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Old 29-09-2009, 06:24 PM   #21
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That equates to a very good slip figure (< 10%)

What are you using to measure the speed?

I calculate that 10% slip, on the 25's @ 7500rpm would give you 85.4mph (this obviously assumes accurately pitched props, and an accurate Tachometer)
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Old 29-09-2009, 06:33 PM   #22
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Phantom 23 bottom pictures

Hi!

I use both a handheld gps and a dasboard gps speedo with recall (see picture) from Livorsi. So the figures should be correct. The cruise speed slip is also very good, running 48-49 mph at 4,000 rpms with the 4bl Hydromotives. However, the Hydromotives are known to be "tall" in pitch and Bravo is known to "low" in pitch. Since the Mirage 25 is labbed, I think that prop is most correct in slip figure. With that prop I ran 47-48 mph at 4.000 rpms.

I see that the Xr2 with cleavers mentioned above do approx. 10% slip. I blueprinted the bottom some years back. I have also wetsanded it with 600 grit paper. It helped on top speed too.

Cheeers, ToffenG
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Old 29-09-2009, 06:37 PM   #23
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Interestingly, I have Bravo1-30's and Mirage Plus-29's for my 28.

One would assume the Mirages would require higher rpm's to achieve the same speed as the Bravo's, but in reality, the boat is a couple of mph faster, and a little bit lower revving (maybe 50rpm) with the 3 blade Mirages over the 4 blade Bravo's. never quite understood that, as I would have thought the 3 bladers had less blade area (and less pitch) so would scream compared to the B's.

Propellers are black magic, that's for sure. Nothing short of Witchcraft.
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Old 29-09-2009, 06:42 PM   #24
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Interestingly, I have Bravo1-30's and Mirage Plus-29's for my 28.

One would assume the Mirages would require higher rpm's to achieve the same speed as the Bravo's, but in reality, the boat is a couple of mph faster, and a little bit lower revving (maybe 50rpm) with the 3 blade Mirages over the 4 blade Bravo's. never quite understood that, as I would have thought the 3 bladers had less blade area (and less pitch) so would scream compared to the B's.

Propellers are black magic, that's for sure. Nothing short of Witchcraft.
HI!

And as we know the props in stock form vary a lot. I remember the Bravo prop to be fast. Taken into account they were in stock form, and the Hydromotives are thinned and labbed, the Bravo 26 could end up beeing faster than the labbed and thinned Mirage 25's. The only problem is that I have to buy to test.....

When you speak to Steve could you ask him about the propshaft height for twin installation mounting the engines directly on to the transom. Maybe I have them too high.... (1 inch - 1,2 inch below now).

Cheeers, Toffen
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Old 29-09-2009, 06:44 PM   #25
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Will do.

Quote:
Originally Posted by ToffenGunnufsen View Post
HI!

When you speak to Steve could you ask him about the propshaft height for twin installation mounting the engines directly on to the transom. Maybe I have them too high.... (1 inch - 1,2 inch below now).

Cheeers, Toffen
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Old 30-09-2009, 01:19 AM   #26
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Interestingly, I have Bravo1-30's and Mirage Plus-29's for my 28.

One would assume the Mirages would require higher rpm's to achieve the same speed as the Bravo's, but in reality, the boat is a couple of mph faster, and a little bit lower revving (maybe 50rpm) with the 3 blade Mirages over the 4 blade Bravo's. never quite understood that, as I would have thought the 3 bladers had less blade area (and less pitch) so would scream compared to the B's.

Propellers are black magic, that's for sure. Nothing short of Witchcraft.
Jon, isn't that down to prop efficiency related to number blades i.e. increased slip?

I was always under the impression that technically the most efficient form for a prop was single blade assuming you could get enough blade area out of it. I guess the gearboxes wouldn't last too long with that design though.
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Old 30-09-2009, 08:59 AM   #27
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Jon,
I was always under the impression that technically the most efficient form for a prop was single blade assuming you could get enough blade area out of it.
Yeah, I understood the same, certainly with a light efficient boat.

However, with this 'Witchcraft', we see multi blade props, such as the Merc Maximus (5 blade) that are apparently super fast. Also, we see Rolla & Herring produce 5 & 6 blade props for class 1 etc that one assumes are pretty speed efficient. yes I hear you say, but they're surface. Yeah I know, more Witchcraft. They should strap the likes of Phil Rolla in a chair and see if he floats, that's how they used to deal with his type.

All I'm sure of, is that I'm sure I don't understand it anywhere near as much as I'd like to.
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Old 30-09-2009, 11:07 AM   #28
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All I'm sure of, is that I'm sure I don't understand it anywhere near as much as I'd like to.
Same as that but then if we did know?



I actually run a 26" Maximus on the Revenger as I was over revving with a 24" Bravo when trimmed and thought this would help.

Did it bollox. Now have too much blade area to get the thing to liven up. Apparently the solution now is to cut the exhaust tube back as a shorter tube gives more effective rake.

So there you go Toff just toss 2000 quid at a pair of Maximus then pay another grand or so to someone who knows what they are doing and it might work. Fun this boating lark ain't it

Actualy that wouldn't work as you need transom exhausts to be able to lose the diffuser.

Toffs problem seems to sound a little like mine in that you want to get rid of stern lift and introduce more bow lift at optimum trim.

That means high rake props like a chopper but then you don't get the dia needed for bravos. So there's no answer that fits all.
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Old 30-09-2009, 01:04 PM   #29
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That means high rake props like a chopper but then you don't get the dia needed for bravos. So there's no answer that fits all.
But 'in theory', at least as far as I can understand it, the high rake prop will only really give this effect if running high, so that the lower blades can exhert their effect during the lower 180 'alone' and won't be contra'd by the upper 180.

Again, only theory, and even worse, my theory, and I'm no witch.
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Old 30-09-2009, 03:02 PM   #30
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But 'in theory', at least as far as I can understand it, the high rake prop will only really give this effect if running high, so that the lower blades can exhert their effect during the lower 180 'alone' and won't be contra'd by the upper 180.

Again, only theory, and even worse, my theory, and I'm no witch.
I think you're right in theory Jon but possibly we get too hung up on how high is high.

The bit that always blows the brains is understanding the majority of a prop's motiveness comes from the front of the prop not the rear. i.e. it is the reduction in pressure in front of you that draws you in not the increase in pressure behind you that pushes you along. That's why you don't lose all forward motion when you start to throw a rooster.

Actually you're right, it is witchcraft
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Old 30-09-2009, 04:08 PM   #31
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The bit that always blows the brains is understanding the majority of a prop's motiveness comes from the front of the prop not the rear.
Dunno about that, though I have heard the theory. Surely that would mean the maximum thrust you could achieve would be prop area x absolute vacuum? Absolute vacuum can't be much more than -1 atmosphere, can it? (15psi) (It's very near the surface afterall)

If I'm right, my 15.25" Bravo's have roughly 180 square inches of area, x 15psi = 2700lbs (1.2 tons) mmmn, maybe it does work...for my boat anyway, but what of a 7000hp drag boat with a tiny prop, surely that wouldn't figure.? would it Fkk knows....oh, and the 180 figure is wrong come to think of it, coz you can't include the hub/bullet, or probably an area larger than that. So the figure should be quite a bit lower than 180 square inches.

Hubble Bubble, toil n trouble....
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Old 30-09-2009, 04:56 PM   #32
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The thing I was always taught is to ignore the principle of it being a screw as it is fundamentally incorrect thinking.

My understanding is that the prop works in a number of different ways but principally the idea is to increase speed of water against the face hence lowering pressure which means the force acting on the prop are now out of equilibrium.

This is achieved by the face of the prop being curved such as an aerofoil to gain maximum advantage of angle of attack without getting ridiculous drag and also by creating vortices at the blade tip which accelerates water away from the face.

The calc you are making is close to correct except that you need too look at the corresponding pressure increase on the back.

Don't forget it's where the majority of the motive force comes from not all.

Your workings of a near vacuum on the face won't do any good though as you would reach the vapour point of the fluid, in this case water, and hence you would generate the fatal cavitation.

In this case you need to now look at producing a super cavitating propellor where you look to produce the low pressure point slightly off the face giving the material a buffer of water in between to protect the steel from superheat damage.

In the case of pitch as well it is not as easy to just look at the pitch. As you probably know the blade is twisted principally because the further away from centre you are the higher the speed at that point and hence angle of attack is reduced the further out on the prop you go to retain the same pitch. In reality nowadays prop designs are progressive with their pitch to benefit that acceleration we talked about.

That hurt, I'm off for a lie down now
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Old 30-09-2009, 05:16 PM   #33
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I've always understood (maybe wrongly) that supercavitating blades don't attempt to fly (as in 'foil') through the water (as in a regular screw) but quite literally slice off chunks of passing water, throwing them backwards (and upwards) almost like a free blade waterjet (hence the fat, straight, transom like trailing edge on the blades, with clean breakaway sharp corners) this type as I see it, makes no attempt to go through the water, with it (the water) coming together again in the wake of the blade. That suggests that chucking the water very quickly backwards, in large volume, is somewhat of an action, and of course, every action.....

Different technology to the seagull you have on that old shead I know. but obviously on a Phantom design, it all works very well.

Q spiteful retort. (+ a barrage of fire from Graham)

You'll have to forgive me, I'm very bored on a conference in Lisbon, and feel a little disruptive.

Lovely place, and I'm stuck indoors! http://www.penhalonga.com/hotel_e.html
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Old 30-09-2009, 05:28 PM   #34
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Quote:
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I've always understood (maybe wrongly) that supercavitating blades don't attempt to fly (as in 'foil') through the water (as in a regular screw) but quite literally slice off chunks of passing water, throwing them backwards (and upwards) almost like a free blade waterjet (hence the fat, straight, transom like trailing edge on the blades, with clean breakaway sharp corners) this type as I see it, makes no attempt to go through the water, with it (the water) coming together again in the wake of the blade. That suggests that chucking the water very quickly backwards, in large volume, is somewhat of an action, and of course, every action.....

Different technology to the seagull you have on that old shead I know. but obviously on a Phantom design, it all works very well.

Q spiteful retort.

I can see your point and the newtonian bit makes sense completely.

and of course, every action.....Have you tried throwing yourself out of the back of your old tub, gotta be worth a good few MPH that

Not spiteful that one, I really think you ought to do it in the cause of boat science
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Old 30-09-2009, 05:31 PM   #35
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You'll have to forgive me, I'm very bored on a conference in Lisbon, and feel a little disruptive.

Lovely place, and I'm stuck indoors! http://www.penhalonga.com/hotel_e.html

Feck me I never knew there was that much money in talking bollox unless............................................ .................... your not Tony Blairs brother are you?
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Old 30-09-2009, 05:34 PM   #36
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I was also under the impression that the best of the pure cleavers (Super cavitating) props, having got the process going at speed, literally chop (cleave) the gobs of water, leaving the forward face of the blades dry (and subsequently drag free) and taking that into account, I've wondered why they don't have a wee step near the leading edge on the front face to help break away the water in said fashion.

Like I said, Witchcraft.
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Old 30-09-2009, 06:08 PM   #37
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You actually had me going for a bit there so I've just done extensive research on the net to check myself (briefly scanned the first item that came up).

I think we are using differing decriptions for supercavitating. As I understand it you are talking about superventilating propellors in a surface piercing environment. These don't have cavitation at all as they replace the cavitation zone with a slice of ventilated air on re entry and as you say reduce the drag across that face. Cleavers can be used in this manner as can some others.

Suoercavitating props in my mind were props that are designed to run fully submerged and allow cavitation to occur across the face but use a boundary layer to protect the base material from explosive damage. Cleavers again can be used in this manner.

I think. p.s. don't mess with me on this on this subject as I live with a fully fledged witch and will have to invoke her opinion.
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Old 30-09-2009, 06:14 PM   #38
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In a similar manner to you wondering about stepping the leading edge I've always wondered what the effect would be of introducing winglets to the ends of blades to control the vortices?
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Old 30-09-2009, 06:38 PM   #39
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In a similar manner to you wondering about stepping the leading edge I've always wondered what the effect would be of introducing winglets to the ends of blades to control the vortices?
Complex math I would think.

Do such things exist on rotary stuff? ie, do they exist on props, or heli blades? I've only seen em on wings used in a linear vibe, not rotary.

As far as Supercavitating terminology goes, I'll read up, I could well have my thumb up my arse, and not for the first time!
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Old 30-09-2009, 07:33 PM   #40
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Complex math I would think.

Do such things exist on rotary stuff? ie, do they exist on props, or heli blades? I've only seen em on wings used in a linear vibe.
I guess in those situations they would be unstable at the tips due to going supersonic.

Come to that what would happen if a prop blade went supersonic. That was a missed opportunity with Pash and his 1:1 speedmasters
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