Why no deep vee V-drives?

Discussion in 'Inboards' started by Myoung42, Jul 18, 2010.

  1. Myoung42
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    Myoung42 Mechanical Engineer

    This is probably a silly question, but as you guessed, I'm going to ask it anyway..

    Why don't we see V-drives or inboards on deep vee boats? (regal, cobalt, etc). Is there a mechanical or structural reason? I see that those with deep vee boats are interested in high speed in rough water, which lends itself to the sterndrive for efficiency/trim, but which also introduces maintenance issues. any reason I'm missing?

    I very much appreciate your responses.

  2. CDK
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    CDK retired engineer

    There is no structural reason, just a mathematical/statistical one.

    Deep V hulls are rare and so are V-drives. I have no data, but assume that within a group of 100 boats 5 have a deep V hull and 2 use a V-drive.
    The statistical chance that a deep V hull also has a V-drive is 1:1000 or 0.1% (for equal distribution). If one large boat builder uses only stern drives for all his hull designs and has a significant market share for deep V hulls, the probability drops even further.

    It also depends on geographical distribution. You in Missouri probably see several deep V hulls around you and know owners with V drives as well, but that is not an objective observation. I live in another part of the world where less than 1 boat out of a 100 has a deep V hull and the number of V-drives is virtually zero!
  3. Myoung42
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    Myoung42 Mechanical Engineer

    Thanks CDK.

    You're right, here in MO probably 50% of the powerboats I see are stern drive or outboard V hulls, with 25% for each V-drive v hulls (ski & Wakeboard boats) and pontoon boats. Most boats are trailerable as well. Catamarans are very rare as are what I would consider a seagoing workboat (round bilge, heavy keel)

    So, vee drives offer lower efficiency than sterndrives due to drag and prop shaft angle. Are there any other reasons?

    For instance, why shouldn't a V drive be installed on a 18°-22° hull?

  4. CDK
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    CDK retired engineer

    Technical merits are not the issue I think.
    A stern drive with two 90 degree gearboxes and lots of seals and bellows is not the ideal product to propel a boat. A V-drive could do just as well or better and has less maintenance plus a far longer life expectancy.

    But from a boat builder's viewpoint it is almost as easy as attaching an outboard but a lot cheaper. The whole package comes from just one supplier and is almost plug-and-play.
    With the proper tools I estimate you can "do" a stern drive in less than 2 hours, wiring and plumbing included.

    Setting up a V-drive system is a much more cumbersome operation, involving parts from several sources. So the choice isn't difficult.
  5. gonzo
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    gonzo Senior Member

    There are plenty of deep Vee boats with Vee drives. They are mainly used to get the engines far aft so the saloon can have more headroom. They are hard to work on because the shaft is under the engine. Quite difficult to align.
  6. Myoung42
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    Myoung42 Mechanical Engineer

    Thanks guys. You bring up some good points.

    "whole package" sounds like the commonality of the sterndrive may be related to production efficiency and reduced labor costs in "plastic boats" ?

    I guess alignment would be more difficult with two separate pairs of coaxial shafts in the same plane....

    How do cost compare between the two?

    Thanks again for all your input!
  7. J3
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    J3 Junior Member

    I found this thread interesting (power lost through drag from running gear vs. outdrive leg, thrust angle, gear loss) and wish there was more discussion of performance difference:

    Were the sea ray hulls quite similar between the 30' express cruisers and sundancers ? That was a while back but might be a case where enough numbers exist for some side by side comparisons (might be before the net though so would be good to find some more modern additional comparisons.) I would favor straight inboards myself, but would also be curious if there are some good examples for a side by side boat with both vdrive and sterndrive power for some good performance comparisons.)
  8. J3
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    J3 Junior Member

    For small dayboats, I suspect power trim is unbeatable where people moving and different numbers of people onboard make the small boat's balance unpredictable in advance.

    How long does the new boat buyer think into the future? Corrosion of the sterndrive can be $$$ long-term if not religiously maintained or kept out of salt water, but how many buyers worry about 5 or 10 years down the road? Not to be cynical, but bigger manufacturers might like the fact that sterndrive longevity isn't forever to get people into new boats (like the car makers until competition has started to demand more...) I think maintenance items like bellows could be made cheaper to change, but would it affect sales any to do so?

    Also brunswick owns both mercruiser and major sportboat builders like bayliner (cough) and sea ray, so the cost of some of these stern drive plug and play units stays in the family so to speak.
  9. Bglad
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    Bglad Senior Member

    I thumbed through my 2009 Powerboat Guide and found that Sea Rays in the 25 to 30 foot range are reported to be approximately 5 to 6 knots faster with outdrives versus inboard V-drive at cruise and wide open.
  10. daiquiri
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    daiquiri Engineering and Design

    There are some good points regarding installation and performance issues in this comparative test of a stern vs vee drive:

    On the other hand, this weekend I've talked with a guy who has nearly lost his boat due to broken seals of his stern drive. A rescue boat has managed to tow him to safety minutes before a complete flooding of the hull. Stern drives are so easy to install but require a lots of love and attention afterwards. ;)
  11. missinginaction
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    missinginaction Senior Member

    Since I own both I thought I'd weigh in.

    1. Sterndrives OK on a trailered boat, but a maintenence nightmare if left in the water all season. Too many nooks and crannies for corrosion to start. Bellows can leak and sink boat. Complicated. At least I think my Mercruiser is!

    2. V-drives low maintenance (but Gonzo's right about engine alignment) since everything but the shaft, strut, cutlass bearing and prop is in the boat. PSS shaft seals make maintenance easier. My boat originally had a packing gland that was almost impossible to service.

    3. All v-drives I've seen are single engine boats.

    All things considered the sterndrive performs better with it's hydraulic trim.

    BUT since my old Silverton lives in the water all season I'm happy to have the v-drive.

    These guys have been making them for decades....


  12. marshmat
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    marshmat Senior Member

    A few related thoughts:

    - The main reason for using a deep V hull is to be able to go fast in moderately rough conditions. For calmer conditions, or going slowly, or seriously nasty conditions, there are other, more suitable hull shapes.

    - Since speed is a main criterion in the choice of hull, it must also be a main criterion in the choice of drivetrain. Forgetting all maintenance concerns for a moment, it is hard to hide that fully trimmable, vectored-thrust drives are better for getting a heavy planing hull to high speeds than fixed, non-trimmable, angled-downward props. (Go sketch out the geometry, and try to find a way to get the shaft angle of a V-drive to be less than 15-20 degrees in a V hull with 15-20 degrees of deadrise- you'll end up with either a too-long shaft, too-small props, or both.)

    - A 30 foot deep V cruiser can easily burn through $500 of fuel in a weekend. Even a 10% improvement in drive efficiency (and thus fuel consumption) makes a noticeable difference on the credit card bill during boating season, and saying "it's 10% easier on gas" is an attractive sales line when gas runs five hundred bucks a trip. The expensive drive rebuild happens ten or fifteen years down the road, far beyond the point where the original buyer (and the warranty department) has stopped caring.

    - "Keeping it in the family", re. Brunswick, is likely a factor in some model lines.

    - There are a LOT of really shoddy sterndrive installations around. Only two or three of the last dozen sterndrive-powered runabouts (new, at boat shows) that I've inspected have had well-designed engine compartments. On some, you have to either remove the deck, remove the exhaust, cooling and electrical systems, or hire an 8-year-old double-jointed midget mechanic with a trained snake to get to such everlasting, reliable bits as, say, a starter motor. One of my favourites is when you see a single 300 gph bilge pump centred under the oil pan, with just two inches of clearance between pan and stringer because the engine mounts are just lag-screwed right into the top of the stringer. This sort of thing is just asking for trouble.
  13. Willallison
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    Willallison Senior Member

    I concur fully with Matt's comments. For me, the improvements in efficiency (which allow a smaller engine, increased range, higher top and cruising speeds etc) and reduced draft that a sterndrive provides, far outweigh any increased maintenance cost. Further, the corrosion protection of the current crop of outdrives is far better than it was ten or twenty years ago.

    And of course it's worth noting that poor installation practices are not confined to sterndrive enginerooms...
  14. Myoung42
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    Myoung42 Mechanical Engineer

    Thanks for your input everyone!

  15. mydauphin
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    mydauphin Senior Member

    Vdrives were used to get propellers deeper underwater and at a straighter angle. Surface drives and propellers have eliminate this application. I had a 45 foot v-hull with big v drives driving propeller from almost mid-ship. It was a fast boat in heavy water, like an old pt boat
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