New diesel-electric hybrid installation - how to size propellers?

If you're that interested in paddle wheels (I'm not btw) and definitions of slip etc, you may find these useful if you wish to analyse further.

 

So was I basically. The formulation for slip vs. efficiency depends on the assumptions made. This formulation comes from basic momentum theory for propulsion, which does not consider slip. But if you make an assumption between slip and velocity increase caused by a propeller, you get something similar. Here is the formulation: http://ceprofs.tamu.edu/jzhang/oe402class/Chp6-part1.ppt

If you assume slip=a, you get your formula. Probably this comes from slip=1-1/(1+a)=a/(1+a) ~a for small slips. This would give efficiency formula 1-s ~ 1/(1+s) for small slips. I think 1-s is the more valid one and at high slips you will actually have efficiencies very close to that (without cavitation).

You can use the same formulation for a paddle wheel and get the same results, if you assume similar dependency for slip and the velocity produced by the wheel.

At (very) high slips you can think the propeller as a gearing. Torque=Thrust*pitch/(2*PI), since propeller "moves" pitch/(2*PI) for each radian. Power needed to turn the propeller is Torque*w and effective propulsion power is Thrust*V. Thus efficiency = V*2*PI/(pitch*w). Slip=1-V/(n*pitch)= 1- V*2*PI/(pitch*w) -> efficiency = 1-slip.

If you compare these to e.g. B-series data, you will find very good match for slip>50% and for slip~30% the error may be ~5% (the real open water efficiency will be lower).

 

Mind that you provide for adequate tip clearance in your installation.
Perhaps the experts can parse the importance (or lack of) at the shaft speeds you are considering..

 

No, not the biggest one. If the shaft rpm is fixed, then there will be an optimum prop diameter, which is not necessarily the largest one you can fit. As has been noted by both me and Joakim in this thread: http://www.boatdesign.net/forums/pr...lectric-diesel-drives-40591-5.html#post502361 . Larger than that will require reduced rpms, and it is still not clear if your propulsion plant allows it or not.

 

I think all these calculations are missing a basic point. The power the op proposes is not adequate to operate the vessel in anything but flat water. By his own admission it will not be able to sail upwind.

 

No, it's not about tip speed. It's about optimal pitch/diameter and loading. If you keep the same rpm and increase the propeller diameter, you must decrease the pitch/diameter ratio. At 18" it is already well under 1. If you would go to say 36", it would need to be well under 0.5 (maybe ~0.25, I don't have any charts for such low values). Such a propeller would be very inefficient even without cavitation (I don't think it would even cavitate at such a low loading).

About 22" is the biggest diameter that can reach 1100 rpm with 33 hp around 8 knots with reasonable (min 0.5) pitch/diameter. Here are some efficiencies I calculated earlier in another thread about this same case: http://www.boatdesign.net/forums/pr...e-between-electric-diesel-drives-40591-5.html

Even a 36" propeller at 1100 rpm would not have bigger tip speed than what is used in many outboards with good efficiency.

 

Joakim said - 'No, it's not about tip speed'

Sorry to contradict, but it is. Having established a pitch/dia ratio that works reasonably efficiently we can use published curves to determine maximum tip velocity to prevent cavitation. From this we get a suitable diameter. P/D ratio first then diameter follows.
Of course if you want to move away from normal yacht auxiliary powering practice you can adopt cavitating props etc but your low end grunt will suffer.

Cheers,
Graham

 

But that assumes you can vary shaft speed. If shaft speed is fixed as it is here then the diameter with a reasonable pitch/diameter ratio may be be less than from one of the tip velocity charts.

 

Graham, I am not saying that you're wrong, but Joakim is surely right in this case.
The fact is - for a fixed rpm operation, there is an optimum (in terms of efficiency) diameter even without taking into consideration the cavitation. It is easily verifiable from propeller charts (either Kt-Kq or Bp-Delta) which have been made and are valid for non-cavitating props.
Cheers.

 

No harm done, but I'll contradict as well.

Cavitation is not determined solely by tip speed, it is very much linked to propeller loading as well. At low loadings the blade angle of attack is very small and thus also the pressure coefficient is small and the pressure will not be that low. When you increase the diameter you will increase the tip speed linearly and the pressure differences to the square. But you are also increasing the area to the square and thus the pressure needed to make the thrust is proportional to the inverse square of the diameter.

Thus at the same rpm cavitation becomes more of a problem at small diameters (and tip speeds) than large.

In this case a much bigger diameter and tip speed could be used without cavitation problems that would be otherwise sensible (the best efficiency will be in the 18-22" range) unless rpm is changed.

A bigger than 22" for this power and rpm would indeed be "away from normal yacht auxiliary powering practice" and thus the curves used normally would not apply, since the loading and P/D of that propeller would be much lower than normally used.

 

Oh well, it seems when I tried to answer a layman's question as simply as possible I opened up a whole new can of worms. At least it got us off paddlewheels for a moment!
Yes, all your comments are true but for on-line propeller predictions and simple software there are limitations built-in including restrictions on tip speed, p/d ratios etc to ensure there are sensible answers for the punters to use.
Propeller choice is an iterative process and the design spiral has many defining points. It does not matter where you start on the spiral as long as you continue to a logical conclusion using realistic inputs.
Rats, now I'm getting all philosphical, better have a lie down.

Cheers,
Graham

 

With the power available, cavitation is a moot point though.

 

Clearly the OP is not too interested in the qualified opinions of those who would question the viability of this project. But I wonder about the legal ramifications of the position he is taking. There has not been a single post that I can recall, made by anyone with any suitable qualification that has indicated that this is anything other than an underpowered craft (at best) and quite possibly an unsafe one. Those opinions are here in black and white, in the public domain. I wonder what the judge would say when this thing winds up on the rocks?
If you will not accept our advice regarding your power requirements, will you at least consider taking the design to a suitably qualified proffessional who's opinion you do trust and get his / her opinion....
Nobody is gleefully taking pot shots at you... we are just trying to save you a lot of money (again, at best) and possibly the safety of you and your crew....
I know you are sick of hearing this kind of advice, so on that note, I will bow out of the discussion....

 

I've already stated a number of times that I am getting advice from many experts, including local boat builders (professional) and naval architects. The vessel will also of course receive a survey. But opinions that this vessel is underpowered are just that - opinions, and not all opinions received agree with your assessment. However, this is a technical forum to examine the best possible approach to the propellers for this application, there is a different thread where you can air your more general opinions.

 

It is not an opinion, but solid engineering and good practice. A converted trawler that can't sail upwind and can't motor upwind is underpowered.