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

Philsweet is right on the money. Good sound advice.

What you're doing, is as noted before to RayT, is putting the cart before the horse.

You need your full SOR, without knowing this, as Philsweet and Mike have noted, what is your target?? Pointless designing for a flat clam fair weather clean hull no loss of power/efficiencies "ideal" world. This doesn't happen in real boats.

If you do not make allowances for the "what if's" at the beginning of your design, via the SOR, you're not going to get what you expect.

Ray's priority is finding someone, anyone, that will give him an answer that he has already arrived at to make him feel comfortable. Not an answer that is suitable for the SOR, which hasn't been defined and what is given changes daily.

 

Who is your reply directed to?

Phil's advice is to jump into optimizing the propeller to optimize efficiency with dynamic effects and the control system. That doesn't sound like first establishing what the basic requirements/desires are. And who is arguring to design for "for a flat clam fair weather clean hull no loss of power/efficiencies "ideal" world."

 

The objective here, is a response to “a seaway”. RayT’s “seaway” is flat calm swimming pool 24hours a day everyday.

RayT’s only SOR is 100nm/day; that is simply 4.1knots continuously. Unless you run your boat in either a swimming pool, or just a few hundred metres from the shore with no exposure to wind, tide etc and as close to an easily accessible safe refuge (when a gust comes along or worse, waves!), this is not a real design objective that reflects reality. To think otherwise, is just foolish. Unless you beg to differ of course.

 

Is the assessment that 100nm/day is not realistic based on any analysis?

 

Ad Hoc, as I already requested, please butt out of this thread. You are doing nothing but muddying the waters. Your statement is completely untrue, I have posted a massive amount of information (everything I have) and just because it doesn't fit your tidy process it's all I have to work with. I have a boat. I have propulsion equipment. I know they are not a perfect match according to modern yacht design, but I just need a passagemaker that will help me power and motorsail at least 100nm/day, though I hope for better. I really don't expect to be driving through big headwinds. If the bit about "hoping for better" translates to you as "continually changing requirements" then I really, REALLY don't care. I am going to make them work in my application with or without your unhelpful comments.

So for the third time, butt out of my thread, you are borderline offensive.

 

A successful boat starts with an SOR. From the SOR arrives the “design” a fixed design, not a fluid one..otherwise the implication is that the SOR is changing, ergo different design solution. Any analysis requires a design to ‘analyse’! Analysis is the easy part.

If there is no SOR, than what is it you’re analysing?

 

Thanks for the answer.

 

I see this has been questioned already. I would also look at this again. How is this "electronically controlled system" going to distinguish the difference of a reduction gear between it and the prop?

 

I just noticed this thread-

ad hoc- does not torque have some relationship to prop size? after all its most definitely included in figuring out steam calcs for props..this is why steam boats had mega over-squared large dia props ...when i look at steam calcs prop sizing i always include the torque rating into the equation (see vicprop)..actually i looked into a d.e. system some time ago--see my threads- i went instead with hydraulics...the d.e. system was too complicated in comparison. but torque was definitely a consideration for a displacment tug hull/prop sizing relationship.

so wouldn't it be fair to say that hp and torque at the prop determines prop size as well?..i.e. a engine with 500 ft -lbs of torque and reduction gear of 3:1(assuming 500 rpms) but only 14 hp @ the shaft still will spin a huge prop --it(the boat) will not go fast...but it will go 5 knots... where as a boat geared 1:1 with 300 hp will go fast but not have a lot of torque in comparison. but the shaft speed is then increased so the boat needs a smaller diameter prop but goes faster as torque increases at higher rpms...??
sorry if this is redundant--just saw the first two posts...

 

analyse--hnmmm its more like "anal- ize"...
i know of a steam tug that has ten hp- the calcs for it said the prop should be about a 25 inch -27 inch dia. but the guy got told by some editor of "steam boat magazine"-that all he needed was a 20x20. well long story short- the small prop works fine- drove it to hull speed fine maybe 5 knots or so- but--i disagreed with his decision to use a smaller dia prop- if the tug does any towing it will not absorb the torque created, effectively- and the small prop will waste all that torque generated.basically frothing up the water and not being efficient. electrics and steam are closely related in torque ratings at certain rpms. electric engines actually have an ideal rpms range. go over that and they lose torque go under and they might even gain torque. ill try to post some ratings for a 18 hp ev motor. it is rated at 72 ft-lbs @ 1361 rpms. and 18 hp. I wanted to use 2 of these --and it would spin at 1361 rpms- a 22 x 19 prop. geared 2:1.

 

Waterline length in feet: 26.4 feet

Beam at the waterline in feet: 10 feet

Hull draft in feet (excluding keel): 1 feet

Vessel weight in pounds: 9500 lbs

Engine Horsepower: 18 HP
Number of engines: 2
Total Engine Horsepower: 36 HP

Engine R.P.M. (max): 1361 RPM
Gear Ratio: 2:1
Shaft R.P.M. (max): 681 RPM

Number of shaft bearings (per shaft): 1
Desired speed in Knots: 7 knots


Horsepower Calculations
This will calculate the maximum horsepower and torque available at the prop(s).

Total available horsepower at the engine(s): 36 HP
Total available torque ft/lbs at the engine(s): 139 ft/lbs
Horsepower loss of 3% per gearbox: - 1.1 HP
Horsepower loss of 1.5% per shaft bearing: - 0.5 HP

Total horsepower available at the propeller(s): 34.4 HP
Total torque ft/lbs available at the propeller(s): 265 ft/lbs

Speed & Power Calculations
Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 6.89 Knots
Minimum horsepower required at propeller(s) for Hull speed: 18.8 HP

Calculations based on desired speed and available HP
HP required at propeller(s) for desired 7 knots speed: 20 HP
Estimated speed with existing 36 horsepower:
This is the speed we will use for the propeller size. 8.38 Knots

At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM, which allows the engine to develop it's maximum power without overloading. The chart below shows typical engine ratings, you can find this information in your engine specifications.
Recomended RPM for continuous operation
Type of engine % of max RPM
Light-duty gasoline and diesel automotive conversions 70 - 80%
Light-duty or high output marine diesels 80 - 85%
Intermittent-duty marine diesels 88 - 92%
Continuous-duty heavy marine diesels 98 - 100%

Propeller Size
Number of blades Diameter (inches) Pitch (inches)
2 Blade 23.4 X 22.8
3 Blade 22.3 X 22.6
4 Blade 21.0 X 22.1

The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch

 

here are specs on the electric motor

 

Ad Hoc listed both RPM and HP. HP is RPM * torque so it is already there.

 

Yes it does, but perhaps not in the way you think, or assume.

Any prop will follow the power law, that is the power is proportional to the RPM^3 (cubed).

If an engine and prop are matched correctly, then the prop curve will pass through the MCR of the engine. But, if the prop is too small it shall reach full RPM at less than full torque, and, if the prop is too large it shall absorb all the torque available before the full RPM is attained.

Thus #2 and #3 are very important, in the list I gave.

 

The design point will not necessarily hit the rated power at the rated RPM and design speed. This is due to the limiting torque of the engine. The propeller torque demand curve must always be below the engine torque limit or the enigne may lug. So the bollard torque may impose a limit.
Getting back to the point of this thread, the difference between diesel and electric drives has to do with the torque characteristics of the two. An electric motor is essentially a constant torque device, so power increases linearly with RPM (2*pi*m*torque), a deisel engine can have a very variable torque curve, especially turbocharged engines.