center of gravity changes with conversion?

Discussion in 'Boat Design' started by modunlavy, Aug 21, 2017.

  1. TANSL
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    TANSL Senior Member

    You are very right, I agree with you. In addition to checking how the boat is navigating, the OP should ensure that the transom is ready to withstand the weight and thrust of the new engine.
     
  2. DCockey
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    DCockey Senior Member

    Two different considerations for CG location of a planing boat:
    1) How the boat floats at rest and at slow speeds, and the corresponding "static" stability. This is what naval architects consider for most vessels. Fore/aft CG location will affect the fore/aft trim and have some effect on stability. Vertical C/G location will affect stability.
    2) The fore/aft location of the CG affects the behavior of the boat and fore/aft trim when planing with the boat support by water pressure against a portion of the bottom. This is more difficult to predict than the static/low speed situation above. Savitsky's method can be used if the boat has close to constant deadrise for the aft portion of the hull. The behavior and trim at planing speeds can be modified using trim tabs if the CG is further aft than optimal.
     
  3. DCockey
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    DCockey Senior Member

    One approach would be do the engine conversion including necessary transom modifications but before moving the fuel tanks, installing the generator, etc. Test the boat with portable fuel tanks. Use movable ballast (bags of sand or similar) aboard to compensate for the missing fuel tanks. Try a range of fore/aft CG locations to see what the sensitivities are. Then make the final decisions on tank location, generator location, etc. If needed add a small amount of fixed ballast forward to move the CG forward.
     
  4. modunlavy
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    modunlavy Junior Member

    thanks for all the input. I am planning on completely redoing the transom and adding a "knee" from the transom to the stringers to be able to properly support both the force and the weight of the outboards. Trying to work on the layout and simply just gutting the boat for now. Once I get the layout settled, then I plan to start make the necessary modifications to make it all work.....

    thanks again for all the help,

    Michael
     
  5. gonzo
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    gonzo Senior Member

    The fuel and water tanks should be positioned close to the CG. Otherwise, the CG of the boat will change as fuel is used.
     
  6. modunlavy
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    modunlavy Junior Member

    I like the idea, but isn't that somewhat of a perfect world scenario. This boats original layout has a 205 gallon fuel tank in the stern of the vessel. My current 1978 24' Topaz inboard has both fuel tanks in the aft part of the boat as well. When coming in light on fuel, with a decent following sea, she wants to bow steer from being too heavy in the bow and we have to shift weight to the transom to try to help offset. No other place for the fuel tank in the boat.

    I think converting to outboards opens up that possibility a lot better since hanging the engines off the back opens up a lot more space to use and therefore more options....
     
  7. gonzo
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    gonzo Senior Member

    That is the reason many boats have saddle tanks.
     
  8. TANSL
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    TANSL Senior Member

    This phrase, which in itself is totally correct, could be totally incorrect and lead to errors of difficult, or at least expensive, solution. It would be better to explain what you are talking about.
    There is no human way to prevent the COG in the boat from changing as the fuel is consumed (a system could be invented to achieve this, but I think it is not worth getting into such complications).
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, if the fuel tanks are situated where the COG is with empty tanks, then it is the least effect on trim with variable fuel load. I had a boat that trimmed well, on plane, with tanks full ( they were forward of amidships, by necessity), but tabs were needed when they were low. If the tanks are rearward, you'd expect the opposite.
     
  10. TANSL
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    TANSL Senior Member

    I think the most appropriate, and it is what I have always done, is to bring the cog of the consumable liquids to the cog of the waterplane. In that way, the moment of the liquids with respect to the axis of rotation of the ship is always null or very close to zero.
     
  11. modunlavy
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    modunlavy Junior Member

    I am not sure I understand this statement in respect to my 24' Topaz. The boat does have saddle tanks, just in the back of the boat. locating the fuel tanks forward at or near the CG would only make the boat more bow heavy and cause the boat to bow steer in all conditions.

    The obvious solution to me would be to move the engine aft, which could only be done with a v-drive transmission. But this is a small sportfishing boat and having the engine in the middle of the fishing cockpit would be a bad solution - so it appears the designer put the fuel tanks aft to help (there is also some lead glassed in the transom to add additional weight). Could always add more weight, but the boat is already a very heavy boat and adding a few hundred more pounds isn't desirable.

    Boat performs fine except when light on fuel... a compromise I can live with on that boat to have the wide open fishing cockpit.
     
  12. Barry
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    Barry Senior Member

    The 29 (really it was called a 28) with the outboards mounted on a transom cut-out were made by the old Blackfin - those are known as the Cary hulls, because they were designed and for the most part built when Cary owned Blackfin (up through early 1982). Cary Blackfins are the 24, 25, 28 & 32/33 - all were relatively narrow and the 28 & 32 came as either outboards or inboards. When Herndon took over Blackfin, the 25 continued as it was, the 28 became the 27 and the 32 was discontinued - he then introduced a new 32 (Blackfin's most successful model) and 29, then a little later added the 36 (which was never blackfin's best - the hull was a little too short - a problem fixed by additing 2' and creating the well respected 38) which had a short life.
    The 29 is the only Herndon Blackfin to live the entire time Blackfin Yachts existed (1982-1998) as the same model; the BF32 was made from 1982-1993 (Combi '89-93')and then evolved into the 33 (Flybridge and Combi) which was made from 1993-1998 to allow for it to introduce the 31 Combi (1993-1997).

    Blackfin 29 Combi with outboards - The Hull Truth - Boating and Fishing Forum http://www.thehulltruth.com/boating-forum/223384-blackfin-29-combi-outboards.html

    The above comments were pulled from a few other forums on the net,

    If you have calculated the cg of both and say that they are within 1 to 4 inches inches of each other loaded. You could leave things alone.

    There are many boat manufacturers that offer 30 foot boats with inboards and outboards. Google some and then pick up the phone and find out if they move the location of the tanks for each model.
    there was a comment that I ran across that stated that when they changed to outboards, they moved the tanks back as well to get more space under the cabin sole.

    Better to be weight back than forward, as DCockey said, install the engines, go for a spin,

    I am sure if you look, you will find people who have done this same conversion,

    Alternatively, phone Blackfin, and tell them what you are doing, a $5 phone call could save you thousands
     
  13. modunlavy
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    modunlavy Junior Member

    Unfortunately Blackfin is long out of business (current builder under same name is a completely different company and boat). I hadn't thought about calling another manufacturer, but I am in the boat repair business and have found most manufacturers to not be very easy to get technical support from on their own products, let alone someone elses..... There are exceptions (just got great help from Boston Whaler on a repair)....but I don't see me getting far with it.

    We do work with some naval architects from time to time (Blount, Ocean 5, EG Designs) so when I am ready to start writing checks I will talk to one of them most likely, but as an engineer type (degree in electrical engineering) I am enjoying the process of learning and doing as much of this as I can on my own.

    I appreciate all the help everyone is giving me!
     
  14. gonzo
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    gonzo Senior Member


    Most boats in that size range have the CG close to the location of the engines. The saddle tanks allow the fuel tanks to be installed outboard of the engines. When the fuel level changes, the CG does not move forward or aft.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    You'd be ill-advised to put fuel tanks at the stern, in a boat running outboards perched on non-lifting brackets. As a general rule of thumb. Bow steering simply does not enter the equation as a result of having tanks amidships, in a podded outboard boat, if it did, the boat would he a poor design, which this one obviously isn't.
     
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