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Thread: 40 OL - Mild Upgrade

  1. #41
    PBN User BajaDan's Avatar
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    No need to jack the boat up KT. Everything is relative to the running surface of the hull. Just measure from that (mount one of your lasers on the hull, measure the offset down from the hull to the beam, then use that as a reference).

    I discovered an error in my calculations regarding water rise abaft the transom. It doesn't make much difference on a standard Bravo with a prop setback of 28 inches, but for your application at 70 inches, it is more significant. I want to review this to make sure I am right, but if I am, water rise and target boat speed become more important.

    The problem I have with the problem in my calc's is that the hole in the water created by the passing of the boat is filling in much too fast. I am ignoring the viscosity of water as it just makes the problem too complicated and that may have a significant effect. The good news is that my calculations are worst case, the water rise will never be more or faster than what I am claiming. The bad news is you need an accurate prediction.

    My current thinking for water rise at your prop is as follows:
    30mph = 3.30 inch rise
    40 mph = 1.98 inch rise
    50 mph = 1.25 inch rise
    60 mph = 1.00 inch rise.

    Since your target speed is 60+, then again we are correct in assuming that variations in water rise are not much of a concern. Your may end up burying your props at slower speeds however. This will be compensated for somewhat by the fact that at slower speeds you will ride more bow high which will require more drive trim and that in turn raises the drive (be thankful that the prop is a long way behind the gimbal pivot). I will run the various scenarios and let you know something by Monday.

    Did you mention that a 2.5 inch rise in drive height was relatively easy to accomplish? Does that require new holes in the transom? That number is probably going to end up being very close.

    Dan
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  2. #42
    PBN User 92nsx's Avatar
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    Dan I think i found a photo of you?



    Ok back to the regular scheduled program
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  3. #43
    PBN User BajaDan's Avatar
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    I like this stuff. I can't help it. Now if I was just better at it.......

  4. #44
    PBN User kidturbo's Avatar
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    Thanks Dan. Stands to reason the faster you go the less water fills in the void at X location after being displaced by the hull. Those estimates look very reasonable. And yes there is enough meat on the boxes to raise them little over 2" if need be. BUT, I got a replacement for my drivetrain angle tool today and verified my fears below.....

    I had planned to hang the drive back on the boat today and gets some very accurate measurements. However none of my helpers showed up, and my buddy who's building and equipment I'm using, was sick on the couch all afternoon. So all I could accomplish was some more light duty work.

    I leveled the bottom up close as I could get it using the tongue jack and tire pressure. Since I'll be working inside mocking up engine mount plates, I'll likely just jack stand it true once I get you these drive measurements. I also picked up a new digital angle meter, and have the bottom within 0.5deg of level over the last 10 feet. Depending where on the running surface you measured, no two spots were the same. Someone check that mold please. LOL.. But I chose an area directly in front of the props to finally go by.

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    After and hour on my back with a straight edge, I realized this boat would gain 5mph with just a good blueprint job. Anyways,,, see the pics below. The transom is defiantly not 12deg. So I found your 2 inches or more. ADFFJASDFJADSGJdshoiwehfdl

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    I hope to get things mounted up again later today just for the exact numbers. But either way these boxes are getting modified, or I need to build some wedge plates to get us anywhere near zero drop again


    -K
    Last edited by kidturbo; 07-22-2016 at 11:02 PM.
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  5. #45
    PBN User BajaDan's Avatar
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    I've got one of those angle gauges. I'll check mine also just for kicks.

    Good work!

  6. #46
    PBN User kidturbo's Avatar
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    Handy little bugger ain't it? I couldn't find my old school swinger I've had since late 80's, so splurged $30 on a new digital upgrade.

    I verified EVERY point we needed last night. While not a physicists, I have concluded that string theory is not accurate science... Between the digital protractor and two laser levels, everything is now clear prospective. The boxes are 12deg for certain, and when hung level off the transom, our prop shaft centerline matches up perfectly with the to bottom.

    The transom was drilled with a true 18.6" X-dim as Merc SSM #6 docs show. Those #6 drives I removed had lower spacers setting the props (about) 1.5" deeper. Weismann set his drives with a 15" X-dim, but the boxes are drilled to put the props at same height they would be if stock #6'. Exactly even with the bottom, but like 20" further back.. But now with the jack shafts in the drives, you end up with a true 21" X-dim on that driveshaft coupler. Still with me?? I hope cause he lost me there... lol. All you need to know, that 3deg mismatch between our boxes and transom is causing our props to run 1.75" deeper than they would if boxes were level, or about the same as the old #6 setup but further back now..

    And that pivot pin you wanted to know about is exactly 36" off the transom when measured at propshaft centerline. 31 & 1/4" off the transom along the actual input shaft centerline, which is 15" above prop line. Our exposed prop shaft starts at 59 & 1/4", and ends at 66" off the transom. Easy enough? Ok cause now I have to cut at least 1.5" off the boxes top face to straighten everything up..

    I got a bunch of pictures but am beat since we worked on this till 11PM and drank beer till 1:30AM. So look for them around noon...
    Last edited by kidturbo; 07-24-2016 at 12:13 AM.

  7. #47
    PBN User kidturbo's Avatar
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    Here ya go Dan.

    Box hanging dead level. Angle diff leaves about a 1.25" gap bottom, that's minimum I'll need to take off the top to get us back on track. Probably round that up to 2" total to after modifications.

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    With boat level, box level, drive level, prop shaft centerline is about about 1/4" below the bottom.

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    Shooting that pivot pin position. 15" X-dim with 36" off bottom or 31.25" down the centerline of drive.


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    Here's one showing drive is mounted up with boxes as is...

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    Last edited by kidturbo; 07-24-2016 at 11:00 AM.

  8. #48
    PBN User kidturbo's Avatar
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    A less bury shot of that last one. Believe there's the 2" you were missing?

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    And for a bonus "How To" on removing a huge ass hatch cover that's mounted seven feet in the air with only two people....

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    Last edited by kidturbo; 07-24-2016 at 11:16 AM.
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  9. #49
    PBN User BajaDan's Avatar
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    Ok. I thought you were going to move the box up 2 inches, but you are talking about taking 2 inches off the surface, changing the angle of the box instead. I am glad we got that sorted out. That will raise the drive significantly. just scaling off your first picture, if the box is off the transom by 1.25 inches, that makes it about 25 inches tall. You can check that number for me to keep me from going too far afield. Just leveling the box (closing that 1.25 inch gap) would be about a 3 degree change (2.8 actually). If you take 2 inches off you get about a 4.5 degree change.

    I am a little confused about your pivot pin location "Shooting that pivot pin position. 15" X-dim with 36" off bottom or 31.25" down the centerline of drive."

    You want to run that by me one more time?

  10. #50
    PBN User BajaDan's Avatar
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    Measuring my transom angle I am just little over 10 degrees. That is a big jump from the 16 on your 40.

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  11. #51
    PBN User BajaDan's Avatar
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    Running through a bunch of pictures, I settled on this one of KeFurr on a smooth day. I wanted one almost exactly in profile and with a sharp horizon behind the boat. Projecting a line off the chime and moving that to the horizon gives a running trim of 4 degrees, about where we thought it would be.

    Thanks for furthering the cause KeFurr!

  12. #52
    PBN User BajaDan's Avatar
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    Well, why didn't it upload the pic?

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  13. #53
    PBN User BajaDan's Avatar
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    Quote Originally Posted by kidturbo View Post
    A less bury shot of that last one. Believe there's the 2" you were missing?

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    Ok. Now we are getting somewhere. I believe I have my model matching your measurements. I am showing a prop shaft depth of 2.26 inches on a 15 inch X dimension with a 16 degree transom and a 13 degree box face angle. Boat and drive at 0 degrees trim.

    Now if I change my model to reflect a 4 degree hull trim and drive trim (drive is parallel to the direction of travel of the boat, not to the boat itself), things get ugly. The prop shaft is now 4.8 inches below the transom. This is primarly due to the long distance the pivot is from the transom (32 inches). If we factor in 3.3 inches of water fill at this prop location, we are looking at the prop center being 8.1 inches under water, or nearly the entire prop based on a 17.5 inch diameter.

    I'm having trouble posting pictures. I will put them in the next post. First picture is what you have now.

    The second picture is if you modify the extension box by taking your 2 inches off on edge. This changes the angle of the box 4.6 degrees. Prop shaft depth at running trim improves to 2.24 inches and prop depth is now 5.5 inches running.

    Still too deep.

  14. #54
    PBN User BajaDan's Avatar
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    Current Setup.

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    Box machined on 4.6 degree angle

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    I apologize for the messy sketches, but I hope you get the idea.

    You need to raise the box or run the boat more level (high drag on a straight hull though).
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  15. #55
    PBN User BajaDan's Avatar
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    And by the way. If you could magically get rid of the 14 inch long extension box you are still left with a pivot 18 inches back from the transom. Dumping the extension only nets about 1/2 inch of prop shaft height.

    Big lesson here, extension boxes or anything that moves the pivot away from the transom are not helpful on boats that do not run flat.

  16. #56
    PBN User emilsr's Avatar
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    Quote Originally Posted by BajaDan View Post

    Big lesson here, extension boxes or anything that moves the pivot away from the transom are not helpful on boats that do not run flat.
    I'm not following your train of thought here....

  17. #57
    PBN User kidturbo's Avatar
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    Thanks Dan that's a great help.

    I'm gonna tear the boxes apart tonight and look at the options then. Without the boxes it would require re-drilling the the transom for certain. At minimum using an adapter plate, which could be an option here that allows me to adjust height also. Or I could put some ^up angle on them, which likely skews our data at slower speeds then correct?

    I know that Adam ran these on a 50 Nor-Tech Vee and they were sub surface on his setup. I believe he told me they were on a big cat before he purchased them. If so, there's you matching run flat design they were likely built for...

    Decisions, Decisions

  18. #58
    PBN User kidturbo's Avatar
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    And on your question about the pin location. With the drive box top parallel to the bottom of boat I measured it at 31.25 where you show as 32". To mount the box level, I would need to cut off at least 1.5" in length, so lets call it 2" to be safe. Moving that point down to 30" off the transom.

    Two other options that come in since we are in mod mode, I could raise the boxes 2" by changing bolt pattern, or put like an 18deg angle on the box face, so against the 15deg transom they would have 3deg up angle now...

  19. #59
    PBN User BajaDan's Avatar
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    Quote Originally Posted by emilsr View Post
    I'm not following your train of thought here....
    The extension box is a long arm attached to a fulcrum. Like a see saw, for a given angular displacement the vertical travel is very large. As the hull needs more trim (hull, not drive), that extension box pushes the drive deeper. If the pivot is right at the transom (such as an Arneson, which is not only at the transom but near the waterline) then the arm is shorter and the effect is less.

    KidTurbo, I am still not satisfied with my water level calculations. Working on it.

  20. #60
    PBN User emilsr's Avatar
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    Quote Originally Posted by BajaDan View Post
    The extension box is a long arm attached to a fulcrum. Like a see saw, for a given angular displacement the vertical travel is very large. As the hull needs more trim (hull, not drive), that extension box pushes the drive deeper. If the pivot is right at the transom (such as an Arneson, which is not only at the transom but near the waterline) then the arm is shorter and the effect is less.

    KidTurbo, I am still not satisfied with my water level calculations. Working on it.
    So less trim authority the further out the pivot point is?

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