Pedal (assist) boat prop pitch

Hi,
I know there's a pedal boat thread, but it's got 146 pages in it, so I fear this might get lost in there! Plus this is three+ times as powerful as the usual pedal boat.

I have been doing some calcs for a lightweight electric pedal assist pedal boat prototype I am working on. Because it is electric assist and 150 kgs including 100 kgs passenger it is a planing design (just about). Perhaps foiling later down the road, but I am taking on a lot here without getting the thing working on foils from the outset too. Plus this is a fun boat, designed to surf, jump, skid turn etc, and foiling isn't the right solution for such activities ( plus there's also already an electric assist pedal foiler).

Target speed in planing format is 15 mph, with ideal shaft rpm at prop of 778rpm.

With a bit of slip into the equation, this suggests a propeller pitch of about 22 inches.

Because of the available gearing parts, layout, obsessive weight control etc. I don't want to have to change the shaft RPM. Or rather, if I do have to change this shaft rpm significantly I need to know early on as it would require extensive redesigning. As it could require additional gearing this could also attract a further transmission loss, AND additional weight.

So is a shaft speed of 778rpm driving a 22 inch pitch prop (probably about 10 inch diameter, high aspect ratio), within the realms of efficient design?

Or would those numbers take me outside the realms of efficient propeller drives and suggest a major rethink?

As always in these situations I have already googled props, and 10x22 aero props are commonly available, which is a good sign. A strong wooden or nylon internal combustion RC aero prop shaved down all over then vacuum bagged in a small hot box with carbon and epoxy might meet my efficiency and strength demands at a very low price, and the project is already getting expensive!

 

Jake, have you got any drawings of this boat that you can post?

It sounds like you have only allowed 50 kg for the boat itself, including the electric motor and the batteries - is this enough? It sounds rather optimistic.

How does a boat like this work? Is it mainly electric, and you have the pedals to give it a bit of extra ooomph (eg to get it up on the plane), or vice versa?

 

Thanks for your interest.

Unfortunately it uses various novel systems and devices that are as yet unprotected so unable to post pictures.

50 kgs is not a misprint. It has to be this light to stand a chance of planing. It's a spaceframe of high modulus carbon tubing holding together an 'arrangement' of hulls which are dropstitch inflatable, but quite modified for planing performance.

In terms of pedalling versus electric. Because I'm working with a choice of motors between 1.5 and 8 KW, it will be a bit 'Suck it and see'.

Lots of playing with prototype boats to be done.

So if it's possible to make the thing plane and have some fun with only using 1.5 KW, ( unlikely) this might be what it has. In that case it will feel largely like an exercise device, with a bit of electric help. I have a 1500 watt eMTB and I do a lot of pedalling on it to get places fast, and it's a good bit of my weekly aerobic exercise. I really feel it's assisting, not doing all the work.

If it's more like 5kw or above from the motor, it's going to feel like an electric boat and the driver's pedal input will be, and possibly feel, negligible.

Because nobody's done this before ( to my knowledge), I'm not sure how it will pan out on motor versus pedalling for a small boat.

With eMTB market, there's a lot of choice in power already. Some people prefer practically a motorbike with as much power as possible, others prefer to do nearly all the pedalling and just use the assist on the bigger hills. The latter creates a lighter bike that's more responsive and more like an 'acoustic' bike to jump, turn etc.

Is all about range versus power. Note I have not said anything about how long it will go for! It uses plug and play batteries though so replacement is a 10 second job.

 

My thought is that you need to start with some idea of how much thrust this craft needs to operate at all speeds up to your planned peak. More thrust might require that more water be accelerated aft than you planned and a larger prop that you don't have the torque to turn. The pressures and forces on the prop need to be checked too.
There are plenty of people with a good grasp of planing hulls that have no intention of ever making anything like what you are talking about. If you are really paranoid, have them sign a non-disclosure agreement and video your "disclosure" ending with them saying "I have no intension of ever doing that". Or they will tell you it's been done commonly.

 

And it is rather unfortunate that you are so paranoid about somebody possibly stealing your idea(s).
If you want to get some useful help on here, a drawing or sketch is worth a thousand words of description.

Do you know what your output power is when you are pedalling at full chat?

 

Good IP hygiene is nothing about being paranoid or thinking someone will steal the idea.
If you publish the idea before it is protected in any way that could enable somebody educated in the same field to make something similar you can't ever license it out in future, you blow the IP and you have nothing to sell- no exit for all your hard work. The third party don't have to make one, or try to sell it etc, they don't have to do anything. Publication pre patent pending blows IP.

This is a conversation I've had rather a lot. People forever take it personally, get upset etc. And accuse me of being paranoid or having an unrealistically high opinion of the quality of the invention. They say things like " if you really think I'm going to steal your idea"...

But please know that it's nothing to do with any of that. It's just how the world works. And it's a complete pain because it gets in the way for people like me who need experts in other related fields to just take a look at the plans and offer advice.

It also leads people to go a long way down dead ends on their own, because they struggle to get the advice they need early on. Hopefully this idea isn't one of those.

 

Jake, if you want help, answer the questions asked of you.
Or is that a secret too?
The average Joe puts out about 100 watts sustained (despite what those lying, ego-massaging pieces of fitness equipment might be telling you).
Peaks are up to 400 watts, but not for long.

 

this is bit weird to me. I just rowed 45 min with 150ish W output. Now, tour de France elites can push some crazy 400w for an hour. But even for them 1.5kW is not "bit of help".
If human can only provide 10-20% of the power (100-300W, from somewhat sustained to burst) I question whether the added complexity is worth the hybrid system.

I also think your trust on IP protection seems bit optimistic. After all you can only defend your patents with a thick wallet. I get your view but think getting the product right is more worthy headache.

I have argued here before over the accuracy of watts in C2 ergometers. All sources I found state that they actually give 20-25W pessimistic reading vs the actual mechanical effort. Same for C2 bike ergometer vs “watt pedals”. https://www.researchgate.net/public...nical_Sensors_vs_Concept2R_Measurement_System

 

Jakeeef, I think you've got your pitch and diameter A about F. Diameter first. 22" x 10" is readily available in model 'plane props but not 10" x 22"pitch.

 

Thanks, I might well have got these the wrong way round.

 

Don't think it really matters whether I'm putting out 200 or 300 watts. As discussed, it's getting a minimum 1500w hit to help things along.

 

Well, it's like eBikes, some don't get on with it as a concept. That's fine, I resisted them myself for years, but it's of growing interest to lots of people. I for instance don't want to be a cyclist, ie., Train on a bike. I enjoy going out riding, and cycle to get about, but don't want to spoil my rest days by exercising in a sport I do not wish to compete in, so an ebike is brilliant, I can get good 'active recovery' level training without having to ride around very slowly.

In the same way that I can cover twice the ground on a 1500W ebike than i can on a normal bike, electric assist just makes a pedal boat a lot more usable. Especially if it allows planing where it was formerly impossible.

The pedalling does make a noticeable difference on a 1500w ebike, especially when going up very steep ground, the torque of a pair of legs is a massive help when the motor has been slowed down out of its efficient rev range. Remember power is one thing, but often it's torque that really counts.

An electric assist boat might have similarities, there will be times when pedalling will significantly help things along. Getting on the plane springs to mind.

I can absolutely predict that electric assist pedal boating is going to become a thing in the next decade. Just look at how it took over mountain biking. A lot of the motor, battery tech will carry straight across too. So in terms of added complexity... There isn't much, as the eBike and e-scooter people have already done a lot of the hard work. There's added weight of course, but that's just a case of trading off how much power and range you want versus the weight you'll tolerate.

The real issue with pedal boats I've found, even fast displacement ones, is most humans don't really have quite enough power to make it enjoyable. It's a lot of cost and complexity to go a bit faster than you could on a $250 inflatable paddle board off Amazon.

It was the same with mountain biking it turns out. Realistically, most people just don't possess the necessary power to weight ratio to make it particularly enjoyable, ( other than on the flat or down hill). And most people don't have the time or inclination to physiologically change this about themselves. But this is less so with road bikes, as any adult can get 15 mph on the flat, which for most is tolerable. So road cyclists have not taken to motors in the same sort of way.

As you can probably tell Im a real advocate of electric assist pedal boating. I'm convinced it's coming and I'm prepared to sink many thousands into being a part of it.

It's never going to be massive of course, it will be niche and expensive, but I think it will be much bigger than pure pedal boats ever got. For starters storage is less of an issue when you can plane, as you no longer need the waterline length.

 

I think that you are comparing the proverbial apples and oranges here - with a bicycle you have a bit of frictional drag from the tyres in contact with the road, and a bit of wind resistance drag - but with a boat you have a lot (relatively) of drag from the boat simply being in a medium (water) that is one thousand times more dense than air.
I hope that I am proven wrong, but I have a feeling that the effect of you pedaling the boat as an extra oomph to help the electric motor is not going to be as great (relatively) as the effect of you pedaling the ebike to give it a boost.

A planing boat needs quite a lot of power to get up on the plane - once you are planing, you can throttle back a bit. Same as an aeroplane taking off - it needs full chat on the engines to do this, but once in the air it takes much less power to keep it going at 500 mph.
Maybe you need to take this one step further, and go for foils on the boat - then maybe once it is up on the foils you might be able to keep it going on the foils just by pedaling?

 

I must quickly debunk this one as its an oft seen misunderstanding of the IP process.

I used to think the same thing until I had it explained to me some years ago by a very rare person- an inventor that had accrued considerable personal wealth out of selling an invention!

Why should I bother getting a patent if I cannot afford the millions it would cost to pay lawyers to defend it?

It's all about having an exit plan. Investors like patents. It's the first question they ask.

It's perfectly true that almost no one man inventor would ever have the finances or the financial stomach to defend his IP in a court, particularly a foreign court.

So to use my idea as an example. I could save the money and not patent, spend it on R & D etc. Instead. But then I'm sort of stuck to making the boats myself, forever, because I can't sell the idea to anyone else. It has little or no value to someone else due to the fact that anyone can do it who wants to now the idea is in the public domain. Interested companies would simply copy the idea, not buy it off me. If it is a good idea, it will be copied by many.

A patent doesn't protect me from a company down the road, let alone in China, ripping off the idea, safe in the knowledge that I'm never going to take them to court. But that's not what they are for.

For a solo inventor, rather than an established manufacturer, building a product in anything more than a handful of prototypes, normally means things have gone wrong and he has missed his exit, or his valuation is too high, or the idea is just not marketable for one reason or another.

So in my example, get IP in process ( patent pending), then a flurry of marketing, perhaps crowd funding etc. Or meet with investors and get an exit, hopefully take some/ any profit, get on with the next one.
Or shelve, take a loss before the costs of maintaining all the patents gets too much to bear, have enjoyed building prototypes and have a very unique/ interesting looking boat in the driveway, exit and get on with the next thing.
Either way any potential stealers of the idea won't have had long enough to work out how to make it work. It's not even on the market so nobody can buy one and copy it.

The real losers are small companies that actually do want to manufacture a widget themselves. They come up with a brilliant new product, and they want to make it and sell it so they protect it with a patent so they won't have any competition. That's when an unscrupulous bigger company can do due diligence on the little company, and steal the idea knowing the little company won't be able to afford to defend their patent. Happens a lot. It does happen to solo inventors too, but less so.

Solo inventors think, if a company can't do anything about a patent infringement by a bigger company what chance have I got on my own?

But this is not a valid reason to give away or blow your IP. If you've got no patent you've got nothing to sell.

You can, equally work for 20 years, making your boat the best one on the market, ( it will have been the first of its type launched, which will have helped). You can do lots of early marketing so you are generally accepted as the originator/ inventor of the concept (which gives your company a slight advantage over competition), you can improve models launched and keep top of the industry. Then you can really have a valuable company to sell. But that's a lot of risk and a lot of time compared with quick exit, get on with the next one.

One of the most important/ earliest questions an inventor of anything needs to ask: "Do I want to end up on an industrial estate making these things, in the case of boats, breathing in styrene and carbon dust every day waiting for the lung cancer to kick in?"

It informs strategy from day 1!

 

Yes, that was an idea I was also considering at one point. A completely liftable long tail prop drive for a foiler. So when foiling there is no prop drag whatsoever.

Problem with foiling is the fragility and impractical nature of the boat you end up with. And the cost of development. Plus not as much fun as planing re jumping etc, planing not as sensitive to different rider weights etc. It's a leisure device not a means of transport, if it was transport, foiling might win the argument for me.

Someone will do it in the near future though. You'd only need a little RC lipo battery because you'd only need a handful of bursts of power in a day out on the water.

I appreciate your views on the power demands of planing. This is a very light boat though.

A 4 hp outboard motor produces less than 2 hp thrust equivalent at the prop ( according to Torqeedo)

One up an adult planes at 12/ 13 mph in any old flat bottomed Zodiac 3 m inflatable with any old 2 stroke 4 hp outboard thrown on the back. My hull is more efficient re planing surfaces, and about the same weight and has more power. ( Depending on level of motor assist). And more torque as it's electric.

Do you wish to revise your prediction on whether it will plane?