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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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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.

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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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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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Old 08 January 2010, 07:05   #11
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Up? Down? - Discuss!
If you apply the converse of jwalker's theory, then if the blade is less 'spoon shaped', it will have the opposite effect, ie lift the stern

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Old 08 January 2010, 07:14   #12
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If you apply the converse of jwalker's theory, then if the blade is less 'spoon shaped', it will have the opposite effect, ie lift the stern

Hehe, Is less spoon-shaped not a spoon shape?
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Old 08 January 2010, 07:23   #13
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Up? Down? - Discuss!

Props give THRUST, which can only go axially relative to the prop shaft.........
Yer reckon? The resultant of the forces may not act parallel to the shaft. Since our props are not deeply submerged, a blade at the bottom of the rotation will have more effect than a blade at the top.
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Old 08 January 2010, 07:34   #14
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if the original poster can't get the bow down enough I would suggest he should be looking at transom angle rather than faff about with a prop. We transferred crew from one of our Humbers on the south coast to a tornado occupied by ex clients of ours a couple of years ago. Running into a head sea we were trimmed down with a decent amount of power on and doing fine whereas the novices onboard the other boat were struggling, so we took them onboard and transferred two of our experienced commercial skippers onto their boat. When we got back to the slipway our two were ****** off with the inability of the outboard to get in to a sufficient angle to hold the bow down. If this is the problem on the boat on question then a bit of shimming by inserting a shaped wooden wedge under the mounting bracket could solve the problem. Also a bit of weight up front in the shape of a second fuel tank would help. Trimming up with a following sea is easy enough as you have loadsa power on-trimming down in a head sea is a tad more difficult as you havent usually got the same amount of power on.
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Old 08 January 2010, 07:51   #15
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I am in agreement with wavelength. When I bought my rib it had a 'high' five prop and it got me out of the 'hole' quick for the skiers but was no good on long cruises so I changed back to the original three blade mariner prop, still gets me up quick enough but the rest is better.
Hope this helps.

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Old 08 January 2010, 08:03   #16
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Hehe, Is less spoon-shaped not a spoon shape?
There will come a point where your 'spoon' won't hold any cawl
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Old 08 January 2010, 08:52   #17
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Yer reckon? The resultant of the forces may not act parallel to the shaft. Since our props are not deeply submerged, a blade at the bottom of the rotation will have more effect than a blade at the top.
Close enough for the proverbial tango in this situation.

Of course the hydrodynamic effects of that extra 4" of absolute head at the blade centre is going to sooooo outweigh the effects of weight distribution in the hull & engine angle on the trim switch........ If you really want to start arguing infinitessimally, remember that the water coming off the back of the boat will be travelling slightly upwards due to the pressure under the hull......

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if the original poster can't get the bow down enough I would suggest he should be looking at transom angle .....<snip>..... amount of power on.
Which is a more illustrated version of what I said at the beginning!
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Old 08 January 2010, 09:32   #18
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The reason I asked the question regrading a prop that will create lift was due to the RI article regarding the RS4 and the "Pimp my Rib" article. In this article it was either stated or implied that a prop was obtained from steel developments which provied lift .

TSM
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Old 08 January 2010, 10:14   #19
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Close enough for the proverbial tango in this situation.
Nope, but you are free to believe whatever you wish.

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Of course the hydrodynamic effects of that extra 4" of absolute head at the blade centre is going to sooooo outweigh the effects of weight distribution in the hull & engine angle on the trim switch........
Well firstly, sarcasm doesn't make you right. Secondly, a prop which creates bow lift is more efficient than running a drive at a steep angle in order to lever the stern down into the water.
Thirdly, a bow lifting prop will also hold the water better when partially submerged so on those occasions when you're running skipping over the waves, the ability of the prop to keep the bow up will also provide a noticeably improved ride comfort. You would also benefit from softer landings if hopping out of the water because the prop will still provide drive before the hull lands and therefore prevent a dead landing.

However, two stroke mick was asking about a stern lifting prop. I'm not sure about this one; in the same way that a bow lifting prop pulls the stern down, I suspect a stern lifting prop is one that doesn't pull the stern down. I think it doesn't actually lift the stern but I'll be pleased to be corrected on this if anyone knows different.
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Old 08 January 2010, 11:14   #20
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Nope, but you are free to believe whatever you wish.
OK. Can we see some data / maths that proves the shape of the blade will create SIGNIFICANT lift?

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Well firstly, sarcasm doesn't make you right. ....<snip>.... the prop will still provide drive before the hull lands and therefore prevent a dead landing.
Yeah, I admit to a touch of sarcasm, and I agree with your lift / speed / comfort comments, but my "significance of prop lift relative to weight distribution / trim" argument still stands.

As I understood it, TSM wanted to sort out his "angle of attack". Yes, the proximity of the surface will have an effect, as will the size & shape of the cav plate, the hull it's bolted to, the fact that the leg supporting the gearbox on the top half is fatter than the skeg will also have an unbalancing effect, BUT compared to altering the angle of the thrust or shifting significant weight around the boat, it's going to be relatively small.

You are talking about bow lift & angle of the hull relative to the water, and I agree with your comments, BUT, you still have a significant thrust vector acting below the fore / aft Centre of Rotation of the hull, so as you open up, by definition, the bow will rise. Boat then "sits" on it's rear end as it skims along, thus the effective pivot point moves aft, and the mass of the bow has a greater effect on the fore / aft trim. Hit the PT switch, tuck the engine in, and the thrust torque around the Centre of rotation is more aligned with the CoG, so the bow drops as the countering force vector reduces. Conversely, trim out, the resulting vector pushes the stern down, but as it's held up by the pressure of the passing water, the bow lifts instead. Eventually you trim so far out that gravity on the bow wins the balance argument & the prop comes out the water.

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However, two stroke mick was asking about a stern lifting prop.
Exactly. will this "surface effect" make that much difference on a boat that weighs in at around half a ton versus some far less subtle changes like trim or weight distribution?

I'm not saying it doesn't exist, I'm saying that it's negligible compared to the other things that can be adjusted, and suggesting that TSM looks at them before splashing out on a new prop. (although changing the prop will alter the thrust & speed charachteristics, and therfore the boat's trim & balance)
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