Prop calculators - is there a difference between electric and diesel drives?

It sounds as though you are getting way more advice than you bargained for (too many cooks!). If you run a dockside test you will not get the correct inflow into the prop and the results will be misleading. The high RPM (1400 at the prop I think) is what is limiting the optimal diameter to about 18". However, the power is quite low, so you will not have a cavitation issue unless the inflow is extremely unsteady (assuming a 0.4 DAR). The pitch must be set to absorb the power and avoid overloading. Pitch can be modified if you have missed the design point slightly. The only other potential issue is the torque curve of the electric motor. If the motor cannot deliver enough torque to get to the design point, you will have an unsatisfactory installation. In my experience, electric motors generally have fairly constant torque curves, so this should not be an issue.

 

I assume your comments are based on a single propeller behind the keel. From a previous post in this thread by Ray:

 

Oops, I have missed that one...

 

You took one part of my note out of context, presumably inadvertantly, and went on from there with at best confusing comparisons. Very unfortunate.

I deliberately did not say anything about what the speed of the boat nor what Va would be. Instead I provides some examples for various Va's to see for what Va an 18" diameter propeller would be reasonable. I have also looked at a wide range of other diameters but only provided the results for 14".

A Va less than 4.3 knots with an 18" diameter propeller is off the charts I have, and presumably not a good situation. We appear to agree on that.

What Va are your thrusts for? I assume 2.5 knots, not 5.5 knots. For Va of 2.5 knots and thrust of 7 kN (your estimate) the propeller is 40%. I wonder if that efficiency is feasible in this situation? For a Va of 5.5 knots and a thrust of 7 Kn the efficiency would be almost 90% which is not feasible. My thrust estimates which you quote were explicitly stated in my note to be for a Va of 5.5 knots with no claim as to whether 5.5 knots Va is "correct".

I completely agree "prop selection is not as straight fwd as plugging numbers into a spread sheet." See the comments at the bottom of my note. What I attempted to do was to make some initial estimates to see if a significantly larger diameter than 18" would be desirable, not suggest a specific prop.

 

Crikey, I just woke up and found stack of erudite comments to digest and two private comments, this is going to take some time to consider and discuss with my team's mechanical engineer!

BTW, I have never "harped on endlessly" (I haven't even harped on once!) about sub 4 knot speeds, the closest I've come is to comment that I'm perfectly happy to average 100 nm per day, which, if my elementary mathematics and abacus haven't failed me, is actually above 4 knots... As a passagemaker, I've spent many months at that rate of speed and can live with it quite happily with it under power alone in this application. Having said that, most of the time it would be motorsailing, which has been mentioned in other posts.

 

It occurred to me to add a picture of this sailing trawler when she's complete:


Ignore the prop behind the keel of course, that's the way we would have gone with a 35-year-old 105SHP Perkins if we hadn't received the donation of this new diesel-electric 66HP twin drive propulsion.

 

You can’t do a prop calculation without the correct Va for YOUR application.

So what is the point of “further examples” when Ray can do that with his odd little free program anyway?? Which is why Mike and myself have repeatedly asked for further information, which seems to be very thin on the ground. This “further” information all helps in gathering the data to arrive at an informed Va and many other aspects about the prop.

Yet everyone seems to want to plug in A+B+C = Y, hey presto.

Oh, my mistake then:

If this is not harping on endlessly, I am mistaken! My apologise..

I have asked for this several times.

So how can you seek advice if the only data you can provide is minimal at best and in the case of the GA, wrong?? Location of the prop and engine is fundamental!

Without a correct shaft line/engine location etc and without resistance curves, engine power data/curves etc etc…I beg to differ!

 

IMO it's vitally important to work this as an iterative procedure and usually compromise something along the way.

We should start with a "Mission statement" (or SOR) for example do you want to be able to motor into a force 7 with a 50 mile fetch to shelter, or get away from an unexpected lee shore at 2AM, or to pick up a MOB ? What do you want to actually achieve and is it even sensible within the constants already set.

Then what's the actual resistance plotted against speed. Then what can actually be achieved with the power available at the prop. Is it even being realistic......maybe change some parameters and start over...

Once you define all the parameters you can drop into a standard series chart and make some sort of reasonable prediction.

But it's not even a rule of thumb issue unless the problem is well defined to start with.

 

Personally I think the answer is quite clear. If you can't change your shaft speed you are going to land up with a small inefficient propeller. So YES 18-20 inches will be the most efficient stock propeller for your systems (AS IT STANDS).
You could improve this dramatically though which is where everyone's advice is coming in.
A simple tranny after the DC motor would only loose you a couple percent shaft power for huge efficiency gains at the prop, and that's only the start of what you could achieve if you spend the time now figuring out the drive line details.

 

Ad Hoc,

Please don't bother replying to this thread, you are not being helpful and just muddying this up. Others have been extremely helpful already and given me excellent advise. You keep asking me for data that I don't have yet, and in some cases I may get it together, in others it just isn't available and I have to go with what I've got. But I'm really sick of your attitude. So please butt out. Your advise isn't welcome.

 

Thanks. I've got this advice pretty clearly, unfortunately I just had a meeting with an ex employee of the manufacturer and the donor of the diesel-electric hybrid system and he has told me pretty clearly not to try gearing it down, it's a highly integrated electronically controlled system and doing that could cause me a lot of problems.

So it looks like I do have to go with what I have, and accept inefficiencies as long as it drives the boat reasonably well, framed as best as I can with some new information from people who have helped me build a hull model over the last few days and run resistance calcs:

What is the power to drive a streamline "trawler" style hull of 1030sq.ft and 54ft LWL at 5 kts? Will twin 18" props do the job? What power will I need? What pitch should I try? The wetted surface area is 1030sq.ft. and waterline length is 54ft.

 

Thanks, I agree this does seem to be an iterative puzzle as I gradually get more information together, though I thought I'd pretty clearly stated the requirement by now, which is to select the most efficient propeller and pitch to drive this boat. I can accept anything better than 4 knots in flat water (from calculations so far, it looks like this system should be able to deliver 6 knots). The boat will be motorsailing most of the time.

One NEW requirement is that the representative of the equipment last night told me I should assume a maximum revs for highest power as 1,100 RPM rather than the 1,400 RPM figure in the manual, that's quite a change to this puzzle. I've also attempted to pose other new information that may make things clearer in the previous post.

 

The advice not to gear down the output of the diesel-electric system is puzzling. What types of problems would it cause? Was the system originally intended for boat propulsion?

I'm struggling to understand what difference it makes to the system if it's output is connected directly to a propeller or to a propeller though a reduction gear.

 

What power or torque is available at 1100 rpm?

Is there a concern that if the system is connected to reduction gear the resulting speed of the system will be too fast?

 

I don't understand it very well myself yet, still working on it. Previous poster said this is going to be an iterative process, clearly it is. So right now on the electric drives, all I fully understand is that I get the full 25 kW (33 HP) from two motors at 1,100 rpm. That might allow for a slightly bigger propeller than the results I've been getting so far, hopefully. No power curves are available yet, but they will definitely be quite different from the typical direct diesel propulsion engine. So frankly the best I've been able to do so far is frame the power/prop calculation this way:

What is the power to drive a streamlined "trawler" style hull of 1030sq.ft and 54ft LWL, hopefully somewhere between 5 to 7 kts? The wetted surface area is 1030sq.ft. and waterline length is 54ft. Will twin 18" props do the job at 1,100 rpm? What power will I need? What dia/pitch should I try? Some new information using Free!Ship and Michlet is a resistance of around 2000N (440lbf) at 6 knots in light weather conditions.