1975 23' Mako Complete Re-Build

Discussion in 'Boat Design' started by DaveD, Apr 24, 2014.

  1. DaveD
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    DaveD Junior Member

    I gutted a Mako 23. I have professional repair guys doing work. Very confident of their abilities. I am concerned about getting the C.G. correct. Also we extended the hull to 25'-7".

    1. Took out liner and disgarded 400#
    2. Popped off Cap and replaced all wood with Divinicell. All glassed in.
    3. Took out transom complete and rebuilt with 3 layers of Coosa 26 Bluewater
    4. Extended hull 2'7" to 25'7". Excellent work. Replaces Extension Stingers with 3/4" Coosa Bluewater 26.
    5. Ground existing stringers and glassed in new Coosa BW26 to correct deck ht.
    6. built temporay deck to install full tower, CC, and Livewell leanpost.
    7. mounted porta bracket and 250 4stroke suzuki and then weighed boat.
    8. Launched Boat and added garbage cans of water to simulate fuel cell placement and additional live load.
    9. took measurements to determine deck elevation while 3 people were in various loaded positions while taking measurements.
    10. have got original dry weight and am cognizant of overall dry weight.
    11. no liner going back in. 400# savings will end up lower in the boat to help counter balance the tower C.G.
    12. Deck will be 3/4 Coosa BW26 which will be lighter than original wood deck.

    My main concern is that the weight distribution needs to be correct bow to stern so the ride is correct. Not bow heavy or stern. trying to avoid Bow steering in rough seas. Boat will be primarily backwater fished but will be some bluewater fishing in the keys. Transom is fully enclosed with a builtin livewell in transom as well. Status: Fuel Cell is being fabbed now. Looking for any comments regarding last thought considerations before we start setting tanks and buttoning the deck up. My builders, while very competent in workmanship are designers so to speak. I am a civil engineer by trade and wanting to be sure C.G. bow to stern is right. Looking for pointers. I have lots of pictures on file. All interested comments welcome. DaveD:?:
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds very methodical Dave. Fuel tank placement as a measure to get LCOG "right" is limited by the fact the weight varies according to how full it is. The right place for the fuel tank is where changes in fuel load are least noticed in the boats running attitude. Since you have a "new" hull now different to the original, and with an outboard sitting out on a bracket, you are in uncharted territory to some extent, and how the original hull behaved, only a rough guide. I would be inclined to keep my options open to be able to move some weights back and forth, as experience dictated, before being "locked in". But with trimmable outboards and the possibility of trim tabs, you have a fair range of LCOG that can be considered satisfactory. Bow steer is obviously not corrected by trim tabs, though. I might be more concerned about your tuna tower, and the vertical COG.
     
  3. gonzo
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    gonzo Senior Member

    The center of gravity should be easy to find. Install either the motor or something of equal weight. If the trailer is double axle, take off a set of wheels and add weight to make it balance. Put the boat on and shift it until it is balanced, that is the center of gravity.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Of course, that will show you where it is, but the on-water test is the only way to find out if it is too far forward/back. Planing boats are reasonably forgiving of alterations in LCOG, by reason of the "leading edge", an area of high dynamic lift, moving forward or back as you shift weights. It is probably better to err on the side of having weight too far back, than forward, as you can trim the motor, or apply trim tab, or both, but lifting the nose is not so easy, and if it is inclined to broach, just bad news.
     
  5. DaveD
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    DaveD Junior Member

    1975 23'Mako 23' Complete Re-Build

    Great thoughts with Trim Tabs and trim. The porta Bracket yields a high level of options also.

    Keeping in mind the original weight of the 23 and what was supposed to be the weight of a 25 Mako. I installed a partical board temporary deck (almost 500lb.) Composite deck will be about 40% of weight. Then installed the tower. Loaded a garbage can with water (simulating half tank of fuel) and then three of us simulating fishing at stern (630lb. plus 250lb bait well and transom not yet complete) to get a feel of how it sat at rest. Also want to make sure it self bails in this condition when I set the deck elevation. It is an extreme stern loaded condition. I have picture attached to show the profile. It seems a little stern heavy to the eye. Can not run boat yet so I got to go with my assumptions. I know there will be a little more glass work (bulkheading and fish box which will add a little more weight forward, but not much. The deck is fairly evenly distributed so wont change overall profile when replaced with permanent lighter version, just a little safety factor in the deck height which is good.

    Last thing I did was lift boat with a strap and gantry to find Dry Weight LCOG of boat without the fuel tank or transom livewell enclosure. LCOG was 92" from transom. 307" LOA.

    Also some random pictures of construction.
     

    Attached Files:

    Last edited: Apr 28, 2014
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  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, looking at the pictures, it looks as though it does not need any more weight shifted aft. The motor looks very high, is that the height you are stuck with ? But only the on-water test will settle both issues. Also, that tower worries me, unless your boat is wider than trailable width.
     
  7. DaveD
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    DaveD Junior Member

    Mr. Efficiency

    The motor is on a hydraulic "Porta Bracket". Lots of flexibility with mounting height. Tons of range of motion vertically. Will be able to adjust when water testing. I'd like to hear more about your concerns over the tower C.G.. This tower came off a 23 ft. Dorado. Dry weight of about 1900 lb. It was definitely too much for that boat. That is why he took it off. We will be 2900 lb with a lower C.G. - boat for boat. (excluding the tower). I assume you are concerned about affect the Vertical C.G. of the tower will have on the boat. Do you know a way to calculate the stability or resistance the boat will need to have in order to counter the "moment arm" created by the tower. Are there any empirical calculations (publications) which can be applied as a "rule of thumb". I have seen boats with similar towers on boats considerably smaller than this one. Comments welcome. DaveD
     
  8. TANSL
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    TANSL Senior Member

    For transversal stability (also for longitudinal) is very important to know the height of c.of g. and, with this method, I'm afraid you can not determine. Things, if you want to do them well, are not so simple. With the boat in the water, you can make a very simple inclining test and, with light calculations, determine the position of c. of g.
    This boat, with that tower and engine located very high, would be very important to have as much correct data as posible.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't think it is possible to make definitive calculations about the effect of the "tower" on stability, especially as it will vary according to the weight of whoever goes up on it, but I'd not want anyone up there in rough water, crossing barred inlets etc. Your boat is a moderate vee with good initial "stiffness", which can give, perhaps, a false impression of a boat that can handle it, but as a rule I would not want "high rise" in any boat with an 8 foot beam.
     
  10. TANSL
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    TANSL Senior Member

    DaveD, the calculation is exactly like the case of weight lifting.
    For a certain displacement, you have to trace the curve of righting arms of the ship and heeling arms at different angles of heeling due to the weight located at that point. The two curves intersect at a point that represents the equilibrium heel angle. The regulations indicate the maximum value that can achieve this angle.
    Also, if you want, you can calculate the effect of wind and waves and add all these elements as destabilizing elements.
    For all that, formulas and procedures are perfectly well described.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You are right in theory TANSL, but in practice some big bozo climbs up there and all bets are off, you have too many variables to factor in, like where the rest of the crew are at the critical time, etc, I would just not bother with that tower, which does look like a work of stainless steel art admittedly.
     
  12. TANSL
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    TANSL Senior Member

    In ships there is always a large number of variable factors, however, the calculations performed are very reliable. Everything is, as you most know, to study the worst case and take some appropriate safety factors.
    No talk of risky theories without any basis; I talk about the normal practice in naval architecture calculations.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    You are used to bigger vessels, this is a "toy" by comparison, where non-fixed weights are a much bigger percentage, I would even regard 2 or three people hanging on to overhead ironmongery as likely to be destabilising in some situations, compared to say rails at waist level. This boat has a raised sole ( I assume) which also does not help stability.
     
  14. DaveD
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    DaveD Junior Member

    Tansl

    Thank you for your comments. I have been looking into inclining testing to understand our stability. Understanding your foe is the first step in overcoming a hurdle. A tower is imperative to our application and quitting on this facet is not acceptable so I will break down the potential issue until we can come up with a solution. Any references to other information regarding calculations, formulas or procedures are welcome. When understanding VCOG of my application it would be important to understand this is not a stainless steel tower but rather aluminum with a specific gravity approx. 2.8 times less mass. Also this tower can be collapsed to about 7 ft. if needed. I appreciate constructive input with a notion there is a solution we can reach. Thanks again.
     

  15. DaveD
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    DaveD Junior Member

    TANSL

    That is the approach we are looking for. Thank you for that input. DaveD
     
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