Building Oars

Hello All,

I decided to build the Oars for my 10 Foot Row Boat, and have also decided to change the design again, allowing for a small 2.5 hp outboard. This blog is about the Oars.

When first starting this project, the first thiing that came to mind is "where can I find information on Building Oars"? I have already gone through a number of books on boat design, and I recalled from my readings that an author had discussed this subject. It did not take me long, and I found the information; the author was Gavin Atkin He addressed the subject in his book Ultrasimple Boat Building: 17 Plywood Boats Anyone Can Build
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The perfect solution for what I was looking for was on page 65 of his book. The oars are actually designed by Jim Michalak. The oar depicted in Gavin's book is 83 inches long or 6'-11". This is approximately the right length for my boat. These are not double paddle oars; they have a 5 inch handle at one end and a paddle at the opposite end (see image at right). I will explain how the length should be derived below.

For calculating the length required for your boat use the following formula:

1. Determine Beam at the Oar Locks.

Then, Oar Length equals approximately (Beam @ Oar Locks/2) * 3 + 6 inches (West Marine)

The oar design looks very nice from the picture in Gavin Atkins book; being a traditional style oar. If you haven't done any woodworking for awhile, like myself, building the oars is a good way to get accustomed to any new tools you have purchased for your boat building project. Total material cost for 1 Oar is under $5.00 Canadian for a nice piece of 1 x 8 spurce, plus finishing cost. The book calls for using a 1 x 6, but a 1 x 6 (nominal) will be too small for one oar while keeping the dimensions the same as what the author calls for. I found this out the hard way.

I finally determined that a 1 x 8 (nominal) would be about the perfect size board for building one oar. The tools I used to make the oars were:

1. Hand Saw
2. Surform
3. 4 Foot Straigt Edge
4. A wood file
5. Tape Measure
6. Powerful Jigsaw (with a blade that does not bend easily)
7. Electric Orbital Sander
8. Electric Planer

I find that good power tools are worth the investment. You need a good powerful Jigsaw and Electric Planer to shape the oar. I used DeWalt and Bosch in that order. The small Electric Handheld Sander was inexpensive but did a good job.

To me, the first time you approach this job there is nothing very easy about it. You have to spend a bit of time laying out all the measurements. Then, you have to cut the three pieces of wood with a jigsaw, being sure to cut on the high/wide side. Finally, I did the final edge trimming using my Bosch Handheld Planer. The Electric Plaer allowis you to achieve very accurate final cuts.

Day 1
Being new at this, it took me about a day to do all the measuring and cutting of the oar. If you plan on building more oars down the line, it is a good idea to make a template of the oar. Then all you have to do is use the template to lay out the outline of the oar on wood. This saves a great deal of time you would be spending on making all the measurements for the oar.

Day 2
The second day you spend gluing the three oar sections together. I used West System Epoxy, by the Gougeon Brothers. They have a great book out called Gougeon Brothers on Boat Constructon 5TH Edition. They also have free User Manuals you can use with their products. I made the appropriate epoxy mix, applied it to the oar surfaces that needed to be glued together, applied clamping pressure, then let the oars set for about 24 hours.

Day 3
Day 3 is spent shaping the oar. With an Electric Hand Planer, Surform, Wood File, and Electric Sander, this is done very quickly; and is no more difficult than any other part of the project. For positioning of the Oar Locks, see page 107 of Gavin Atkin's book.

Day 4
Finish the oar using a method of your choice. The five inch rowing handle should be left as bare wood.

Thre references provided will get you started on building your own oars; especially the book from Gavin Atkin. The Gougeon Brothers on Boat Construction is an excellent book to have if you are new at boat building, are interested in Epoxy on Wood Construction, and would like to learn how to use Epoxy in Boat Construction.

Selecting The Best Motor For A 10 Foot Fishing Boat

Selecting the correct Motor and Propeller for your boat is critical, since the motor and propeller must perform well during adverse weather conditions. My previous post was about Mercury's 2.5 hp motor. 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 your weight. Regulations in my area call for an adult to weight 160 pounds. The motor weighs 38 pounds, so the number look OK from this standpoint.

So now I am left with selecting the smallest (cheapest) 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.

A very popular book for selecting the correct propeller is the :
Propeller Handbook : The Complete Reference for Choosing, Installing, and Understanding Boat Propellers, written by David Gerr.

FourStrokes 2.5 - 3.5 HP | Mercury Marine

Mercury Marine has two small outboard motors. Lightweight at 38 pounds, the 2.5 hp Mercury Motors should work well on a small boat used for freshwater fishing. Reverse Thrust is very easy, and reduced tiller vibration makes for smooth operation. The following link takes you to the Mercury Marine website, and to the page with the specifications for these motors.

I am going to do a preliminary check on how the 2.5 hp motor should perform on my 10 ft boat. A 2.5 hp outboard seems like a better solution to an electric battery operated motor, especially in larger lakes where you would likely run out of battery power on your trip back.

FourStrokes 2.5 - 3.5 HP | Mercury Marine

10 Foot Row Boat - Linesplan and Hydrostatics

This boat was designed with the following factors in mind:

1. The boat needs to be car toppable, so the lightweight of the boat is at 102 pounds.
2. The boat is being built with marine grade plywood that is 1/4" thick.
3. The hull will be encapsulated with West System Epoxy - 3 coats on the hull. The bottom of the hull will be fiberglassed. The interior of the hull will have fiberglass tape at all joints, inside and out with extra reinforcement at the chines. The weight of the fiberglass cloth, fiberglass tape, and fillet dimensions was determined using Dave Gerr's The Elements of Boat Strength: For Builders, Designers, and Owners.
4. Additional Construction Images will be added to this blog as the boat is being built.

10 Foot Row Boat - Linesplan and Hydrostatics using Free!Ship :

Hydrostatics calculated using Free!Ship :
Project : 10 Foot Row Boat
Designer : wayne@freeboatresources.com

Design length : 10.000 [ft]
Length over all : 10.000 [ft]
Design beam : 4.364 [ft]
Beam over all : 4.364 [ft]
Design draft : 0.460 [ft]
Midship location : 4.846 [ft]
Water density : 62.500 [lbs/ft3]
Appendage coefficient : 1.0000
Volume properties:
Displaced volume : 6.177 [ft3]
Displacement : 0.172 [tons]
Total length of submerged body : 9.592 [ft]
Total beam of submerged body : 3.984 [ft]
Block coefficient : 0.3514
Prismatic coefficient : 0.5702
Vert. prismatic coefficient : 0.4642
Wetted surface area : 30.735 [ft2]
Longitudinal center of buoyancy : 4.680 [ft]
Longitudinal center of buoyancy : -1.731 [%]
Vertical center of buoyancy : 0.325 [ft]
Midship properties:
Midship section area : 1.129 [ft2]
Midship coefficient : 0.6162
Waterplane properties:
Length on waterline : 9.592 [ft]
Beam on waterline : 3.984 [ft]
Waterplane area : 28.924 [ft2]
Waterplane coefficient : 0.7569
Waterplane center of floatation : 4.307 [ft]
Entrance angle : 45.584 [degr.]
Transverse moment of inertia : 29.514 [ft4]
Longitudinal moment of inertia : 155.85 [ft4]
Initial stability:
Transverse metacentric height : 5.103 [ft]
Longitudinal metacentric height : 25.557 [ft]
Lateral plane:
Lateral area : 2.923 [ft2]
Longitudinal center of effort : 4.924 [ft]
Vertical center of effort : 0.277 [ft]