Motor Selection for 300 LB Displacement Hull

Hello All,

I need to determine the motor size for a 10 foot fishing boat that will be used in small freshwater lakes. The following information outlines how I determined the best motor size for the 300 pound displacement hull. You can view the complete design parameter in one of my previous posts, or at my blog : http://freeboatresources.blogspot.com. Any comments would be appreciated.

I found information on a 2.5 and 3.5 HP Mercury Motor at http://www.mercurymarine.com/engines/outboards/fourstrokes/2.5-3.5/. This particular motor has 2.5 BHP and 1.9 SHP. At full throttle, the propeller is rated at 4500 to 5500 RPM, and it weighs only 38 pounds.

My 10 foot fishing boat is a Displacement Hull, and it's estimated weight is 102 pounds, just making it car top-able. This along with a lightweight, 38 pound motor makes it very easy to take your boat and motor just about anywhere your car or truck will take you.

Determining the exact motor size you need is not an exact science. Your may must want to ask the dealer what size motor you need. This approach is really not the best unless the dealer has design information about your boat. Your best approach for selecting the best motor is to do a little research yourself; determine what motor size you feel will be best, and then contact a dealer and make your final selection.

My small 10 foot fishing boat has a displacement of 300 pounds. The boat will weigh approximately 100 pounds, leaving 200 pounds for boater, gear, and motor. This is more or less a personal use boat depending on the weight of those using it. Regulations in my area call for an adult to weight 160 pounds. The motor weighs 38 pounds, so the number look OK from the standpoint of one adult user (with motor), or one adult and child for rowing.

So now I am left with selecting the smallest (and lowest cost) motor and propeller possible. The Mercury 2.5 hp motor has a 6.5 inch prop, and is not interchangeable.

Visit http://www.youboat.net/boatPower3.aspx to follow the example below:

My 10 foot fishing boat has an LWL (length on the wateline) of 9'-4". Using a graph as a very simplified method for determining HP, the graph at YouBoat.net tells me about 2 HP will work in calm conditions. To this number, add 35% for adverse conditions. Adding 35% puts me at 2.7 HP, so the 2.5 HP motor is just shy of it's required value. Since all of this is not an exact science, I am going with the 2.5 HP motor; the next size up is the Mercury 3.5 HP motor, which costs more and weights more.

Next, you need to determine the propeller size. Propking is a nice program for this. Propking is a Microsoft Excel Spreadsheet program. If you have all your design information, enter the information into Propking, and the program will calculate the Minimum and Maximum size propeller your boat motor should use. For a displacement hull, you will normally want to go with the largest diameter propeller you can use.

The Mercury 2.5 HP motor has a 6.5 inch diameter propeller. Propking tells me that the smallest propeller I should use is 5 inches, and the largest/best propeller I should use is 7.6 inches. The 6.5 inch propeller falls into the midrange, so Mercury's 6.5 inch propeller should work well.

 

Welcome to the forum.
You will find all sorts of participants here. Many of them are very accomplished and experienced boat designers. I do not include myself in that category.

I looked at your blog. I will be the first to refute your claim that boat design is simple or easy. The single book that you recommend barely scratches the surface for competant boat design. You show a tabular sheet that contains Cp, displacement, etc.... Some of those numbers have signifigance, several of them are pure show business with respect to a little dinghy.

The ten footer that you have drawn will float. Beyond that positive virtue it has some points to criticize. Use one of your programs to heel the boat 5, 10, 15 degrees. Observe that the transom corners will drag. Not good. Perhaps you do not intend to heel the boat while operating it. Wave action will do that for you whether you intend it or not.

You blog is obviously well meaning but it is somewhat misleading for the casual reader. Boat design is not to be taken lightly and 176 pages in a single book does not an education make.

About the outboard; Dont sweat it! I have and old Merc 2.2 (actually a Tohatsu with Merc branding). That 28 pound motor pushes a 19 foot FD, a 20 foot inland lakes scow, and a 16 foot flattie quite satisfactorily. Consider the hull speed of your dinghy. You might get the boat to do about 4 knots with the smallest of motors, even a mid sized electric troller. Overpowering the boat will not gain measureable performance.

Do not take these comments personally. They are certainly not intended in that way. Some of the really smart ones may weigh in here with more encouraging commentary.

 

Hello, thanks for the reply. I am green at boat design as I can be, so should choose my words carefully when talking about the boat design process. I'm positive I wouldn't get a job designing boats with a Boat Design firm without a degree in Naval Architecture. Any newbee out there should feel the same way unless they have the right connections. I think I'll open a thread on this subject. At any rate, I think the 2.5 hp motor will work fine for this 300 lb displacement hull. A speed of four knots is what I came up with also. I am not looking for speed anyway, just to get from point A to point B safely and in a straight line as often as possible. I prefer to run half-throttle to conserve gas. If the boat will move as fast as I could row it would even be fine with me (I spend all my time fishing when I'm on the water). The smaller the boat is, the harder it seems to find information on motor and propeller sizing. I suppose motor and propeller selection becomes even more important when purchasing an Electric Motor. I would consider an electric motor, but don't like the prospect of the battery going dead (one lake in my area is 5 miles long).