question for the engineers: oar length

[davebrown]

i am finishing a whisp, which is a rather narrow-beamed skiff that many ofyou are familiar with (i think the beam is about 36", without the plans in front of me here). i want to use a set of 8 ft. oars with it, and obviously the in-board length is severely truncated.

in my rowing club, we have a neat, cheap solution for some lightweight extensions outboard (which i would photograph if i could ever get my secretary to figure out how to download a photo on this #&@** forum). in essence, it is an alloy metal triangle with a 90 degree bend in one foot, which slides down into a wooden slot between the inner gunnels, supported on each flat edge by an extra frame. the oarlock then fits into a drilled teflon bearing at the far point of the triangle, 10 or 12" outboard.

i could get about 12" or so on each side of my whisp with that setup, which would substantially increase my effective beam, for rowing. hope that makes sense.

but here's the issue: except for the awkwardness of rowing with so little of the oar inboard, doesn't that effectively shorten the oar? it seems we would be reducing the leverage of the oar, which is the whole purpose of using a longer oar. this is kind of a mental exercise really, because either way that is the setup i am going to use. but i would be curious to hear from the engineers about this idea.

[Thorne]

Outriggers don't "shorten the oar" -- they just move the fulcrum to the most efficient location. Personally I'd go for the longest oar you can fit on the floorboards of the boat, unless you plan on doing a lot of rough-water rowing. I've moved from 7' to 8', but find I prefer my one pair of 8.5' oars even more...

You can use bent plywood to do the same thing. I've showed some old pics of the system used by a number of local boats, which is all wood except for the fasteners and oarlock. Here's a newer pic from the recent Petaluma River Row -

http://www.luckhardt.com/pet09-14.jpg

Here is a good shot of the base for the outriggers on another boat at the same event -
http://www.luckhardt.com/pet09-6.jpg

[Bobcat]

The bent plywood for the outriggers: Do you bend plywood or laminate up the shape out of plys?

[davebrown]

thorne: your photos and comments are always helpful. i sent you a reply yesterday regarding your herreshoff, which i did not notice for several months.

regarding the bicycle analogy, that being my second favorite sport, that is very well put. with rowing, i tend to do 2-3 hour rows at fairly slow speed, which means that i would say i am not a torquer, but a spinner. half marathons, not sprinting, to bring in another analogy. as i said, either way, i am going to use this sytem. thorne's photos look pretty similar, conceptually, to the ones used at my rowing club, except we use a lightweight alloy with a fiberglass bearing. they are very quick and easy to use.

well, maybe i will someday master the black art of posting a photo so i can send some pictures for others like-minded.

as an aside, it is amazing how much extra time i can take building a simple skiff than necessary.

[Thorne]

Bobcat -- don't know as I'm just the guy behind the camera. I've always assumed that they were built from single plies that were first bent, then epoxied together.

Dave, if you are really a spinner you might like shorter oars, hard to say. They are easy to shorten but can't be made longer...

[Cuyahoga Chuck]

The Shaw& Tenney calculation for a boat so narrow (42") is only 6.25 footers. Round down for pleasure round up for speed.
I assume these folks are familiar with how much continuous HP the average rower can come up with and established their lever arms accordingly. Accepting their assumptions you would need to overlap the grips several inches to maintain the same ratio with 7 footers.
If you reverse the S&T model and calculate how much distance you would need between the oarlocks to accomodate 8 ft. oars it figures out to about 54 inches with no overlap. You would, therefore, only need 6 " per side to stay in the comfort zone.
If you moved the pivot out 12 " you could use an oar nearly 10 ft. long. I don't think I would want to be in a boat that narrow with that much lumber hanging over the sides.
The S&T oar-length calculation is;
½ the distance between the oarlocks divided by seven and multiplied by 25. The only caveat is the pivot point should always be at least 1 " inside the balance point of the oar. That doesn't effect you.

[davebrown]

chuck: good info. so something between 8 and 10" would be suitable. however, you may have the actual beam (42") for whisp. i posted this without having the plans readily available. i seem to recall the beam is narrower. either way, the ratios make sense.

[davebrown]

btw thorne, what is the white double-ender in your second photo?

[Thorne]

Dave - I think it is a Natoma Skiff, a Sac/SF TSCA member's design that is based on the Herreshoff Rowboat. Here's a few more pics, including the same outriggers (so we sorta stay on topic) -

http://www.luckhardt.com/natoma12.jpg

http://www.luckhardt.com/natoma14.jpg


My recommendation is to start another thread with a more specific title, perhaps something like: "oar length & outriggers on Whisp?" I know that there are several Whisp owners on this Forum.

[gofish]

I grappled with this same issue sxactly a year ago. Cayuga Chuck is right on the money with the calculations but a call into Shaw & Tenney proved invaluable to me.
An important factor is the height of the oar lock above the seat. The inboard part of the oar needs to clear thighs and knees (parts on the rower-not parts of the boat) on the back sweep.
They'll help

[Steve Lansdowne]

I tried 6.5 foot oars on my Whisp after using the Shaw & Tenney formula and found them to be too short, then read that 7' oars are recommended. You need to carefully plan the height of your oarlocks, as on my Whisp I tend to hit my knees with my hands when rowing unless my legs are more straight out than I want them to be. The solution would be higher oar locks than I have. Mine are about 1.5" above the gunnel. Take this into account when planning your setup, and also carefully plan where your rowing foot braces will fall or make these adjustable -- too close to the seat raises your knees higher and thus can interfere with rowing.

[Chip-skiff]

One reason for using longer oars is if the height of the rowlock above the water gives you too steep an angle, which in turn leads to digging the tips into the sand or rocks when you're in shallows.

Good to put the boat in with the planned load and use a piece of bamboo or fishing rod to eyeball the angle. (Before buying a new set of oars.)

[davebrown]

i also tend to row while sitting on a flotation cushion. this effectively raises the seat height a bit too--more chance to whack my knee. i didn't realize this post would raise so many different responses.

[Thorne]

... i didn't realize this post would raise so many different responses.

LOL -- and welcome to the Forum of many differing opinions...

The knee-whacking can be reduced in several ways -- the low-profile gel cushions available for kayaks and canoes are more comfy than the floating cushions and 1/4 the height. I use one and love it.
http://ecx.images-amazon.com/images/I/51MuYhREhdL._SL500_AA280_.jpg

You can also raise the oarlock bases by replacing them with something taller -- I'm doing this right now on the Cosine Wherry I'm restoring.

[boylesboats]

6-1/2 or 7 feet oars is more likely for your Whisp's 3ft beam.. 8 feet oar is bit of wasted overkill..

[Tom Hoffman]

Just have to chime in here. My Whitehall, has a 54" beam. When I made my oars, they ended up being 9'6" and they are a tiny bit on the heavy side outboard.

What is the distance between oar locks on sculls or shells with the outriggers. I had been thinking about makeing some extension as shown in the previous posts.

What distance should I shoot for, for the longer oars/sweeps. I have already cut the oars down once, I really hate to do it again.

Thanks,

Tom...

Here is a shot of the Whitehall.

http://home-and-garden.webshots.com/photo/2409270020070055750gsfUnX

[johngsandusky]

My 2c: shorter oars also alleviate the knee-knocking. The longer the oar, the closer it is to level. I love long oars for ease, speed, straight tracking. But I quickly found that my narrow, shallow, melonseed rows best with shorter oars (maybe 6.5', I'm not with the boat right now) I sit on two cushions on the bottom of the 'seed for rowing, she has no thwarts.

[Thorne]

Tom -

Try adding some lead to the handle end to balance those oars, or see if you can shave / sand off some of the blade or shaft thickness if they have any to spare.

Otherwise I'll guess that there is no hard and fast ratio for the oarlock width / oar length / oarlock height calculation -- too many other factors make it pretty complex. 54" beam should be plenty to give you the necessary spread on the oarlocks, don't think outriggers would help much on that 20' boat.


http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=002gp1

[Ben Fuller]

I have found the Shaw and Tenny formula gives me oars that are about 6" short for my rowing style.

Height off water is a real issue. Example single sculls et al are rigged at 60 inch between outriggers and row with 9'6" oars. I tried a pair I had with my Harrier and found that they were too short. I had the 60 inch beam fine but I was probably a foot too high. I escalated to 10 footers made using the normal shape long blade. These were much too heavy. So I went with a serious blade trim. As it happens my faering oars are about that lenght. Blades are only about half the size of the " normal " blade, in fact they were about the size of the blades on the 9'6" racing oars. Once I reshaped the blades to match the faering blades and reshaped the loom to take out weight ( oval it with max dimension at 90 degrees to the blade, the oars became a pleasure to row. This tells me that there is no reason for a rowing boat ( rigged for sculling with two oars per rower) to have blades bigger than what is normal in racing circles. Sweep oars would have bigger blades.

[davebrown]

ben:is your faering an IO, or ??

[Ben Fuller]

Faering is a real one. If you moosh around on the Small Reach image pages you may find a pic. I don't have a pic of the oars handy, but could take one and get dimensions.

[Paul Maselli]

6-1/2 or 7 feet oars is more likely for your Whisp's 3ft beam.. 8 feet oar is bit of wasted overkill..

I built whisp over twenty years ago and have been rowing her with a pair of 7 ft long ash oars. This is with the oarlocks placed as Redmond designed them and have had no problems with the handles knocking. There are times when I row with the oars pulled in a bit and row with a cross over of the handles allowing one to pass over the other.

More iportantly the boat pulls beautifully with 7 ft oars and is a pleasure to row. FYI I built mine using 5 mm plywood, cedar and mahogany and she weighs in around 70 Lbs.

Hope this helps Have a great row......

[boylesboats]

I built whisp over twenty years ago and have been rowing her with a pair of 7 ft long ash oars. This is with the oarlocks placed as Redmond designed them and have had no problems with the handles knocking. There are times when I row with the oars pulled in a bit and row with a cross over of the handles allowing one to pass over the other.

More iportantly the boat pulls beautifully with 7 ft oars and is a pleasure to row. FYI I built mine using 5 mm plywood, cedar and mahogany and she weighs in around 70 Lbs.

Hope this helps Have a great row......

I am surprised.. it did work.. It a things for people.. different stroke for different folks

[Tom Hoffman]

Thorne,

I can definitely shave off some of the weight. The designer called for 3 - 1/8th in plys laminated with fiberglass and epoxy over the mold for the spoons. Later on, I finally found to late that he then thinned those down to 1/16 plys and then also changed the thickness of the staves for the looms and shafts. They are definetely to heavy on the outboard end. I will get to work with a spoke shave and sand paper and may drill the looms and add lead shot to add wight. The looms are hollow.

Thanks for the suggestion.

Tom...

[Thorne]

Tom - I think you want to add the lead into the center of the grips, as you want the max leverage effect with the minimum amount of actual weight in the oar.

[rbgarr]

I rowed the Whisp available at the WB School's waterfront quite a bit when I was there for an 'alumni weekend'. I don't remember the oar lengths for their boat, but suspect they were seven feet. In rough water the blades hit the wave tops quite a bit, despite lowering my hands on the recovery, and the seat being so close to the bottom of the boat meant my legs had to be locked pretty straight all the time so the hands could clear my knees. That wasn't a very comfortable position to row from, at least for me over long stretches of time, but the boat was light to carry and launch and swift in a straight line.

[davebrown]

thorn is correct, weight on the handles is what you want. i like the way those traditional norwegian oars look that have the big rounded ends at the handle, which serves the same purpose as a drilled and weighted handle.