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Old 07 January 2010, 09:47   #1
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improve lift and repitching of prop

Are there props which inmprove lift in the stern i.e. help keep the bow down

Regarding the repiotching can a prop be re pitched 0.25, 0.5 0.75 & 1" increments?

TSM
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Old 07 January 2010, 10:58   #2
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If you got an Etec I guess it's got power trim?

Prop in itself won't lift, but the angle of the engine will. Two things to check - move stuff around in the boat (fore / aft balance) - that is likely going to be the easiest & cheapest thing to do first. Next check the stop pin is in the lowest hole on the clamp brackets - your trim might not be trimming in far enough.

Have a trawl around this section's posts - lots of prop pitching questions.


Standard props tend to come in inch increments of pitch, I'm suire there must be racing props that can be that finely tuned.
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Old 07 January 2010, 11:53   #3
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Originally Posted by 9D280 View Post

Prop in itself won't lift,
Yes it will
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Old 07 January 2010, 12:02   #4
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Supposedly, a 4 blade prop will "lift" the stern more than a 3. Dunno, never tried a 4 blade. It will give a better bite on spin-up, which means more low end power (assuming the engine will spin it) which means you'll hit planing speed faster, which I assume would give the impression of more stern lift.

As to repitching, from what I recall the prop shop telling me, they could alter SS pitch by about 2"; aluminum by 1", both dependent on the condition of the metal in that particular prop. Best bet would be to call the shop you plan to use and ask them what they feel comfortable doing.

jky
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Old 07 January 2010, 12:08   #5
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You can get props that offer stern lift, but in a fairly low speed application that'd be an expensive and probably ineffective way to go about keeping the bow down!

Weight distribution, trim and maybe doelfins would be my first steps. We put doelfins on our arse heavy boat and they made a fantastic difference.

Harry
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Old 07 January 2010, 13:11   #6
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Originally Posted by two stroke mick View Post
Are there props which inmprove lift in the stern i.e. help keep the bow down
Well many props tend to give some degree of bow lift, it's actually the stern being pulled down, so you could choose one with less bow lift. I'm not sure what shape a prop blade which actually lifted the stern would be.

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Regarding the repitching can a prop be re pitched 0.25, 0.5 0.75 & 1" increments?TSM
I guess a prop can be but whether it would be worthwhile and whether the actual pitch would be that accurate I would doubt. Repitching has to be a compromise in pitch because the angle of blade at and close to the root won't be changed. If the prop wasn't originally, after repitching I would expect it to be a variable pitch prop. I'm not suggesting that is necessarily a bad thing.
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Old 07 January 2010, 16:10   #7
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I guess a prop can be but whether it would be worthwhile and whether the actual pitch would be that accurate I would doubt. Repitching has to be a compromise in pitch because the angle of blade at and close to the root won't be changed. If the prop wasn't originally, after repitching I would expect it to be a variable pitch prop. I'm not suggesting that is necessarily a bad thing.
I was told that all a SS prop could be repitched to was 1" and that was by 3 prop shops. What would benefit from a rework at a specialist would be the fact that it will be balanced properly and hand finished by someone that knows what they're doing.

As for lift, I thought that the larger blade area of a 4 or 5 blade prop gave more grip at lower speeds which translates to thrust and thus pushes the stern out of the hole quicker. The boat would also respond better to trim too and thus this is where you get the stern lift and bow down. However you will suffer with loss of permormance at top end due to more blade resistance though the water.
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Old 07 January 2010, 18:27   #8
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I was told that all a SS prop could be repitched to was 1" and that was by 3 prop shops.
I don't doubt that but since it's not possible the twist the blade where it attaches to the hub you're gonna have variable pitch if you twist the rest of it. Some props are variable pitch, of course.
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As for lift, I thought that the larger blade area of a 4 or 5 blade prop gave more grip at lower speeds which translates to thrust and thus pushes the stern out of the hole quicker. The boat would also respond better to trim too and thus this is where you get the stern lift and bow down....
Yep but blade shape can make a big difference too. As a VERY loose but general description, if the blade is more spoon shaped, it will haul the stern down. Also, if the blade is of this type it tends to hold the water better in adverse circumstances.
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Old 08 January 2010, 04:31   #9
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if the blade is more spoon shaped, it will haul the stern down.

Quote:
Originally Posted by 9D280
Prop in itself won't lift
Quote:
Originally Posted by Downhilldai
Yes it will .

Up? Down? - Discuss!

Props give THRUST, which can only go axially relative to the prop shaft......... The interactions of the hull with the water and the resulting vector relative to the hull as a result of this thrust & your trim angle will raise or lower the stern.
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Old 08 January 2010, 06:41   #10
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9D280 - True, if you take the simple model of a propeller acting fully submerged in water.

And probably near enough true for all our cruising rib applications, however, when the speeds increase and surface piercing propellers come into the picture things change and you can get stern lift from a prop.
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