jfm
Well-Known Member
I don't get that part MapisM. Happy to be corrected, but right now I don't get how there are large "shock type" thrust loads, even on re-entry of a jumping boat
How is the thrust of the props converted to move the boat?
- Thread starter Pinnacle
- Start date
MapisM
Well-Known Member
What makes jumps at fast speed tricky for the throttleman is the need to get the props spinning as close as possible to the same speed at which they were spinning before the jump.
Obviously, it's not just a matter of keeping the throttles steady, because that way the engines would overrev one second after the take off.
Therefore, you must release the throttles quickly upon take off, and re-accelerate right before re-entrying, calibrating both the timing and the throttle position.
Which is easier said than done, of course.
Now, if you're spinning 4500 rpm upon take off, and you re-enter at 4000 or 5000, you've made a helluva good job.
The propulsion can probably take also a higher difference, depending also on the boat rigging, power, weight, etc.
But if you take off at 4500 rpm, and you re-enter either at 2000 or 7000 rpm, what happens to the prop/transmission is a sudden axial peak load - backward or forward,respectively.
Now, I suppose you wouldn't call it "shock type" load because the water is - to use your wording for the rope/nets - "stretchy".
Arguably, much more than ropes... We can even drink it!
But at 100mph, it actually becomes quite hard. And even more so, since we are not talking of the torsional load on the blades border, but of the axial load either on the front or rear side of the blades (again respectively, according to the previous example).
Does that make sense? I'm not sure to have used a good enough wording...
Apropos, no worries, your previous post didn't come across as meant to argue with my previous one.
Maybe my reply made you think that it did, so I guess I'm the one who should apologise...
Obviously, it's not just a matter of keeping the throttles steady, because that way the engines would overrev one second after the take off.
Therefore, you must release the throttles quickly upon take off, and re-accelerate right before re-entrying, calibrating both the timing and the throttle position.
Which is easier said than done, of course.
Now, if you're spinning 4500 rpm upon take off, and you re-enter at 4000 or 5000, you've made a helluva good job.
The propulsion can probably take also a higher difference, depending also on the boat rigging, power, weight, etc.
But if you take off at 4500 rpm, and you re-enter either at 2000 or 7000 rpm, what happens to the prop/transmission is a sudden axial peak load - backward or forward,respectively.
Now, I suppose you wouldn't call it "shock type" load because the water is - to use your wording for the rope/nets - "stretchy".
Arguably, much more than ropes... We can even drink it!
But at 100mph, it actually becomes quite hard. And even more so, since we are not talking of the torsional load on the blades border, but of the axial load either on the front or rear side of the blades (again respectively, according to the previous example).
Does that make sense? I'm not sure to have used a good enough wording...
Apropos, no worries, your previous post didn't come across as meant to argue with my previous one.
Maybe my reply made you think that it did, so I guess I'm the one who should apologise...
MapisM
Well-Known Member
Apologies to Pinnacle for the o/t, but did you choose those numbers based on actual experience?
I'm asking because I had a gut feeling that it shouldn't take as much as 2k ponies even for a big boat as M2, to cruise at 20kts.
And since I'm bored by the long recovery after the flu I got returning from warmer climates (aaarumph!), I just gave a look at the C32 charts.
According to the prop demand curves of the 1600hp version, it takes 2000 rpm out of 2300 to give a 1000hp output, with a fuel burn of 200 lph.
Now, did it really take that much to cruise at 20kts on M1?
Btw, I checked the Acert version of the C32, so the numbers are maybe slightly better than the non-Acert which you had on M1.
I suspect that you actually said 1000hp just as an example, therefore my numbers are meaningless, but as I said I was bored...
rafiki_
Well-Known Member
Hmmmmmmm. Of course, you are right. Should not pen hasty posts in the small hours. Sorry.
jfm
Well-Known Member
Yes it was just a rough guess. There are many other simplifications in my calcs as wellI suspect that you actually said 1000hp just as an example, therefore my numbers are meaningless, but as I said I was bored...
. I was only trying to get a broad approximation (or even just an order of magnitude) of the thrust load on each engine mount, ie is it 150kg, 1,500 or 15,000kg? (Apols again for stating forces in kg not Newtons)I'll come back to you on thrust loads on re-entry later. I have a busy morning here...
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Peak load on engine mounts / bearings etc on a shaftdrive boat occurs when you hit the bottom at planing speeds 
Bilgediver
Well-Known Member
Peak load on engine mounts / bearings etc on a shaftdrive boat occurs when you hit the bottom at planing speeds
Time to put the wheels down
D
Deleted User YDKXO
Guest
Mmm, water is actually almost incompressible so I'm not sure I agree with the stretchy analogy but I agree with Mapism on shock loading. A lump of metal spinning at 5000rpm re-entering incompressible water at 100mph is going to experience huge loads axially, torsionally and shear
jfm
Well-Known Member
1. If its pitch happens to be such that 5000rpm is exactly right for 100mph?
2. Higher loads than if it were doing 5000rpm and 100mph in a steady state ie not jumping and re-entering?
You're just basing this on gut feel, not proven physics imho
The incompressibility of water aspect is very much only 10% of the story. It's a fluid. If your thinking were along the right lines then jumping into a swimming pool would hurt like jumping onto concrete
Pinnacle
Well-Known Member
Pinnacle
Well-Known Member
As an aside, may I ask why you're interested?
Are big power engines tempting you, P?
Curiosity only......
D
Deleted User YDKXO
Guest
Jumping into a swimming pool is not analogous to a 100mph boat. Perhaps cliff diving would be a better analogy where divers do reach entry speeds of 60mph or so and if they get their entry shape wrong, the result is broken bones or even death. Belly flopping from 70ft high into the sea would indeed be much like jumping onto concrete.
Also its not just the spinning prop which re-enters the water at 100mph but also the P bracket/shaft in the case of shaft drive or the drive casing in the case of sterndrive
jfm
Well-Known Member
But it would be if the incompressibility of the water were the issue. Hitting concrete at 10mph hurts, and hitting water at the same speed doesn't, so the incompressibility of water has nothing to do with all this.
Sure the loads are high when hitting the water at 100mph, moreso than at say 20mph, AOTBE. But it's the fluid drag, not incompressibility, which causes this. Drag increases roughly cubicly with speed.
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rafiki_
Well-Known Member
The water is displaced, not compressed. As JFM indicates, set concrete does not displace when you land on it.
MapisM
Well-Known Member
Nah, re-entering at the prop speed exactly consistent with the boat speed is just not going to happen.
Well, probably once in a thousand attempts, and only by coincidence.
A very good throttlemen can only get near to that ideal point, but there will be always some difference.
And the higher that difference, the higher the axial load.
I'm not willing to squeeze enough my poor school memories to try to explain that in "proven physics" terms. Also because it's unlikely that I would succeed...
But there's much more than gut feel behind what I'm saying: I've seen outdrives destroyed, and s/steel cleaver props left with half of their blades.
And to my simple mind, it's pretty obvious that only a high shock load can produce these results. Otoh, what other reasons could you envisage?
Besides, even if I see your point re. being drag (rather than incompressibility) the rationale behind what we are discussing, I would think that in this context water is actually even worse (!) than concrete.
Just ask any waterski racer (and I happen to know one who won some of these crazy races) whether they would rather fall from a monoski or from a bike, at 100 mph.
In fact, on a perfectly rigid surface you can only slip and/or rebound, while on the water if one of your shoulders hits the water first, the water drag grabs it very strongly, while the rest of the body mass continues to travel at 100 mph due to its inertia.
The effects on joints, bones and tendons are often devastating.
Some poor chaps also happened to hit the water with their head first, and I heard stories about the consequent conditions of the corpse which you wouldn't want to hear at dinner time.
Back to props, If upon re-entry the prop blades would hit a concrete surface, I think that all blades would detach from the hub in a second, even with a minimal difference between the "ideal" and actual prop speed.
With a water re-entry, even if the prop rotation is very far from the ideal one, the whole blades surface grabs the water, and that's the reason why the drag can introduce a much higher load than in a steady cruising state.
D
Deleted User YDKXO
Guest
Of course it is but that doesn't alter the fact that the incompressibility of water contributes to the shock loading that an object colliding with it at 100mph will experience
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It's not just about incompressibility: water minding it's own business travelling at 0mph doesn't suddenly want to do 100mph just because it was hit by a boat, and something has to give. Normally something inside the gearbox.
Water is funny stuff. With a plane, if you get the angle and the speed just right, the plane will skim like a stone, and come to a gentle halt (Captain Sully). 5 degrees out or a bit faster, and investigators are examining aircraft and body parts the size of a tea tray.
Water is funny stuff. With a plane, if you get the angle and the speed just right, the plane will skim like a stone, and come to a gentle halt (Captain Sully). 5 degrees out or a bit faster, and investigators are examining aircraft and body parts the size of a tea tray.
jfm
Well-Known Member
That is such nonsense. Sit on your bathing platform and put your leg in the water. It goes in, with little resistance, proving that your incompressibility of water is a non event. The surface of the water actually rises by a millionth of a micron, to compensate for your leg, which is what rafiki called displacement. It may well be incompressible, but as it is a fluid and isn't constrained to move (upwards), its incompressibility has no material effect
Insert your leg at 100mph and it will hurt. But that is ALL to do with surface tension and drag, and zero to do with incompressibility
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Although surface tension does cause the surface of a liquid to resist an external force, it is a very small force (e.g. it stops small insects sinking, or water to form drops) and it is not significant in explaining why your foot parted company with your ankle when you put your leg in the water at 100mph.
4/10.
See me.
.
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MapisM
Well-Known Member
Ermm... I don't think so.
According to my slide rule here, the leg volume would be compensated by a lowering in Deleted User's Ferretti w/line of exactly 3.7 hundredth of a micron.
LOL, how did we get into this...?!?
