Curious

[floater1]

I know this is a simple question, but the answer is a little more complicated.

On any vessel, why is the propulsion always in the rear. ( a pusher)

On a smaller scale, I have a canoe with a front mounted motor (trolling, 56 pound thust)), and this little booger can spin on a dime and give change, and tracks much straighter than a rear mounted system. It got me thinking, as in the automotive field, a front drive might be somewhat more efficient to pull the vehicle vs. push, can this apply to the boat/ship design industry as well?

I remember Yipster had a craft design with the outboard mounted fore of the passenger compartment.

ANY input would be appreciated.

Mike

[Tim B]

Forward-mounted propellers are around, particularly on ferrys using a Voith-Schneider setup. They do have incredible manoevrebiliy. The problem usually comes in putting a propeller (and kit) at the bow and not having the bow slamming horibbly or getting stuck in every wave. There is also the point of protection, as propellors at the stern rarely take the brunt of any damage from ship-ship collisions.

There are increases to be had in propeller efficiency in "open water" as opposed to behind a hull, but until anyone can think of a way to stop the slamming/burying problem, and get rid of exhaust gasses / coolant or whatever, and keep the boat nicely balanced and manoevreble (because hydrodynamically, we're pushing volume ever further aft)

Interesting idea though, and it would have application in certain ships.

Tim B.

[BulBob]

Motors mounted on the front are for control but the thrust hitting the hull has a thrust nuetralizing affect. In other words there are opposing forces because the thrust is pushing the boat backwards at the same time it is trying ot pull it forwards.

If there was some way to use the thrust hitting the hull to help move it forward you would have something new. Probably impossible though.

[yipster]

Volvo brought us the pulling IPS, who knows what 2006 brings...

[floater1]

Thanks for all the input. Bulbob...As far as the "nuetralizing effect",
something that never occured to me, what about a concave style hull. I have a Jammer Canoe that has a concave hull that seems to be effective by giving it a catamaran profile in the water. Would a hull along these lines work?

Thanks guys

[BulBob]

It may be possible a single or dual hull could be designed to generate additional speed from the dispersing thrust. It would kind of be like riding your own prop's wake to help move forward. It's pretty hard to visualize any significant savings in energy consumption but any is better then none.

[yipster]

tom kane suggested a displacement cat for the idea
or slow turning forward surface props may be able to ventilate the hull?
its as you said before "the answer is a little more complicated"
and it would be intersting to see some push and / or pull comparisons

[BulBob]

If there is current a boat will go the speed of the current but if there is an anchor line the current can make the boat plane to the side at a speed much faster then current. A continuously moving anchor could be the train of thought needed to proceed.

We're getting pretty far out there...I'm getting dizzy...

[BulBob]

If you can ride a self made wake (wave or or under water thrust) and a use a planing or surfing action to excelrate beyond the wake speed you should be conserving some energy. I just find it hard to believe that the energy needed to create the wake can be less then the energy it provides. The perfect prop would move the boat not the water. The perfect boat would use all the energy of the prop to move forward and leave no wake or turbulance.

[floater1]

Paul

Forgive me, could you please elaborate on your last post? I'm not sure I understand.

Mike

[yipster]

visit BulBop's site and you see what he means floater.
something else, if you have a canoe with a front mounted motor; how hard would it be mounting it astern?
i would be very interested in the different data :cool:

[safewalrus]

Apart from the obvious damage point of view, plus for larger vessels the comfort for the crew bit (Modern Oil Rig workboats tend to have everthing at the front - tends to be a bit uncomfortable at times but has other advantages, and like a lot of tugs, they tend to run around backwards a lot!) I believe the main reason is the fact that in the beginning, when propellers where first invented they were put on the back! AND being a conservative lot ( seafaring people) we tend to like it like that!

tis the good old cry

"if it was good enough for me dad it's good enough for me"

Yes but....... So will the inovative youngsters have a go and see what they can do (if it falls apart there'll be lots of oldies sucking their teeth and muttering "we told you so"). But ****! man never got to the moon without trying! Actually, now he's there it don't look worth a damn, but you gotta try!

[floater1]

The first Jammer Canoe I had a stern mounted motor, a 40# thrust, it scooted through the water at about 4 knots but didn't track worth a @#$%!
probably due to the concave hull. The one I have now is foot operated bow mount, travels about 7 knots and easy to put just where I need it, but it is a larger motor. Steering is vastly improved and I can hear what sounds like airation(sp?) under the concave part of the hull, and bubbles exit the stern area. By the way, this hull design is the most stable canoe I have ever used.

[BulBob]

OK - I like dreaming - You never know when you'll find that acorn - if your looking!
If I can come up with a patent anyone can. www.bulletbobber.com
One of the uses for my patent is for a life preserver but I am just trying to make fishing bobbers because they are small and reachable with my budget. I sent numerous letters to the Coast Guard and never got a reply.
I know that a life preserever with directional control could save lives.
A 1 million dollar grant would be good, after all what is a life worth.

Anyhow, I am just thinking out loud and if I need corrected please educate me. I'm not an engineer and calculs is beyond me but I tkink I can grasp concepts and find that acorn as well as anyone.

First off props are not as fuel efficient as wheels because they move water and a wheel does not move the road. This problem and the fact that there is a lot more drag in water then in the air. The basic problem with props is they are not positive displacement like a wheel on the road. We will not improve on efficency beyond the wheel but we can try to get closer. I was wondering if a better paddle wheel could be figured out and it brought back an idea I had for a windmill. A powered paddle wheel like this would provide more positive displacement and maybe easier to capture the wake or dispersing thrust to increase velocity. It is vertical shaft windmill with tall foils. This shows the changes to the limited swing foils as it turns. I did a hp conversion once and it looked like a 5mph wind could generate 1/4hp if foils were just 4'x8'. It also looked exponential gain for larger sizes. I don't trust my calcs.

[yipster]

did you read the modern pacclewheels (http://www.boatdesign.net/forums/showthread.php?t=2637&highlight=modern+paddle) thread?
looking for electricity windmill people told me clumsy (multiblade = less noise) normal windmills give way more efficiency.
drawing looks like a variable pitch savonius (http://en.wikipedia.org/wiki/Category:Turbines) design?
so i agree but again think its more complicated than it looks :mad:

[JonathanCole]

While bow drive may have some benefits over stern drive, the ultimate in maneuverability, reliability and versatility would be a drive at both ends. this would allow the boat to turn 360 degrees in place (by facing the drives in opposite port/starboard directions). This would work best on tunnel hulls and catamarans specifically designed to accommodate such an arrangement. Forward facing props would be advantageous. A forward facing vane for prop protection from debris impact would be straightforward to implement.

[BulBob]

Having more props is good for control and could improve efficiency as demonstrate by Volvo IPS. I think there is still room for improvement in efficiency.

My windmill is more compilcated then the one below but uses a more positive diplacement concept and requires less wind speed. It is more positive displacement because the wind is always perpendicular or near perpendicular to one foil. Most standard windmills do not generate any electricity untill the wind velocity is high. I was hoping that my windmill would allow us all to have a generator in our back yard. I once built one out of tinker toys and put it front of fan and it turned quite erratic as the foils swang in and out. I tried to come up wih a restraining system to limit the free swinging foils to smooth it out but could not come up with anything. I still think some means of connecting opposing blades could smooth it out.

[yipster]

"to have a generator in our back yard"
or a cake drum generator on the boat arch :cool:

[BulBob]

OK what's a cake drum generator on the boat arch.

[yipster]

http://www.munderkinger-city.de/realschule/themhp/nanu/nanu02-03/Bilder/4.htm7.jpg
http://www.iboats.com/sites/sanddollarys/site_page_2611/images/l_100433radararch.jpg
something eastically looking a bit beter than a windmill to keep the battery's charged, a solar panel would be bigger i understood

[yipster]

Ok, must perhaps say ive only seen a "cake drum generator" in my fantasy

[BulBob]

Yipster - I'm understanding your name a little better all the time.

Your cake drum generator is very interesting, never saw that before!

My design is not nearly as asthetic but has less resistance on the side rotating into the wind. My blades would probably need be shaped different then a simple foil but still free swinging. 2 or 4 What's better?

I thought the thing on top of the boat was radar.

[yipster]

and with fishingrod holders :D

as you know symetric vertical rotors may need a little starthelp and need keep their concave side upwind. remembered reading something bout using a vertical vortex inside, thought about a parabolic inlet and first site i looked (http://www.sunwindconcepts.bizland.com/id10.html)today has a story on it. yes thats probably a radar sending electricity out, my poor mans thought was inconspicious getting it in.

the convex side of the blade creates a drag which must be subtracted from the power that is created by the concave blade surface. Even though the concave surfaces catch the wind quite effectively the convex surfaces deflect it. Power generated is a 'combined or net effect' of cohesion between the air molecules and ALL of the turbine surfaces -- traditional Savonious rotors are only capable of achieving 15% efficiency as compared to 30 to 50% efficiency for horizontal axis machines.

HOWEVER, the drag decreases and the power delivered to the turbine increases DRAMATICALLY if an augmenter is utilized. To achieve this, a vertical slotted funnel is added to concentrate wind pressure to the concave surfaces and to block the wind from the convex impeller surfaces of the turbine. Accelerated air to the impeller's concave receives more power while the augmenter blocks wind to the convex side. Output can be increased by up to several hundred percent. This requires a sturdy...

[BulBob]

The augmentor/scoop would have to rotate as the wind changed direction and that makes it more complicated then my design.
I still think my design deserves some review by someone that can calculate it properly.

If you do not want it in your back yard we'll put it on your boat and use it as a sail and to drive a prop, and to charge your batteries. We'll call it the cake drum generator and mount it next to the radar.

[Gilbert]

Paul, I thought I invented the vertical vane windmill in your diagram back in about 1980. I made a little model of it with vanes that were (I think) 4" x 36". (I could go measure, but I'm lazy) Four vanes like your diagram but the vanes were hinged about 8" from the axis of the frame. The model was mounted on top of an upright 2x4 about 15' off the ground for about 14 years until it had deterioration problems. So I had a lot of opportunity to observe it. As you suggested it would turn in very low wind velocities. I can't remember exactly how low but if memory serves me it was in the 6 to 8 mph or thereabouts. It also was self limiting as to rpm. I also don't recall now exactly what that rpm was, but I am pretty sure it was less that 60 rpm. I do remember that for the bulk of it's speed range I estimated (rather accurately) that the speed of the hinge was about half the speed of the wind. In order to try to estimate the torque it might apply to the shaft at various points along it's path of rotation, I demounted three of the blades and measured the force generated in front of a fan with little weights tied to threads and passing over a little pulley. In this way I could graph the resultant force of two opposing vanes. The vector diagram approxiamated a morphed figure 8 which luckily didn't approximate it so much that it ever got to zero. I concluded that six vanes would make the graph very smooth but thought perhaps five would be better in case aerodynamic interference became a problem with six. This windmill has two very positive virtues; the self limiting speed and no bird would ever be killed by one. It perhaps could make a bird very dizzy.
Cheers!

[Gilbert]

I forgot to mention that my model had plastic torsion bars for hinges. I have no idea if they served any good purpose. The idea was to make the flop less violent, theoretically. Also, properly tuned it might provide a lower wind speed start up, and perhaps even some benefit in torque after the flop.
After I made the first post it came to mind to comment that whenever I watched the model in action, the image that it suggested to my mind was that of dutiful soldiers marching briskly around in a circle.

[BulBob]

Gilbert,
We probably came up with it about the same time but you have a lot better understanding of the math and had a much better model.
Just looking at 4 blades - if opposing blade tips were connected directly by a rod it could prevent the over swing and smooth it out so higher rpm could be reached?

We need some high tech help.

[floater1]

Hi guys,
Is this still applicable to my original query? Just the same, it has been a most intriguing conversation. It has me thinking.....(of which I am not equipped), is this not all the same as the Voith-Schneider system (http://www.voithturbo.de/vt_en_pua_marine_vspropeller.htm)stated earlier? Fluid is fluid. No?

[BulBob]

I'm glad you are OK with your high-jacked thread.

I just looked at the Voith-Schneider system.
How the heck did Voith Schneider ever figure that out.
They musta had better beer.

OK see if you can find me one of these.

I want a direct drive back yard wind turbine generator that charges fuel cells and pays for itself in 3 years.

[JonathanCole]

I want a direct drive back yard wind turbine generator that charges fuel cells and pays for itself in 3 years.

http://www.renewabledevices.com/swift/photo1.htm

The fuel cells are the problem, not the wind generator. Your design has already been patented and unsuccessfully developed. Wind generators must be able to rapidly reorient themselves in strong, gusting wind, otherwise they tear themselves apart due to unbalanced centrifugal/gyroscopic forces. To understand the power of gyroscopic disturbance, hold a spinning bicycle wheel by its axle shafts and try to change the axis position suddenly. With your style of vertical axis windmill only part of the blade system is making power while the other side is out of balance. A large machine of that design would be extremely expensive compared to straight forward rotors and in high winds could self-destruct.

If a turbine goes slowly it won't have much torque unless it has a huge surface area. Much more materials makes it more expensive to build and engineer. If its gonna big big, I say make it real BIG. Here's the system I like!!
http://www.enviromission.com.au/project/video/video.htm

[jehardiman]

Just to drag this topic back to the original question....

I know this is a simple question, but the answer is a little more complicated.

On any vessel, why is the propulsion always in the rear. ( a pusher)

ANY input would be appreciated.

Mike

The real reason the propellers are mounted in the rear for ships is that they are overall more efficient that way. While there are maneuverability and yaw stability issues, most ships are designed to go from A to B as efficiently as possible so a rear mounted propeller is the best.

While TimB referred to "open water" efficiencies being higher, modern (i.e. post Lerb's 1953 paper) wake adaptation design renders forward propellers obsolete. The reason for this is the adaptation of manipulation of J (Advance Coefficient) so as to recover energy from the viscous slowed wake behind the hull (the so called Taylor Wake efficiency), which may reach 30% recovery. Additionally, as pointed out by BulBob (Paul), the forward propeller wake causes a pressure increase on the front of the hull leading to increased resistance.

Your original problem with an aft mounted propeller was cause by lack of adequate directional stability, a trait common to canoe hulls designed to be pushed-pulled by a paddle, and easily solved by the addition of a skeg.

[jammer]

http://www.carbonconcepts.co.uk/product_pages/carbon_concepts_wind_energy.htm