folding prop

hello everyone. todays question is .if i replace a folding prop with a fixed 2 blade to improve reverse thrust will the 2 blade naturally stop with the blades behind the deadwood due to water flow or would i have to fit a shaft lock. it is a volvo md1 with mechanical gearbox.

 

No. Most likely it will stop due to a compression stroke of the diesel and that could be anywhere. You would need to shift into neutral, stop the shaft in the right place, and put it in gear to hold it there. You need some way to sense shaft position. Fit a magnet to the shaft coupler maybe.

 

thanks phil, how much drag would a small 2 blade cause when sailing if i just let it freewheel.

 

White point,

I haven't seen the numbers for a two bladed, but for a three, the drag is about the same as 1/2 the total drag of the rest of the boat. While the prop is free wheeling the drag is cut by about half.

In other words if the hull drag is 100 units, then the drag from a fixed three blade is 50 units, and the drag for a spinning prop is 25. For a feathering prop it is around 2 units, and a folding prop wasn't measurable.

 

It depends if you can let it free wheel. Check with the tranny specs.
Then, do you want it to free wheel. Wear and tear on the cutlass etc.
Next, what is your shaft drop angle. If it is a typical 15-20 degree sailboat set up, the freewheel info based on zero drop shafts is very, very suspect in my opinion.
You just have to experiment. I have no idea what your prop aperture looks like or what the prop pitch ratio is. It is no big deal to run the test yourself. Dive the prop and put it up-and-down. Mark the shaft with a silver marker. Let us know what you find out. Can't find any reported info on drop angle shafts. Just anecdotal stuff.

What is your folder like. Mine was installed in '82 and is looking a bit rough

 

thanks stumble. there is quite a bit of drag then, even with small props.

 

phil, the shaft angle is very shallow, around 4 or 5 degrees. are you saying that if i open the prop by hand it will stay open for a test sail. i have never had a folding prop before.

 

I meant with the fixed prop. The low shaft angle is good news, though.

 

no worrys, i will get hold of a fixed pitch and try it. the boat needs to come out for antifoul soon, i can put it on then.

 

hey I donot know about props can anyone guide me please?

 

you need to explain what you want to know .

 

"In other words if the hull drag is 100 units, then the drag from a fixed three blade is 50 units, and the drag for a spinning prop is 25. For a feathering prop it is around 2 units, and a folding prop wasn't measurable."

This is only true IF the shaft has NO friction.
With most stuffing boxes and shaft bearings the spinning prop will absorb more power than the fixed prop.

Locked behind the deadwood is the best.To set the prop lock position , sail at your "normal speed" and use two pipe wrenches in opposition to hold the shaft.

There WILL be a spot where the prop does not wish to rotate, mark it.

When dragged behind the deadwood only the surface creates drag , not the prop form or worse the huge power absorbed by spinning slowly.

When locked put the engine start key on the prop lock handle.

FF

 

Fast,

These were experimentally measured numbers, not computed. There was a pull test by yachting monthly using a beneteau 331 (I think) where they put a load cell on the tow line and actually measured the drag difference between no prop, a folding prop, feathering prop, and a fixed. In total they tested 16 or 17 props and reported there results over a number of different test.

I will try to get the paper, but the Flex-o-fold was the clear winner.

 

http://www.flexofold.com/upload_dir/docs/Test_YachtingMonthly_low.pdf

That is the full report. And the idea that a spinning prop causes more drag seems more like an old wives tale than experimental results from what I have seen.

 

All of the tests were done on the sailboat except the drag test. The drag test was done with the props mounted on an outboard leg trailing behind a little lightweight skiff.
So once again, no stuffing boxes or tranny or cutlass; and no shaft drop angle. The drag test is meaningless; but the rest of the test seems excellent. Pity Gori showed up with the wrong prop.