a prop's rake

Discussion in 'Props' started by Sindel, Aug 7, 2009.

  1. Frosty

    Frosty Previous Member

    Fwd or aft??? Ive never come across negative rake!!!

    Rake is meant to keep the prop operating in cavitating conditions.

    Its not normally used on a shaft boats, I cant think why it should be raked or what effect if any it would have.

    Rake is of course extremely effective on surface propellers where just the submerged blades rake will make a big difference as it has non to oppose it.

    Something is wrong here Sindal. I am trying to balance you obvious experience of 10 years owning the boat and its seamingly suddenly mis behaving itself for no real reason.

    I am thinking increasingly worse reasons for this such as bad storage and the bottom of the boat being distorted some how, hooked.

    Are the bilges flooded and water rushes to the back???
     
  2. Sindel
    Joined: Jun 2009
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    Sindel Junior Member

    Rake?

    ...to improve tip clearance, and sometimes aids flow into the prop to provide a more uniform flow, nothing else.

    Exactly what I'm getting at...

    As for pictures:

    at rest
    [​IMG]

    moving 5 to 7 mph
    [​IMG]

    wot
    [​IMG]

    a peek around picture of the bows bottom
    [​IMG]

    and aft
    [​IMG]

    an oldie showing the bottoms flatness
    [​IMG]

    another
    [​IMG]

    That wot picture above is the only shot I have at high speed, it's hard to take pictures when your driving :D

    So, anyone...
    How would one go about measuring a prop's rake?
    Can it be done by hand or do I need some computer to do it?
     
  3. Sindel
    Joined: Jun 2009
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    Sindel Junior Member

    This is not something that has happened recently, it has happened everytime I've used either of the two 13" props.

    Whenever I have messed up my 12 x 14 prop, I've used the 13" as a spare to use during the repair of the 12" prop. Because of the 13" prop's poor performance with trimming out my boat, I always pulled the boat and reinstalled the 12 x 14 as soon as I get it back from the repair shop.

    I have 3 props now (2 are in for repairs)

    I'd like to be able to not have to play musical props...
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "..Fwd or aft??? Ive never come across negative rake!!!.."

    I assume then you've never heard of the Classic paper "Propeller Design Model Experiments" by Emerson and Sinclair?

    I use them frequently, well, when a prop is required....these experiments were using a prop with a 10 degree aft rake.

    I know that rake of SC/CP props can theoretically ahve an effect on the trim of a boat owing to vertical forces exerted, but, the amount and for what size of prop, power delivered, boat size, displacement etc, I've no feel for since, i don't use SPs/CPs so can't confirm this myself to their real effect.

    But from these pic's, the prop is a normal 3bladed submerged prop.

    When you chnage the prop's over...does the shaft move in anyway or move any weights accidentally???...all seems very odd.
     
  5. Guest625101138

    Guest625101138 Previous Member

    Sindel
    Does the boat want to steer to port noticeably more with the bigger diameter prop?

    Also when you have done the 90 degree lose in a good breeze has it always been in one direction or has it gone either to port and starboard?

    Rick W
     
  6. Sindel
    Joined: Jun 2009
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    Sindel Junior Member

    The prop pictured is the 12 x 14 prop

    The explanation I got about rake from an old geezer who used to race these old wood boats is:
    Forward rake draws more water from under the boat's center line helping to keep the boat drawn tight to the water then also the force of water going out the back of the boat is spread out more and since there's no water above the water the resulting force helps to push the nose back down...

    Clearance do change a little...
    13" prop on the shaft shows 3/4" before the strut - 1 1/4" clearance from bottom
    12" prop on the shaft shows 1" before the strut - 1 3/4" clearance from bottom
    The 13" props do have a beefier hub than the 12", but everything else about the same...

    I don't have the 12 x 14 here yet to look at, but I'll try to keep myself from changing it back right away to get some good photos and maybe a plaster cast or something...

    And here's my dilemma, when I get the prop back from the repair shop I like to put the prop back on asap (boating on the brain syndrome) and I lose my ability to physically handle the prop, then all I'm left with is the 13" prop wondering how to make it preform better... (like the 12 x 14)
     
  7. Sindel
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    Sindel Junior Member

    The boat pulls noticeably more to port with the 13" prop and there's actually not enough slip during acceleration (you can notice the engine working really hard)
    The newest 13 x 13 will become a 12 x 15 when it needs repaired, but I'd like to add the proper rake also

    Now that I have 3 props, I should be able to have a prop on the boat, a spare, and one in the repair shop being modified...
    I always keep a spare shaft, coupling, lots of brass keys, stainless cotter pins... Pretty much the whole drive-train...

    The quick unexpected turns have happened in both directions (always 28+ mph)
    We ride mostly on the river with spots only 40' wide in spots, plus there are bridge columns, trees, rocks, other boaters, people skiing, tubing, etc.
     
  8. Sindel
    Joined: Jun 2009
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    Sindel Junior Member

    Also, a note on efficiency...
    Every night we go for a ride to the damn, let the dogs out, and back home.
    The trip takes 1 to 2 gallons with the 12" prop.
    With the 13" prop it takes 2 to 4 gallons of fuel.
    (averages dependent on time 1 1/2 to 2 1/2 hour rides)
     
  9. Frosty

    Frosty Previous Member

    Ok nice pics. A lovely boat and what I expected. these boats don't trim you get what you got and you got what you get.

    This boat is designed for 4 people you are bit of a fly weight yourself so if you were to seat 4 people in there your trim would be perfect.

    I would be very happy with that trim. It would handle a bit of chop or another boats wake.

    As far as blowing about in the wind these kind of boats usually have a fin under neath amidships, I mentioned it before, if it is missing it will blow about. If it never had one then!!!!

    Lovely boat --I like that a lot.

    I would be more concerned about the rudder, that big lump where the shaft is welded in is too big and right in the way of prop thrust. A higher aspect ratio (longer one ) might help the blowing about, certainly increase control.
     
  10. Sindel
    Joined: Jun 2009
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    Sindel Junior Member

    Does no one read?

    That's the original rudder used during production. (kept as original as posible)

    The boat preforms very well with the 12 x 14 prop (trim and steering at all speeds)

    Even does well with another old brass 12 x 12 prop borrowed from my brother-in-law's boat years ago (runs high rpms, pegs the tach, breaks the needle, money out the wazoo...)

    I don't use the 12 x 12, nor can I modify it because, my brother-in-law still uses it...

    I can get them in brass for the same cost as the nibral ones, but one usually wants the best materials for the drive train to help with any hits... Besides each additional prop would be a gamble on how it will preform.
     
  11. Frosty

    Frosty Previous Member

    Well then get your 12x12 prop back off your brother in law. Find a prop you like and fit it. I doubt if thats the original rudder but if you say so.

    Fit the 12x14 if it performs well ???

    I cant help you any more my magic wand is at the menders.
     
  12. Guest625101138

    Guest625101138 Previous Member

    Pulling to port is to be expected with the shaft inclination you have. The bigger prop has quite low slip. You can do the comparison based on the numbers you provided:
    14" pitch 3200rpm 28.6mph gives what slip?
    Compared that with:
    13" pitch 2900rpm and 31.1mph?

    The lower slip with the larger diameter (17% larger area) means the down-going blades on the starboard side are working very hard in thrust and the up-going blades on the port side are giving little thrust or even acting as a brake. It means the prop is providing a lot of vertical lift force and also a significant steering moment, forcing to port.

    Ski boats with this sort of set up that we used in Australia many years ago had a deep brass skeg (I have seen up to 16" deep) located slightly ahead of midships to maintain good control.

    Reducing the prop diameter increases the slip so both sides of the blades provide thrust. Also the smaller radius results in slightly less steering arm from the unballanced forces so it is a double benefit.

    As far as fiddling with rake and the like to get things to work better it is probably a matter of trial and error. The fundamental problem is the angle of inclination of the shaft. It limits the efficiency you can get by increasing the diameter of the prop. As you have seen the smaller diameter is actually better. A prop with smaller blade area should give better efficiency. The prop is spinning quite fast and the boat is reasonably easily driven.

    Going even smaller diameter and increasing pitch may work better for controllability as the loading on each side of the prop becomes better ballanced as the slip goes up.

    The prop efficiency is quite good anyhow - up around 75% if it was on a horizontal shaft. Would take a lot of effort to work out what is gets with the inclined shaft.

    Rick W
     
  13. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics


    Naah, Rick, that's not correct! Let's look at the inflow velocities (for the sake of simplicity using Va without asymmetrical corrections for induced speed), and check the situation with a shaft inclination of 15 degrees. The vector sums will show that it is the upgoing blade that has the max angle of attack and thus does most of the job, in particular with the 13"x13" prop!

    For the 0.7 radius case on the 12x14 prop we get the following:

    Downsweep: peripheral velocity (U) =35.75 m/s, advance vel. (Va) =12.79 m/s, relative vel. (W) =40.97 m/s, flow angle (fi) =17.57 deg.

    Upsweep: U=35.75, Va=12.79, W=34.71, fi=20.85.

    The blade angle at 0.7 radius is 27.95 deg., which leaves us with an angle of attack "upsweeping": 10.4 deg., and "downsweeping": 7.1 degrees.

    Doing the same for the 13" prop we get the following:

    Downsweep: U=33.89 m/s, Va=13.90, W=39.82, fi=19.72.

    Upsweep: W=33.13, fi=23.90.

    Blade angle at 0.7 radius is 24.45 degrees for this prop, resulting in an angle of attack of 0.55 degrees for the downbound blade and 4.73 when rising on the port side. This propeller is producing its thrust asymmetrically; the starboard side actually running idle. The small prop is working with high angles of attack (probably close to stalling) all the way around and with less asymmetry in pressure loading.

    This explains why the 13x13 propeller is "digging down" the boat aft, (not creating lift, Mr Willoughby!!!), which in turn results in the nose-up attitude. The asymmetry of the propeller backwash is also creating a side-paddling effect to stbd. If this velocity check were done with correct inflow velocities (takes some iterating time) the tendency would be even stronger.

    A flat bottom like this needs a fin to avoid skidding around as has been said earlier in this thread. In order to find the optimum prop and trim, use an inclinometer to avoid guessing on trim angles!
     
  14. Guest625101138

    Guest625101138 Previous Member

    Well done baeckmo. You agree with my diagnosis of the shaft inclination and the fix I proposed.

    (I will leave others to check the vectors and get the right answer)

    Rick W
     

  15. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    baeckmo

    I'm surprised you haven't been hit with the usual childish negative points that I get for pointing out the usual errors in Ricks arguments!.

    However, nicely put.

    I don't normally go into this much detail when i design a prop, i just check the basics, run a few worse case scenarios and then hand over the data to the prop manufacturer, as it then becomes their warranty, not mine!

    Not having done the vector forces analysis, well except when i was a student some 25years ago but not since, is the vertical force, on a 100-ish HP motor really that high and really that different between a 12x14 and a 13x13 prop to influence a some-1.1 tonne run-about? In other words, would the increase/difference really be in the order of magnitude sufficient to create a large trim, assume a large trim exists, even at a shaft angle of some 5~10degrees (looks like)?

    I can image on a very light mono running at some 80~90knots with a huge amount of power, then perhaps yes, but on a small run about? Since such a slight difference in prop size, relatively speaking, would have been notice when the vessel was designed, again I'm assuming "normal design practice" here.

    Bearing in mind I'm speaking from a normal fast boat commercial field, as opposed to the pleasure-run-about market, design with props.
     
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