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Thread: Need Prop Advice for a 73 X18

  1. #16
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    I spoke with my mechanic today and he was referring to the original specs from the booklet I gave him. He's said that Chris Crft 350 produces 320 hp and is supposed to run at 5700 RPM. He's said it was built or that and has a special cam etc. Interesting... I have the original 21 pitch aluminum prop and he gave me a 22 aluminum that runs great at 5700. Problem is it only goes 52 or 53 mph. I also have a brass prop but I have no idea what it is.

    I am trying to get this guy to join his forum. He has all kinds of great Donzi history I am learning. Like for example, why they wanted that many RPM's for racing back in he day.
    Ted

  2. #17
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    73x,

    the real thing that makes the solas a great prop is how efficient they are , or how well the prop grips the water . your numbers are at about 28% slip which is aweful
    . in a perfect world a 22 pitch should travel 22 inches in one turn but water is not a perfect world. so at 28% slip in one turn your prop travels about 16 inches.

    a good prop is around 10% a solas runs between 5-8 % in slip so in one turn the 21 solas travels about 19 .5 inches.

    plus diameter is a factor too I would say your setup will turn a 21 solas to about 4900-5000.


    I know this all sounds a tad crazy until you try a solas
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  3. #18
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    What prop diameter is appropriate?

    I also have an X-18 (1972), and I am thinking that a Solas 21 prop might give me a bit more speed.
    My boat has a bronze prop and runs in the mid-50s at a bit under 5,000 RPM.
    Engine is a Ford 351 with Cobra heads. No idea what horsepower, but previous owner built it for reliability more than output. The reason that I think a new prop might help is because the prop hub is a lot smaller than the drive diameter, so I am guessing that I am getting a bit of cavitation and slip. However, the boat is really stable at speed.
    No idea of the pitch. Here is what it looks like. Any advice would be very much appreciated.
    - Don
    Donzi Transom Starboard View.JPGDonzi Transom Stern View.JPG

  4. #19
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    Working from an iPad (smaller image) , it looks like you have an Ultra prop. That was Volvo's high speed prop, and they commonly came in 22, 24 and 26 pitch. Most people run the 24 on the 18s. They aren't a bad prop, but tend to have a higher than average slip.

    It looks like you also have power trim. Nice, and somewhat rare. With power trim, and the Ultra prop you can really get the nose of the boat in the air at full power. It will be more squirrelly. As Matty said the Solas 21 will run the boat flatter, without so much bow lift to get top speed, which gives a much more stable ride.

    On my 16, when it had around 270 hp, I could turn the 24 Ultra about 200 - 250 rpms more than a 21 Solas. It has been my observation that an 18 with the same power and out drive will be a few mph faster than a 16. If that prop is an Ultra 24, and you spin it to 5,000 (at a lower altitude, with medium fuel, by yourself, not towing a skier, blah, blah, blah), you should be at around 59 - 61 mph. At 5,000 rpms my 16 was at 57.4 GPS mph.
    “Oh right, because you walked into strippers discount warehouse and said ‘Help me showcase my intellect.’” - Archer

    Bill
    Grand Rapids, Michigan
    1985 Donzi Criterion SS
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  5. #20
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    Not any ultra that I have seen. The hub is too narrow. Plus looks like the cav plate has been customized never seen one that flat and the exhaust chamber is gone. You'll need to check what the max dia prop you can run before ordering a solas. The water pickup is covered up in the lower leg. Make sure it is smooth and perfect the Volvo hates turbulent flow

    Never mind the first pic just loaded for me someone just bolted an extension on the cav plate

  6. #21
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    73 x18 prop

    I have a 73 x18 with 270 T (working). It has the original Chris Craft Century LT1 motor (rebuilt to stock carb and al)l. I tried N2o's 19 against my 21 (both Solas) the 21 out performed the 19 at every RPM. I hit 4700 at about 55 MPH. Horsepower is unknown. I have not tuned the motor and the carb is a rebuilt Carter. Not sure what a new carb would do but I am leaving it all original. Good Luck

  7. #22
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    Ralph
    yes you would need to spin the 19 to 5-5100 rpm's to get near 57, the difference might be a motor with no low end torque so a 19 might plane quicker but then fall short on cruise and wot to the 21. For the most part with stock engines in a classic the 21 solas or the 24 ultra seems to be the right fit, the big HP guys are at the 23 solas and the 26 ultra.

    A few more thoughts on the solas the pitch is understated add 1-2 inches of pitch to the number due to the cup and the wider diameter.

    donzidon the solas doesn't like to be bunched up Jay Lurie had one on his 200 lower which has the cav plate closer to the prop than a 250 or 270 it had handling issues once he went to a 250 lower this went away. So that extension on the cav plate might cause some adverse handling issue .

    the solas likes clean water so a nose cone might get you a bit of an improvement in top end , the lower leg on the early aq's didn't like a turbulent flow that's why the inner strakes are so far forward on the 16 and 19.
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



    Contact Us

    www.lgdonziclassic.com

  8. #23
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    Awesome info guys. @Mattyboy, l loved the explanation about efficiency. I had no idea and it makes so much sense! It answered a lot of my questions too.
    Ted

  9. #24
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    Okay, I am ready to spring for a 21 inch Solas. I know I need a LH, SS 21 inch. Is there anything else I need to specify? Does anyone know a good place to buy one and what the correct product code number might be? I just want to make sure I order the right one. Thanks again for all the help.
    Ted

  10. #25
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    21 inch Solas arrived today. It's a beaut. Seems big though. Please tell me I ordered the right one. It's LH of course bu the diameter is 15 5/8. Is that right? Fingers crossed...
    Ted

  11. #26
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    That's the baby i run.....

    Picture 002.jpg

  12. #27
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    Nice looking boat. Thanks for the info. I see you used the cone. Did it require any special washers? Not sure if my 270 is different from your 280 in that regard.
    Ted

  13. #28
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    73
    yes that is what I meant by wider diameter the solas titan is nearly at the max of what a volvo aq will spin.
    When the sky is grey,look out to sea.
    When the waves are high and the light is dying,
    well raise a glass and think of me...
    When I'm home again,
    boys, I'll be buying!

    My Ride

    Come Join Us on The Queen Of American Lakes



    Contact Us

    www.lgdonziclassic.com

  14. #29
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    Quote Originally Posted by badbird View Post
    That's the baby i run.....

    Picture 002.jpg
    beautiful boat, badbird..
    Charter Member - WAFNC, SBBR, KWOSG
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  15. #30
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    Quote Originally Posted by badbird View Post
    That's the baby i run.....

    Picture 002.jpg
    Very nice! Are you running a 351?

    I've got the Solas 19 mounted up to my 302. As soon as my new outdrive helmet shows up I'm going to drop this thing in the water and try it out.
    Patrick

    1968 Donzi Ski Sporter C16-512

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