Hull extension..now outboard ventilation issues.

Discussion in 'Boat Design' started by Brother Bill, Jun 27, 2020.

  1. Brother Bill
    Joined: Jun 2020
    Posts: 5
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    Location: Alaska

    Brother Bill Junior Member

    Hello all,
    I'll continue to search through the forum for more info, as I've just stumbled across this site, but I thought I'd let this soak while I looked.

    I have a 28' x 9'6" custom aluminum boat in Alaska. I've probably got around 2000 hrs of seat time in this boat fishing and exploring all over the SE Alaska panhandle.

    We had a scupper issue with the self bailing deck that led to some water ingress issues that were not favorable (mostly an original fabrication flaw that did not fully manifest itself for several years).

    I worked with a local fab shop that does a lot of boat work and decided to change the scupper design. The owner does a lot of hull extensions and we discussed the extension to increase buoyancy to help keep scuppers further above water line; which we ended up doing.

    My first trip out on the boat I knew I was going to have problems. There was way too much bow trim and even in mostly flat seas, I couldn't keep the water from the windshield. She floundered hard in any semblance of a following sea. I could not trim the motors up more than an inch or so without experiencing serious ventilation on both outboards and a very wet (can't see through the windshield) windshield. It was like I had the outboards and the trim tabs both in the full down position. (Full fuel, water, ice for the weekend, etc.)

    Here you can see the original design and the hole where I had pulled the stbd scupper off.

    [​IMG]

    And mid fab where you can see what was added:

    [​IMG]

    Final product (before the jack plates were added to move weight further aft)

    [​IMG]

    [​IMG]

    We decided we needed to shift the CG aft a bit more so installed 12" jack plates, which also lowered the outboards 1".

    [​IMG]

    The next trip out (again fully wet), I thought we had fixed it. I could trim the outboards and could find the "sweet spot" while on plane (~25kt cruise at 1.8 mpg). We were flat seas though.

    As the day wore on and the wind kicked up, I noticed I still couldn't trim the outboards up enough to keep the windshield dry and the outboards would want to ventilate periodically, that was into a 2-3' chop head on. Heading back to port for the afternoon, in a following sea, the ventilation issues got worse. I had to back off to a 15kt cruise in order to overcome the ventilation issues and (prior to extension she could have easily handled those seas at 24kts). The matter simply got worse as the weekend wore on and I burned up more fuel (140g fuel tank aft center line).

    The outboards appear to be mounted "correctly" in that the anti vent plates are aligned with the bottom of the hull.
    I had him cut the new strakes off the extension for fear they were somehow causing turbulence n front of the props.
    The boat sits proper while on the hook or at the dock, with a slight aft rake enough that the decks drain as they should, even when sitting light.

    I'm frustrated and ready to have him cut it all back off, but hoping someone here may have some thoughts on what I'm experiencing and ideas on how to address.

    The options as I see it:
    Cut all or most off and start over.
    Lower the outboards more (currently as low as they can be on the jack plates).
    4 blade prop
    Add ballast (not my preferred option)

    Anyone have any ideas, thoughts, comments?!
     
  2. Barry
    Joined: Mar 2002
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    Barry Senior Member

    The pictures that would be important to provide would be as follows

    Attach 2 straight edges to the bottom of the boat, the straight edges parallel to the keel, the straight edges within 1/2 inch of the ventilation plate ( one on either side of the ventilation plate) and with the engine trimmed so the Ventilation plate is parallel to the straight edges.
    Take pictures from the back showing each straight edge and it relation ship to the ventilation plate
    Take pictures form the side showing the relationship from each of straight edges to the ventilation plate

    The center picture shows that the ventilation plate is extremely trimmed up, I realize that it is perhaps the way that you trailer it,

    As a first round guess, the engines are mounted too high. The extension as it is on plane with the bottom of the hull, makes the extension merely a longer hull, (ie not a pod issue with the props being a long way from the bottom of the hull.)
     
  3. Brother Bill
    Joined: Jun 2020
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    Location: Alaska

    Brother Bill Junior Member

    That video was very helpful, thank you for that. I'll be home in a few days and check this again. The guys who did the work indicated they were at the proper height, and just eyeballing it, they looked correct; but I will use this to verify.
     
  4. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Note that they had the ventilation plate above the bottom of the hull. The merc installation manual shows that the ventilation plate should be 1/2 to 2 inches below the bottom of the hull on the inside of the ventilation plate.
    If you are ventilating in a straight running situation, then the ventilation plate is too high. On a corner as the boat tilts, you further run the risk of the ventilation plate grabbing air
     
  5. Brother Bill
    Joined: Jun 2020
    Posts: 5
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    Location: Alaska

    Brother Bill Junior Member

    Neither of us had (have) immediate access to that install manual. I'll see if I can find one and show the fab guy. I bet you are 100% spot on with that.
     
  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I just pulled the manual off the internet,
     
  7. Brother Bill
    Joined: Jun 2020
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    Location: Alaska

    Brother Bill Junior Member

    Your "google fu" is stronger than mine! If you have a link to share, I would be grateful.
     
  8. Brother Bill
    Joined: Jun 2020
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    Location: Alaska

    Brother Bill Junior Member

    That seems a bit contradictory to the video, if I am understanding the video correctly.
     
  9. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    Barry gives the recommended height of the ventilation plate which cannot be exact for a deep V hull. His number, if I interpret correctly, is for the inside edge of the ventilation plate, with the greater distance for a deeper V and less for a shallower V. Although that seems low to me, I'd think Mercury would know. Adding brackets have messed up running attitude of a multitude of boats, assuming the designer was correct in drawing for the middle of the better running range. A hull extension is another matter entirely as it reduces the trim angle lower than the designer probably set it.

    Lots of factors influence the trim angle and an operator only has control of some of the inputs that determine trim.
     
  10. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    My first reaction is to say that the motors are too high, but also due to significant changes in actual hull configuration, you may need to re-examine propeller specs.
     
  11. Barry
    Joined: Mar 2002
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    Barry Senior Member

    It was because of the distance from the trailing edge of the transom to the ventilation plate. If the distance is large, then the water is beginning to come up towards the normal level of the water. And hence I believe in the video, the
    narrator said that for every foot of distance from the transom to the ventilation plate, you should be able to raise the engine an inch.\

    But the manual shows views that have it below the transom, as your transom is close to the ventilation plate, ie it is not a pod
     
  12. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Many jack plates have a lower position and an upper position. I see the plate all the way down, but check to see if you have an optional holing for the plate that allows you to use the old holes and still get some vertical adjustment. That plate is being underused as an extension only and sort of misused bottomed out.

    I am curious why they mounted you on a plate and then stuck you at the bottom. That plate should give you lotsa options for vertical adjusting unless you bought the plate for an extra long shaft motor in which case you'd be at the top and have plenty of room to head down...

    It looks to me like you bought the wrong jackplates, but like I said, they may have a second position.

    I have 0-6" Vance plates on my rig as I switched to extra long shafts, but I am sorry to say I am not a jackplate expert except to say those Vance plates are wrong as done for your 20" shaft situation. Barry is spot on.

    The good news is a plate issue is not too bad to correct. The optional holing is usually 2". I can't remember up or down at 4am here, but Vance may also allow new holes machined of needed.
     

  13. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Well obviously if you add a hull extension, and it isn't that heavy, then the bow likely will trim down compared to what it was previously, and especially if it ran flat beforehand. He says he can't trim it out enough to raise the bow, sounds like a trim gauge reading is needed, to make any meaningful assessment, 'blow out' will occur before you get to the limit of the trim range, but an actual figure might help here.
     
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