What Length Oars?

 You built/bought a boat that needs oars. How do you determine what length of oars?

A rule-of-thumb is to double the span (the distance between the oar locks). However, this may not be the best length for the oars. For example, if the freeboard is greater than normal, then longer oars may be needed.

Having struggled with deciding what length of oars to make for my Ross Lillistone Flint, I created this worksheet.

Rowing Geometry Worksheet (Click on image to enlarge)

(You can download or print a full-sized JPG version of this worksheet from Dropbox.)

An Example:

For this Oar Cruiser, the span is 52" (1321 mm). The distance from the oar lock to boat's centerline is 26". The distance from waterline to bottom of the oar lock is 20 " (508 mm). I placed a dot at the intersection of these two dimensions on the worksheet above. 

Using an 8' 6" (scale) oar with a pencil mark at 7/25ths from the end of the handle. I placed that pencil mark on top of the dot and the blade just under the waterline, as in the photo below.

Rowing Geometry During the Power Stroke

This shows that when I'm rowing, the handles will almost touch (about 1" (25 mm) apart.

I then changed the oar to horizontal for the recovery stroke. Now the handles overlap about 1" (25 mm) which means on the recovery stroke, I'll have to cross my hands.

Rowing Geometry During the Recovery Stroke

Using this Rowing Geometry Worksheet, I've been able to confirm that 8' 6" oars will work.

See this post for additional information on rowing geometry. 

Another Oar Cruiser based on the Pacific Troller Dory

For this Oar Cruiser, I used an unmodified hull of Paul Butler's Pacific Troller Dory. I added decking and another alternative for weather protection while sleeping aboard.

Another PTD Based Oar Cruiser Buttoned up for Sleeping

Covering the cockpit are two hatches with fixed skylights. Two mooring/anchoring cleats and a fairlead at the bow facilitate anchoring without climbing on the foredeck. See writeup at the end of this post for details. 

While Rowing, the the Two Hatches are Stored on Deck

The hatches are held in place while rowing by side coaming extensions and a tie-down strap (not shown). The reverse reading compass is on the after hatch with a sight line from the rowing seat.

Top View Showing Slatted Floorboards, Rowing Seat (on left) and Foot Rest 

The outriggers hook onto the side coamings and are adjustable fore and aft and can be easily removed by sliding them forward and lifting them off the coaming. A detailed description of these outriggers is here.

Each side of the Hull is Fitted with Two 4" (102 mm) Clear Deck Plates

Two ports on each topside provide ventilation while sleeping. Oars are shown in racks would include tie-downs (not shown). The coaming is 4" (102 mm) high and includes oak strips enabling movable outriggers.

Specifications:

• Length:15' 4" (4.7 m)
• Beam: 48" (1219 mm)
• Estimated Water Line Length: 13' 9" (4.2 m)
  
  
• Est. Water Line Width: 24" (610 mm)
• Est. WLL/WLW Ratio: 6.9:1
• Est. Hull Speed:  5.2 knots, 9.6 kph, 6.0 mph
  
  
• Cockpit length: 6' 6" (2.0 m)
• Cockpit opening: 4' 6" by 24" (1.4 m by 610 mm)
• Cockpit headroom with hatches: 24" (610 mm)
  
  
• Two watertight end compartments: 2' 3" long (686 mm) by hull width
• Two storage compartments (inboard of the two end compartments), with hinged openings, are 2' (610 mm) long by hull width
• Span is 52" (1321 mm) suggesting 8' 6" (2.6 m) oars.  

Summary-Pros:

• Plans are in the form of a booklet that describes every step for building the hull and is easy to follow.
• The full decking and coaming add to the sea worthiness of an already sea worthy hull.
• The two hatches, when closed, provide a weather proof sleeping cabin that will stand up to high winds from an evening thunder storm. 
• She has sufficient protected storage for supporting a multi-day cruise.

Summary-Cons:

• Too heavy to car-top.
• Windage (top hatches stowed on decks) will be greater than a plain hull. Leave the top hatches at home when you use it as a day boat.

This PTD Oar Cruiser is, in my opinion, an effective compromise between a fast day boat, such as Gavin Atkins OarMouse, and other Oar Cruisers with temporary shelters.

Andre-Francois Bourbeau's Gorfnik

Gorfnik  is an 8' (2.4 m) cruiser that is primarily sail propelled and uses a 'stand-up' paddle as auxiliary power. She has been extensively cruised in a wide variety of waters (see the write-up in plans description referenced in "Plans:" below.) Andre-Francois used the PDR hull design and then added a 'cabin' and decking to enable cruising with sleep-aboard accommodations and sufficient storage to be totally self-sustaining for at least a week.

Gorfnik with Spars, Paddle and Anchor Stored. Leeboard in Raised Position.

Classic PDR Hull with Rudder Raised in Michael Storer-Style Holster

Birdseye View Without the Two Hatches of the Cabin Top

Cabin with the Designed Access Port to Side Storage Compartments

Plans:

Free plans are available on the PDRacer site.

Specifications:

• LOA: 8' 4" (2540 mm)
• Beam: 4' 2" (1270 mm)
• WLL: 7' 3" (2210 mm)
  
  
• WLW: 4' (1219 mm)
• Two side compartments full length of hull, full height of hull and 6" (152 mm) wide
• Freeboard: 13" (330 mm) to deck
  
  
• Sail: Sprit rigged, 66 Sq. Ft. (6.13 sq. m)
• Mast: 12' 6" (3810 mm)
• Sprit: 11' 6" (3505 mm)
  
  
• Boom: 8' 0" (2438 mm)
• Cabin top without hatches: 2' 5" by 2' 5" (737 mm by 737 mm)
• Headroom from sleeping platform: 36" (914 mm)
  
  
• Headroom from removable seat: 24" (610 mm)
• Fore Deck: 4' 0" by 3' 4" (1219 mm by 1016 mm)
• After Deck: 4' 0" by 1' 3" (1219 mm by 381 mm)
  
  
• Side Decks: 8" (203 mm)
• Rudder Blade: 3' 3" by 10" (991 mm by 254 mm)
• Paddle: 7' 9" (2362 mm)
  
  
• Leeboard (starboard side only) 3' 0" by 10 " (914 mm by 254 mm)
• WLL/WLW ratio: 1.8:1
• Hull speed: 3.6 knots, 6.7 kph, 4.0 mph
  
  

Possible Modifications:

• Make the side compartments 8" (203 mm) wide and make them open topped rather than accessible by a port. This would eliminate the 'flotation' that the closed compartments provide, but would provide significantly more accessible storage.
• Add oarlocks to the forward end of the sides of the cabin at the top (see example in Skow 244) in order to row standing up facing forward, eliminating the 'stand-up paddle'.
• I would make sure that the removable seat would be low enough and risers long enough that I could sit in both the aft end and the forward end of the cabin without hitting my head on the top hatches. This may require building the hatches with a greater arch than plans show.
• I would add skylights in the two top hatches so that I could see the sail when the hatches are closed.
  
  

Summary-Pros:

• A mini-cruiser that I can sleep, cook and eat in, protected from rain, without having to set up a temporary shelter.
• Capable of at least a week-long cruise.
• Can be sailed and rowed (with the modification).
• A full set of (free) plans that make it a straight-forward build.

Summary-Cons:

• Short WLL will keep speed low.