Weight Distribution in single inboard 24* high speed boat

Discussion in 'Inboards' started by mgray127, Sep 7, 2012.

  1. mgray127
    Joined: Sep 2012
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    mgray127 New Member

    I'll preface myself by admitting that this is very unusual for many reasons and that much about this boat will be less than ideal...So what I'm working with is an 80's 30' 24* high speed hull, very similar lines to a Wellcraft Scarab. A large gas engine is going under the console and I want to move it amidships to reduce my shaft angle. At 10* my math has the front of my engine at ~11' fore of the transom but I haven't yet verified this with a mock-up. Goals for this are 30 knot cruise and 40-50 knot top speed so I'm trying to move the shaft angle as shallow as possible.

    With the velvet drive, the CG of the power unit (50" longitudinal length) is ~2/3 aft of the front of the engine which would put it ~8'-4" fore of the transom, not too bad, is it? I'm interested in how the handling will change as the engine is moved fore. More specifically, I'm interested in how far fore I can move the engine without creating a bow-plowing pig. I do understand that the most ideal location is to put the engine right on the transom but this boat is not running 90 mph, anymore.

    Opinions?
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Without a set of lines, pictures and a reasonable accurate weight study, any speculation as to what you need, preformance predictions, preferred locations etc. are pure guesses.

    Simply put, keep your shaft angle below 10 degrees, preferably below 8 and you'll likely do fine, but I have to admit, I'm completely sure I understand your questions. Are you attempting to put a straight shaft setup in a 24 degree deep V hull? As to handling, she steer like a pig in comparison to an outdrive, jet or outboard. Speeds under 40 knots will be fine, but you may run into a porpoising problem, if you drive this hull much faster with the CG moved forward so much.
     
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    A Vee drive will lower your shaft angle while keeping the engine in the same place.
     
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  4. mgray127
    Joined: Sep 2012
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    mgray127 New Member

    Yes, I am trying to convert a 24* deep-vee hull to a straight inboard. The boat has been stripped down to the laminate skin of the hull, liner is out and stringers are coming out. I want to build a substantial grid-like system of frames but I can't do that until I locate the engine longitudinally.

    Naturally, this center console will be purely a fishing boat so I don't expect it to handle like and run like it did when it had two big gas stern drives. I understand the limitations of low-speed maneuvering with a single straight inboard. My other complication is reduced prop diameter with a decreasing shaft angle.

    Here is a picture of where this boat is going, minus the outboards. Im hoping to fit 200 gallons of fuel along side the engine and underneath the deck, behind the engine.
     

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  5. gonzo
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    gonzo Senior Member

    That boat will be really heavy by the bow with an inboard and handle badly, namely oversteer and probably broach. It is a bad idea.
     
  6. mgray127
    Joined: Sep 2012
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    mgray127 New Member

    Notice that boat's console is ~35% LOA. I have been in two boats of similar hull design, both 24* deep-vee's, and with twin gas engines located at 38% LOA and with comparable towers; both were converted from stern-drives to straight inboard and they do not broach in moderate offshore seas. However the bow is noticeably heavier in a moderate sea than a stern-drive and speed is limited. So I must believe that a single engine inboard, with it's lower CG than a twin engine inboard, must handle at least similarly. No?

    From my anecdotal evidence my assumption is that I could at least operate safely with an installation under the console at 35% LOA, though I would have a slower boat with a steeper shaft angle. Suppose I used a vee-drive either direct attachment to transmission or even further fore with a short jackshaft to attain a shallower shaft angle. I would be increasing speed at the cost of moving weight inappropriately fore, a move whose undesired effects are made worse with increasing speed. I'm getting the idea that I need to keep my engine back and accept the accompanying steeper shaft angle and slower speed in the name safety.
     
  7. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The vee drive can be installed forward with a jack shaft. However, if the boat is designed for outboards, the CG is supposed to be way aft. Also, the propeller may need to be of the surface piercing type if you install the shaft out the transom.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The inboard engine location will work, but you'll have speed restrictions from the new CG location. As long as you keep it under 50 MPH, you'll be fine, unless you have a bow full of half naked ladies and attempt to run high speeds. At high speed you're going to run into CG/CB relationship issues and longitudinal instability will occur. As long as you are aware of this, keep the bow light and keep speeds reasonable, you'll do fine. Of course you'll need to move the tanks and batteries aft as much as possible to help offset the engine move.

    Deep V's don't like straight shafts much. There's a number of issues that come up. Draft is the biggest one, as the prop will be well below the boat and this can make trailering, launch and recover a pain in the butt. A notch under the transom can help. Steering by a rudder will really suck, compared to what you're used to, but you'll get use to it. Backing down will also suck, so I hope you will not need reverse much.

    You have a few choices to make, but it can work, understanding (as it appears you do) there will be some limitations and handling compromises you'll have to eat. Why do you want to go to a straight shaft?
     

  9. mgray127
    Joined: Sep 2012
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    mgray127 New Member

    No naked ladies, just an anchor and line. The draft and lack of maneuverability are of little concern because of where the boat will be operated and its year-round location in the water, very little trailering.

    I want the reliability of straight shaft and a cleaner transom. It's very possible that a V8 diesel might end up in place of a large big block and I'm not too confident in a Bravo sterndrive behind either of those two engines. I refuse to pay the cost of any Speedmaster outdrive. And with a single screw, and my experience with 90 degree gearing, I don't feel comfortable going offshore with an outdrive. I will admit that for no particular reason I have neglected to consider Volvo outdrives which I'm told are suitable for such torque.
     
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