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Thread: Fitting low water pick up to outboard engine

  1. #71
    Administrator Aeroliner's Avatar
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    A pressure of 1 atm can also be stated as:
    ≡1.013 25 bar ≡ 101325 pascal (Pa) or 1013.25 hectopascal (hPa) ≡ 1013.25 millibars (mbar, also mb) ≡ 760 torr [B] ≈ 760.001 mm-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available [B, C] ≈ 29.9213 in-Hg, 0 °C, subject to revision as more precise measurements of mercury’s density become available [C] ≈ 1.033 227 452 799 886 kgf/cm² ≈ 1.033 227 452 799 886 technical atmosphere ≈ 1033.227 452 799 886 cm–H2O, 4 °C [A] ≈ 406.782 461 732 2385 in–H2O, 4 °C [A] ≈ 14.695 948 775 5134 pounds-force per square inch (psi) [D] ≈ 2116.216 623 673 94 pounds-force per square foot (psf)

    I beleive that 33.83 ft of fresh water is the corret number.

    Alan

  2. #72
    Team Member BJuby's Avatar
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    Thanks Alan, I was going to state 30 feet (which I know in passing from my father who dives). I knew 6ft was not 2 atm. You gave the actual calculations, much better, haha.

  3. #73
    phillnjack
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    Glenno
    i understand exactly what your saying, and yes i understand that surface props can be larger picth due to only being half in etc at speed, that makes good sennse ofcourse.
    And yes your right, when i do fall of ski's (very often ai do sink almost straight away.
    what i cannot understand is the blade causing drag while its actualy turning and pulling through the water.
    From my understanding of surface props and i have only run them on a small scale is that they are made to pierce the water far better than a submerged prop.

    Im not being thick or trying to be smug, its just that this all seems to be going completely against
    everything ive been told about propellers before.



    phill

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    Hi Phill,

    I dont think your being thick or smug, I ran through all the same questions when I started racing, as the way race boats props run was also against everything I had learned over a life of being around outboards.

    Quote: "what i cannot understand is the blade causing drag while its actualy turning and pulling through the water."

    I think maybee another word to use would be load instead of drag - deeper gearcase definately drag but prop more load when deeper. So what I reckon is - if your getting enough thrust with only 1/2 prop in, if you were to lower prop into the water more it would load it up more (more potential thrust - bite, if the motor could spin it yes).

    The whole idea of surfacing the prop is to unload the motor so it can rev more and spin a greater pitch prop.

    On one of my boats with low pick ups the centre of the nose cone is around 5/8 above the water when the boat is at full go and the prop is only 7inch diamiter by 10inch pitch- I will try to upload footage to you tube as you can see full nose cone out and at least 1/2 prop out, still with great bite. When I bought that boat it was doing only 56mph and then I raised the motor 3/4 inch and it felt like it freed the motor and released its shakels and now it reaches 71mph.

    If there was a motor that could have the power to turn that 7 x 10 prop fully submerged at the revs I am getting while srface piercing, i dare say it would hit the same top speed, if you ignored the extra gear case in the water drag. But you dont need that much bite, thrust, load on the lite tunnel hull its on. But if you where to put my motor (surface piercing) onto a regular every day fishing boat it would be totaly useles, not enough thrust to get it on the plane.

    Any way I warned you I have been told I can't explain things, I will try just the same.
    The Glenno

  5. #75
    phillnjack
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    glenno, you explain things very well, i do understand what your saying about loading up etc.
    it makes a lot more sense than saying drag to me, maybe different language thing, we might all
    talk supposedly English but many things are very different.
    It took me ages to find that wrist pin is what we call gudgeon pin, ha ha .

    Now i know this might sound daft to some, but if the engine is pulling 6,000 rpm (i think its limited to this)
    with a 13.25 x19 pitch fully submerged, what sort of pitch and diameter would i need if i go up to surface ?


    phill

  6. #76
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    Hi Phill,

    Now we are talking!
    What is make and HP is your outboard? And what sort of boat are you running it on?
    6000 is good rpm for fully submerged, sounds like a good start!

    After we all find out what HP and make your motor is and hull there will be some great input for you I am sure.

    Standard motor examples I or friends have prop surface pierced,
    1) 125hp mariner 4 cylinder ended up with a 24pitch 4 blade chopper
    2) 150hp merc V6 - Low pick ups ended up with a 28pitch three blade cleaver.
    3) 50hp Yam ended up with a 26pitch three blade chopper (very very very light mono hull)
    4) 90hp Yam ended up with a 26pitch cleaver and same speed with a 24pitch chopper- different revs
    5) 30hp tohatsu ended up with a 19pitch three blade chopper on light tunnel
    6) 18hp tohatsu ended up with a 16pitch two blade chopper
    and so so many more!

    It does come down to trial and error a bit as believe it or not, one prop may work awesome on one boat and crap on another with the same motor.

    but let us know make Hp and maybee even a pic of the boat from the side and a transom shot where I can see the under side hull shape.
    Have a good one Glenno

  7. #77
    phillnjack
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    My boat is not a race boat by any standards,its a bit too heavy as it weighs 500 pounds empty on its own.
    The engine is a evinrude 56ci 60hp.with power trim so again is running heavy at around 260 pounds.
    The shape when you look at the boat is very similar to boston whaler, but the underneath is very different.
    The boat is like a cathedral when stationary ,it is deep v at the front but runs on the v and
    2 sponsons untill around 30mph, then when more power is applied it runs on the just the back
    3 feet and that is shallow v.
    Recently i done some repair to the underneath and made the back pad a flatter about 1 foot wide.
    this seems to have made an improvement in the handling of the boat and stop it from trying to
    torque steer.
    Here is a pic of when these boats were first tested out with a 2 cylinder 50hp back in the
    early 1970's, youl see the boat running on about the last 4 feet of hull, this is what it runs at
    around 35mph.
    The more power you apply the less boat is in the water at the back end.
    when i give full power with my 60hp and the nose kept at about the same level as in the pic, only
    the last 2 foot of the boat is in the water.

    When these boats have a 50hp on them and a average deep v 13ft speedboat is fitted with same
    engines props etc, the orkney will out accelerate them but be around 2-3 mph slower top end.
    Hope that makes sense.

    Anyway here is the pic of the orkney 4 metre, its the original prototype and mine is the same.
    Also a pic of one from the back and the underneath of mine during repairs.
    the repairs were sanded and done propely in the end but this shows shape of underneath.
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    The deep v centre finnishes at around the point of engine control box, then tapers to shallow V

    The transom is exactly 20 inch from top to bottom.
    i run with engine equal to 3 holes up at the moment with cave plate about 1 inch above water on full throttle.

    This is why i was thinking about low water pick up if i raise the engine bu say 4 inches etc.
    like i say its not a race boat but i want to go a bit quicker.


    phill..
    Attached Images Attached Images  

  8. #78
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    You'd be surprised how high those cases will run..




  9. #79
    phillnjack
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    Wow yeah that is high compared to mine.
    And that runs fine with no extra water pick up ?

    if the engine wasnt on a set back and jujst raised would you still go as high without extra water pick up ?
    what i mean is, im gonna try and a get thing fabricated to raise my transom instead of having a setback,
    Now by staying close to transom would the 4 -5 inch lift still get enough water ?
    or is it just best to get the setback with the jack up all in one piece like the one pictured above ?

    That engine looks very nice
    can i ask what prop your running ?
    and what sort of speed you get compared to it being at "normal" height
    is the hole shot still not too bad ?

    once again that engine looks very nice.


    phill

  10. #80
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    24 raker, holeshot is terrible as these motors have very little low end torque, even with the vent holes in the prop opened way up. I made a plate than bolted to the set back that helped it a great deal for getting out of the hole. Once the motor hit 3k the party started like turning on a light switch. Stock it ran 65-68 mph @ 7000.

    I had a 1500 xs on it for awhile but that was a little over the top. Boat and motor are painted to match now, fun rig.

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