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Boat trim - am I missing something?

Discussion in 'Post Yacht' started by Greg Page, Sep 8, 2021.

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  1. Prospective

    Prospective Senior Member

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    I have a 43' Ocean and they are somewhat notorious for poor following sea performance. A combination of narrow transom relative to mid-ship beam, small keel and flat aft section built for speed are all reasons I've heard. Interestingly, I find the handling in following/quartering seas much worse with small wakes in otherwise flat water. If I get caught with one of those while the bow is trimmed down, the boat can do some pretty surprising rolling and yawing. Her nose will get pushed down by the swell and want to bow steer. In bigger seas the motion is not great but not nearly as crazy. And I don't get caught by surprise by prevailing sea conditions so I know not to be trimmed down.

    On my boat I've learned that in following seas, I remove all trim tab and get the ass of the boat to sit down. I play with the speed to match the swell as best as possible. And I generally have much better handling with the AP ON. Unless I'm in a narrow channel or for a brief period of time, I'm better off with the AP steering.
    cleanslate likes this.
  2. Greg Page

    Greg Page Active Member

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    I have never tried the AP in those conditions so can't say how well it does or does not work, but I can read the waves and react before they get to the boat which the AP can't. I probably will try the AP in more moderate conditions and see how well it does. It does well in normal cruising conditions once we turned down the agressivness of turns a little by setting the pre-turn distance farther in advance.

    On my first run in the boat, offshore (very different wave type) and with an experienced professional SF captain, in a more perpendicular following sea, he had me trim the bow down a bit more to get some directional "bite" (my term not his) which improved stability. That is why I tried both bow down and bow up to see their effect or lack there of with the shallow angle following sea. In my CC I would trim up and throttle up, the 43 doesn't have the same trim (tiny trim tabs not 36" long K-planes) or throttle response ;)

    In the end changing the angle to a more perpendicular following wave worked better than any change in trim or speed, so that is what I used when course and topography allowed.
  3. Capt J

    Capt J Senior Member

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    no, I mean it’s totally normal for a 42-46’ post to do this. They, oceans, and some Carolina sf are not good sea boats with the waves a aft the beam.
    cleanslate likes this.
  4. Slimshady

    Slimshady Senior Member

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    There is a fine line between sharp entry bow with good down sea handling and outstanding head sea performance and one that doesn't. Change to much and ride suffers in one sea state or the other. The newest sf have really dialed it in. The original single engine Carolina boats that needed sharp entry to fish typical outer banks conditions had small keels running considerable length to reduce broach.
  5. motoryachtlover

    motoryachtlover Senior Member

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    Could larger/longer rudders help with the OP’s situation?
  6. olderboater

    olderboater Senior Member

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    Yes, but....and that's a big but. They'd also have negatives and change the handling and performance in other ways. His situation doesn't call for such drastic action. Just learning, adjusting and accepting.
    cleanslate likes this.
  7. boatpoor

    boatpoor Member

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    Aft cg will greatly improve downsea handling in a Post. Unless we have to top off for a long trip we typically keep 100 gallons more fuel in the aft tank than in the forward tank. I believe the 43 has the fore and aft centerline tanks like the later 46, you may want to give it a try.
  8. Greg Page

    Greg Page Active Member

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    I normally top the aft tank along with the fwd (the engine computers keep good track of exactly how much fuel they will take, I have to guess a bit for the GenSet run hours from the aft tank), so it normally has an "extra" 150 gallons in the aft tank giving more aft weight.

    I possibly didn't run the tabs full up when experimenting with more bow up/down trim, will make sure to run them long enough to be at the retracted stop next time. I miss the tab position indicators from my performance boats.

    I think the key thing (which I did include) is I was trying to run a very shallow angle to the waves because that was where I wanted to go. Running a larger angle to the waves improved the handling. Ultimatly I adjusted the angle to the waves to where I could run normal cruise, and accepted a sailboat like tacking course across the bay and back to end up where I wanted to go. A longer distance but less time than slowing enough to run a direct course.

    Thanks - Greg
    cleanslate likes this.
  9. cleanslate

    cleanslate Senior Member

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    Another thing that helps with our sized boats is that magic 15' water depth. Anything less my boat starts to pull a long big wave off the aft quarter. The shallower it gets the more the wave lengthens and the bow tends to start rising up. I draw 3.5 feet. I would say you are close to the same.
    Anything under 12 feet I pull her back down off plane etc. for me 1400/1600 rpms = 8/12 knots depending on the current. It's not deep enough IMO to hammer along and create all that drag .
    Others here can explain the water forces here better than I.
    I guess I'm talking '' displacement'' ?

    Anyway take notice when you go over and under that 15' mark to see how your boat reacts.
  10. cleanslate

    cleanslate Senior Member

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    I know that's right!

    BUT, I'm boating, with out payments......:)
    chesapeake46 likes this.
  11. Tight Lines

    Tight Lines Member

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    @Greg Page - if I recall your vessel was repowered by the prior owner. Is there a significant difference in weight between the outgoing DD and the new motors? How about placement of that rotational mass fore and aft? Where are your batteries what sizes and how many?
  12. Greg Page

    Greg Page Active Member

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    15 ft is deeper than I usually consider limiting. I have my MFD color set to start at 12 ft MLL, so anytime it is "white" is safe to run (but do keep an eye out for uncharted changes and unusally low tides that would impact my guideline). I wait till at least 9 ft before trying to get the boat up on plane. I'll try seeing how the 12-15 ft range affects running and wake.

    Book draft for the 43 is 3' 6", but with the lighter CATS I do notice my boat sets maybe 2" higher (using the exhaust exit height above the static waterline).

    Depth wasn't an issue the day in question, it was usually at least 19-20 and often 30+
  13. Greg Page

    Greg Page Active Member

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    Engines are about 1000# lighter each (DD 6-71 2850# book, CAT C7.1 1676# book). The new engines are also about a foot shorter in length, they are mounted at the same aft transmission flange location to mate with the shafts.

    Batteries are 6x 12V, (a 24v bank for each engine, 2x 12v in parallel for the 12v house loads) and are all located at the forward end of the engine room.
  14. Tight Lines

    Tight Lines Member

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    Has anyone noticed a performance difference in a following sea running w/o curtains on the bridge? A big spinnaker like that would certainly push a garvey around on a windy day.
  15. Tight Lines

    Tight Lines Member

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    Are you sure she drinks diesel and not Mic Ultra or Truly?

    Do you keep your freshwater tanks full? Anything below the cockpit between the fishboxes?
  16. Greg Page

    Greg Page Active Member

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    I am continually impressed that the post is 8 feet longer and nearly 20,000 lbs heaver than our prior Viking. It runs 30% faster on the same gallons per hour, and diesel costs about 80% what gasoline does here.

    Fresh water tanks are full heading out, but not neccessarily on the way back. Segment last week was about mid-way, and about 3/4 fresh water load. The only heavy thing in the fish boxes is the 50A shore power cord, which weighs a ton ;)

    -g
  17. Greg Page

    Greg Page Active Member

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    Wind was about 16 knots off the port stern beam. The 3 side enclosure was up, with the front cracked a bit to get good ventilation. I don't usually see much difference with the enclosure, but the house side area is significant in cross winds. Especially coming in to the fuel dock.

    -g
  18. Tight Lines

    Tight Lines Member

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    This issue fascinates me b/c I am always running in the slop that @Prospective describes above as the jet stream crosses Lake Michigan. Consistent 2-5’ chop with a high wave frequency - steep on both sides.

    We took the boat out Sunday afternoon in 4 footers for an hour or so before going to the gas dock for a fill up and pump out.

    Less than 1/4 in both diesel tanks, freshwater tanks were also nearly empty and the s**tter was full Cousin Eddie style. I have a pair of 8Ds between the motors that I wish I could relocate to the bilge as close to the rudders as possible.

    The uncontrollable bow steer and sudden keel catch in following seas seemed to be even more pronounced and less predictable on Sunday and I am of the opinion that this is more a matter of weight distribution and balance before speed and trim tabs.

    We are running to Chicago this weekend (sans children) 108 miles straight shot each way. We’re expecting 2-4 head seas outbound and 2-5 following on the return. We’ll be fully loaded with 550 gallons of Diesel and full freshwater. If the NOAA and Windy forecasts are accurate, I should have a good block of time to test these variables at cruising speeds. I’ll let you know how it goes.
  19. Tight Lines

    Tight Lines Member

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    The jury is still out - NOAA forecast changed Friday morning. Consistent 20 kts SW wind and we crossed in unforgiving 6’ head seas with a high frequency. Changed direction a few times get a better ride and ultimately had to out run a thunderstorm that welcomed us to MKE for the final leg south. The return trip was almost perfectly flat. 20-21 kts cruise ~120 miles. I am asking Santa for an autopilot and a new MFD for Xmas. Hopefully we’ll have a chance to experiment with sketchy following seas on the next trip.

    That said, I am curious about how the trim tabs can positively (or negatively) impact efficiency at cruising speeds. How far can one go on 500 gallons with 671s?

    @Greg Page - with the repower, what stats are you pulling when the conditions are right? Does the bow naturally settle when the boat is fully on plane? Will she skim on glass like sn overpowered bay boat?
  20. Greg Page

    Greg Page Active Member

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    The CATs normally cruise 23 kts at 2400 RPM (2900 is max, up from 6-71's) and 36-37 gph at 70% load. On glass smooth it is slower (same as my offshore performance boats), need a slight chop to get some air under the hull. Without indicators I can't tell tab position but they usually are not fully retracted when I have had the boat out of the water. In good conditions I trim the tabs to whatever gives best speed at my cruise RPM (throttles electrically sync the two motors and hold RPM with throttle position pretty well).

    The current 3B props (24x26.5) I believe are too large diameter as they run up to 100% load at 29 kts, but the 4B (22x28) we also ran on (before purchase) ran the same WOT speed but at only 90% load and 1 kt faster at cruise at 2400 and 68% load. They also may not be quite balanced (although just came from prop shop when we got the boat) as the port side runs 3% more load at the same RPM across the board. Some of that is probably due to efficiency differances from one trans running "fwd" and the other running "reverse".

    -greg