Question about props in racing boats - thoery

Discussion in 'Powerboats' started by RipSlider, Jul 17, 2008.

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RipSliderJunior Member

Hello all.

Short version of question:
Is there a reason that high speed racin boats use multi-bladed props, when on paper a single bladed prop is more optimal?

Long version of question:
I'm currently trying to teach myself the maths and physics behind boat design in order to develop a model boat that goes as close to the limits of what is possible as I can make it.

I have been researching and reading as much material as I can get a hold of. Some of it makes sense, other bits of it I'm not so sure of.

One thing I have come across is proppelor theory.

From phot0 references that I have, it would seem that most of the high speed racing boats seem to use a three bladed supercavitating/ventilated combination prop. I therefore assume that this makes for a very good design.

On paper however, this design is les than optimal. Setting aside the whole argument about "Is trans-cavitating more efficient than super caivitating" - and so we can ignore the question of the blade design itself, I wanted to ask about the blade count itself.

As I understand it - when you are talking about getting the most speed out of a prop, the "ideal" is to use a single (properly counterbalanced ) blade, as this reduces turbulance ahead of the blade, and allows you to use a large blade size, and cuts down on drag by a power order.

The downside of a single blade is that it needs higher RPM's than a double, an a double needs higher RPM's than a triple.

But in a racing boat, this shouln't be a problem, as the gearbox is already a major component where lots of money is spent.

So - apart from "Thats the way we always do it" - is there a reason that in the real world 3, or 2 bladed props are used rather than single bladed versions?

Thanks

Steve

2. Guest625101138Previous Member

Airplanes go faster than boats. Just make a plane that flies close to the water and you will beat any boat that relies on the water for thrust and lift. Maybe turning will be a bit tougher.

A surface prop has best efficiency around 70%. The reason they perform better than submerged props at high speed is that they minimise appendage drag. An air prop can get more like 90% efficiency at high speed so you are a long way ahead.

If you want to get into conventional prop design then have a look at JavaProp. You can set the parameters in the options page to suit air or water. It does not apply to surface props though.

Rick W.

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eponodyneSenior Member

I think part of it, maybe even a big part of it, has to do with loads. More blade area=lower loading per blade. I'd think that at 100MPH, a single-bladed prop would have to be like a foot thick or something to take the stain.

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marshmatSenior Member

Blade loading is definitely a factor. The ratio of thrust to blade area can only go so high before the blades simply shear off (or bend, as the case may be). You can reduce blade loading, for a given type of propeller, by increasing the diameter or increasing the number of blades. Increasing diameter brings other problems- tip clearance, will it even fit, etc.

Vibration is another factor. Counterbalanced or not, a one-bladed prop will vibrate like hell at the speeds we're talking about here. (It would also bend the shaft into a pretzel.) Two-bladed props have been used, fully submerged, for high speed craft with some success. But when you get into the speed ranges where appendage drag starts getting nasty, and surface piercing props become appealing, you pretty much have to go to three or more blades to keep the loading uniform enough to not shake the driveline to pieces.

For these and other reasons, we just don't see single-blade propellers very often, and we usually see three or more blades on anything designed for high speed or surface-piercing effects.

As to the details of design, and why particular shapes are developed to be what they are... that's a long, long story that is quite interesting to research.

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VerytrickyLarge Member

I would guess that a single blade would be required to have a larger diameter, and this would increase the paddle wheel effect, and make handling at high speed 'interesting'

At high speed, water makes things happen to the dynamics of a boat about 250 times faster than air dynamics change an aeroplane. so seemingly insignificant changes at low speed in a prop becomes critical at high speed.

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dregszJunior Member

I'm not an expert at this But...

Drag boats, usually V drives, still run 2 blade props, they may be submerged in theory, but from the pics I've seen there isn't much or any boat in the water.
I'm not sure what unlimited Hydros are running but I think they too are running 2 blades. (I can't shake the image of a one bladed prop, sorry).

Large offshore boats have other issues besides just speed. Prop selection for these boats is also based on the need for bow lift and stern lift etc.

The current trend in big offshore racing cata marans, w surface piercing props is 5 blades.
I "Think" the reason is, the blades stay on better with reduced impact on water re entry from the blades being closer.

This is a good question to ask in the prop section at offshoreonly.com and do a search for Herring propellers.

I think the trend is one piece CNC props for \$1000- \$2000 USD each.

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1/4 MilerDesigner/Crew Chief

Re Drag Boats ... Hydro's typically run 2-bladed props (both the single & twin screw boats, though I've seen a few singles, slower Bracket boats, 9 sec and below, run a 3-bladed prop) that are set up to run 1/2 above the water surface and 1/2 below ... one blade in the water while the other is out.

Flattie's (flat bottom) drag boats run 2-bladed props that are set up to be totally in the water ... I don't think I've ever seen someone with a flat try a 3-bladed prop ... I would think there would be too much lift which typically catastrophic in the case of a flat bottom boat.

In my opinion, by far the best drag boat props for professional class boats are the new drop forged, hand finished props made by the HONDO-MENKENS Joint Venture.

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