Why are shafts so much less efficient?

[Nick_H]

I'm no engineer, but I can't see why they are 30-40% less efficient than outdrives/IPS.

The gearbox i'd have thought would be more efficient, 'cos it only needs to change the ratio, it doesn't need to transfer the turning moment through 90 or 180 degrees.

The drag on underwater gear shouldn't be much greater, if you look end on then there isn't much more leading surface area than sterndrives, although for sure there is more total surface area with shafts, supports and rudders so more friction. Still, as a percentage of hull surface area it would be minimal so shouldn't have a great effect.

That leaves the angle of propulsion (unless there is something else i'm missing all together). I can certainly understand an efficiency loss there, but 30-40% seems too much, and if it is all down to this surely some method of universal coupling could be used so that the drive was parallel to the hull.

 

[gibbowolfie]

Where are you getting your figures from? Single shaft vs duo-prop outdrive? Well I'd have thought a Duo-prob is more efficent that a single prop regardless of power transfer method.

I doubt if angle of propulsion should be underestimated, but again depends upon the figures, just straight line (agree shouldn't be that much different), worsecase (high angle from centreline) or some sort of average.

 

[StuB]

I'm no expert, but when my out drives are on the plane the only bit of the legs actually in the water is the bit below the cavitation plate... so that the Props plus 25mm.

Surly boats with shafts must have 2 meters of shaft, P brackets rudders et al, all dangling in the water. Isn’t that the drag that makes them so much less efficient?

 

[Deleted User YDKXO]

Interesting point. I think you're right about the down angle of the shafts. I don't think it makes all that much difference, maybe less than 5% efficioency loss so that leaves other factors. I wonder if one other factor is the position of the engines and hence the ability of the boat to lift ie plane. With shaftdrive, the engines are near the middle of the boat whereas with sterndrive, the engines are right aft so maybe shaftdrive boats need more power to lift the forward part of the hull out of the water. Probably rubbish
I think the main reason is the drag of the sterngear and what the sterngear does to the smooth flow of water to the props. Not only is there drag but also disruption of water flow to/from the props

 

[Nick_H]

I think the figures i'm using are fairly well discussed and acknowledged. What particularly raised the question is that i'm looking at spending more money than I really should on a big hard top sport cruiser, and a forumite made me aware of a similar boat with IPS that does the same speed with engines half the size! In this case the 30-40% may even be understated. That got me questioning what makes shafts so inefficient

 

[Channel Ribs]

What is the league table of drive efficiencies then?

At the risk of being irritating, I will have a guess...

1) IPS
2) Outdrive/outboard
3) Shaft
4) Jet

 

[Nick_H]

I think that would be true if the shafts were at 90 degrees to the hull, but they're not. I'm thinking Mike might be on the right track, and it may be to do with disruption to the water flow in front of the props. That would explain why IPS is more efficient than sterndrive is more efficient than shafts.

 

[Nick_H]

I think it changes with speed, at high speed jets may be the most efficient, although this is a guess not a statement.

 

[rickp]

So we'd expect the Zeus stuff to come in at about the same sorts of efficiency as a sterndrive?

I wonder where the Yellow Fin stuff will fit in too. They claim 50% improvement in fuel consumtion, but not sure what they're comparing with...

Rick

 

[hlb]

Shafts were very efficient till they invented the infernal egg whisk thingy, which was a bit more efficient if not broken.

Now they have invented the IPS doinz which has the prop infront.

Course if some one just had the wit to turn the engines round and stick the shafts out the other way. The shafts would actually be facing about horizontal, cos the nose is up. Rudders could be done away with, just slow an engine down or speedy trim tabs would turn the boat.

 

[Nick_H]

would make a bit of a mess if you hit anything underwater though, i think the inboard would end up as an outboard /forums/images/graemlins/grin.gif

 

[hlb]

Nah. The trick is to buy special Volsplutter steel for the prop and shaft. If it hits anything, it just falls off!!

 

[Nick_H]

What, a system where the props face forward and the whole lot just falls off if you hit something, like anyone is going to buy that..

 

[hlb]

Ahh but, my design has a patented bungy cord attatched to the prop shaft, so if it does fall off, it springs back and drops on the bathing platform, ready to be bashed back into shape and refitted. Anyway being made from Volsplutter steel, it only costs three quid.

 

[Deleted User YDKXO]

LOL at that one Haydn /forums/images/graemlins/smile.gif

 

[hlb]

I honestly dont know why some one does not try it.

 

[Hurricane]

Hey Haydn

Look - Roger has already done it!!!
Does this infringe your patent?

 

[alb40]

This arrangement on my boat (shafts) must deal with most disruption to water flow infront of it:

That probably explains why it was made like this.

 

[hlb]

/forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif <span style="color:red">X </span>

If the principle works. My boat should go faster backwards than forwards. I'll give it a go next time I back out the marina!

 

[hlb]

Ah, it's working now.

Buggger has coppied that from my blue print that I posted on here some time ago. /forums/images/graemlins/mad.gif