# Is there a simple relationship between prop rpm and boat speed?

Discussion in 'Props' started by Diamond Cutter, Jan 12, 2012.

1. Joined: Apr 2011
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### Diamond CutterJunior Member

Hi guys;

here's a question that's puzzling me, though the answer to it is probably obvious to all here; namely, how to calculate the required rpm of a propeller to give "X knots" boat speed, for "Z tonnes" displacement?

I intend to build a 12m multihull with electric drive (never mind why I want electric drive or how I'm going to achieve it) and I'd like to have two props driving, one at each end of the bridgedeck (they'll be 7m apart - yes, I know that's close to the hull ends). The drives will be outboard type with long drive legs, steered in the usual manner, cables rather than tillers (and fastened down properly!). This way I can easily spin the boat around in its own length, or move it sideways etc. All obvious.

What I need to know is how fast the prop needs to turn. I understand there are many variables, but surely there is some formula or at least a rule of thumb?

Intially, I'd like the prop to be as large as possible under the assumption that bigger is better; I'm thinking 50cm diameter, and seven blades, under the assumption that more blades is better (though the number of blades isn't something I'm wedded to). My motors spin optimally at 1200rpm, and have adequate power to shift the boat (I know they do, or will, because they will be sized appropriately once I know the answer to the question posed here).

However, while I've noticed that answers to similar questions often devolve to comments about engine rpm and reduction ratios, surely the prop speed required to drive the boat has nothing to do with engine revs, except what is caused by the limitations of the particular engine and drive?

So, with a 50cm prop, what prop rpm will make the boat travel at 5 knots? Is there a simple answer to this?

Hoping there is; thanks to all in anticipation, and regards to all in the forum.

PS; I've been reading this forum so long that I can't remember if I've ever actually posted before. If not, hi everybody! Nice to be here, love your work, and thanks again.

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### DCockeySenior Member

No, there isn't a simple, one equation/curve type of relationship. A good, though not perferct, reference is Dave Gerr's Propeller Handbook. It walks through how to do the calculations and read the propeller charts.

3. Joined: Oct 2009
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### DCockeySenior Member

For any given boat and speed there is a propeller diameter, pitch, shaft speed, number of blades and area which give the highest efficiency. That is requires the least power to the propeller to drive the boat at the desired speed. Diameter, ptich and shaft speed are the major factors. Frequently the propeller can't be the optimum diameter due to lack of space so a smaller diameter has to be used. To maintain reasonable efficiency the shaft speed increases as the diameter decreases. Or the shaft speed range may be restricted due to the engine and reduction/reverse gear to be used which then leads to the desirable diameter. Other considerations such as cavitation will help determine how many blades and blade area should be used.

4. ### Submarine TomPrevious Member

Hello DC,

Yes, you've posted before, four times.

No, there is no simple answer.

No, there is no simple relationship between boat speed and prop speed (RPM).

Yes, there is a correlation to boat speed and propeller speed (RPM) but it's certainly not a direct correlation.

In your case, the props should spin at the same speed as your motors, 1200 is fine although slower would be better. So, you need a prop designed to your boat speed and 1200 RPM.

To figure out your boat speed you'll need to do a multitude of calculations. An easier and most accurate alternative would be to use JavaProp. It'll take you a while to figure it out but it is doable and well worth the effort.

-Tom

P.S. Dave Gerr's Propeller Handbook is a must read for someone in your position.

5. ### CatBuilderPrevious Member

Diamond Cutter: I know people's creative ideas often get trashed here, but I was wondering if you could start a thread that describes your electric system. I had gone through it all a bunch of times and every time I did it, I ended up with regular outboards being much lighter weight. I couldn't get my electric drives to weigh less than diesels.

Also, where are you sourcing those drive legs? I looked at Sonics and they were something like \$6000 USD per leg!

6. ### Submarine TomPrevious Member

I missed the part about the outboard style drive legs. Then you've got some gearing. The slower you can turn those props the better providing you have the room to swing big props. As CB said, it's a trade off: prop diameter to RPM.

-Tom

7. Joined: Apr 2011
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### Diamond CutterJunior Member

So, that's a NO then? Ah well...

Thanks for the fast response, folks;

I'd suspected the answer would be "no" but it's good - well, informative - to have that confirmed. perhaps I'll get a copy of the Propeller Handbook and go from there. I've had a brief play with Javaprop, but it asks me all sorts of questions that just don't apply. It's starting from existing motor and reduction ratio, rather than from required thrust vs rpm. Not that I'm complaining, it's free after all. And I've seen posts from Rick W where he's used it to do what I'd like to, but I don't see how he got there. More practice needed, I guess...

@ Submarine Tom; thanks for your straightforward answer; I will of course gear the motors down to whatever is the ideal speed for the props (whenever I can find out what that is), and I'm pretty sure 1200rpm is too fast for a big prop, pushing a 4-tonne boat at 5kts. Gearing down gives me more torque, so that's good. Having no aptitude for a multitude of calculations, I guess I'll just build the sucker and play with the variables.

@ CatBuilder; yes mate, I've had my ideas trashed on enough forums to be cagey about details. It's tedious to ask a question then have every issue in the post queried without any attention to the original question. However, to your question; my boat is intended strictly as a live-aboard cruiser, so speed is not at all an issue. 5kts is fine for me, and well below the hull speed, so doesn't require much power. Also because it's a multihull, I can hang multiple motors off the bridgedeck, adding more motors and props as required to achieve adequate power. And the prop diameter can be relatively enormous since it's nowhere near the hulls. I'll probably go with a surface-piercing prop eventually (and I don't want comments about that, thanks, fellas) just because it'll easily fit.

You're not going to make a ski-boat with electrics; the power density of generators and batteries just isn't there. Well, actually you could if you didn't mind being limited to 30-minute runs. If you've got the bucks, you can get Lithium Iron Phosphate batteries, which have very impressive power densities with very light weight. I'm not wealthy, though, so I'm going another way.

For me, it's not a question of the weight of the outboards, so much as unhooking myself from the shore and gas pumps. I can make electricity, but I can't make gasoline, diesel, petrol, or anything like that. There are several sources for electric drives, and I've only looked at outboards since I don't want an inboard. Electric Vehicle sites are a good place to start looking for the parts and design tips. There is also the Torqeedo outboard - around \$5000 for a 10HP equivalent - or a DIY kit with everything but the drive leg from http://www.thunderstruck-ev.com/electric-sailboat-kits.html.

Note that electric motors are most efficient when they are run at their optimum design rpm, so rather than putting in one or two giant motors and huge controllers (expensive!), use many smaller motors and switch them in as more power is needed (maybe using an electrically-operated sprag clutch). You can have several small motors on each drive leg. Cheaper, better. The controllers are usually the same cost as the motors, and smaller is disproportionately cheaper, and it's easier to keep them cool. Heat is the enemy of electronics, and they have to be sealed away from salt water, so that's a bit of a problem area.

The drive legs can be found by searching scrapyards, asking people, keeping alert for abandoned or neglected boats and asking the owner if he'd like to sell; that sort of thing. You can find them for a couple of hundred bucks; need a bit of fixing. I never buy new; why pay somebodys advertising budget?

I expect the electric system - motors + batteries + generators - will be about the same weight as the IC engine, gearbox and fuel tank, but it's way more convenient; fewer moving parts, easier to fix (if you're an electronics tech, which I am) never need to buy fuel, and the noisiest part is the prop churning the water.

I hope that's helped, but I don't want to start a thread on this, it's bound to degenerate into armchair theorising.

How am I going to generate the electricity? Well, it's like the propeller question; complex; buy the book. Clue; I'm not relying on the batteries for anything but emergency backup manoeuvring and to run my coffee maker.

Anyhow, you've all confirmed what I suspected was the case, namely that I'm on my own, so I'll go work it out somehow. When you're breaking new ground, I suppose it's unreasonable to expect a path to be there in front of you, hey?

Thanks for the input, and good luck with your builds.

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### eyschulmanSenior Member

Another sorce is the boat diesel site they have a prop calculator. You have to join site to use not very expensive.

9. ### Submarine TomPrevious Member

"@ Submarine Tom; thanks for your straightforward answer; I will of course gear the motors down to whatever is the ideal speed for the props (whenever I can find out what that is), and I'm pretty sure 1200rpm is too fast for a big prop, pushing a 4-tonne boat at 5kts. Gearing down gives me more torque, so that's good. Having no aptitude for a multitude of calculations, I guess I'll just build the sucker and play with the variables."

You're welcome, my pleasure.

It appears you may have misunderstood my second post, I'll try and clarify. The props should be as large as space allows. No surface piercing though, the deeper the better.

I hear your frustrations with JavaProp, I went through the same thing after Rick W suggested it for what I was trying to do. It drove me nuts (it's a short drive) but I eventually figured it out and it is stellar!! You are selling yourself short if you don't master it. Initially, you do need to make some educated guesses for input variables, but the more you f&#@ck with it the more you'll get it and it really starts to work. The hand book / manual is a huge help and was posted here not long ago. I can't remember where but I'll find it for you.

You are on the right path and you know the electrical side already which is a huge plus. Like running those motors at speed and forgetting the electronics of speed regulators. It's ideal for propeller design as a single speed propeller is the most efficient.

You'll end up with such a unique prop design you'll make them yourself in about two hours each simply twisting and grinding stainless steel.

If you PM me some details I can start you off with some pretty good educated guesses on input perimeters for JavaProp.

I can understand your reluctance to expand on-line, the nay sayers abound.

Gerr's book will help you immensely in this process.

-Tom

10. Joined: Apr 2011
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### Diamond CutterJunior Member

Thanks, ST; I've realised you're right, and I've bitten the bullet and downloaded JavaProp and the associated manuals, tips and tricks, and now am gritting my teeth, inhaling coffee (to get my aging brain fired up) and diving into it.

At least my electronics background has given me the mentality to persist until success is achieved, if not the calmness with which to proceed quietly.

Listen for the sound of thunder from beyond the sunset...

11. ### Submarine TomPrevious Member

Those more knowledgeable than me can give you some pretty good educated guesses for drag on your displacement at 5 knots.

Prop diameter: how big can you go?

RPM: what's the biggest reduction you can get from that 1200 RPM? 200?

If JavaProp tells you "can not compute" then something is wrong. Your loading's too high or some other variable is not doable. Be tenacious, take the appropriate breaks and keep having at 'er and you'll be dumbfounded by not only what you get, but what you learn.

-Tom

12. Joined: Apr 2011
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### Diamond CutterJunior Member

Yes, I've already had the "overloaded propeller" message, but I persist...
The prop can be a metre (39.29") in diameter; draft is the main limit, boat draws 40cm, but I'll arrange to lift the drives for beaching or shoal water.
The boat hull has about a 12:1 fineness at the waterline; easily pushed.
I can gear the drive down as far as I want, no problem; a Poly-V serpentine belt drive is a wonderful thing. I was initially aiming at a prop speed of 180 to 360 RPM.
(I presume this is why you advise against the SP prop? since no way is such a slow prop going to cavitate)
The spinner can be large, or there can be a void at the prop centre, and use rim drive, and so on and so forth.. these and all other decisions are interlinked, and not really possible to decide upon
until the basic "how fast is the fershlugginer PROP going?" question is answered.
I'm a-gettin' there.
One of the advantages of hoeing your own row, is that the only limitations are those laid down by the laws of physics. Boat parts catalogues don't enter the picture. Onward...

13. Joined: Oct 2009
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### DCockeySenior Member

For a given boat speed and thrust there is a combination of diameter, pitch and speed which yields maximum efficiency. Smaller diameter than optimum and the efficiency goes down, which is the usual situation due to geometric constraints. But larger diameter than optimum and efficiency also goes down.

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### Diamond CutterJunior Member

Definitely an engineers response

Hi DC; you are absolutely correct in what you say. However, the information takes me no further towards my goal. Being in the engineering field myself, I'm aware of optimum solutions and resonant points etc. I just don't have info on the crucial node. Namely how much thrust for what RPM. I know the answer exists, since it has been programmed into Javaprop. I was hoping the equation didn't have too many variables, and I've learned that it does have too many. Javaprop to the rescue, and I'll just have to learn another piece of software tooling. First programme I wrote was in 1976 (computers generally had like 7K - not 7Meg - of RAM back then). Seems like my whole life has been lived behind a keyboard. Thanks for the input.

15. Joined: Oct 2009
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### DCockeySenior Member

How much thrust for what RPM depends on vessel speed, propeller diameter, pitch and blade area.

First pick a target speed for you boat. Then you need some sort of estimate of resistance for your vessel at that speed. That's the target thrust. Without an estimate of the thrust needed for the desired vessel speed it's somewhat pointless to talk about diameter and shaft speed.

You will also need to estimate some factors relative to flow around the hull.

There are an infinite number of propeller diameter, pitch, blade area and shaft speed combinations which will provide the target thrust at the target vessel speed. With the particular charts and/or software you are using you pick a diameter and a shaft speed and determine the corresponding efficiency and pitch. Also check if loading is excessive. With efficiency, shaft speed, vessel speed and thrust the shaft power and torque can be easily calculated. Repeat the calculations with a different shaft speed and/or diameter and see if you like the results better. Continue to iterate until you are either satisfied with the results or decide to change the target vessel speed and corresponding resistance.

Gerr's book does a reasonable job of walking through the calculations. It also covers the basics of propeller design and selection considerations. Amazon.com lists new copies for \$13.15 and shipping to Australia appears to be very reasonable.

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