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

Actually, I'm quite happy if I can get 4 knots (about 100 nm per day) out of this system. I have been creating a Linesplan in Free!Ship and am pretty satisfied with the model now, but at this point don't know how to run a resistance calculation (and at different displacements).

To Tom's question (and thank you both for joining in the discussion) I could put in 36"+ propellers if called for, but I'd be very concerned about mis-matching the DC electric drive motors. The instructions do say that I should design to allow the motors to reach 1400 rpm, and as this is a highly integrated diesel-electric hybrid system with touch-screen electronic controls, I'm trying to go with the original instructions! I'm rather doubting that a 33hp electric motor could swing a propeller of that size - - but then again, I don't know! One thing that's occurring to me is to do a dockside experiment before installing, and try to turn various size props to reach the target shaft rpm. As the motors are raw-water cooled, I'll be monitoring the temperature too. Actually it sounds like a very interesting experiment.

 

You're under the impression that everything has been provided. So why don't you supply the answer?

 

Can you place the engine, the shaft angle and location of your prop onto this lines plan, preferable a GA of sorts.

 

Why are you paranoid and constantly making incorrect assumptions about posts being made by others?

You did exactly the same here:
http://www.boatdesign.net/forums/metal-boat-building/grinding-welds-36802-3.html#post486909
http://www.boatdesign.net/forums/metal-boat-building/grinding-welds-36802-4.html#post487271

You seem intimidated by people that wish to provide a full answer for some reason and accuse them of being pedantic or in this thread unhelpful, when the response is not to your liking/expectations??

 

No-ones chortling at your expense.

One of the commonest queries with boat of the size you are working on is how to get more thrust for the fuel burned. A lot of small props 'thrash' the water and the boat goes nowhere trying to motor into weather. Thats usually when you need it most and its that thrust that's needed to get you out of trouble the one time you really need it.

But if anyone wants a definitive answer to prop size area and pitch from a professional approach then you need to post your torque power curves available at the shaft and you really need to post some vessel resistance curves. Hull Cp , expected sea states, projected area for windage..... then you start to see it's not down to a simple one line query.

Your setup can always be changed. It depends on your mechanical aptitude.
A marine diesel is a diesel engine suitable for use in a boat with heat exchangers shielding and electrical safety. I just see you throwing away a lot of your engines already diminutive power output in an electrical conversion. Hence the advice to fit mechanical gear boxes.

But your choice, I really thought you might benefit from the advice.

 

Happy to provide the details you've requested. It's just that they were not requested before to this point so I didn't know what was needed except for vague demands for "more SOR" information!

But it does look like you haven't understood what I've already stated a number of times - I don't have any traditional diesel engines. I have a twin system of generator and DC electric drives, it's a dedicated diesel-electric hybrid system, this is NOT a "conversion", and I'm not trying to "throw away" anything, I'm simply trying to figure out how to ensure I get the best efficiency out of the existing set-up.

I don't know how to get torque power curves for these two DC motors, but the manual states the following:

Power output 33HP
Torque 103 ft-lbs (140N-m)
Max RPM 1,400
Input current: Up to 137 Amps
Voltage: 240 DC

If we look at the formula for power Watts = Volts x Amps, we get a maximum figure of 32.8 kW, which can be used in short bursts, but the maximum rated continuous power is 25 kW which equals 33 horsepower. I don't know Synchronous Permanent Magnet DC motors well, but my understanding was that torque curves are flat, whatever that means...

Ray

 

Not a choice as the diesels and electric motors are married as I understand it.

Ray, you disregard my advice and my offer because you have preconceived ideas. I take it you don't want my help then?

Best of luck my friend.

-Tom

 

No, I don't know these yet. I can't give a shaft angle until I know the prop size. Constrained by internal placement of tanks etc. in the hull, propellers will be positioned approx 8 feet forward of the transom.

 

Tom, I was about to reply, and think you've been very helpful so far. Unfortunately I have to runn to a meeting right now but will be in touch

Ray

 

Without showing the shaft line and approx location of the prop on the hull, it is all guess work. The reason why this is important is because it affects the flow of water into the prop.

Those "prop calculators" that everyone seems happy to use until an odd answer pops up and then cry foul, is because these assume open water efficiency and flow into the prop, generally.

With a hull and skeg and poor shaft location and a high angle, this seriously affects the flow into the prop, thus the speed of advance (Va) into the prop, which defines the props characteristics, is reduced significantly. Which is why engine power curves and resistance curves of the hull become important to ascertain the drag/thrust and if the prop shall be spinning in aerated water from a poor wake owing to the lines/skeg or simply overloaded.

In the absence of this all you can do is guess. Which means it could be a good guess or a bad guess. If it ends up a bad guess (for whatever reason), you'll blame the messenger! Which is why you're getting little response from those that can actually provide an answer and those that can't constnatly asking why not?

 

This is a good question. The propellers will not be behind a skeg, but either side of the box keel in very clear water flow under the aft section of hull, the props will be hung in clear water from v struts, I'm guessing around a 10 degree for smaller prop, 15 degree for larger. I'm anticipating 20% of diameter clear of the hull as Gerr suggests in the prop book, and they will probably clear the keel plate by about 2 feet.

 

For what it's worth, and I'm sure some here would claim very little, I've done some calculations. I don't claim to be a naval architect, and I don't have any experience since college and graduate school doing propeller calculation.

For a propeller with 30 HP at the shaft, turning at 1400 rpm and advance velocities, Va, from 3.5 to 5.5 knots the Power Factor, Bp, ranges from 335 at Va of 3.5 knots to 108 at Va of 5.5 knots. Va is the approaching water velocity the propellers see and my understanding is it's usually less than 1. The corresponding advance coefficient, delta, with a shaft horsepower of 30 HP for an 18" diameter propeller ranges from 600 at 3.5 knots to 382 at 5.5 knots.

Based on the propeller charts in Gerr's book for a 4 blade propeller, disc area ratio of 0.40 and Va of 5.5 knots, the efficiency be around 43%, and a pitch ratio of 0.55 corresponding to a pitch of 9.9 inches. The thrust would be about 765 lbs (per propeller).

The maximum Bp on the propeller charts I'm looking at is 200 which corresponds to a Va of 4.3 knots for an 18" propeller. Efficiency at that Va would be a little less than 36%, and pitch ratio is around 0.5 for a pitch or 9 inches. Thrust would be 820 lbs per propeller.

Going up to 36" diameter for the propellers Bp stays the same but the advance ratios double which is off the charts.

Going down to 14" diameter reduces advance ratio to 467 to 297. At Va of 5.5 knots the efficiency would be around 35% and pitch ratio is around 1.1 for a pitch of 15.4 inches. Thrust would be around 620 lbs.

I also had a quick look at what happens if shaft speed is reduced. Efficiency and thrust can be significantly improved with the appropriate reduction ratio and larger propeller diameter.

Here's what I get from this:
- 1400 rpm shaft speed is marginal at best with 30 HP for a vessel speed of 5 knots or higher. At lower vessel speeds it is too fast.
- Propellers larger than 18" diameter won't provide much if benefit at the combination of power, shaft speed and vessel speed contemplated.

Bottom line appears to be that you should seriously look at a way to reduce shaft speed.

There is also a fundamental need for at least a rough estimate of drag vs speed for the boat which you have some confidence in. Without that it's only guessing if the speed range considered is reasonable given the thrust which would be available.

Back to the disclaimers. I've checked the math but there may be a mistake. Also, I haven't made various adjustments for shaft angle and the like which should be made if the calculations were to be used to specify the reduction ratio and propeller diameter. And presumably an "off the shelf" combination of diameter and pitch ratio would be desired. However unless there is an error in the math it looks like the shaft speed is fundamentally too high for the combination of power and vessel speed.

 

The comments below are basically "first principles" type arguments.

Personally, for an underpowered boat, I would rather see a prop closer to square, say a pair of 17X14 rather than 18X12 (assuming an 18x12 is the correct book answer) The steady state efficiency might be a tiny bit lower but underpowered boats never operate steady state. This is what I would do for straight diesel drive. However, I don't know how the control system of the electric drive compares to the diesel in terms of mimicking the diesel's governor. That, to me, is the big unknown. Are the electric motors beefy enough that the genset governor doesn't know the difference? Do the electric motors surge in response to torque changes? Do electronics control shaft speed and the gensets run constant RPM regardless of load? Do the electronics control amp draw rather than shaft speed, or some more complicated scheme?

Underpowered vessels tend to get pushed harder and that implies an extra level of confidence or higher safety factor; and a bit more slip and less diameter will help to reduce the size of the load cycles in a seaway. I also believe it better suits motorsailing operations, at least for straight diesel drives. Versatility is always fighting with efficiency. For versatility, there is a certain desirable amount of slip. For efficiency, you want slip to be minimal to the point that the skin friction of the prop becomes as big a problem as the potential for better efficiency through less slip. When you are minimally powered, go for versatility, the loss of efficiency is less of a cost than on a bigger engined boat. So it comes down to what the electronics can do and what happens when they crap out, and I would go back to the geset manufacturer to see if he has any finangling factors for you vis a vis a straight drive. (and please post them here if he responds)

 

Firstly the Va is clearly wrong since Ray keeps harping on endlessly about sub 4knots...ie circa 3knots..therefore your Va just by inspection on the lines wont be any better than 2.5knots. That to one side...

Your figures show a diameter of 14" or in English = 355mm
on a BAR of 0.4
You state the thrust is 620lbs, or in English 2.7kN.

With those figures the area is 395cm^2

With the thrust values you provide says that the pressure on the blade is 68KPa.

However looking at the lines and guessing where the prop shall be and the overall PC, I get a thrust value in the region of 7kN. (Thrust => PdxPC/[v(1-t)]. This gives a pressure on the blades of 177kPa. This is well over the cavitation region of 65-70KPa. Thus no good, too much load on the prop.

With the 18” or in English = 457mm
on a BAR of 0.4
You state the thrust is 765lbs, or in English 3.4kN.

With those figures the area is 656cm^2

With the thrust values you provide says that the pressure on the blade is 52KPa, whereas using the thrust I calculate the pressure is 106KPa.

As you can see, prop selection is not as straight fwd as plugging numbers into a spread sheet.

Your conclusions are half right.
1) Needs a large reduction ratio
2)Larger diameter prop is required not small. Larger diameter helps to reduce the loading on the prop.

 

So true. Which is also what Mike Jones correctly said in his very first reply to this thread: http://www.boatdesign.net/forums/pr...lectric-diesel-drives-40591-2.html#post501731

Plus, I'd add that the trailing edge of the box-keel (picture in this post: http://www.boatdesign.net/forums/pr...lectric-diesel-drives-40591-2.html#post501486) in front of the prop has to be tapered more gently, instead of being so grossly squared-off. The current solution will give a highly disturbed and turbulent inflow to the propeller, which will significantly degrade the propeller performance, and possibly lead to vibration (and possibly even cavitation) problems. It also makes the correct estimate of the inflow velocity to the prop disc very difficult.