Symmetrical Power Cat Trim at Rest?

Discussion in 'Boat Design' started by Bullshipper, Apr 18, 2018.

  1. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    I measured the static waterlines forward and aft on a 26' Power cat I recently purchased and the differences at rest surprised me.

    The rear of the hull drafts 13.5" (as advertised) but the sponson bows are are drafting 23.5". This is a symmetrical center console planning cat.

    Shouldn't the hull have slightly negative trim at rest and under power if balanced correctly?
     
  2. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    wow 10" difference in the wrong direction does sound odd.
     
  3. fallguy
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    fallguy Senior Member

    This isn’t that aluminum cat that hogs in down in Australia is it? Seem to recall that was about a 25’er or so.

    In order for trim to be that far off; you would almost certainly have either a weight for’d issue or water for’d issue/ingress. Water is especially trouble as once you power up; it moves; if it can.

    Maybe you have a bow full of gold bars?

    Did you walk to the front to measure? That would not be accurate if you did. The bow cannot take your weight the same as the stern.

    Got a picture or brand?

    We had a guy posting about a bad hull design here awhile back. Aussie if I recall.
     
  4. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    This is a fiberglass production cat produced in the USA with a lot of overhang on the bow to make it look fast.
    The gas tanks look to be mounted about mid ships and the 4 heavy batteries are near the transom near the 580 lb (each) 175 hp outboards but the console and front deck are oversized and laid up too thick imo.
    The sponsons are 35 3/4" wide at their water line insead of the 32" I would like to see so I think their added bouncy is also lifting the rear and depressing the bow.

    But again, is about 2% lift on the bow + their rocker about what I should shoot for if I want her to run more efficiently?

    The hull came with trim tabs, which I took off, prior to adding Permatrims to the twins to see if I could trim her up right after I got her, but their props will blow out before any real lift occurs, and these fins are large and stiff.

    3 blade props, so I doubt if prop lift is a contributing factor, especially at rest.

    I am measuring the water lines on the hull out of water on her trailer.
     
  5. fallguy
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    fallguy Senior Member

    You’d need to upload a picture to better explain how you are arriving a the 10” delta. Something seems off and I gotta be honest; I doubt it.

    Are you at max horsepower? If your boat is rated for heavier engines; you really need to adhere as engine weights change rapidly on the biggies. Not to suggest 580 isn’t heavy!

    The layup of the console and foredeck are not adding enough weight to change the trim 10”.

    Also, if you have aft well; make sure you load it.

    Prop blow out? Care to expound? You mean they are cavitating at speed or are you truly blowing out? Some hulls are worse for blowout, but you need to know if you are getting green water or truly blowing out. If you aren’t getting trimmed properly; you might not have water/clean water. Did you blow out with trim tabs?

    Throw those foils away. That boat is meant for a finicky trimout. You can’t do that without tabs. If you had the problem with tabs; then I’d put all the weight in the back of the boat; anchors; and maybe a couple extra hundred pounds and see if you still blow out.

    I noticed you are avoiding naming the boat maker. ?

    Also; are you certain the boat isn’t saturated? A boat full of water behaves oddly, too.
     
  6. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    Props blowing out does sound like your digging the bows in. Stupid question but have you trimmed the motors up? See if you can trim them high enough to start porpoising and then bump the trim down, that will be your most efficient planing angle and you will feel it and rpms will go up.
     
  7. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    If you have collision bulkheads on the hulls, check to see if they're full of water. Drill a small hole and plug it up after.
     
  8. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    I have inspected the hull underdecks and forward and there is no water in the hull on the trailer, nor colission bulkheads. However the hull was in a slip for 6 months and I cannot say for sure if that was the case during all that time period as there are forward and aft bilge pumps. The time in slip produced the scum water line that I am measuring.

    So I guess I will have to launch and check that again.

    But can anyone answer my question? Should the cat sit level in the water AT REST, referencing its keel profile and the water line? I know what to do under power guys.

    Thanks
     
  9. fallguy
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    fallguy Senior Member

    The design waterline (DWL) is typically almost always the same as the sole. If you get in the boat at rest in calm water and get close to the console; the soles would have been designed at DWL.

    Take an accurate! 4’ level and find a section of sole when you are on the boat at the console. Use some pennies to make the level true. Place the pennies on one end; then extrapolate that error. It can’t be 10” in 26’. That would be a 1.5” error on your level test.

    Kind regards.
     
  10. fallguy
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    fallguy Senior Member

    After that test; take all removable objects from the bow and place them at the stern. See if it changes anything.

    Then do it again with a full 5 gallon bucket of water aft. You’ll get some ideas about how susceptible the hull is to weights.

    Honestly; you haven’t even said what the core is or if...

    A saturated core would drip out of any garboard drains, but only if so equipped.

    Saturated core won’t dry with a bilge pump.

    Most likely you have a wet core; but it is a big guess here.
     
  11. JSL
    Joined: Nov 2012
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    JSL Senior Member

    I assume your draft was measured at hull centerline. "If" the hull form is a modified V (aka. warped bottom), measure the drafts at the chine from the transom - for'd about 5'. and see if there is any difference. Quite common for a warped bottom V to float 'level' (designed trim) and yet have a deeper draft at the forefoot.
    As others have mentioned, some photos would really help to reduce the amount of assumptions and mind reading.
     
  12. Bullshipper
    Joined: May 2008
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    Bullshipper Bullshipper

    I will be launching the hull next week to see if this condition exists again. Not much point in trying to help me until we test SamSams theory on water in the hulls.
    Thanks.
     

  13. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member


    One way to test if core is saturated would be to weigh boat and trailer at a truckers scale.
     
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