Propeller Gear + Efficency

In the past to match the engine to the boat usually took 2 or 3 attempts to get it just "right".

The CPP is infrequently used to optomize underway .

There have been lots of claims by Volvo and Bravo about a 30% efficiency gain with their contra rotating setup.

And the Brits with the Yellowfin make great claims , but don't yet seem to have a product on market.
Or demonstrated efficiency , tho their video is great on manuverability.

IS there NOW a more efficient prop than the CPP (Hundstat or similar)?

Thanks!

FF

 

The best efficiency of fixed pitch propellers is better that the best efficiency of controllable pitch propellers. This is due to the fact that fixed pitch propellers have thinner blades and hub and are optimized for one speed.

Contrarotative propellers do have better better hydrodynamic efficiency. The real issue is does the increased hydrodynamic efficiency cover the additional mechanical losses in transmission.

Probably yes for volvo and bravo transmissions, with very short shafts mounted on ball / needle bearings, and power division made at the very last step with conical gears.Note that similar classical legs would already use conical gears, so the additional penatly is not that high. Note also that propeller efficiency goes down with speed contrary to exposed shaft and strut drag that increase with speed, and theses systems are planned for planning boats, not displacement ones.


For slow boats,say up to 20-25 kts, the best efficiency is fixed pitch prop with straigth shaft (minimum mechanical losses). That is the setup of most commercial boats where there is a paid accountant to count ALL the beans.

For higher speed, you should probably look something between those contrarotavive legs and jets, depending on speed.

 

A fixed pitch prop 10% bigger and turning 10% slower than a controllable pitch prop will have a obvious better effiency. It is also way cheaper and easier to maintain.

Have you checked the price of the hundested prop you quote ? Also do not forget the hollow shaft and specific gearbox. I fear you should expect over 15 000 $ for the smallest size. Not including the engine.

 

Fred
Big diameter and big pitch (low revs) will give the most efficient prop.

Rick W.

 

Big diameter and big pitch (low revs) will give the most efficient prop.

But , always that But , if the diameter is limited swinging that big wheel at 100 rpm is a problem.

Since it seems that the craft I'm working on is limited to about 20 inches (fine with me as at about 24 inches the price soars!) the question becomes efficiency for $$$ spent.

AS noted a CPP is $15K US or so , a second choice could be the use of a 2 speed transmission instead.

This (a ZF90 aint cheap) could allow the engine to be much heavier loaded at lower rpm to bring the engine efficiency up.

Using the std prop curve , the engine at 1/2 load is no where as efficient as at close to full throttle.

The cheapest would be to select an engine that ran 20K at full tilt and was still well within the max efficiency range when pulled back to 18K cruise.

Might be able to get to the "ideal" 80% load at 90% of rated RPM.

Of course any adverse seas or headwinds or slight fouling would reduce performance in proportion.

The 2 speed tranny could allow say a 10K cruise with good low fuel burn.because the shaft speed could be sped up 24% or more.

IS there a conventional contra rotating drive (not like Zeus) but run from a straight shafts setup?



FF

 

CPP Power Curve

It is my understanding that the FP will be more efficient than the CPP at one particular rpm, but less efficient at virtually all other speeds. What I would really like to see is a propeller power curve for a CPP setup. Has anybody seen such a thing? My expectation is that unlike the convex curve (looking from the top of the graph) of a standard FP, the CPP curve should somewhat follow the shape of the HP curve and be some small distance below it. If anybody has something to share, I would be very interested in seeing it.

Regards,
SeaJay