Here's a Thought About Propellors

[Lakesailor]

In the US, lots of fishermen use bow-mounted electric trolling motors.

Now this is interesting. I wasn't thinking of just hanging a motor off the the bow of a boat however, but the thought had run through my mind of making a small cat with the motor between the hulls and ahead of the crew.
The advantages of hanging the motor from the transom as outlined earlier are really disadvantages.
Weight on the stern. Helmsman sitting not only in the stern as well, but offset.
The bow rises way too high flat out in a displacement boat.

I'm thinking the idea has legs, but finding an outboard that you could easily turn the gearbox through 180° on the end of the leg may be a problem.

 

[DownWest]

Pusher a/craft are more efficient, look at Rutan's efforts (the guy behind Virgin's space flights) Lots of records set. Party because the airframe is not in turbulent air and it is easier to streamline without a big donk up front.
As for boats with outboards, how many times have you bumped the leg? With front mounted prop it would probably be once only, not to speak of ropes etc.
The original new Mini was going to have a rear mounted engine under the back seat, why? because it was easier to arrange the crash protection with deformable structures than have the engine there.
And finally: I was at a crash site of a Cessna 337, which has an engine at both ends of the fuselage. It was the front one that got the the two occupants.

LS, there are small hire boats on the local river, that have a electric O/B under the foredeck.

 

[Lakesailor]

Second mention of electric. So it looks like an idea that works for low power, low performance boats.
But is the prop at the front or on a shaft as per normal?

Lots of pedal powered craft seem to favour the idea. This shows a pusher prop, but that wasn't important really. I wasn't particularly looking for tractor props but more the placement of the prop further forward.

 

[DownWest]

These are standard electric o/bs, prop on back. Apart from the risk of hitting something, I expect they are less likely to attract weed.
DW

Steering is much quicker if you have the prop at one or other end, if you turn the prop on your canoe it will go sideways, not turn. On such a long thin boat the rudder is not very effectve.
Extra edit: I like the bulb bow, worth a knot or two.....

 

[Lakesailor]

Good point on the steering. Maybe need the prop well forward.

 

[LittleSister]

Stop messing about, just get yourself a Voith Schneider propellor at either end!

http://en.wikipedia.org/wiki/Voith_Schneider_Propeller

 

[Lakesailor]

I've a feeling that may be a more expensive deal than messing about with an old outboard.

 

[Bilgediver]

My somewhat indistinct memory of A' level physics suggests there is very little difference in effect between a 1000lb engine hitting a 100lb crew at 100kts, or a 100lb crew hitting a 1000lb engine at 100kts. Both are likely to take the smile off of your face.

Unfortunately the pushing engine behind might catch you up and hit from behind..Could have dire effects if you are between the engine and ground!

 

[Twister_Ken]

Unfortunately the pushing engine behind might catch you up and hit from behind..Could have dire effects if you are between the engine and ground!

I fail to see the difference between collecting a pusher engine in the kidneys or catching a pulling engine in the belly, if they're both coming at you at the same speed. Either way, you're the crush zone.

 

[LittleSister]

I fail to see the difference between collecting a pusher engine in the kidneys or catching a pulling engine in the belly, if they're both coming at you at the same speed. Either way, you're the crush zone.

Yes, but with the motor in front the belly is decelerating your 100lbs (?!) body from 100 knots to zero, whereas with the motor behind your kidneys are decelerating the 1,000lb motor 100 knots to zero. I know which I'd prefer less!

p.s. Having thought about it further it's worse than that: because with the engine behind, your belly (hitting the ground or whatever) decelerates your 100lb body 100 knots to zero before, and in addition to, the motor hitting your kidneys!

 

[LittleSister]

Think of it this way - you're flying down a hill on your bicycle at 30mph and a car is doing the same speed down the hill. Given the choice, would you rather the car hit a brick wall, then you run into the back of the now stationary car while you're still doing 30mph, or hit the brick wall on your bike at 30mph and then have the car run into you now you're stopped and the car's still doing 30mph?

 

[Twister_Ken]

Yes, but with the motor in front the belly is decelerating your 100lbs (?!) body from 100 knots to zero, whereas with the motor behind your kidneys are decelerating the 1,000lb motor 100 knots to zero. I know which I'd prefer less!

p.s. Having thought about it further it's worse than that: because with the engine behind, your belly (hitting the ground or whatever) decelerates your 100lb body 100 knots to zero before, and in addition to, the motor hitting your kidneys!

Whatever, you wouldn't be around to complain either way.

 

[Twister_Ken]

Think of it this way - you're flying down a hill on your bicycle at 30mph and a car is doing the same speed down the hill. Given the choice, would you rather the car hit a brick wall, then you run into the back of the now stationary car while you're still doing 30mph, or hit the brick wall on your bike at 30mph and then have the car run into you now you're stopped and the car's still doing 30mph?

Not the same thing.

Imagine you're strapped in a chair thats fixed to the ground. You have the choice. Would you rather have a 1000lb weight doing a 100kts hit you in the belly or the back. I submit it makes no difference. Neither is survivable without a huge chunk of luck.

 

[LittleSister]

Not the same thing.

Imagine you're strapped in a chair thats fixed to the ground. You have the choice. Would you rather have a 1000lb weight doing a 100kts hit you in the belly or the back. I submit it makes no difference. Neither is survivable without a huge chunk of luck.

But it's not a 1000lb weight hitting you in the belly!

The 1,000lb forward weight/engine is as good as stationary when you catch up with it, and it's your 100lb body hitting you in the belly from inside. In fact it wouldn't make any difference if the weight/engine in front of you weighed 10lb or 100,000lbs (assuming the obstruction stopping it was strong & heavy enough to stop it instantly), you'd still hit it with the same force, which derives from your 100lb body.

In practice you'd actually be better off with something really, really heavy in front of you, as it would deform more whatever it hit and decelerate more slowly so your closing speed with the engine/weight would be less than if it were lighter and stopped faster.

And yes, if you actually came to an abrupt halt from 100 knots you'd be very unlikely to survive anyway, but I thought we were just musing about the principle of the thing. But if by chance you did survive your belly decelerating you from 100 knots, the last thing you'd want is a fraction of a second later your kidneys having to bring a 1,000lb engine to a halt!

Fortunately Lakey's not allowed to do 100 knots where he sails!

 

[Twister_Ken]

But it's not a 1000lb weight hitting you in the belly!

That's where I have to rely on my physics O'Level. My recollection is that it makes no difference whether the heavy object hits the light one, or vice versa. The effect is the same.

 

[Seajet]

Aircraft with pusher engines

As explained long ago on this thread, in a hard landing / crash it's easier to restrain a relatively light crew from going forward to a tractor propellor & engine, than to restrain a powerplant, and fuel swilling forward onto a crew may well also be a factor which would spoil their day...

 

[Pye_End]

That's where I have to rely on my physics O'Level. My recollection is that it makes no difference whether the heavy object hits the light one, or vice versa. The effect is the same.

I guess it is a matter of inertia. It is easier to slow down a light body than a heavier one, so in a crash it may well be that the heavy lump will keep going forward once the light one has already slowed down. In which case it will be better to be behind a heavy body.

 

[davidej]

That's where I have to rely on my physics O'Level. My recollection is that it makes no difference whether the heavy object hits the light one, or vice versa. The effect is the same.

Correct!

What matters is the size of the crumple zone -ie the space in which you can decellerate (spelling?). A car has a bit of one - a brick wall none.

 

[Heckler]

My somewhat indistinct memory of A' level physics suggests there is very little difference in effect between a 1000lb engine hitting a 100lb crew at 100kts, or a 100lb crew hitting a 1000lb engine at 100kts. Both are likely to take the smile off of your face.

+1
Stu

 

[LittleSister]

That's where I have to rely on my physics O'Level. My recollection is that it makes no difference whether the heavy object hits the light one, or vice versa. The effect is the same.

You are mistaken, I am sure. Would you rather be dropped 6 inches onto a car, or have a car dropped 6 inches onto you?