Multiple Gear Boat w/ Propeller

Thanks for the drawing. My location is NY, USA so I don't even expect you to be able to hint any specific stores. Regarding the permanence of the setup, if I realize that the whole idea is worthless, I'd still like to be able to use my boat the normal way, without the bike on top of it. So if this mod can't be done as a put-on, take-off mechanism, then at least it shouldn't do any serious permanent damage to the boat so that I can still use it normally.

I guess you could say that I am an adult male, I'm 19 years old and ride my bikes often, especially in the summer, I ride them for miles every day.

If you could please, go on and show me how I can mount the gearbox.

 

Rick Willoughby is correct. There's little or nothing to be gained from gearing a pedal boat. Below is message I posted on rec.bicycles.sci a long time ago which addresses this in more detail:



Subject: Lake runner impressions

To answer some of Dan Gold's questions...

Variable gearing makes little sense for a propeller
on a HP boat. The terrain is usually quite flat .
But seriously, a prop of fixed geometry designed for
a certain boat-speed/power/rpm combination will remain
nearly-optimal for a very wide range of operating
conditions. The prop's power absorption capability
varies as V^3, while its operating rpm varies as V.
This closely matches the power demands of a displacement
hull (assuming skin friction dominates), as well as the
cadence requirements of the rider. For example, if the
Lake Runner at 6 mph requires 150 Watts (0.2 hp) at
65 pedal rpm, at 10 mph it will require 690 Watts (0.9 hp)
at 108 pedal rpm. Both power/rpm combinations are
reasonable, and in each case the prop will be at its
design condition.

The biggest reason for changing the gear ratio is to
compensate for different-sized riders. A bigger rider
will inherently put out more power and go faster, requiring
a larger gear ratio (i.e. a "bigger" gear) to keep the rpm
the same as that for a smaller rider. In this situation,
the gear ratio wants to vary as Power^1/3, which is a
pretty weak dependence. With a hydrofoil the dependence
is even weaker.

Another reason to increase the gear ratio is when going
downwind, in order to compensate for the higher speed
and rpm at a given power level. I can't see such an
air-drag effect being too significant on a relatively
slow vehicle like the Lake Runner.

In theory, there is little difference between water
and air props. An air prop must be about 30x bigger
than an equivalent water prop to compensate for the
density difference. The Decavitator's 10 foot diameter
air prop is equivalent to a 4 inch water prop. The
advantage of the air prop is that it turns about 30x
slower, and hence requires much smaller gearing step-up
from the pedals. This translates to smaller drive
losses, and also avoids the draggy surface-piercing
drive shaft and prop-mounting strut. Disadvantages
of the air prop include the big overall size, nose-down
moment from the high thrust line, and nasty side moments
from crosswinds.

The prop disk area (i.e. diameter) is strongly dictated
by the operating speed, power, and fluid density.
The non-dimensional power coefficient

Pc = 2 x Power / (disk_area x rho x V^3)

should not exceed ~0.2 for good efficiency. The Lake
Runner's prop looks far too small for the claimed speed.
It may be blowing off an additional 10-20% power as a
consequence. I guess they compromised performance to
get a small draft.

Using a hydrofoil only makes sense above ~10 mph.
Below this speed a good displacement hull like a racing
kayak has less drag.

 

I see. But as I do not have access to a wide range of propellers and am on quite a tight budget, I won't be able to pick out the most ergonomic propeller for me. Hence, if I'm stuck with one that requires a lot of power, I'd be able to utilize it at an easier gear. Like you said, heading downwind would make it reasonable to switch gears to a more powerful gear. And vice versa.

 

If you do this well you will have something that is very pleasant to operate and you will never want to use paddles again.

The most serious permanent consequence for the hull will be a small hole that will need to be plated over if you want to remove the drive. This can be done quite neatly and will certainly not bother you from a rowing perspective.

For this reason I think the shaft tube will be the most demanding for you.

There are a few ways to do this and you will need to find compatible bits. The tube has to accept a bearing and a lip seal. Their size will depend on the prop shaft you use.

The first step is to determine what shaft you can get. Spring 8mm or 5/16" steel is the best but you may have trouble finding this. The next best is 10mm machinable aluminium.

Once you find what shaft material you can get you select the bearings and seal. I have attached photos of the sort of things I use regularly. I get these from a firm called "smallparts" and they have warehouses in the US. They provide good service and prices are OK. If you have a general power transmission firm near you they might have these things on the shelf. Get bearings with stainless balls and metal shields.

The OD of the bearing and seal will set the ID of the shaft tube. You can use various materials for the tube such as rigid PVC, HDPE or aluminium. The tube will be glassed to a wooden flange that is screwed to the hull. A small hole will be drilled though the hull to enable the shaft to pass through. The wooden flange will be approximately 4" wide and 6" long and will have half rounds on either end that are a neat fit in the stiffening ridge of the hull.

The shaft tube will come up out of the flange at an angle of 20 degrees.

Setting up the shaft tube will be the hardest part. It has to be done neatly with the miniumum permanent changes to the hull.

This link shows the sort of prop that you can use:
http://www.hobbycity.com/hobbycity/..._15x13_/_381_x_330mm_Poly_Composite_propeller
The pitch is a bit low but will be OK to get you going and is not expensive.

To start you need to find a suitable shaft. Let me know what you come up with. Bright steel will be adequate for a test but will not last indefinitely. You need a length of 6ft if they will cut it. I can only get the heavier spring steel in minimum order lots of a few hundred dollars so I do not expect you will be doing this. Anything but mild steel will work.

Rick W

 

The project is getting more and more complicated the more you describe the parts and work needed for it. To be honest, I don't understand more than half the terminology you're using. I will either have to resort to doing it the simplest, thought probably least efficient, way possible, or drop the project altogether. I don't have much time and resources, and when I thought about it at first, it seemed like something that I could fashion using scrap from the garage and a few days of my time at most. The Mitrpak that I had my eye on ended up going for quite some money still on ebay, and I just don't have such a keen eye in this exact field to be able to see things in the hardware store that I could use to substitute the right angle gear box.

 

Pedaler
"...The project is getting more and more complicated the more you describe the parts and work needed for it..."

This is because your focus and RW's focus is totally different. Your original idea is sounds, no problem, it is fun and have a go. But if you wish to pursue to the n'th degree for very small increases in efficiency etc, which on your project will never be realised, you can.
But as a project, a nice little exercise for fun, do it. Ignore the "..if you do XXX you gain better efficiency..etc".
This is a small fun project, all you want is the means of how to make it work. Whether it works as the most efficient machine built by man ever i think is not your objective, unless I am wrong. Your objective is to put your idea, on the water..regardless of its flaws. All boats have flaws, it is called a compromise.

If you build it....it will float

 

Yes, Ad Hoc, you've got the right idea. I appreciate Rick's professional and detailed posts greatly, but they are from his world record winning background, and I'm just after the proof of concept for myself, without any need for competition, or market. So Rick, can you simplify the instructions by say, 100 times?

 

Paddles are simple and convenient. They are still competitive with more modern technology.

If your build is not thought through using suitable parts it will fail often and just be a source of annoyance and great frustration.

There is some interesting pictures and commentary on HPBs down the linked page:
http://www.wisil.recumbents.com/home.asp?URL=wisil/hpb/boat.htm
You will see race results and pictures for the various years. Some of them have to be rebuilt after each race day because they break down regularly.

These boats tend to be fun for people who like to tinker. There are some successful commercial boats like the Hobie Mirage but most people stick with paddles.

Even if you decide not to proceed then you can take some satisfaction from the fact that the fastest man on water took the time to respond to your thread and provide some advice on gears:
http://www.youtube.com/watch?v=l2UOH65QOI4

Rick W

 

professional advice is a subjective term too.

A professional naval architect (which RW is not) would not be giving advice in this way. A details person on one subject, as RW is, is helpful in that very narrow band and he can provide lots of little details of just that subject.

So you need to decide yourself first what advice in which area you wish, overall design/concept, or the details of how bits fit together to get the deisgn to work.

The person giving the advice must also be able to recognise your limitations and provide guidance accordingly. No point blinding you with science. Just as no point explaining spin/charm and colour of quarks to a high school student just starting physics, is it?

Advice/guidance is only helpful to the precipitant if it is clear, concise and answers the question asked.

 

Rick I do apprecieate your help, and I do understand why you're stressing me to go with the proper materials, tools, and building for my project, but the circumstances just don't allow that for me. Think of it like a hobby with a very minimal amount of money set to spend, quite a bit of time, and not many tools for it.

Would you say that even my boat, with the proper gears and propeller setup can be made more efficient and convenient than when it is used stock with the oars? If so, then how come we see so many row boats and so few pedal+propeller boats? My point is that it's probably still not such a no-brainer to use pedals and a propeller on boats, especially non-competition boats, so I just want to try the idea for the fun of it, especially because I'll realize that going with good ol' oars is still better.

 

I agree, but I also value and appreciate the time Rick put in answering my questions and sharing his wealth of knowledge. Though he probably lost track of the simplicity that I desire in my project, his advice provided great knowledge towards my idea either way.

 

The concern that I have for the shaft tube is that it requires some permanent change to the hull. If it is not done neatly then you will not be able to remove the shaft and make a simply repair.

The alternative of sticking the gearbox on the stern or side of the boat creates much greater complexity in setting up the transmission.

First step is to find suitable material for a shaft. Can you get 8mm or 5/16" spring steel? If not then can you get 10mm or 3/8" machinable grade aluminium? If neither of these then what can you get. Mild steel will not be good enough.

Rick W

 

I have no idea where I could get either spring steel or machinable aluminum off the top of my head. The shaft is the "stick" going from the gears to the propeller, correct? Are there any online sources where I could cheaply acquire this material? Or perhaps some sorts of local businesses that are bound to have this kind of material?

 

How about you email me and I will give you some leads. It will speed the process.

rickwill@bigpond.net.au

Rick W