Shadowcast 18 Prop Thread

Discussion in 'Prop Shop' started by TwoKids, Jun 12, 2013.

  1. TwoKids

    TwoKids Well-Known Member

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    Motor: Tohatsu 30 4-stroke
    Load: 330 lb of people + 5 gal fuel + gear (no TM) + size 24 rear mounted battery

    Stock aluminum prop 9.9 x 13
    * Top Speed 26-27
    * Excessive blowout/cavitation on smallest of turn, trim, reverse, wholeshot. Almost unusable.
    * Frequent slips, cruising straight, on plane at any speed
    * Weak holeshot

    Powertech SS 3 blade SRA3R11P
    * Top Speed 27-28
    * Good bite forward and reverse
    * Cavitation greatly improved, blowout issues 95% resolved
    * Holeshot greatly improved
    * Trimability now exists
    * Compared to stock prop boat has the feel/response like I gained 10-15HP

    Wish I had a tach to provide RPM data. The performance difference of a prop with some rake and cup is night and day.
     
  2. MUD_MINNOW

    MUD_MINNOW I Love bumping bottom early in the morning

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    Did you get a four blade on it yet? Waiting on some great numbers and how it handles!!!
     

  3. TwoKids

    TwoKids Well-Known Member

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    Thanks to a fellow forum member I had the oppurtunity to test a 4 blade 10 pitch powertech SRA4R10PTN30.

    Load: 320 lb of people/tools + 5 gal fuel + bow mount TM + size 24 battery under front deck. No trim tabs.

    Unfortunately I dont have a tach to provide RPM deltas but this should be some useful data for SC18 owners. Ran multiple runs using the 4 blade 10p then switched back to my original 3 blade 11p (SRA3R11P) making multiple runs under the same conditions. Calculating the average speed with & against the current the 4 blade 10p had an average speed of 28. Contrary to popular belief, the 4 blade 10p was 1 mph faster than the 3 blade 11p. As expected the 4 blade had slightly better hole shot. The 4 blade allowed the boat to plane at a slower speed. The 4 blade produced a noticable increase of stern lift (reducing tunnel squat) while on plane which is desirable on the SC18 hull (I believe this was the key to the speed increase). Most importantly the 4 blade provided the grip to allow the boat to turn without sliding. With the 3 blade if you dont lean hard into the turns, trim it down, and slow down, the skiff's backend slides in the turn, same type effect as an airboat turning. The 4 blade eliminated the slide to produce more of a carving type turn. In summary, the 4 blade 10p slightly increased speed, slightly improved holeshot, and significantly improved turn performance. All performance gains with no negative impacts. After this test I firmly believe that the 4 blade is the optimum prop for this skiff.

    I am very interested in testing a SRA4 in an 11 pitch next...
     
  4. el9surf

    el9surf Well-Known Member

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    That was my prop used for the test, it's for sale for anyone interested. Just figured I would throw that out there. It's in perfect condition and since I sold my SC I have no use for it.
    I believe power tech can adjust the pitch up or down to some extent.
     
  5. TwoKids

    TwoKids Well-Known Member

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    Tested a 4 blade 12 pitch powertech SRA4 today.

    Sluggish out of the hole. Better than stock aluminum but noticed reduced trimmability and frequent blowout when compared to 3 blade 11p SRA3. Motor sounded as though it was working hard.  Once it got up to speed the top speed was 28 with 250 lbs people/gear+ 8 gal fuel + TM.  More speed sensitive when increased weight, speed dropped to 25 with 450 lb of people/gear. Bottom line is 12 pitch 4 blade Powertech SRA4 was too much prop for the Shadowcast 18 with a Tohatsu 30 hp 4 stroke.

    Out of all the props I've run the 4 blade 10 pitch was the best. Still need to test the 4 blade 11 pitch.
     
  6. coconutgroves

    coconutgroves Well-Known Member

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    Without a tach, I don't know how much difference you'll find in the 11p versus the 10p. A tach is really crucial to find if you are operating at the desired RPM range of the engine.
     
  7. TwoKids

    TwoKids Well-Known Member

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    Great timing, I agree. I ordered a tinytach and it was delivered today. Looking forward to retest and capturing the data.