Prop selection

Discussion in 'Prop Shop' started by bullsix, May 5, 2009.

  1. bullsix

    bullsix I Love microskiff.com!

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    I recently added a CMC jack plate to my 18 Sea Ark MV jon boat. I have a 60 hp 4 stroke yamaha. I now know I am not propped properly for using the jack plate. When I drop it dowen to "normal" position, all is ok, but when I jack it up I get almost instant blow out. I can tease it up to almost the max with trim and tilt, but blow out is pretty much a given. I actually gained 2.5 mph on top speed with a lot less trim tab. I know every prop mfg wants to look at rpm, but with the jack plate I know I am getting every RPM I have out of it. BTW, my top speed with only me on board with 14 gals of fuel is GPS 38.2. I really want to pick up even more top end speed, but want to also have some better bight with the jack plate raised.

    I do not really know of any body local to me that is willing to help me dial in my prop, so it loks like I have to try to get the closest I can with the info I have.

    BTW, I love this forum....very imformative.
     
  2. Tom_C

    Tom_C Well-Known Member

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    Without a tac how do you know that you are gettingn every RMP out of it? [smiley=1-doh.gif] If you are getting every RPM out of it then you are prop properly. [smiley=1-whoops1.gif]

    If you are running a 3 blade prop, going to a 4 blade with cupping may help hole water on your prop, but you will lose some speed, or adding a Bob Stabilizer Plate, or other fin to your cav plate may also help. [smiley=1-hypno.gif]
     

  3. bullsix

    bullsix I Love microskiff.com!

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    By all the RPM's , I meant to say 5000-6000 from the tach depending on load, trim, water conditions, etc. I am propped correctly for normal operation when the cavitation plate is at its proper position, it is when I raise the engine that I am getting blow out, which leads me to believe I would need a prop with more cup? I know all the calculations, and prop determination is not new to me, I just have never propped out a smaller, lighter boat like this and am searcning for a starting point as to what everybody else has experienced with the micro skiffs. I guess mainly since I have lost touch will the guys that used to loan me props for my boats in the past, I would have to ask around.
     
  4. iMacattack

    iMacattack busy, too busy

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    As a general rule, more jack = more cup. You need the prop to bite the water and hold it.

    What size, style, pitch prop are you running now? How far above the bottom of the hull is the cav plate on the motor in the full down position?

    The RPM is very important. To say it runs between 5000-6000 unfortunately won't help. What are you spinning under normal loaded conditions? This should be narrowed down to the closest 100 RPM WOT.

    Also one thing to keep in mind, you might not be able to go max out on the Jackplate. More data will help folks here help refine your choices for a prop.

    Welcome to the site!

    Cheers
    Capt. Jan
     
  5. bullsix

    bullsix I Love microskiff.com!

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    The prop I am using ow is a 10 1/4 x 14 coated stainless. At full down on the jack plate, the cavitaiton plate sits at about 1/8" above the bottom of the hull. Yes, you're correct, I might have to get used to the fact I may not be able to use the plate at full height. But I think just a slight bit more cup would get it. As for RPM's, I probably need to chart where they are at different times....to the 100 rpm or so, to get it closer to what is happening.
     
  6. iMacattack

    iMacattack busy, too busy

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    1/8" above the hull bottom... hum that actually sounds a bit low. I'm not jackplate expert... but I did stay at a Holiday Inn once... ;D

    What is the setback of the jackplate?

    Getting the RPM will REALLY help folks give you better data for the prop. Just think of it as an excuse to get out on the water more for field research. ;)

    Oh, and listen to Tom C... he might know a thing or two. :cool:

    Cheers
     
  7. bullsix

    bullsix I Love microskiff.com!

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    Yes, I have a few more slots to play with on the lowest setting on the jack plate. Good advise, I need an excuse to leave the family on land and see what rpms I get playing by myself. They never want to go in the big boat any more. This boat was supposed to be all mine, but we leave the gas hog behind on calm days and use "my" boat mostly every time now! I'll get it in the water this weekend and get some better numbers for the folks here.....

    I just took a look at Toms website and....Tom you make some nice looking products.

    BTW, the setback on my plate is 5.5"
     
  8. bullsix

    bullsix I Love microskiff.com!

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    Well, after monkeying around a few days with my prop delima on my small boat, I called a few old friends, who pointed me in the right direction and I think we have it mastered. I didn't need a prop change, just a little reworking. I was runing 38.5 on GPS, now I have increased to right at 41. I minimize trib tab and increase jack once on plane....it took a little practce to get them right, but I am pretty pleased with the results. I can still get the prop to blow out in some circumstances, but it is much less sensitive thant it was.

    Thanks to all who helped out... ;D