Savage Hull Extension

Discussion in 'Boat Design' started by savagescout, Mar 28, 2011.

  1. savagescout
    Joined: Jun 2009
    Posts: 37
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    Location: australia

    savagescout Junior Member

    Hi Guys,

    I wanted to get some advice on modding my hull (which i have been planning/changing design for the better part of a year or 2!).

    I have a 23 foot savage half cabin as per the photo below.

    [​IMG]

    I also have a 220hp mercruiser turbo diesel sterndrive that i wish to install in the boat. Unfortunately the engine is slightly to long for the current boat and takes up a bit too much deck space, but considering that i have a decent powerplant, i have decided to lengthen the hull by approx 3 feet.

    I would also like to toy with the idea of running the diesel as a mid mount engine and using a jackshaft to transfer the drive to the bravo 2 leg at the transom. Ideally i would love to be able to incorporate the engine box as a back to back helm seat to provide a clear deck space that is free of clutter whilst still providing the reliability and fuel economy of a diesel power plant.

    I have roughed up a design below.

    [​IMG]

    I was wanting to get some advice on whether moving the engine this far forward would severly compromise the performance of this hull. Would love any advice that anyone can offer.

    CHeers,

    Nick
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Have you ever sat on a 225 HP diesel engine box before? I hope you are a really cold weather boater.

    Your ideas can be done, but you're making things a lot harder then they need to be. I can't see the bottom of your boat well enough to tell what type of shape it is, but I'll assume it's a warped bottom. If you have enough deadrise at the transom currently, you could continue the warp or you could just go monohedren from there on back, but damn, this ain't going to be easy Nick.

    I don't think the engine in the middle of the boat is as good an idea as you might think. Place it aft, maybe with a transom "bump out" so the drive and part of the engine extend out past the current rear of the boat, which will free up a bunch of cockpit space, without requiring you to build much of a new hull extension. Trust me, you don't want to sit on this thing without 10" of some of the finest insulation you can get.

    It would be a whole lot easier to just find another derelict hull, say in the 26' or better 28' range and stop screwing around with this extension idea. There are old hulls a plenty with the economy the way it is. People are giving them away. My best advise it not to attempt to make a silk purse out of a sow's ear and find a more suitable hull for that monster turbo diesel.
     
  3. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    I like the idea a lot. But I agree w PAR ...looks like you need a boat in better shape and there's lots of boats available like that cheap. But the boat may be in better shape. Looks like the spray rails are damaged. But I disagree w the much more experienced PAR on the modification. The ideal single engined planing boat would have an IO in it's usual place, a tubular propeller shaft running fwd to the thrust bearing of an Aqua Drive and the engine just fwd of mid-ships. A lightweight high performance diesel like a Yanmar of about 100hp may be enough power. An old used OB would of course be the cheapest formula. No offense PAR but I like the mid-engined boat. But it would'nt make it as a production boat as the long shaft would take up too much space. I think that's where the engine (weight) belongs. This boat has enough volume fwd in the hull to support it too.
     
  4. savagescout
    Joined: Jun 2009
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    Location: australia

    savagescout Junior Member

    Hi guys,

    I am actually from Australia and don't quite have access to the range of hulls like you guys do in the states... And definitely not as cheap... These old savage hulls were Australian classics and very solidly built and I know this hull is solid as a rock.

    I already have someone in my who will be able to assist with the hull extension but was wanting to know whether I would be producing a pig of a boat with mounting the engine in this location (weight wise)?

    I am getting conflicting advice like the two replies already (and I really appreciate the insight from both of you). Some people seem for it, some against it. But surely the situation must work as big name manufacturers like shamrock boat, albemarle and Carolina classic all have mid mounted engines on similar sized boats...

    Re the concern with heat par: I am sure that with suitable ventilation and insulation I can make the engine cover work as a seat... Other boats have it so why not this???

    Really appreciate every bit of feedback.

    Thanks heaps,

    Nick
     
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Again, I'll ask, have you sat on a well ventilated and insulated 200+ HP, turbo diesel engine box? Other then an old air cooled Lister, I can't imagine a hotter engine package.

    Eric and I have debated the merits of a centered engine, shafted drive before. It's not a common solution for several reasons, most of which have little to do with CG location, as this can be trimmed as desired with other equipment (tanks, batteries, etc.).

    In a nut shell you want you CG in a fairly narrow window or ill handling will result. If the engine is aft, on the drive, you'll have tanks under the sole near midship to counter. The reverse is true if the engine is midship. The problem with a midship engine, particularity that one, is the physical foot print it has. In a centerline box aft, especially if bumped out of the transom a bit, you have minimal intrusion into the cockpit and wide open accommodations space. On the other hand if the engine is in the middle of the boat, then you'll be spending your time working around this is every regard, accommodations, cockpit/cabin intrusion, etc. It just seems much more logical, to not trip over the engine and shaft enclosures all over the most valuable real estate on the boat (midship).
     
  6. Easy Rider
    Joined: Oct 2009
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    Location: NW Washington State USA

    Easy Rider Senior Member

    Well I'll break it down a bit more. The configuration is great (I think) but there is a possible/probable problem. The connecting inboard propeller shaft will turn at engine rpm and will probably be sensitive to balance. That's why I recommended the tubular shaft as there are lots of automotive places that can balance that shaft. Then if you attach the fwd end of the shaft in the normal way (even with a flexible coupling) the engine will shake the shaft and induce vibration. But with the Aqua Drive (or Python Drive) the fwd end of the prop shaft would be fixed solidly to the thrust bearing and minimize vibration. This is an expensive and complicated system but it optimizes weight distribution and maneuverability. A spline shaft and universal joint may be possible along with a pedestal bearing to save the cost of the Aqua Drive. Probably be iffy.
    As to the boat ...it does have metal framed windows and I see you have plans to get rid of the bonnet style top ...good. Most boats in the US have the bottom full of longitudinal wood hull stiffeners like 2x4s, 2x6s ect all glassed in and most boats have at least some rot in the wood. Also the transom. A fairly big job to fix. Extended or not I still think an outboard would be fine.

    "Eric and I have debated the merits of a centered engine, shafted drive before." ...We Have?? Well, I agree about the space problem but he is adding 3' to the boat (over 20 sq ft) ............Oh I just saw that this is a diesel he's got. That would be big indeed ...and the heat ..yup. I think that engine is too big and maybe too heavy too. Sorry dude it's not a good idea. With a 450lb 4cyl Yanmar it could be a good project but not w what you've got. PAR, we were posting at the same time.
     
  7. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Yep, it was a few years ago Eric and you were a fan of midship mounts then too. About weight distribution, particularly in full plane craft. It's either where it needs to be or you experience progressively ill handling issues. In other words, it's on or your moving stuff around until it is, regardless of engine location.

    Personally, I think this is too small a boat for that power plant. I also think that an extension is a difficult proposition for a skilled back yard builder and very difficult for a novice. I agree with Eric (Easy Rider) in that an outboard, possably on a bracket, would free up all of the cockpit, no engine box, no 100 mm shaft spinning at 3,000 RPM right next to your kids and a wide open cockpit/cabin accommodations layout is possible. The price you could get for that turbo diesel would probably cover the outboard and bracket.

    Don't get me wrong, you can do it, but wow are you making things difficult for yourself with the current approach, which still leaves a huge, hot box in the middle of where you want everything else.
     

  8. Pericles
    Joined: Sep 2006
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    Location: Heights of High Wycombe, not far from River Thames

    Pericles Senior Member

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