Stopping the Prop.

[bobconnie]

Guy , Your wrong! there are lots of these setups out there ! we ran one on our colvin for 10 or 12 years, and it would keep the batts topped off when we were cruiseing ! I don't have any pics, but it was made with a GM 30 amp alt. In fact there was a pic shown a few mos ago on this site, with a working set up shown very well !! just sayin they do work

[biltong]

My Amel Sharki has a factory installed alternator which runs my third alternator via a belt on the shaft and generates about 10 Amps while cruising. There is a spare belt on the shaft but not touching anything in case the first belt should break. There is then no need to separate the shaft from the gearbox to install a new one.

[Fuss]

Originally Posted by Guy

Quite frankly I don't believe this works. You may be able to turn an alternator but not at the speed to generate any power. It would, after all your trouble, stop the prop from turning if you turned it on at max rpm. Perhaps if you could find a little tiny alternator generator you might get something for your trouble but it would cost more than that wiz bang thing shown in the other post. Maybe one of those things you used to see on old bikes that were the size of a large pill bottle that made perhaps a few watts at full speed.
On top of all that the drag from a turning propeller is worse than a stopped prop so you loose whatever you might gain from this lame generator idea.

Ok, you don't believe this works.

But it does work and some boats do have it.

Do you believe it now?

[GILow]

I just use piece of VERY fine cord with a loop in the end, hooked around one of the bolts on the shaft coupler. If I forget it and start the engine, the cord would snap easily.

I was surprised how easy it was to prevent a prop from turning, even mine which is a 22" I diameter three blader, right up to seven knots so far.

[Hoohaa]

Originally Posted by GILow

I just use piece of VERY fine cord with a loop in the end, hooked around one of the bolts on the shaft coupler. If I forget it and start the engine, the cord would snap easily.

I was surprised how easy it was to prevent a prop from turning, even mine which is a 22" I diameter three blader, right up to seven knots so far.

This might be what I am looking for. Do you have any pics?

[stevensuf]

Guy,

If you do some digging you will find most studies on boats show a spinning prop having far less drag than locking the prop, its easy to prove, lock your prop, reset average gps speed, measure a few minutes, then reset again, free the prop to spin and remeasure average speed.

[rebel heart]

Originally Posted by stevensuf

Guy,

If you do some digging you will find most studies on boats show a spinning prop having far less drag than locking the prop, its easy to prove, lock your prop, reset average gps speed, measure a few minutes, then reset again, free the prop to spin and remeasure average speed.

Yeah I don't see how a fixed prop would generate more drag than a spinning one.

[skipmac]

Originally Posted by scoobert

if its in reverse and still rotating, something is broken. .

Sorry but this is not correct. Whether or not a prop will spin while a transmission is in gear, forward or reverse, will depend on the kind of transmission. If it's a hydraulic transmission, at least all that I've ever had on a boat, it will spin.

[biltong]

Originally Posted by biltong

My Amel Sharki has a factory installed alternator which runs my third alternator via a belt on the shaft and generates about 10 Amps while cruising. There is a spare belt on the shaft but not touching anything in case the first belt should break. There is then no need to separate the shaft from the gearbox to install a new one.

I must add that we achieve the 10 Amps while sailing at 5 to 6 knots. Also the pulley on the shaft is around 200mm in diameter to the alternator turns a whole lot faster than the shaft. For those who don't believe this system works.

[skipmac]

Originally Posted by Guy

Quite frankly I don't believe this works. You may be able to turn an alternator but not at the speed to generate any power. It would, after all your trouble, stop the prop from turning if you turned it on at max rpm. Perhaps if you could find a little tiny alternator generator you might get something for your trouble but it would cost more than that wiz bang thing shown in the other post. Maybe one of those things you used to see on old bikes that were the size of a large pill bottle that made perhaps a few watts at full speed.
On top of all that the drag from a turning propeller is worse than a stopped prop so you loose whatever you might gain from this lame generator idea.

Well others have already pointed out that there are systems like this that do work. Here's a couple of reasons why.

First, the speed the alternator turns is dependent on the relative size of the pulleys. So a very large diameter pulley on the prop shaft driving a very small pulley on the alternator can spin the alternator at high rpms.

Also, an alternator can be designed to put out at relatively low rpms.

[rigormortis]

I used the pipe wrench for a year. I had to head up to slow down to fit it (it always seemed to be slightly bigger or smaller every time) and tap it on with a rubber hammer. didn't like the whole deal. Next iteration was a string attached to engine mount with a loop to loop over the set screw on the shaft coupling. I stopped the shaft (1 1/4") with a medium sized pair of vice grips and found it to be very easy, even when doing 6-7 knots (which I don't do very often). So, 3rd iteration, simply put the vice grips on. As safety measure I use red flagging tape under the rubber splash guard on the key insert (Yanmar 4 banger). 3 years later no mistakes. Yet.
Pete

[model 10]

Originally Posted by stevensuf

Guy,

If you do some digging you will find most studies on boats show a spinning prop having far less drag than locking the prop, its easy to prove, lock your prop, reset average gps speed, measure a few minutes, then reset again, free the prop to spin and remeasure average speed.

Ask any pilot and you will find that a stationary fixed pitch prop has less drag than a spinning prop. The first thing you do after an engine failure is slow down the plane and stop the propeller. The drag from a spinning prop is about equal to a disk the same diameter as the prop. There is nothing like the ground rushing up to convince you this is true. The sample you get from doing 5 knots is just to small to be meaningful.

[deckofficer]

Originally Posted by Guy

Quite frankly I don't believe this works. You may be able to turn an alternator but not at the speed to generate any power. It would, after all your trouble, stop the prop from turning if you turned it on at max rpm. Perhaps if you could find a little tiny alternator generator you might get something for your trouble but it would cost more than that wiz bang thing shown in the other post. Maybe one of those things you used to see on old bikes that were the size of a large pill bottle that made perhaps a few watts at full speed.
On top of all that the drag from a turning propeller is worse than a stopped prop so you loose whatever you might gain from this lame generator idea.

You seem to forget that many moons ago Steve Dashew and other cruisers would run a dedicated prop and shaft for hydro charging while underway. When the Max prop came out that would feather when placed in neutral after being in forward but would stay locked in astern pitch if placed in neutral from astern propulsion, made it unnecessary to run a 2nd driveshaft with a prop pitched astern to generate power. There still are many cruisers that employ the Max Prop for both features, and my old Cal 40 was also set up like that. I would not consider 200~400 watts at 6~8 kt as "not producing".

[skipmac]

Originally Posted by rigormortis

I used the pipe wrench for a year. I had to head up to slow down to fit it (it always seemed to be slightly bigger or smaller every time) and tap it on with a rubber hammer. didn't like the whole deal. Next iteration was a string attached to engine mount with a loop to loop over the set screw on the shaft coupling. I stopped the shaft (1 1/4") with a medium sized pair of vice grips and found it to be very easy, even when doing 6-7 knots (which I don't do very often). So, 3rd iteration, simply put the vice grips on. As safety measure I use red flagging tape under the rubber splash guard on the key insert (Yanmar 4 banger). 3 years later no mistakes. Yet.
Pete

On a previous boat I used the vice grip technique and it worked just fine. Doesn't hurt a thing if you forget to take them off and start the engine, they just pop off and land in the bilge (as long as they don't smack into anything but the side of the hull).

You can probably guess how I know this.

Think I will try the string technique on the new boat. Bet it doesn't make the same loud noise that the vicegrips make when you forget.

[Cheechako]

What gear do you have? You are way over propped. I would want a 4-108 to peak out about 2800 rpm or so wide open. good cruising rpm 2000-2300