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

Discussion in 'Props' started by RayThackeray, Nov 14, 2011.

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### RayThackeraySenior Member

I'm trying to calculate for my boat, which is likely to displace about 55,000 pounds and with full cargo up to 75,000 pounds, 57 foot full displacement, turning 1,400 rpm max. All three of the top prop calculators come up with the same answer, about 18 inch prop, but everyone I speak to says I need a 36 inch prop!!

I don't understand what's wrong with this.

Perhaps more pertinent - electric drives present torque differently from diesel engines, but I can't see what difference it makes when you key in the shaft horsepower and revs - surely it's the same either way, or are the calculators taking into account multiple rev ranges or something?

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### Ad HocNaval Architect

There are only 3 things that govern the propeller size:-

1 Speed of water flow, iwo the hull as if the prop is not there. (Wake reduction)
2 Horsepower at the prop
3 The propeller shaft RPM.

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### baeckmoHydrodynamics

Your calculation (18" @ 1400 rpm) corresponds to roughly 50-55 hp shaft power, which is certainly not what is needed to move a 30-tonne boat!

A propeller can only produce a certain maximum thrust per swept disc area, which is to say that there is a link in size between displacement ("size of ship") and propeller diameter ("size of disc area).

You must specify reasonable values for the three input parameters, as Ad Hoc has pointed out. Some of those s.c. propeller calculators are predicting speed as a function of power or power asf of speed based on some kind of algorithms. It is most likely that the snake is hiding there (unless you are the culprit feeding sh-t into the programme....).

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### Ad HocNaval Architect

Indeed!

To be in with a half decent shout of a speed faster than swimming, its gonna be around 350~400Hp on that length and displacement.

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### RayThackeraySenior Member

Actually, you are exactly correct. In this particular application, we are putting in dual 33hp electric drives from a hybrid diesel-electric system. The design requirement is to gain the optimum speed from this power in long-distance passagemaking. There is very little requirement for close-in docking maneouvres. In fact, the primary propulsion for this medium-heavy displacement sailing trawler is actually sail, the motors will only be used when the vessel drops to below 4 knots.

So the big question remains - is this correct? By the way, this amount of power still potentially offers 8 knots in calm conditions, if the propeller can be chosen well. Or does it?!

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### RayThackeraySenior Member

So how is it that I have spent thousands of hours with a little Ford Lehman in a similar sized 54,000 yacht, ticking over at just above idle speed for days on end at 5 knots, using just over 1 gallon of diesel per hour? And by the way - my day tank in the engine room held 24 gallons, and it would run like this almost all day before I pumped fuel up from the main tanks - so I couldn't have been fooling myself.

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### Ad HocNaval Architect

As i noted unless you want to go above swimming speed, which these below 4and 5knots are not, you'll need more power. If you wish to approach 10knots, then you shall...if you're loitering around at circa 4knots, its fine.

Most tides run at those speeds, so not sure how much head way you'll make going less than 4 knots!

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### RayThackeraySenior Member

I see you're a marine architect. That tells me that you're designing for the usual rich guy who wants to be able to cruise at hull speed burning fossil fuels with get-home-itis.

Why on earth would I want to do that? I'm quite happy doing 100+ miles per day on a passage. I hardly ever spend any time trying to stem a tide in an inlet, and in that case I'd read the tide tables first.

So put aside the preconceived market perceptions and work with me on the aforestated application of a passagemaker that's determined to operate in a conscious "green" mode.

Will it work?

Can I make two 33 horsepower props drive this medium-heavy displacement sailing trawler at up to 5 knots under power alone? Are the propeller calculators determining an 18" diameter propeller with pitch around 12" all correct in this application?

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### Ad HocNaval Architect

Nope, i design for anyone. Their SOR dictates what the final design shall end up looking like and the power required to achieve the SOR. If a client wants to run on chip fat/oil....or Vodka, doesn't bother me one bit. The SOR dictates the design and if it is achievable.

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### RayThackeraySenior Member

So answer the question then. Will it work as specified?

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### Ad HocNaval Architect

What is your SOR?

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### RayThackeraySenior Member

What's an SOR?

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### Ad HocNaval Architect

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### RayThackeraySenior Member

Forget it. Way too complicated for me at midnight. I'll just go ahead with my plan and will post the sea trial results in February on launch. I'm fairly confident I'll get approximately what I want out of this configuration as stated in earlier posts, just wanted to check if there were any reasons why it might not work in this stated application, didn't want to write a dissertation.

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### DCockeySenior Member

It appears that Ray has provided the relevant part of his SOR:
1) Two 33 HP motors
2) 5 knots throught he water

He also provided some basic information about the boat:
a) Displacement of 55,000 pounds, and with full cargo up to 75,000 pounds
b) 57 foot LOA
c) "Full displacement" type hull

His question is "Can I make two 33 horsepower props drive this medium-heavy displacement sailing trawler at up to 5 knots under power alone? Are the propeller calculators determining an 18" diameter propeller with pitch around 12" all correct in this application?"

Nothing about if his speed requirement is correct.

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