surface piercing props in action

Just goes to confirm what I always thought, surface drives are simply flawed designs, at all but displacement speed half the prop is out the water.... so I am sure they won't catch on as its clear to see they just don't work... :rolleyes:



....BTW, Howard you indeed get nerd video award of the month.
 
That's just 7.44 mins of which the first few are at displacement speed, they really look like they're having fun crashing across lumpy seas:rolleyes:, probably be passing blood in their urine for the next couple of days, oh to be young again!:D:D
 
Don't understand: what are the advantages of having the prop blades exposed to air for half a rev, and thus not contributing to the thrust ? It's clear that the props are not working full time, and that the revs have to be dropped to take account of tiume when they are almost completely in the air.

Also I think it's astonishing that fast mobo seats are not like tractor ones. Those guys are bouncing around onto the seats, and IMHO they should be IN the seat, which then has a bounce-rebound shock absorbing system.

I suppose pain is pleasure ?
 
I cant give you the technical details on how they work but im sure Cookee will be along at some point as he has them on his raceboat. the obvious advantage would be having less drag as the drives are not "in the water" i would guess that as they are set further back than conventional drives they would also allow less running surface in the water and therefore even less drag??

As for the seats, many of the endurance boats do have the "tractor" seat set up. there is a company in Canada (shockwave) that have developed the seats to have a suspension console. there are some videos on the web somewhere. Of course Dr Ullman has developed a range of suspesion seats for many years also, as have others....

if i were doing long distance endurance racing a suspension seat would be pretty high on my list of must haves for sure!
 
sarabande said:
Don't understand: what are the advantages of having the prop blades exposed to air for half a rev, and thus not contributing to the thrust ? It's clear that the props are not working full time, and that the revs have to be dropped to take account of tiume when they are almost completely in the air.
Click to expand...
Sarabande, so long as they're asorbing the engine's power, that's ok. I mean if you took any prop and magnified it so it was 2x too big in terms of blade area, and put only half in the water, that would (conceptually) be ok, right? The benefit, as shown nicely in that video, is that the shaft is above the water and creates no drag. Only the bottom 40% or so of the "prop circle" is under the water. It definitely works on the low drag score, and is very tried and tested for 20 years ++

Incidentally, see the picture MapisM put up on Mavric's Mariner 50hp outboard thread, of the Mercury 2.5 litre. That's a surface piercing outboard :-)
 
sarabande said:
what are the advantages of having the prop blades exposed to air for half a rev, and thus not contributing to the thrust ?
Click to expand...
Aside from what jfm correctly said, just think of how much less drag you can get by not having ANY fixed structure (shafts, p-brackets, outdrive cone) fighting against the water resistance.
There's simply no way, mechanically, to transfer power (usually a lot of it!) to a fully submerged prop without having some fixed parts also submerged in the water.
Btw, you would think that this reduced drag advantage only justifies surface props at extreme speeds.
But that's not true. Theorically, any boat at any speed could benefit from lower drag.
There are other advantages (seakeeping, maneuverability) of traditional transmissions which can offset the efficiency advantages, though.
Otoh, whenever efficiency is the name of the game, surface props are the answer.
If you look at this video from 3:20 onward, you can see which sort of surface props pushed this animal to circumnavigate on solar power alone, 100% restricted by full displacement speed:


As an aside, you're right re. seats, though. Those in the video are plain vanilla bolster seats which you can find on most speedboats, requiring your legs to work as dumpers whenever necessary.
But many serious racing and endurance boats are equipped with safety belts and a suspension system. This is one example:
tecno-g10.jpg
 
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thank you both for the explanations and links. Makes sense. I can now see why the profile of SP props is so different from the fully immersed ones.

One slightly less confused raggie. :)
 
Something about both videos above showing surface drives just makes them look inefficient to me.

The props seem to spend half their energy just splashing the water round and aerating it, (or shooting it upwards) rather than pushing the water straight backwards.
They look like modern day paddle steamers as a result.

That just can't be efficient can it? I get the bit about reduced fixed parts in the water and less drag, but the props themselves just don't look efficient.
In fact thinking about it, paddles would be more efficient, as far more of the water moved is propelled straight backwards ;)
 
Firefly625 said:
thread drift, on MapisM video 4m 18s, that rib just went straight under the planetsolar boat!! :eek:
Click to expand...
thread drift2: what an awful dash that planet solar boat has, in the still pic in the youtube link above. Looks like it's made from an old plywood packing crate with a thin lick of cheap varnish
 
jfm said:
thread drift2: what an awful dash that planet solar boat has, in the still pic in the youtube link above. Looks like it's made from an old plywood packing crate with a thin lick of cheap varnish
Click to expand...

And a distinct lack of scatter cushions. ;) :D
 
Firefly625 said:
thread drift, on MapisM video 4m 18s, that rib just went straight under the planetsolar boat!! :eek:
Click to expand...
Yes it did, it was one of the marina ribs.
If you think that was a crazy maneuver, you should see what they can do to sort out hairy situations with strong wind moorings... :)
 
jfm said:
thread drift2: what an awful dash that planet solar boat has, in the still pic in the youtube link above. Looks like it's made from an old plywood packing crate with a thin lick of cheap varnish
Click to expand...
Agreed, but that's nothing.
Would you believe that they have no A/C in that small p/house, right in the middle of a football field of solar panels?
There isn't a fan for nothing, in fact.
This bit of strictly efficiency-oriented design was a bit too much even for my tastes, I must admit...
 
Trundlebug said:
The props seem to spend half their energy just splashing the water round and aerating it, (or shooting it upwards) rather than pushing the water straight backwards.
Click to expand...
Yep, of course you can argue that with surface props some of the energy is wasted trasversally, throwing in the air a big rooster tail as a result.
But don't be fooled, submerged props do exactly the same.
The difference is only that with them you don't see the water displaced sideways, because, ermmm... they are submerged.

Otoh, I can't think of an obvious reason why paddles are less efficient than propellers, in principle.
But there must be a bloody good one for sure, considering how many boats/ships with paddles are still around (not!) :)
 
MapisM said:
ed.

Otoh, I can't think of an obvious reason why paddles are less efficient than propellers, in principle.
But there must be a bloody good one for sure, considering how many boats/ships with paddles are still around (not!) :)
Click to expand...


The paddles would have to be going through the water faster than the boat speed. The idea of the size of a paddle wheel big enough to propel M2 through the water at 30+ knots is challenging. With a boat displacement of around 50 tonnes, the paddles would have to move at least 50 cubic metres of water over the length of the boat (25 meters) every 1.6 seconds. Perhaps a paddle wheel is a thought for M4 ? It would be a machine wonderful to behold.
 
I think the reason why a paddle on say a lake steamer type boat is less efficient at " speed" ( youcan define that yourself)
- is that as the the speed increases there becomes a point whereby the forward blade actually causes drag as it re-enters the water .
The hull is sliding forwards and the front forward facing area on the blade - for a moment as it enters/ dips in drags - slows it down
Once past that point then it creates more of a pushing - hence the forward " speed" - But as soon as its doing this another blade - the next one is about to and a split second later enter and drag
This limets the speed to arround displacement
This system ihas a trump card - fouling or lack of it
 
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Following on with the OP Q,s it's back to drag agian
At the N th speed There reaches a piont where ALL the submersible gear slows the boat
( given hull shape and enough BHP ) -even the prop I have called this N speed - next step start removing the prop ie 1/2 in = less drag worth it compares to all in = more speed
Although prop design is different to conventional props to cope with this
 
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