivetrain drag is substantial. Even on my "slow ***" small block boat, the difference in speed from simply raising the outdrive while under full throttle is not only apparent, it is substantial. The GPS indicates an increase in speed from about 43mph to somewhere between 50 and 51mph by simply raising the outdrive from "all the way under the boat" to "pretty much still all the way under the boat".
02-06-2017, 11:10 PM
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