|
| How do I find out what outborad engin... |
| Author |
Message |
    
John McQuilter
| | Posted on Friday, April 12, 2002 - 05:30 am: | 
|
My Friend has just purchased a 24feet, 3tonne yacht, it has a 6HP outboard 2 stroke engine, on flat calm water it will only travel at 3 Knots maximum, I have been trying to find out what size of HP outboard He would have to purchase to bring the speed up to 8 Knots.
Suggestions or advice welcome
John
Scotland UK |
Rickyd (Rickyd)
| | Posted on Wednesday, April 17, 2002 - 12:20 pm: |
|
John: looking at my copy of Skene's "Elements of Yacht Design" (chapter IX if you can find a copy) there are first, the calculation for maximum speed on displacement-type boats: the square root of waterline length times 1.3; for a 24' WL the maximum speed (in knots) thus calculated is not quite 6.4 knots. Skene's further notes that a potential range of resistance for a displacement yacht of that speed to length ratio is 45 lbs per long ton (tonne). Since you note the displacement of your friend's boat is 3 tonnes, the resistance is therefore 45*3=135 lbs. the formula for effective horsepower is EHP = resistance X speed X .003, so 135*6.4*.003=2.6hp. If you assume 50% efficiency from the propeller on the motor (a round number for argument's sake) and divide the 2.6 hp by .5 propulsion coefficient, you get 5.2 horsepower needed to drive the 3 tonne 24 foot displacement yacht to its hull speed. So...the 6 hp motor is probably sufficient, but--is the hull clean? is the propeller in good shape? Is the motor working well? is the propeller deep enough in the water to be clear of the water distubed by the hull? could the design of the hull include a deep, full keel and a large outboard rudder that will increase wetter surface therby lowering maximum available hull speed? Unfortunately, it may not be possible to get 8 knots from the boat, no matter what horsepower is behind it, unless you get the boat up on plane--unlikely with a sailboat. (Note: Skene's "Elements of yacht Design", 1973, Dodd, Mead & Company, editor Francis S. Kinney). Good Sailing!  |
Rickyd (Rickyd)
| | Posted on Wednesday, April 17, 2002 - 12:29 pm: |
|
Follow up--recalculating using 8 knots as the goal--first you need a speed to length ratio of 1.64--probably not attainable, but let's assume it is. thus, EHP=135*8*.003=3.24, and using the same 50% propulsion coefficient the horsepower needed is 6.48. That is probably not enough of a change to warrant selling a used motor and spending 1200 sterling on a new one. maybe borrow an 8 to 10 hp and try it out? again, good luck. |
|
|
|
Product Categories |
| Engine Parts · By Brand |
| Engine Manuals · By Brand |
| Books |
| General Repair and Maint. |
| Care and Repair |
| · Adhesive and Sealants |
| · Bottom Cleaners |
| · Cleaners Wax and Polish |
| · Mops Brushes and Sponges |
| Covers and Tops |
| · Bimini Tops · Hardware |
| · Cover Accessories |
| · Motor Covers |
| · Universal Boat Covers |
| Electronics |
| · Gauges and Compasses |
| · Mounts and Accessories |
| Electrical |
| · Batteries & Chargers |
| · Battery Accessories |
| · Dockside Electrical |
| · Electrical Terminals |
| · Electrical Wire, Cable |
| · Fuses, Fuse Holders |
| · Switches |
| · Wire Ties, Clips, Tape |
| Fuel Systems |
| · Fuel Tanks |
| · Fittings, Fills, Vents |
| · Hose & Primer Bulbs |
| · Filters, Senders, Caps |
| Hardware |
| · Deck Cabin Hardware |
| · Molding and Rub Rails |
| · Rails and Fittings |
| · Snaps Shackles and Hooks |
| · Windshield Hardware |
| Lighting |
| · Cabin Lights · Light Bulbs |
| · Navigation Light Parts |
| · Navigation Lights |
| · Spotlights · Parts |
| Mooring and Dock |
| · Boat Fenders · Boat Hooks |
| · Buoys and Markers |
| · Deck Cleats · Dock Edging |
| · Dock Parts and De-Icers |
| · Fender Covers and Holders |
| · Keelshield / Toon-Tectors |
| · Ladders and Platforms |
| Prop / Trim /
Transom |
| · Jack Plates, Wedges |
| · Mounting Brackets |
| · Outboard Stands |
| · Trim Tabs, Stablizers |
| · Prop Nuts, Locks, Pins |
| · Propellers · Wrenches |
| Pumps |
| · Aerator Pumps |
| · Bilge Pumps |
| · Drain Plugs and Tubes |
| · Hose · Through Hull Fittings |
| Rope and Cordage |
| · Anchor Line · Dock Line |
| · Fender Line · Bulk Rope |
| · Shock Cord and Bungee |
| Safety Products |
| · Horns Bells and Whistles |
| · Mirrors |
| · Vents Blowers and Hose |
| Sanitation |
| · Fresh Water Pumps |
| · Marine Toilets |
| · Sanitation Chemicals |
| · Sanitation Pumps/Hose |
| · Water/Waste System Parts |
| Seating |
| · Deck Chairs and Tables |
| · Fishing Seats |
| · Lounge Seats |
| · Pontoon Furniture |
| · Seat Bases and Pedestals |
| Steering / Control |
| · Control
Boxes |
| · Control
Cables |
| · Hydraulic
Steering |
| · Mechanical
Steering |
| · Rigging · Steering
Wheels |
| Tools / Shop Supplies |
| · Lubricants & Additives |
| · Motor Flushers |
| · Tools & Test Equipment |
| Trailering |
| · Boat Guides/Motor Support |
| · Hitches Balls Couplers |
| · Rollers Brackets Springs |
| · Tires Wheels and Carriers |
| · Trailer Hubs & Bearings |
| · Trailer Jacks |
| · Trailer Wiring and Lighting |
| · Winches Straps Tie Downs |
| Manufacturer List |
|
