I gave this problem some thought and I think this approach makes sense.
Figure 1 is a schematic of a drift boat floating in water with two oars held horizontal. The letters A, B, C, and D mark points along the length of the oar. The overall oar length is AD. The length of the oar that is submerged in water during a stroke (i.e., the paddle section) is AB. The length of the oar inside the oarlock is CD. All dimensions are relative to the centerline of the oar shaft.
Length AB is based on the design of the particular oar being considered. If you are purchasing the oars from a manufacturer, then this dimension is known based on the characteristic of the paddle.
Length CD is based on the design and ergonomics of the boat, and the preferences of the oarsman. For example, length CD can be determined by sitting in the boat and simulating rowing with two wooden dowels. Figure 1 shows a gap between the ends of the two oars; I set up my drift boat with about a 3-inch gap as not to run the risk of pinching my hands between the oars. In this approach, length CD can be calculated based on length CE, which is the distance between the two oarlocks at the centerline of the oars. However, some oarsmen may decide to overlap the ends of the oars.
Therefore, length AB and length CD can be easily determined by some very straight forward considerations. However, at this point length BC remains unknown.
Figure 2 is a schematic of the same drift boat floating in the water, but now the left-side oar is dipped into the water such that the paddle is fully submerged up to point B. This is the assumed typical stroke position of the oar. The desired location of point B is probably a few inches above the shaft from where the shape of the paddle ends, such that the paddle is fully submerged in the water during the stroke.
Figure 3 is a schematic of the of the same drift boat floating in the water, but now the right oar is removed. The letters F, G, H, and I mark key elevations along the length of the oar. Vertical distance GI is based on the design of the boat. It is the vertical distance from the bottom of the boat to the elevation of the centerline of the oar shaft positioned in the oarlock. Vertical distance HI is the draft of the boat at the position of the oarlocks. Of course, for any given boat draft will vary as a function of the cargo loading. Draft can be easily measured while floating the boat in calm water. Vertical distance FG can be determined along with length CD as described earlier with the two wooden dowels used to simulate rowing.
Therefore, with a little ingenuity, it is a straight forward task to determine lengths AB and CD, and vertical distances FG and FH.
Figure 4 is a schematic diagram of the oar and associated dimensions from Figure 3. Also shown is the trigonometry and algebra used to calculate the oar length based on the dimensions described earlier.
The bottom line is that the oar length can be calculated based on lengths AB and CD, and vertical distances FG and FH. With minimal effort, each of these dimensions can be determined for any combination of drift boat and oarsman.
Another method: 16 or 17 foot boat = 9 foot oars.
Two additional variables to consider: angle A, the angle of your oarlock and height S, the height of your seat. Angle A can be important as pointed out by Robb Grubb and others and I have to concur. In certain situations an oarsman needs to push the handles of his oars down enough to keep them out of the water to avoid obstacles or such. If your oarlock angle combined with the diameter of your oar wraps precludes full extraction from the water it can be problematic. The height of the seat in the drift boat can also create issues if it is too high causing issues with impingement of the oar handles upon the oarsmans knees or thighs.
While neither of these issues can relate directly to oar length they can indirectly affect the use of the oars thus impacting the length of oars that feel right to a particular person. Secondarily rowing styles and the rivers and the rapids a boat travels through or down also greatly affect selection.
An example would be rowing a shallow river with fairly steep gradient i.e. the Middle Fork of the Salmon requires an oarsman to not allow his downstream oar to interact with the river bottom causing broken oars, sprung oarlocks or an oar being shot from the boat like an arrow. Also because your are in shallow water it can be difficult to get enough bite with a partially submerged blade if the oar angle is steeper rather than more shallow. In this case I prefer a longer rather than shorter oar so I can get a full bite. Sawyer's Shoal Cut oars are another method of doing a similar thing to get a full bite in the water.
My point is there is no substitute for trying a variety of oar lengths in your own boat in the water you most often row in to find what works best for your style, the river(s) and your boats design. Multiple paths often lead to the same destination, each of us is free to find that path which works best for us.
Let me explain further. Please forgive my nature, but I’m not a rule-of-thumb kind of guy. And there seem to be plenty rules-of-thumb out there regarding how to calculate/estimate oar length. So I figured that I’d try to develop a more rigorous approach to this problem that made sense to me. Certainly I’m not the first to come up with these equations; but that said, I’ve not seen them elsewhere when I was looking. They are probably just buried in the mountain of information and suggestions on this topic.
Here’s how I see this working…
Step 1: Locate two straight dowels (or some equivalent) long enough to reach the oar locks for this exercise. Now sit in your land-locked boat with the two dowels and pretend you are rowing. Based on what feels comfortable to you during this exercise, you should be able to take reasonable measurements of CD and FG. Some people like the ends of the oars to overlap; I prefer a gap as not to create a pinch-point for my fingers and hands. You probably don’t want to row so low as to knock your knees, or so high as to hit your face. About mid-chest is probably a comfortable position.
Step 2: Next, standing next to your land-locked boat, measure GI and subtract a few inches to account for draft in order to get an estimate of GH. Alternately, you can float the boat and measure GH directly. But an estimate is probably good enough. Then add FG to GH to get FH.
Step 3: Next, presuming you are purchasing oars, select the oars of interest and determine the paddle length, which is AB. In other words, AB is the length of the oar below the water line during a “normal” stroke.
Step 4: Next, use AB, FH, CD, and FG to calculate oar length. It may be best to take this calculated measurement and round-up to the nearest ½-foot increment. This should give you a pretty good estimate for oar length. Consider it an unabridged rule-of-thumb.
Of course, the best advice comes from Rick and Chris, which is basically “try it, before you buy it”.
Guy, when I get a chance I will check your ideas against the oars I use just to see how they measure up. While rowing a raft down the Colorado I was plagued by an oarlock stand that would not stay in place and it was rather strange to have it move in the big waves.
Chris, Robb's boat has quite tall sides and is also fairly wide. I hope he will chime in and alert us to: a. what the current oarlock angle is and b. what he changes it to.
Guy, how has your boat been working out? It is working as planned? Any changes you would make?
PS I chose my oars based on price. Fifty dollars for a pair of Sawyers couldn't be beat. Of course I had to rebuild one and refinish both of them! My 16' Tatman, measured from stem to stern rows well with 8' 6" oars.
I'm pretty pleased with the way the boat turned out. I did add a cleat (see picture) in order to better secure the anchor rope, particularly for those times when the anchor is raised and there is a Labrador retriever bouncing off the sides. Based on my experience so far, I wouldn't change anything regarding the design. It seems to handle well, but I'm a novice and haven't run anything approaching difficult.
Otherwise I've been enjoying the summer in Idaho.
How are things with you?
Things are going well. I really enjoyed my trip on the Colorado, it was a challenge and a dream come true. I just returned from fishing Kelly Creek and the North fork of the Clearwater River. Caught some nice cutthroats and rainbows. A couple of weeks ago fished the North Fork of the Coeur d' Alene and the St. Joe. Had better luck on the St. Joe. Didn't take the drift boat but still had fun.
Headed to Livingston, MT soon for a Project Healing Waters conference and the IFFF activity then float the Yellowstone for a photographic expedition for a client. We will do four days on the section the Lewis and Clark traveled upon.
Will take a veteran fishing in our annual Two Fly contest on the Yakima River where 25 teams fish for a day. A week later travel to the Seattle area for Salmon for Soldiers a one day salmon fishing trip for veterans. Hopefully some of the 1.6 million Chinook Salmon that are supposed to arrive soon will make it in my freezer.
After boating big water I want to deck in the bow and stern areas creating buoyancy chambers so I can do some more big water again. I realized how much I missed it.
So overall really busy and enjoying life and my boat, rivers and fishing.
Sounds like a pretty awesome summer, Rick! You're a good man to be helping our veterans. I picked up a new hobby this summer, shooting IDPA pistol matches. It's great fun!
Holy smokes, that is some serious work.
Just a minor problem with the idea. I run white water and class III fishing stuff with about 10 other boat builders. They all like different oar configurations and oar lengths. I am the odd fellow in that i pull 9 foot and 9 foot 6 oar lengths. All the reset are using 10 foot oars. Some use 10 foot ash oars that are like steel beams in that they have zero flex.
This is not science but preference and that makes many experienced rowers wrong most of the time.
Don't try to make this science. Row a bunch of different oars, aluminum, composite, wood, heavy and light and find what you like. This is not physics, is rowing. You might find that the oar you like doesn't fit the math.
Now matter, what I bet I am always wrong.
The boat width is also a major factor here and must also be considered, i have rowed many different size boats with alot of different oar length combo's over the years and i always tend to lean toward the longer oar length with exception with my current boat. I am going to have to agree with Larry though that you really need to test your theory and row a few different oar lengths and type, everyone has a little bit different rowing style and that also plays into oar length as well. As far as oarlock angle, i have always kept the angle the same as the sides and never had a problem but would recommend that you go with the biggest diameter oarlock possible, this will allow for a greater angle to the oars when needing to lift your oars up over rocks and such and will give more flexibility to your rowing than the standard oarlock. Last year i bought some of Ken Helfrich's custom oarlocks and dont think i will ever use anything else ever again...........the range of motion that you have with a larger oarlock is pure freedom on the water
Gents, I can appreciate the math to take up a rainy Saturday afternoon, but as some others voice, it really is all about your individual needs, techniques, capabilities, boat width, side height, seat height, etc. and what and how you are going to row your boat down the river. The main elements that I can see from my 35 years of rowing flatwater and whitewater rivers (yes I will boast on this one, since I am very passionate about this subject), are to:
_ make sure you have enough leverage from the oarlock to your hand to pull or push effectively (this means more or less oar length from the oarlock to the blade to determine good leverage),
_ your seat height / distance from oarlock matches the oar length / angle to comfortably row without wearing yourself out,
_ enough oarlock inside diameter to feather and ship your oar in and out / up and down to avoid obstacles,
_ enough spacing between your oar doughnut / stopper and your body so it can't sweep by your body and pin you (this means, keep your doughnut closer to your hands) This element is always dependent on individual taste on rowing against the doughnut stop or not. I personally "free float" and row not touching the stopper.
Overall, sometimes you need a short oar for tight, technical, boney water and sometimes you need a long oar for cruising down class 1-11 rivers and lakes, sometimes you forget to pack your own oars and trade "all" of your beer to borrow some at the boat ramp...
Happy rowing, Robb