How sailboats sail

[tartansailor]

This post has been deleted by the author.

[Neil Gordon]

Did you ever consider what the resultant force goes when you brake the shaft?

It heels the boat.

Sure, but it always heels the boat the same way (assuming the boat is moving forward). That means more heel on one tack and less on the other.

I think this needs to be factored in when deciding whether to store the beer to port or starboard (and to be precise, how far off the centerline in either case).

[darmoose]

[quote="Neil Gordon"]


>>No. The water doesn't simply push endlessly against the locked prop. It flows around it. Since the water needs to change direction to do that, there's an energy cost that slows the boat. So there's an energy cost to keeping the prop locked and an energy cost to letting it spin. (Each as compared to not having the prop at all.) The question is which costs more.<<


Neil,

Be sure to hold onto the above thought, you are dangerously close to being set FREE.

Forget the rock for a moment

Now, you have evaded my question to you FOR THE THIRD TIME.

Last chance, or I am gonna assume that you are REALLY afraid to answer it. Look back to see what it was and try again. Don't be afraid.



Darrell

[Neil Gordon]

Last chance, or I am gonna assume that you are REALLY afraid to answer it. Look back to see what it was and try again. Don't be afraid.

Fear? Me?

>>Answer me this.....when i let go the the driveshaft, does the shaft stay at the same speed, slow down, or speed up <<

If the boat is moving and the prop is otherwise unlocked, and you let go of the driveshaft:

(i) The shaft will rotate. If you ease up a little, it will rotate a little. If you ease up more, it will rotate faster. So if RPM of the shaft is your measure of speed, the shaft speeds up.

(ii) The energy that was used to drive everything forward is now being (partly) used to turn the shaft. So if you measure the speed of the shaft by its forward motion, it slows down. Since the shaft is fixed in position relative to the boat, if the shaft slows down, the boat will slow down, too.

[M. R. Bober]

Toss that into the problem set.

Mitchell Bober
Sunny Lancaster (where prop locks and bagels can be hard to find), VA

[Oswego John]

Sunday morning.

Fresh bagels. Yum yum.

Thinly sliced lox. Pant, pant.

Maybe a small portion of minced onion and chicken liver paté on the side. Lust, lust.

A pox on you, Milton. Have you no mercy? Don't you realize that we're in the middle of a blizzard accompanied with 40 MPH gales and drifting snow up to my elbows.

Now I have to, *JUST HAVE TO*, go out to the deli for some lox and bagels.

Thanks a lot pal. You had to bring it up?
O J

[Neil Gordon]

Toss that into the problem set.

Having your prop stolen is a drag but sailing without a stolen prop reduces drag.

Sailing with a stolen prop will increase or decrease drag depending whether you steal a two blade or a three blade prop and whether you lock it or not.

Getting caught with stolen good, props or otherwise, is always a drag.

[darmoose]

Darrell,



If you're applying energy to squeeze the prop into slowing/stopping, what's wrong with the notion that the energy applied goes to making the boat go faster?

Neil,

look at this quote of yours wherein you say that slowing the propeller makes the boat go faster.

now just wait, dont answer yet.

[darmoose]

[quote="Neil Gordon"

>>The water flowing past the freewheeling propeller at any sailing speed is going to cause the propeller to spin at an RPM that is relevent to that boat speed. If you artifically slow the propeller, the additional force created by the water trying to get the propeller back up to the proper RPM, (a force you can feel if you grab the shaft) will slow the boat.<<

Either that, or since the force can't make the locked prop turn faster, the energy has no place to go but to make the boat go faster. [/quote]

Neil

once again you said that slowing the prop speeds up the boat

hold, dont answer yet

[darmoose]

If the boat is moving and the prop is otherwise unlocked, and you let go of the driveshaft:

(i) The shaft will rotate. If you ease up a little, it will rotate a little. If you ease up more, it will rotate faster. So if RPM of the shaft is your measure of speed, the shaft speeds up.

Neil,

and finally here you say that if we release the shaft it will speed back up.

So, how can you explain this, if the shaft speeds its rotation back up after we release it from our grip isn't the only reason it would speed back up is because the boat speeds up and the water rushes against our propeller faster and harder causing it to speed back up.?

Darrell

[Neil Gordon]

So, how can you explain this, if the shaft speeds its rotation back up after we release it from our grip isn't the only reason it would speed back up is because the boat speeds up and the water rushes against our propeller faster and harder causing it to speed back up.?

For this example, secure the boat firmly in any sort of contraption that will hold it firmly. Suspend the boat over a six knot current. Lower boat so that it's (i) still held firmly and (ii) the prop is now in the water and (iii) the prop is unlocked.

With the prop in the water now, will it spin or not? I contend that (i) the prop will spin and (ii) the boat speed will remain the same, i.e., zero. the only variable would be the water flow. With the propeller now spinning freely in the flow, would you contend that the water is now flowing past the boat more quickly?

I contend that the water will be slowed by the prop in the above example whether the prop is free or locked. The question has always been which will slow it more.

[darmoose]

So, how can you explain this, if the shaft speeds its rotation back up after we release it from our grip isn't the only reason it would speed back up is because the boat speeds up and the water rushes against our propeller faster and harder causing it to speed back up.?

For this example, secure the boat firmly in any sort of contraption that will hold it firmly. Suspend the boat over a six knot current. Lower boat so that it's (i) still held firmly and (ii) the prop is now in the water and (iii) the prop is unlocked.

With the prop in the water now, will it spin or not? I contend that (i) the prop will spin and (ii) the boat speed will remain the same, i.e., zero. the only variable would be the water flow. With the propeller now spinning freely in the flow, would you contend that the water is now flowing past the boat more quickly?

OF COURSE NOT

I contend that the water will be slowed by the prop in the above example whether the prop is free or locked. The question has always been which will slow it more.

I DONT THINK THE WATER WILL BE MUCH EFFECTED AT ALL, IT WILL STILL FLOW AT 6KTS

How any of that proves anything is way beyond me, hopefully you can explain cause i dont get it.

But on the other hand, aside from the fact that Newtons 3rd law of physics requires Vigors helicopter to slow its rate of descent when the pilot slows the blades because that puts the opposing forces out of balance, and also requires the sailing boat to slow down when I slow its freewheeling propeller for the same reason, the questions I asked you and your responses show that even you understand that the speed of the sailing boat and the speed of rotation of its freewheeling propeller are corrallary and relevent to each other.

you said you believed the following

1.when i slow the propeller shaft (as the boat is sailing forward), THE BOAT SPEEDS UP

2.when i release some of the pressure on the shaft to allow it to freewheel once again THE BOAT SPEEDS UP (as evidenced by the shaft rotating faster which can only be caused by the boat speeding up)

These two statements are diametrically opposite and both cannot be true, obviously. The fact is that number 1. is untrue.

Isn't it time for proplockers to reasess


Darrell

[Neil Gordon]

you said you believed the following

1.when i slow the propeller shaft (as the boat is sailing forward), THE BOAT SPEEDS UP

2.when i release some of the pressure on the shaft to allow it to freewheel once again THE BOAT SPEEDS UP (as evidenced by the shaft rotating faster which can only be caused by the boat speeding up)

These two statements are diametrically opposite and both cannot be true, obviously. The fact is that number 1. is untrue.

No, that's not what I believe and not what I believe I said.

The first statement is true... slow the prop (and shaft) and the boat speeds up, with maximum speed attained when the prop stops.

The second is false since it's the opposite of the first one.

The rotating shaft doesn't make the boat speed up anymore than a pinwheel makes the wind blow harder.

As long as there is flow over the prop and the prop is free to rotate, the prop will spin. Say it starts spinning (when it's unlocked) at 6 knots. To be extreme, say it slows the boat to 4 knots. It will spin at 6 and will continue to spin as it slows the boat down to 4. (If the effect was that dramatic, it would be easy to measure... we're talking relatively small props vs. all the other forces involved.

[Neil Gordon]

>>I DONT THINK THE WATER WILL BE MUCH EFFECTED AT ALL, IT WILL STILL FLOW AT 6KTS<<

Can I ask where the energy to spin the prop came from, then?

As the blades turn, do you think that a windmill slows the wind, makes the wind go faster or is there no change?

[darmoose]

you said you believed the following

1.when i slow the propeller shaft (as the boat is sailing forward), THE BOAT SPEEDS UP

2.when i release some of the pressure on the shaft to allow it to freewheel once again THE BOAT SPEEDS UP (as evidenced by the shaft rotating faster which can only be caused by the boat speeding up)

These two statements are diametrically opposite and both cannot be true, obviously. The fact is that number 1. is untrue.

No, that's not what I believe and not what I believe I said.

The first statement is true... slow the prop (and shaft) and the boat speeds up, with maximum speed attained when the prop stops.

The second is false since it's the opposite of the first one.

The rotating shaft doesn't make the boat speed up anymore than a pinwheel makes the wind blow harder.

As long as there is flow over the prop and the prop is free to rotate, the prop will spin. Say it starts spinning (when it's unlocked) at 6 knots. To be extreme, say it slows the boat to 4 knots. It will spin at 6 and will continue to spin as it slows the boat down to 4. (If the effect was that dramatic, it would be easy to measure... we're talking relatively small props vs. all the other forces involved.

Neil (my friend),

You arent making sense(you ever heard the term FUBAR). I must help you (maybe its the cold weather up there, hopefully your mind will clear when you get into some of this Florida sunshine)

Sooo.... based on what you're saying above if I understand it right, is that the faster the propeller rotates when freewheeling, the slower the boat must sail (is that right?)

...and maximum speed (hull speed perhaps) can only be obtained if the prop is locked (right?)

So, the SLOWER I sail, the FASTER my prop spins (if I am freewheeling). When I stop the boat, it must be going like hell. So we can never unlock our props at anchor (it would be way too noisy)

...and you dont believe that if I unlock the prop BEFORE I start sailing, that it will begin at ZERO RPMs and increase RPMs as the boat gains speed, hhmmm?

FUBAR once again comes to mind, but good luck with that.

Darrell