Planing Hull Slow Speed Performance Improvement?

Discussion in 'Hydrodynamics and Aerodynamics' started by Bing, Mar 2, 2014.

  1. Bing
    Joined: Jun 2006
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    Location: New Hampshire, USA

    Bing Junior Member

    I’ve studied older posts on this topic, so I think I have an informed question here. I own a 24ft, 1973 fiberglass SeaCamper, which is more or less a cross between a houseboat and a speedboat. I’m in the middle of a complete overhaul of the boat and have replaced all of the wood structural members. The boat has new stringers, transom plywood, and transom knees, all properly glassed and tabbed and ready for up to a 200 hp outboard (per original SeaCamper designer notes and other owner experience). The old GM I/O engine is gone. I am re-powering with an outboard for all the typical reasons people do this (not the topic of this inquiry).

    Here's a couple of photos of typical SeaCampers:



    The straightforward thing to do is to install an outboard bracket on the reinforced transom. No problem there, as many SeaCamper owners have done so. However, I only need to power the boat to achieve 5 to 7 mph as our cruising grounds are the NY State Barge Canal. We just don’t need to go 25 mph when the speed limit is mostly 10mph and lower. We like life in the slow lane. I will hang just about any good condition, used 40-60 hp 4 stroke outboard on the transom if the price is right. I can’t afford a new or used 200hp motor, and I think an engine that size would be a fuel hog at slow speeds. Here’s my question:

    Is there anything I can do with the design of the outboard bracket to improve slow speed performance of the boat? While I doubt we will achieve the .75 gal per hour fuel rate at 7mph of say a Nimble Nomad (similar size, weight, and power), I can’t help but wonder if a relatively inexpensive bracket modification could help.

    Here’s a sketch of the aft section of the hull showing a conventional outboard bracket dimensioned closely to those one can buy off the shelf for a 25 inch shaft engine.


    This is what the bracket manufacturer recommends when repowering an I/O with an outboard for planning hulls. I can get this bracket in aluminum, delivered and ready to mount for a tad under $1000.

    I could also have the bracket made to extend the shape of the hull bottom, more or less as shown here, in the interest of easing the flow of water from the deep vee stern, perhaps causing a bit less water turbulence and maybe better fuel economy and handling:


    The up-charge would be about $250. Would this be worth the expense?

    Either way, the bracket is rated up to 300hp. When I sell the boat a new owner could mount a bigger engine and plane away (which is one reason I chose not to build the bracket myself out of ply and glass). Would the fuller bodied bracket negatively affect planning performance?

    Or should I just bolt on the conventional bracket on and enjoy the boat for what it can and can’t do? And no, I can’t buy a different boat. I’ll have about $10K in the SeaCamper when I’m done, so it’s a pretty cheap houseboat, all things considered.

    Thank you,

    Tim Jennings
    Enfield, NH
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    The second design won't decrease drag compared to the first design, and might even increase it. The angle between the hull bottom and the bracket is acute enough that the flow will separate at the junction between the hull bottom and the bracket.
  3. Stumble
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    Stumble Senior Member

    If you are positive abut giving up planning speeds switching to a 9.9 or 25hp high thrust outboard would probably be a bigger savings than any bracket. They swing a much bigger prop and are intended for sailboats restricted to displacement speeds so you take a lot off the top end. On the other hand they have a lot more low speed power.
  4. Bing
    Joined: Jun 2006
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    Bing Junior Member

    It has crossed my mind to use a "kicker" motor for now, hung on a small transom bracket for 25hp and under motors. I wonder though it that's enough power on a windy day with river currents in a tight maneuvering situation.
  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Adding small brackets etc to the hull wont improve the hulls performance at the low end as you seek. Since the hull is a planing hull form to maximise the slow speed range requires a radical change. It would also limit your resale.

    Stumble has put you on the best path. If you're serious about only wishing to potter along, then ditch all that is associated with the higher speed. The engine is the most obvious and costly and thus first on the list. The only thing you may need to address is the change of trim, since a 200HP engine weighs considerably more than a 10-20Hp.
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Leverage man, leverage. Make a light aluminum angle stock bracket for your 10 HP outboard and space it 10' aft of the transom. This should offset the several hundred pounds of trim adjustment with the lose of the engine, tanks, etc.
    1 person likes this.
  7. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    You're going to need some power just to steer this thing. It is not like kicking a sailboat along that has a huge keel and rudder. I think your 40 - 60 guess is a good one. Power steering, too, if this is for canals. Electric start and a decent alternator required. It's tough to judge whether or not an integrated bracket could be made to help. You certainly need to retrim the vessel first. I think finding the best trim will garner a bigger savings than the bracket could ever manage. It's difficult to recover pressure from the area just in front of the prop.

    I wouldn't bother trying for a fancy bracket straight off. The missing hull extension might cost you $20 in gas the first year. If that bothers you, you can always spend the next five years tinkering on it to see if a hull extension works. Door skins and Styrofoam only take 2 hours to cut and tape on and relaunch. You need to look at the wake behind the boat to even guess at what might work. Good luck with project.
  8. dougfrolich
    Joined: Nov 2002
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    dougfrolich Senior Member

    "The second design won't decrease drag compared to the first design, and might even increase it. The angle between the hull bottom and the bracket is acute enough that the flow will separate at the junction between the hull bottom and the bracket."

    This MAY not be true...A Japanese designer Kotaro Horiuchi designed a number of boats with transom extensions very similiar to the one above, and claimed significan drag reduction at slow speeds. He credits the drag reduction to limiting excessive trim and or squat. In his designs he incorporates a slight step at the transom before the extension to ensure the water separates from the end of the running surface at planing speeds.

    ref. Locus of a Boat Designer 2 pp.234. Horiuchi, Kotaro
    Last edited: Mar 5, 2014
  9. Bing
    Joined: Jun 2006
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    Bing Junior Member

    Thank you gentlemen for all of your good comments and suggestions. I'll study what you've offered, talk to my bracket fabricator, then get back with you with what I plan to do.

    Joined: Oct 2002
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    FAST FRED Senior Member

    I would buy or borrow a transom lift bracket and borrow an outboard.

    What ever you cobble up will give a good idea of what you require.

    When you decide and purchase , yes remote start , alt , and high thrust gear/prop.

    I would Guess a 20 Hp Honda would be all that is needed.
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