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Rubbish to stop or go into Reverse


robtheplod

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4 minutes ago, robtheplod said:

it might be best if i put this on hold until she's next out the water then i can be sure of what she's got and see if a bigger prop might be viable.... what gaps are needed between the prop and sceg/weedhatch to be safe?

I think a minimum of 2” top and bottom is considered the norm.

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15 minutes ago, nicknorman said:

I think a minimum of 2” top and bottom is considered the norm.

 

But I doubt 1.5" would make a significant difference to fouling or hydrophonic noise, 1" might, but it is all a black art so I am sure the 2" thing is somewhat flexible.

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10 minutes ago, nicknorman said:

I think a minimum of 2” top and bottom is considered the norm.

 

The rull of thumb I've heard a few times is is a shaft diameter of clearance top and bottom.

 

I think its highly arbitrary though. I ran a 25" blade on one of mine for a few years with 3/4" clearance top and bottom and had no problems. It mashed up bits of rotting wood off the bottom just the same as any other gap.

 

 

 

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Just now, Tony Brooks said:

 

But I doubt 1.5" would make a significant difference to fouling or hydrophonic noise, 1" might, but it is all a black art so I am sure the 2" thing is somewhat flexible.

 

IIRC 10% of prop diameter is the minimum recommended by most marine engineering sources to reduce prop noise. A minimum of 2" is also often recommended on canals/rivers to reduce the chances of a prop jam with a piece of wood or branch.

 

As Tony said using smaller clearances won't suddenly lead to disaster, but is not recommended unless there's a really good reason for doing it.

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4 hours ago, Tonka said:

Your bank balance is healthier too

As I said earlier, it may have much less to do with the prop than the hull design or at least its interaction with the prop. Until you have confident assurance that changing the prop will indeed make a difference you could well find yourself poorer and unhappier!

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I still think the dropped skeg is interesting. 

 

Why else would a skeg be dropped like that if it was not to fit in a larger diameter propeller? It clearly isn't needed for the propeller which is currently fitted.

 

 

 

 

 

I had a narrow boat once with a dropped skeg. It was a bit of a nuisance if the pound dropped overnight or going under bridgeholes with old motorbikes under them. 

 

Big propeller but relatively shallow draft. 

 

 

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25 minutes ago, Mike Todd said:

As I said earlier, it may have much less to do with the prop than the hull design or at least its interaction with the prop. Until you have confident assurance that changing the prop will indeed make a difference you could well find yourself poorer and unhappier!

It may also be to do with the propeller itself; depending on the blade shape and skew, most marine props trade off better performance (quieter, more thrust) when going ahead (most of the time) for poorer performance (noisier, less thrust) in reverse (very little of the time), and some do this to a greater extent than others.

 

You can go to the other extreme and make the prop symmetrical so it's just as good (or bad...) both ways to get better stopping power, but this didn't work for Axiom... 😉

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14 minutes ago, ditchcrawler said:

In theory my prop is to big for the gap but it doesn't cause me problem that I am aware of

 

Same on Parglena and Idleness, both had no more than 1" between the tip and the counter, both worked well.

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14 minutes ago, GUMPY said:

Same on Parglena and Idleness, both had no more than 1" between the tip and the counter, both worked well.

Bear in mind that worries about prop noise with a small clearance are kind of pointless if you've got a big trad diesel thumping away -- the 10% rule is really targeted at ships and boats where the engine is well silenced... 😉

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Our sharedboat had an 18" prop and very little clearance, certainly less than 2". Prop whine was a constant source of frustration.

 

My current boat has a 17 x 12 according to the builders specification. At first blacking I was intrigued to find that the propeller is stamped as 17 x 10. I contacted first owner who confirmed it was still on its original prop so builders docs clearly not always to be trusted.

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10 minutes ago, IanD said:

Bear in mind that worries about prop noise with a small clearance are kind of pointless if you've got a big trad diesel thumping away -- the 10% rule is really targeted at ships and boats where the engine is well silenced... 😉

One was a trad slow revving JP3  the other a 6cylinder Ford that came out of a fishing boat, two very different kinds of engine.

Both however were slightly overkill for the boats they were fitted in.

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9 minutes ago, Cheshire cat said:

Our sharedboat had an 18" prop and very little clearance, certainly less than 2". Prop whine was a constant source of frustration.

 

My current boat has a 17 x 12 according to the builders specification. At first blacking I was intrigued to find that the propeller is stamped as 17 x 10. I contacted first owner who confirmed it was still on its original prop so builders docs clearly not always to be trusted.

 

Do you mean prop singing? That's nothing to do with clearance, it can usually be fixed by some "adjustments" to the edges of the prop blades with a file...

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