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Old 14-07-2013, 11:36   #31
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Re: Stopping the Prop.

i tried with shaft brake vs spinning--i didnt save any boat speed nor did it noticeably do anything with my brick..i also was advised to let the damnable thing spin, and that was from repair guy..... is all up to you.

but , then , i only have around 2500-3000 miles on this perkins engine, so what would i know...rodlmao.....
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Old 14-07-2013, 12:20   #32
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Re: Stopping the Prop.

To : biltong
I also have a Sharki. A previous owner removed the prop shaft driven alternator and brackets. Could you please post pictures of the Amel installation ?
I would be interested in recreating the factory set up on our boat.
Thank You.
Mark
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Old 14-07-2013, 13:27   #33
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Re: Stopping the Prop.

a spinning prop will, at the very least, prematurely wear out your output shaft bearing in the transmission, as well as your packing and your cutlass bearing.
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Old 14-07-2013, 13:45   #34
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Re: Stopping the Prop.

Quote:
Originally Posted by Hoohaa View Post
This might be what I am looking for. Do you have any pics?
Sorry, I won't get down to the boat for a week or more, darn it, otherwise I would post a picture.

But really, it is just piece of cord, tied at one end to a handy bit of boat, I think is a bit of framing timber that supports the cockpit. Then I fiddled around putting a loop in the other end of just the right length so that when the loop was put over one of the bolts on the coupler, it was as close to perfectly tangential to the coupler at that point.

Downside is that it IS a fiddle to attach on my boat, you have to climb downstairs, pull open the engine cover and attach the loop. Also, pays to do it before you get to 2 or 3 knots on my boat as that is when the prop starts to spin. Stopping it once it is spinning is a bit of an effort, mainly as the inertia in the prop is pretty impressive.

Also, and this might be contentious... but it does FEEL like there is more torque on the prop once it is spinning. (hard to eliminate the inertia from the equation, but once you manage to stop the prop, it feels as though the torque suddenly drops off more than just what is consistent with lost inertia.) This sort of is consistent with the old (and debated) wisdom that a free wheeling prop might create more drag than a locked prop, one of the explanations I read was that the locked prop "stalled" in the water and the flow around it was messier and subsequently less drag. Hardly a scientific argument I know.

I just lock it as I hate the noise of a spinning prop when everything else about a sailing boat is lovely and quiet. (in the right weather.) I don't notice any difference in boat speed and a 22 inch prop must create fair drag.

I have an idea for a "real" brake mechanism, but there are more important jobs right now, and the cord, like many solutions, is good enough if not ideal.

Matt
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Old 14-07-2013, 13:53   #35
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Re: Stopping the Prop.

Quote:
Originally Posted by scoobert View Post
if its in reverse and still rotating, something is broken. unless the engine is spinning backwards, and it will not be.
.
No, freewheeling in forward or reverse is normal for my velvet drive, which I believe is quite a common transmission. According the workshop manual it freewheels in forward, reverse and neutral.

According to the manual this is normal behaviour and is not bad for the transmission.

That last point is debatable I guess, as logic dictates the extra revolutions will cause some wear over time, but from what I have read of others' experiences the wear on the transmission itself is minor.

I just don't like the noise of freewheeling prop.

Matt
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Old 14-07-2013, 14:31   #36
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Re: Stopping the Prop.

I have a Perkins 4.154 with a max prop. I usually ran it up to about 2600 rpm. I wanted to see how it did if I adjusted the pitch up one notch.
I ended up with stats much like yours. My max was 2100 and it was struggling. I could cruise at about 5 knots at 1500 rpm.
I reset it back to the original.
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Old 14-07-2013, 17:03   #37
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Re: Stopping the Prop.

Quote:
Originally Posted by NTD View Post
I have a Perkins 4.154 with a max prop. I usually ran it up to about 2600 rpm. I wanted to see how it did if I adjusted the pitch up one notch.
I ended up with stats much like yours. My max was 2100 and it was struggling. I could cruise at about 5 knots at 1500 rpm.
I reset it back to the original.
This describes my current situation perfectly.
So if I get a new prop and it bumps my cruise rpm up to say 2100 rpm as opposed to 1500 rpm. Is this more efficient and will I gain some speed? I currently have no trouble motoring at 6.3 knots doing about 1500rpm.
So, what do I stand to gain?

Regarding the prop break, I will try tying it back with some builders stringline. Sounds like a simple cheap method that could work, as I have easy access to my prop shaft, Will let you know how it goes.
Thanks for all the great advise.

Just to ad my 2 cents worth on the spin or not to spin debate, I saw utube video somewhere which tested both theories using a small runabout with 2 outboard motors, one running and the other allowed to spin and then not spin. There conclusion was that the spinning prop was slightly quicker than the (more efficient) than the fixed prop.
Someone should send this in to Myth Busters.

Jeff

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Old 14-07-2013, 17:15   #38
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Re: Stopping the Prop.

I know that Guy is correct about airplane propellers creating more drag when free wheeling that when stopped. I used to fly powered hang gliders. We would use the engine to get to altitude, shut the engine off and glide down. Our engines had centrifugal clutches that allowed the props to spin very easily. We fitted brakes to the shaft to stop the prop. Our gliders were equipped with very sensitive vertical speed meters. When we stopped the prop you could feel the glider accelerated and the rate of decent would go from about 700 feet per minute to less than 500 feet per minute.
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Old 14-07-2013, 17:27   #39
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Re: Stopping the Prop.

Quote:
Originally Posted by HopCar View Post
I know that Guy is correct about airplane propellers creating more drag when free wheeling that when stopped. I used to fly powered hang gliders. We would use the engine to get to altitude, shut the engine off and glide down. Our engines had centrifugal clutches that allowed the props to spin very easily. We fitted brakes to the shaft to stop the prop. Our gliders were equipped with very sensitive vertical speed meters. When we stopped the prop you could feel the glider accelerated and the rate of decent would go from about 700 feet per minute to less than 500 feet per minute.
The problem with aircraft vs. sailboats is I wonder if freewheeling aircraft propellers are not so much creating their own drag as interfering with air flow over the wings? Unless your gliders were pushers?
I seem to recall reading an article a year or two ago by some well educated scientist type showing research in favor of freewheeling boat props having less drag than fixed props.
Sorry, no clue where I came across it.
I always figure it's easier to push through a revolving door than through a wall but I'm simple that way.
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Old 14-07-2013, 18:09   #40
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OP as your box is hydraulic , in will not generally work without the engine running. Hence the inability to lock it. , secondly yiu should check the specs , many such boxes should not be allowed to freewheel.

You need a shaft brake, there are one or two commercial units about , pricey though

Dave
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Old 14-07-2013, 18:19   #41
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Re: Stopping the Prop.

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You need a shaft brake, there are one or two commercial units about , pricey though

Dave
Piece of cord isn't too expensive...
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Old 14-07-2013, 18:26   #42
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Re: Stopping the Prop.

Well, IMO the best (and likely most expensive ) means of stopping the prop from freewheeling is to replace it with a Flex-O-Fold three blade folding prop!!!

Your boat will love you and reward you with better sailing performance, and your bank manager will smile... how could you go wrong?

Cheers,

Jim
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Old 14-07-2013, 18:36   #43
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Quote:
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Piece of cord isn't too expensive...
How's that going to work then, big cord for that prop !!

Dave
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Old 14-07-2013, 18:44   #44
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Re: Stopping the Prop.

I have been in touch with a prop company thismorning, the only info I can't give them is the reduction ratio for the gear box. Can anyone help me out here? The boats a bit far away at the moment for me to read it off the plate.
Jeff
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Old 14-07-2013, 18:45   #45
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Re: Stopping the Prop.

Quote:
Originally Posted by Jim Cate View Post
Well, IMO the best (and likely most expensive ) means of stopping the prop from freewheeling is to replace it with a Flex-O-Fold three blade folding prop!!!

Your boat will love you and reward you with better sailing performance, and your bank manager will smile... how could you go wrong?

Cheers,

Jim
Am looking into this, hopefully Kill 2 birds with one stone.
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