Transmission nuetral or locked while sailing

[Vipe6]

If a locked prop induces more drag than a free rotating one, then how would one explain the autogyro (or autogiro). I know some say that there is a difference between airplane propellers and those of boats but I can't really see that difference.

[Wotname]

Originally Posted by Vipe6

If a locked prop induces more drag than a free rotating one, then how would one explain the autogyro (or autogiro). I know some say that there is a difference between airplane propellers and those of boats but I can't really see that difference.

First of all, read and understand the stuff in the links below and you will understand how / why autogyros work; then compare all of that to a 3 blade fixed pitch boat prop directly linked to a piston engine via a gearbox. You should then be able to see the difference

Autogyro History and Theory
aerodynamics of the autogyro

[StuM]

Originally Posted by Vipe6

If a locked prop induces more drag than a free rotating one, then how would one explain the autogyro (or autogiro). I know some say that there is a difference between airplane propellers and those of boats but I can't really see that difference.

1. Shape/pitch and cross sectional area of a boats prop as a proportion of the swept circle compared to autogyro, helicoptor rotor or fixed wing prop. Even a two bladed prop takes up a lot more area than the others and has a greater pitch.

2. Relative density and resultant drag between water and air.

3. Completely different airspeed/boat speed and rotation speed of the propellor

You just can't draw a valid comparison between the two situations.

[sartorst]

https://youtu.be/Sg_lwiFSUZo

[Vipe6]

First of all, I'm not trying to start an argument, I just want to have a better understanding on the subject.
At the beginning of the text in the first link provided i find this: "Think of those seed pods that spin as they fall. As they fall, the air makes them spin. (Because they're spinning, it's as if the mini wing was moving faster, so it creates more lift, and the seed pod doesn't fall as fast as it would if it didn't spin.)" Does this not mean that a spinning propeller creates more drag than a locked one? In adittion, the seed looks very much the same as one blade of a ships propeller.
In the past I had a boat with a fixed propeller that I always locked in reverse (Volvo Saildrive) just to keep the wear and tear as low as possible. Never noticed a difference in speed however when leaving it in neutral on the rare occasions that I forgt to put it in reverse. Now I have a boat with a Maxprop so after shutting down the engine I put it in reverse for the prop to feather and then back in neutral.

[missourisailor]

Originally Posted by StuM

1. Shape/pitch and cross sectional area of a boats prop as a proportion of the swept circle compared to autogyro, helicoptor rotor or fixed wing prop. Even a two bladed prop takes up a lot more area than the others and has a greater pitch.

2. Relative density and resultant drag between water and air.

3. Completely different airspeed/boat speed and rotation speed of the propellor

You just can't draw a valid comparison between the two situations.

Spot on mate, spot on!!!!!

Lots of folks think that a prop is a prop is a prop.
The only real similarities between a a/c prop and a marine screw is that they both spin and both propel. Beyond that, they are two totally different animals in completely different environments.
It hard for folks to get their head around this.


Now if I may bring this thread full circle and back to the topic at hand (sailing with tranny in gear or neutral), I have been following this thread (along with many others on the subject) very closely.
Because, as it is now becoming very obvious, I have been taught wrong and have been given a healthy dose of misinformation.

All the way from great grand pappy, through all of my mechanical repair and design education and personal experience's, it has been drill in my head that you don't spin what you don't need to spin!!!
I.E. keep the shaft stationary when not motoring.
I have always done, observed and even instructed folks to do the same.

It seem that I was/am wrong!

Okay, I can accept that a few blown, locked up, striped..etc, trannies could just be operator error or lack of maintenance. I think that over the last few weeks I've read or heard of, about a hundred and a half that have failed. 150 out of how many ten's of thousands that were built, seem to be a small amount of failures.
Okay I can live with these numbers. After all I would like to think that I take care of stuff better then most.

But... Yanmar has issued SB after SB on this very subject. I think that they are up to three or four now. At first, I thought they were just SMA's (save my ass's). But after reading, what the third one, it is apparent that Yanmar has a design flaw! To be fair, it's not only Yanmar, but all of the manufactures seem to be drinking the same Kool-Aid.

Okay, I've spent $6k on a new tranny, and now you want me to spend another couple of grand for a shaft lock, just to keep the wear & tear and noise down. Why the hell is a marine transmission not designed to allow it to stop the shaft spin in the first place?!?!?


My question is why do we as consumers keep putting up with this crap.

I still think that it is a bad idea to let the prop shaft free spin for hour's/day's/ week's at a time. That can't be good on the bevel gears. So I guess I'm now looking for a shaft lock.

----END of RANT-----

[Littlechay]

Originally Posted by missourisailor

Okay, I've spent $6k on a new tranny, and now you want me to spend another couple of grand for a shaft lock, just to keep the wear & tear and noise down. Why the hell is a marine transmission not designed to allow it to stop the shaft spin in the first place?!?!?

Many are designed to spin of course. You pays your money and you takes your choice.

Depends on if you have fixed or folding or feathering prop; whether you want the shaft to spin because you have a shaft alternator attached... decisions decisions

[transmitterdan]

Originally Posted by missourisailor

I still think that it is a bad idea to let the prop shaft free spin for hour's/day's/ week's at a time. That can't be good on the bevel gears. So I guess I'm now looking for a shaft lock.

----END of RANT-----

If a transmission is designed to free wheel then it is not a big wear and tear issue. The amount of wear depends on several issues but the reality is that a free spinning transmission (up to the clutch) isn't under very much load. Some transmissions don't have any way to remove heat if the main engine is not running and this becomes the main limiting factor in free spin mode.

The "best" solution is not a shaft lock but a feathering propeller IMO. This solves several problems at once. They have excellent reverse thrust and no stress on the transmission when engine off. There is very little down side from a technical/mechanical point of view. Obviously they cost more but...that's how it is with boats.

A shaft lock requires some interlocking with the engine (or transmission) to prevent damage or worse. It slows down the boat too.

[reed1v]

Originally Posted by Wotname

Yep, you probably will regret it.

In essence you have pretty well nailed it so I will just make a minor correction if I might.

Try this "The reason that rotor-craft spin their rotor on a unpowered decent is to use the momentum (energy contained in) of the spinning rotor to create lift after the collective has been pulled up just before touch down."

Well said. There however are two other reasons to spin rotor on unpowered decent(have done this once too often myself). Spinning rotor, like a spinning top, will keep the plane reasonably stable on decent. Otherwise, it might go into a turn, which you do not really want. Plus spinning provides an equally distributed drag from prop. So many reasons to keep spinning. Its always exciting to land without power. My excuse was an old, really old Piper with only a dip stick to figure out how much fuel in tank. Never let someone else take the measure.

[missourisailor]

Originally Posted by reed1v

will keep the plane reasonably stable on decent. Otherwise, it might go into a turn,

I've not noticed this, but then again the prop only stops turning because of a catastrophic failure or stall conditions. When these have happened, I've had other things on my mind, so I may have been hypersensitive to other issues and not noticed.

Also, to be honest, I don't think that I have ever been part of, overheard or sat in on a conversation about this. By this I mean the above quote.

[missourisailor]

I should clarify a bit on my last response.
We are of course discussing single engine aircraft here.

[missourisailor]

Originally Posted by Littlechay

whether you want the shaft to spin because you have a shaft alternator attached... decisions decisions

Okay.. I'll bite, because I've always wondered about a shaft driven alt..

How many amps can you reasonably expect to produce from this setup?

Granted, I know that pitch, speed, pulley ratio..etc.. all need to be factored in. But generally, are we talking an average of only an amp or two or 25+ amps?

[transmitterdan]

Originally Posted by missourisailor

Okay.. I'll bite, because I've always wondered about a shaft driven alt..

How many amps can you reasonably expect to produce from this setup?

Granted, I know that pitch, speed, pulley ratio..etc.. all need to be factored in. But generally, are we talking an average of only an amp or two or 25+ amps?

The dimensions of a sailboat propeller are no where near optimal for extracting significant power when acting as a turbine to generate electricity. A typical sail boat averaging 6.5 knots cannot generate many kwh from the rotating shaft. It would be senseless to even try this on a power boat.

There are purpose made water powered generators that you can fit to the stern of a sailboat that will do a decent job but they are really expensive. They have optimized turbines and gear ratios for maximum power extraction. But some (most?) cruisers don't want another appendage in the water that can hit something and break. But these machines can really shine when they are behind a fast sled like an Open 60.

[reed1v]

We had a water powered generator. 2 amps. Over a day of cruising not too bad. Setup cost about $300. Not expensive. Now if you want more amps, like 10, then need a larger, and way more expensive setup.

[crazyoldboatguy]

Originally Posted by reed1v

We had a water powered generator. 2 amps. Over a day of cruising not too bad. Setup cost about $300. Not expensive. Now if you want more amps, like 10, then need a larger, and way more expensive setup.

The key is to keep expectations in the reasonable zone. There is no way I am going to spend 5 boat bucks for another appendage hanging off my taffrail. If I can get 2 amps out of a jury rigged prop shaft alt I'd be tickled pink. I figure I can do this for about $300, like you did.

Yes, the prop isn't optimal. Yes, the output is not great. But for the amount invested I think it's a no brainer. The prop is already there. It is already spinning. It will turn an inexpensive auto alternator. Where is the downside?

Can you tell me how big was the pulley on the shaft? Where did you find such a pulley? I found some but if you had a better idea I'm all for it.