Why inboard shaft drive is popular in ski boats but not in smaller CC sportfishers

Discussion in 'Powerboats' started by Northeaster, Aug 27, 2012.

  1. Northeaster
    Joined: Sep 2011
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    Northeaster Senior Member

    Hi Folks,

    Please help a novice powerboater with a bit of info - I am used to my 30' displacement sailboat / inboard shaft drive.

    I am looking to build an approx 18 - 21' aluminum CC boat, and almost all of the designs are for outboard power.
    Yet, when I look at ski boats, they are mostly inboard / shaft drive.

    I have read some articles on shaft vs I/O or outboard, but I have not found an explanation as to why shaft drive is so popular in ski boats, yet not in other similar sized non-ski boats.

    Of course, not knowing the answer, I am wondering how hard it would be to have an outboard design modified to place an engine forward and add a shaft and rudder. Other than ski boats, I have only seen shafts in something like the Shamrocks with (keel drive or pocket drive).
    I have read that it is really hard to create an efficient pocket / prop tunnel and therefore wonder if the small keel and skeg / rudder would be feasible on designs other than ski boats.

    Please give me some of your explanations / thoughts on this!
  2. jonr
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    jonr Senior Member

    IMO (based on many hours behind them), inboard ski boats (at least prior to wakeboarding) are designed for minimal wake. So better to have the engine weight forward.
    Last edited: Aug 28, 2012
  3. Three Ts
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    Three Ts Junior Member

    This is anecdotal info only, and could be far from the complete reasons but here goes:

    Boats are far simpler to manufacture for outboards - no alignment issues, through hulls, no complicated wiring (its all harness and steering cable), easier shipping without the engines, and no direct warranty liability for the engine installs when install done by dealers.

    Inboard center consoles USUALLY place the engine under the center console - it's a pain in the $%^& to work on the engine under a fully wired CC than under a simple engine cover. In a cc of about 28-30' or longer, the engine could probably be located under the helm seat instead, which would be far simpler for servicing.

    The center consoles in an outboard setup are usually placed farther forward than would be practical in a smaller inboard with center console over the engine. Otherwise, the engine would be too far forward in the boat for proper weighting.

    The aft-placed center consoles in a smaller inboard reduce the size of the usable cockpit in the stern, and place more of the open deck forward compared to a typical CC with outboard.

    Inboards are great for DIY types, but for someone (most boaters these days) who will take it to the shop for any and all service, the outboards represent a simple proposition in that all engine repairs are done from outside the boat. Repowering is simple (although very costly for new 4-stroke outboards vs. inboard) since it's a bolt-on application, reconnect steering and wire harness and done. And, the new outboards, being efficient, quiet and much more reliable than in the past, are a more carefree power-plant for someone that doesn't want to spend time thinking about the maintenance. Just flush the motor (if salt water) on the washout port without even running the motor and you're done after use.

    That's my take on it.

    By the way, I own a small 20' inboard center console.
  4. Northeaster
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    Northeaster Senior Member

    Thanks you both for the responses.
    Three-Ts - you possible explanations make a lot of sense.
    When you say your boat is an inboard, I assume you mean with a shaft drive and not with a stern drive / leg? Can I ask what model boat you have, and if it was a production inboard boat, or if it was a custom or modified design?

    I would love to hear some thoughts on how difficult it would be to have standard aluminum outboard design modified to incorporate a small keel / skeg /rudder and prop shaft / strut.
    Would there be drastic effects on handling, performance, etc.

    My father-in-law and I will be building a small (18 - 21' or so) boat this winter, and as we have alot of experience with diesels, maintenance, etc, we would like to explore the possbilities of doing this. We are not looking for top speed and would like to use used marine diesel, or modfify a used tractor or car diesel to power on the cheap. (I am aware of steps required to marinize, exhaust elbows, cooling, etc. We weld steel, stainless and aluminum and would look at doing most custom parts ourselves). Would purchase new or used prop shaft and prop.
  5. Three Ts
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    Three Ts Junior Member

    I have a Shamrock keel drive (model No. 200, Conwalk.) Its a 1986 production inboard, shaft drive with a shallow keel. Power is a PCM 302 c.i. V-8, 1:1 gear (BW Velvet Drive) and 13 x 10 cupped 3-blade Michigan prop. It used to have a 13 x 13 prop, but I couldn't hit the correct WOT rpms and figured it was overloaded a bit so I went to a lower pitch. I cruise at about 22kts typically, but can get up to 26-27 kts at WOT. It would be faster without the keel and with a 350 motor turning the 13x13 blade, but sea conditions usually limit comfortable speed around here to about 20 kts anyway.

    I like many things about this boat, but it does also have some drawbacks as mentioned above. Also, exhaust noise is greater since the helm is farther back in the boat than on most similar size cc. But, it rides well and doesn't pound and bounce as much as some outboards cc's do, since it has the low weight/ center of gravity and has the helm farther aft. Since this boat has a keel, it tracks really well at slower speeds, and manuevers very easily for a single inboard. I can spin it in just over its own boat length when needed. I really like the handling of it versus the manuevering of a directional drive like an outboard or outdrive setup and much prefer the inboard.

    Feel free to send me a PM if you have lots of other questions.
  6. CDK
    Joined: Aug 2007
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    CDK retired engineer

    In countries around the Adriatic sea, traditional 16' wooden fishing boats used to have a single cylinder diesel with a short prop shaft through the keel. Although the shaft angle is far from optimal, the fuel economy is amazing.
    For ease of maintenance, lots of owners converted their boats for outboards or left the diesel in place and added a small outboard. Because that triples the fuel costs, more and more revert to the old diesel.
  7. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    An observation regarding ski-boats is that they all use 1:1 gearing, which results in a high propeller loading (thrust/(disc area), and transcavitating operation.

    I would say, that with a properly cupped propeller this may give a better bottom speed acceleration than you would have with, say a Bravo drive with a 1.5:1 gearing and correspondingly larger propeller. The 1:1 gearing allows the engine to rev up in order to deliver higher power during acceleration, while the cambered/cupped propeller blades are able to retain their "grip" at low advance ratios and partly cavitating.

    In addition to this, the propeller position below the hull will reduce propwash and rooster tails that may disturb the skier.
  8. Northeaster
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    Northeaster Senior Member

    Thanks for the additional input. I hadn't thought about ski boats having a different gear ratio.
    I would really like to know if any of you guys with design knowledge / experience think that a stock design (meant for outboard power) could be modified to include a small keel, skeg / sfhaft and rudder to allow the use of an inboard diesel, or if the results would be completely detrimental to performance, etc. I realize these parts would increase the wetted are and this reduce speed somewhat, but would it be significant?

    Of course, there would be alot of work, and likely trial and error to get a decent propeller / power combination, but I am wondering if it is worth a try.
    Stock design I am looking at would include"
    - Cope Nanoose 22
    - Specmar Orca 1200 loa 20'6"
    - metal boat kits Trident walkaround 20 loa 21'2"
    - cnc marine 6.1m loa 19' 10"

    I was looking at a Boden Boat plans Bollard 144 loa 25' which is designed for an inboard / shaft drive. However, the price significantly higher as they don't normally come with cut files. It could be built to 90% and have cut files based on this figure.
  9. dem45133
    Joined: Aug 2006
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    dem45133 Junior Member

    Not sure you've covered it... Keep in mind the difference between a displacement hull and a planning hull. A couple of your comments made me wonder. Obviously the biggest performance difference is the cruise speed. As a 30 ft sailor your used to 6 knots or so hull speeds. Some fisherman displacement hulls can get up to about 12 knots I think, but thats maybe pushing it. 8-10 might be more realistic in a 21 footer and it would be plowing most likely.

    I believe your really looking for a mild performing planning hull. Now planning equates to power... the initial to get up and over the bow wake and something a little less to achieve an ideal economic plane for a given hull. A planning hull has to be able to handle much more pressures and shock than any displacement I think. Slamming into water is hard on any hull.

    You speak to alum...so it will be light... good for getting up on plane but harder to design for pounding the dead weight of the full inboard mid hull. That's one of the reasons they like to keep that weight near or on the transom... one of the strongest parts of the hull. Years ago I asked a aluminum kit manufacture the exact same question in converting one of their outboard designs. I was told they would not sell me one to convert and that the hull is simply not designed for it in anyway (main members, skin thinkness etc.).

    If you like the simplicity of the inboard... maybe go with a V-drive setup... that keeps the center open, but not so much the stern. Keeps the weight aft. It'll be like most of the stern drives that way. I would opt for a V-drive over a sterndrive (IO) for the simplicity of mechanics and suitability to pretty much anything you want to power with. Most of your 1:1 ratios are for either very slow turning engines or fast turning racers... for all round performance... I think a reduction and a larger prop makes getting on plane easier. Remember speed on water costs $$$ in fuel. You want to be at the hull's least drag plane at 2/3s available hp (or so). Remember, just as in sail design... everything is a compromise of ideal.

  10. tom kane
    Joined: Nov 2003
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    tom kane Senior Member

    If inboard shaft drive were all trimable (easy as pie to do) they would be more popular and a lot more user friendly and no more expensive than fancy expensive drives.
  11. tunnels

    tunnels Previous Member

    One of the requirements for Ski boats is there trackability and holding the direction the person behind the steering wheel has pointed the boat . Inboards track better than sterndrives and outboards .
    My local ski club have a purpose built outboard boat that has 3 fins attached to the keel to keep the boat going in a straight line and the out board is set a little deeper than normal .
    When a skier goes out the side and is pulling really hard the pull is trying to change the direction of the boat ,then hes on the other side and wanting to pull in the other direction . Inboard motors are usually heavy and the rudder and prop and shaft all help much better to keep the boat going in the direction the driver wants the boat to go in . And most time the wake is smaller because of the planning attatude of the hull .
    Sterndrives and outboards have there weight all right at the transom and so bigger wakes and a skier pulling hard is able to affect the boats trackability and change its direction . :D
  12. keysdisease
    Joined: Mar 2006
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    keysdisease Senior Member

    Inboards have more torque, turn bigger propellers and pull a skier up faster and smoother. As mentioned they also track better and maintain speed better than an outboard would in tournament conditions.

    Believe it when a 200lb skier is turning back and forth through the buoys an inboard "feels" it a lot less than an outboard powered boat would. An outboard would be much more erratic and difficult to track straight in tournament conditions.

    Same with wake boats that use water ballast to throw huge wakes. The small propeller of an outboard would work much harder to move the boat as it need to move to create the wake and go the correct speed, it=f it could at all.

  13. michael pierzga
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    michael pierzga Senior Member

    Outboards are best for small craft for a variety of reasons. Cockpit Space is one. High power to weigh ratio, Ease of installation, easy trailering

    If your CC is big enough and you are not concerned with high speed, then go inboard diesel. Inboards always last longer in salt water.

    A friend has twin Suzuki's on a centre console fast commuter ferry. 2500 hours and the engines are dead. 30 thousand for a new pair.
  14. Frosty

    Frosty Previous Member

    Its all about skiing and the pull. With a 250HP inboard you will only get 45MPH but the pull is everything . You don't need any slack even if your 80 kilo on a mono just wait till its gone and go. It will pull you like you not there.

    An outboard is inferior in every respect. Exept--- barefooting when you need a good 42 I prefer a 200HP reason is your not pulling the boat about like a mono skiier on a course.

  15. dem45133
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    dem45133 Junior Member

    Northeaster... forgive my lack of vernacular.... what do you mean by "CC"?. Your wanting a small sport fisherman right? I think you might mean "cuddy cabin"?

    We got off track I think on the ski boats specific needs. I used to drive the operator of my little 35hp 14 footer crazy with cutting back and forth... If they didn't counter steer I could have turned the boat clear around. What they are saying is correct on them.

    I've had several boats over the years. Two outboard runabouts (a 14 and a 17) and two sailboats (27 ft Bayliner Buccaneer which is for sale BTW, and the latest a 30 ft Seafarer. I almost bought a 27 ft Sport Craft 10ft beam sport fisherman 454 full inboard, but decided I couldn't afford to operate it. $2 per minute to operate at cruise and the fish were 12 miles out (Lake Erie). Too bad I didn't though as I would have never let the 454 alone... I'd have made a 572 out of it and had even more fun... and fuel cost. Been a 55 knot boat then on good water (smile). It was a 41 knot boat from the factory. Kinda wish I had, as I could have the boat for the value of the trailer under it. It would have been fun.

    Right now I am setting up the trailer and mast systems to make the 30 ft Seafarer a "trailer-sailor". Marina storage is just too costly. coast 2k this winter to have it pulled ,bottom cleaned, and stored until May (although it does include new bottom paint)... on a 4k boat. It has to live at home when not slipped. But this is a different story for another thread if any one is interested.

    So... have you been able to select a design and start building? If it we me and economy of ops was paramount... I'd elect either a conventional full inboard or a V-drive (with trim tabs) and a small diesel. 20 knot or so plane. (I've decided not to have gasoline on-board anymore).

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