Hull Extension

Discussion in 'Boat Design' started by oldstyle, Aug 18, 2008.

  1. oldstyle
    Joined: Aug 2008
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    oldstyle New Member

    Recently bought a boat that has some nasty handling characteristics. 1981 24' Bayliner Explorer that had the 4 cyl replaced with a 4.3 F.I DuoProp. This has also created a stern down attitude at the dock. Yes I now know it is overpowered from the 4 cyl it was born with. However I own this orphan. It goes onto plane at 18 mph. at 20-23 it will suddenly roll to one side or the other spinning in a tight circle if you do not hold it straight. This is a semi-displacement hull with shallow V and hard chine. The hull is not rounded. However it does have a substantial keel that extends to about 5 feet from the stern. I also have 9" x 18" Bennett Trim Tabs with The automatic controler. This has helped a lot. My research has lead me to believe that one of the chine are "letting go" allowing the boat to flop over. My solution is to extend the hull 16-18" to allow the chine to have more contact with the water, this would also increase the flotation to offset the extra weight of the new motor. My questions are, will I have issues with the leg recessed into my extension? Should I transition the bottom to create a flat planing pad on each side of the leg?
     
  2. the1much
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    the1much hippie dreams

    i'll let the smart dudes give ya a real response,,,(i cant count past 9) but since its a bayliner,,im not to sure extending your transom will help you,,,and "they" know all the legits,,, but im not even sure your boat could handle it without doing a whole lot more then just your transom.
    i cant wait to see the #'s ,, good luck oldstyle :D
     
  3. Grant Nelson
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    Grant Nelson Senior Member

    Any chance you are over trimming, pushing the bow down and basically spinning around the resistance that has moved too far forward of the longitudinal center of gravity?

    with the new power, did you change the props and or shaft (sorry I don't know what a 3.5 F.I. Duo prop is, nor what was there before this change) and perhaps the angle (angling them down more)?

    Try having someone run along next to you and take some pictures. The water line should trim up at the bow by around 4 degrees.
     
  4. oldstyle
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    oldstyle New Member

    Grant, the original motor was a 4 cylinder, the boat was repowered with a Volvo Penta Fuel Injected 4.3 Liter V6 with a matching Dual Propeller Leg. I thought it was my lack of ability showing through, but have some skilled boaters have the same issue.
    Jim, The local yards are putting pods and massive outboards on everything that floats around here. As this extension won't be dealing with thrust issues I believe strength will not be a problem. I am looking at essentially a boxed in Aluminum swim grid. My question is how to make this idea better.
     
  5. the1much
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    the1much hippie dreams

    in the strength,, i would worry about your "stress points" moving,,,when you move your transom,, your "stress points" move along with it,, and by making it "flatter" your moving them too.
    i've always have thought that bayliners are the worst "handling boats there was,,<NOT PUTTING YOUR BOAT DOWN!! ,, but also cause i really dont do much riding in them smaller boats,,,hehe less i was rowing :D,, i was meaning your stringers and such might need to be "bulked up" or moved to handle the difference in stress it will have.,,,thats why i also defere to the "smat ones" hehe ;)
    and not putting any of those boat yards down,,, some may be the best in the world,,,, but just cause they do it ,,,, dont make it right. ,, and they may also be doing it like i mention and doing all that extra work . ,,, and im not even thinking bout your motor,,,,,just the changing of the "design".
    and them "yards" may be right,,, could be as simple as that,,,,but are they also moving the transom and changing the "lines" of the boat?
     
  6. Gilbert
    Joined: Aug 2004
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    Gilbert Senior Member

    I am wondering what exactly you mean by a semi- displacement hull. Do you have pictures you could post or refer us to some online photos of the particular model of hull? The shape of the hull could be a larger issue than the increased weight of the engine, etc.
     
  7. oldstyle
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    oldstyle New Member

    Jim I am not planning to move the transom rearward just add a box section that extends the lines of the hull 16" further back. The top would be the swim grid with a cavity for the leg. This would be Aluminum and bolted to the transom. Gibert I will get some pictures of a similar hull tomorrow
     
  8. the1much
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    the1much hippie dreams

    even "extending" it,,in real life practicality,,,,your moving your transom,,,,,or,, i guess you could look at it like your making oversized funky "trim tabs" hehe :D
     
  9. Ike
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    Ike Senior Member

    It sounds like you have a case of what is called dynamic instability. All that means is that the boat becomes unstable at planing speeds and does something scary like flop over to one side. This is probably because when they installed a much larger and heavier engine they significantly altered the center of gravity of the boat as well as the center of buoyancy and the center of pressure on the bottom of the boat when it is on a plane.

    This is actually more common than most people know. Actually a 1978?? (I think it was 78) Bayliner 25 foot express cruiser did the same thing right out of the box. The Coast Guard has some great videos of it falling on it's side. But not to be out done the Coast Guard 30 foot Surf Rescue Boat, when it first went into service did a similar act, nose diving to the right and falling over on it's side. Fortunately, on the CG boat everyone is strapped in. Not so on the Bayliner. The USCG has since fixed this problem on thier boat. Porpoising is one example of dynamic instability but one that can usually be fixed by shifting weights or using trim tabs.

    The primary reason for this is when the boat is up on a plane the CG rises so far that it is well above the center of pressure on the hull and the center of what buoyancy is left, and the slightest movement to one side causes the boat to go to that side (usually to starboard) until it obtains a stable position. Unfortunately this stable position is not good. Contributing factors are a lot of top hammer, that is high sides and high cabin, a narrow hull (for trailering) and lots of junk up high in the boat. In other words a lot of boat builders have built what are derisively know as "floating condos" with out a lot of thought to what affect it has on stabilty. It isn't only Bayliner either. Almost every major manufacturer in this country has at least one model out there that does this.

    The cure? Put a lighter engine and drive in it, or start removing a lot of weight from the topsides to counter act the problem. I must emphasize, adding weight down low does not fix this. It only makes it worse. You might need a Naval Architect who is familiar with this problem.

    Sorenson's Guide to Powerboats has a section on Dynamic instability.

    A Naval Architect name Steve Cohen published a paper on this through the Society of Naval Architects and Marien Engineers (SNAME) He was also the one who designed the CG boat that nosed dived.

    Cohen, S. H., "Dynamic Instability of a High Speed Planing Craft - An Approach to the Problem," SNAME Power Boat Symposium, Miami, FL. 1985

    The David Taylor Naval Research Laboratory has done a lot of research and testing on this subject.

    P.S. is this the boat? http://www.catfish1.com/gallery/showimage.php?i=3286&c=26
    I don't think this boat was ever meant to go that fast or have that much power.
     
    Last edited: Aug 19, 2008
  10. the1much
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    the1much hippie dreams

    see ,, i knew "smata" people would show up ,,hehe ;)
     
  11. oldstyle
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    oldstyle New Member

    Yup Ike you nailed it. In the beginning it would roll off forward. I installed Bennett Automatic trim tab control, and repaired a sticking control switch which seems to of stopped the bow roll. ( I believe the trim tabs were also wired wrong). I installed 300# of sand in the bow to try to counter balance the heavier engine, and when we sat on the v berth bench (300 each side) The boat handled perfectly. My problem is that we like the boat, paid to much for it and won't sell a dangerous boat. Am I wrong in my belief that by extending the hull form back, which would give it more flotation in the stern I would be possible to correct the CG. The superstructure is minimal as in your picture. no command bridge, seats 2. I see that I have a lot of reading in my future.
     
  12. Ike
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    Ike Senior Member

    I am not a big fan of hull extensions. They create their own problems. It may or may not correct the problem. It would take an analysis by an NA to tell if it would work or not. Actaully moving the engine forward a few feet might be a better solution. But that may not be practical.
     
  13. Village_Idiot
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    Village_Idiot Senior Member

    Hull extensions are quite common and popular on smaller aluminum jon boats, particularly with tunnel hulls since the tunnel creates a loss of buoyancy in the hull. More people are adding them to existing aluminum boats to account for the heavier four-stroke motors commonly available now. That being said, I don't know how that translates to putting pods on a fiberglass boat.

    One trick that some folks use is to angle the pods upward a few degrees so that they don't force the bow down while the boat is on plane. OTOH, if they are straight across the bottom, they have the effect of lengthening the planing surface of the boat, adding to overall efficiency of the hull, at the possible expense of handling maneuverability in tight quarters.

    Check out some of the tunnel hull jon boat manufacturers to see what kind of pods they are using. Two that I can think of off-hand are G3 boats and Boatright Marine. Again, don't know how that info would cross over to a larger fiberglass hull...
     

  14. FAST FRED
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    FAST FRED Senior Member

    Could be the boat is bow steering and needs a couple of strakes forward to get more of the bow out of the water.

    FF
     
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