PVC Pipe Oar Design

Completely agree, which is why I am building this dinghy of my own design out of wood. So far, the most expense part of it will be the 4 oz glass cloth.

 

pdfithian,

You have gotten suggestions from both practical oar maker/ users and also engineers (I recognize several in this thread and some are probably both practical and engineers).

What you are proposing will work poorly from both standpoints.
One of the very first engineering lessons in a degree shows this will not work well.

You need stiff material on the outside, not on the inside. You need stiff material around the entire circumference, not just in the very ends of an x shape. The dowel in the middle will provide very little stiffness.

If you proceed with your idea, please do a test when you are finished. support the oar at the handle and where it will pass thru the oarlock. attach 20# at the blade. measure the difference in deflection between no load and loaded.
Report back so we can do the same to a different set. I have both carbon fiber and all wood oars and would be glad to compare to your results.

Good luck.

 

Great idea, I'd be glad to do this. Before assembling the oar, I'll do this test on just the PVC pipe with and without the X section inserted into it. I will also weigh each and report the results.

I agree with your statement on the external tube stiffness, which PVC does not have. But my thinking here is that it is very difficult to bend a 1.6" piece of plywood in the direction of its plane. The the cross section should prevent it from buckling.

 

look like a short dingy, maybe 10ft

the oars u have will be too big... been there done that
suggest making the oar blades half the width

polite suggestion... i guess u can do ur own thing, then cut them back later

 

There have been well reasoned answers to your project made by engineers and those who have been foolish enough to try this approach. We get a lot of proposals to use PVC pipe for various purposes in boats. Almost all are ill advised. PVC is weak and heavy compared to good wood. It will take much longer to build to your proposed design and the result will be inferior to wood.

Woven sleeves of fiberglass or even carbon fiber are not good for making a stiff tube either. That is based on experience, not conjecture. The fiber alignment of sleeves is wrong for that purpose. We do see sleeve material on spars, but that is for hoop strength to encompass the interior longitudinal material, not stiffness.

No need for birds mouth complexity in such a small job either. A shaft of spruce or, better yet, selected lightweight fir, is simpler, easier, more rugged and infinitely more pleasing to use and look at.

Your first basic design is pretty good but the PVC is wrong for that use. Many structural beams and columns are built in similar concept but the material on the "outside" or "edge" is always of high tensile and compressive strength and PVC is none of these.

Adding lead to a paddle? Please say that was a typo.

 

The dinghy is 8'8" in length. The distance between the oar locks is 53".

I arrived at the 8' oar length by the following:

53"x25/14=94.6" Next closest increment is 96", or 8'

I also want to be able to stuff these inside the boat.

 

A few things I can think of. Ply wood is weak compared to real wood. The wood will want to rot quickly. With any movement between the wood and the pvc, as when the shaft flexes, the wood will wear away and at the same time eat through the pvc creating more and more play while the pvc gets grooves wore in it and becomes weaker and weaker...an insidious vicious circle.

The most stress will be at the oarlocks, a solid chunk of wood there might prevent buckling.

 

Lead ballast is rather common in round shaft oars. If you make square looms you don't need lead because the extra wood balanced the oar..

The very simplest oars are square shaft with ply blades. They work OK. some detailing needed at the lock.

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seriously, my 18ft dory had 8ft oars with the same blades you have,

The blades were way way too big, when the wind came up they just were a pain to use.
So much force was required to move the oars through the water that it was not funny, even in zero wind, such that the number of oar strokes per minute dropped way down, because the force needed was so massive. Smaller oars will move your boat faster. If the wind comes up... you wont be able to move your boat

Dont know which formula your using, you can do your own thing,,,,, is your boat,,,, you can experiment....... but it wont work. My dory had 53 inches between the rowlocks and the oars just were too big.

I have built Michael storer oars,, they did not work

 

You need to let go of the idea that PVC has anything more than shape to recommend it.
the internal stiffener shown does not put the "meat" where it needs to be, at the outside against the inside of the pipe. This is based on attempting to save the original idea of using PVC pipe at all.
Luckily, you don't have to save that idea. It was imaginative but clearly not practical.
Even if you glassed the outside of the PVC, as said, now that you've used the PVC as a form to mold the outer glass sleeve, how do you get the PVC out?
All the time, you have suggested to you the idea of using solid wood instead. Ask the most experienced boatmen here and the answer is consistent. Wood, available everywhere; cheap, strong, light, easily worked with unsaophisticated tools, it is so perfect for this job, anything else pales in comparison.
Making a set of really special oars using a birdsmouth construction would be nice but its probably beyond the capabilities of a lot of people. They would be noticably lighter but that's not something anyone but a seasoned rower would appreciate after a couple of minutes rowing. Like a super-light bicycle, the cost in time or materials, even if it quadruples the price, never pays for itself unless one is in competition.
Your design is problematic not just in terms of strength, but also in the details. For example, attachment of the pipe to the blade and handle ends. Lots of careful work and glass-taping. Imagine inserting and gluing in the stiffener!
A great set of wooden oars would cost ten dollars for all materials but the leathers and the varnish. For thousands of years, fine oars have been made of solid wood and although new materials like carbon fiber could make a stiffer oar, the cost in no way can be justified under normal conditions.

 

Well this is a fun exchange, isn't it? There are many valid points brought up by the other posters.

The Storer blades can be made smaller without much effort. And the length can be easily shortened if necessary.

Since I've started this idea, I will make up at least one section and check the 20 lb load deflection on the end. I'll post these results so they can be compared to those from Upchurchmr.

If I'm wrong, that will prove it, right? It will take all of about 15 minutes to make one of these to test and only a few dollars for a section of pipe.

If the deflection when loaded looks good, I'll make up a set of these and post user experience. No harm can come from that, can it?

 

Deflection is one thing. More important is endurance. Half of the grain of the ply is going in the wrong direction!

 

Pcfithian,

More power to you as long as you are willing to make the test. We all learn in different ways. hopefully I was not too abrasive.

The testing will help others who have a similar idea understand the actual value. Many times we don't get to the point where others can understand the results. Then we start the discussion over again, and again, etc. There is just about as much value in a failure as a success, if you understand what happened. Sometimes more.

Waiting.....

Marc

 

OBTW, I bet the cost of epoxy is more than the glass (for what you actually use) - beware.

 

I made a really light kayak paddle out of wood, the shaft and most of the blades were hollow. Most who pick it up are supprised, it weights less than a carbon graphite kayak paddle. the same construction method would also work for oars, you might try this as an experiment next to your PVC shafts;

I used two 3/8"x 1.125 battens of clear doug fir that went on the side of the shaft that took the major loading, on the "front" and "back" side of the shaft. I cut two 1/4x 1.5" low density clear red cedar battens and used them to form the two other sides of the box shaft. I used polyeurethane glue and than rounded the all the corners so the shaft was almost oval (kind of a squared off oval). I sued thin sheets of western red cedar to form the blades with a doug fir center web and dough fir layer at teh perimeter (mostly for durablility, so the tougher fir was the exposed edge around the profile of the blade). I made the blades high aspect ratio elips shaped.

Once it was final sanded I put 5 layers of marine varnish to finish off smooth and to seal the wood. They are almost as strong as a solid doug fir paddle, but only weight about 10 oz each (it is about 8 foot long with one paddle at each end).

The same construction method would work for oars, you can make the shaft larger as required, and I would put some one inch cedar blocks (about 1.5 inches apart) inside the shaft where the oar locks will bear on your shafts. On the kayak paddle I had no such concern for crushing my hollow shaft since it is hand held.

The wood I used I salvaged from stuff I had laying around my shop. It is not hard to rip some clear tight grained small strips from almost any lumber (by working around the knots and run out and other defects). You can also use white oak and spruce. the idea is to place the high strength wood in the highly loaded direction, and the light weight low density wood everywhere else to complete the shaft and blade length. Both doug fir and western red cedar are abundant and available where I live and easy to find salvage, or buy at the local lumber yard.

This kind of shaft would cost less, weight less and be much stiffer and stronger than a pvc shaft, and not a lot more work to make than what you are proposing. and it will look like a nice wood oar too, rather than a home made plastic one. It will match your boat much better too. It is fun to see the look of suprise when people pick it up for the first time, it feels like balsa wood and does not feel like it is possible that is is strong and stiff. I demonstrate its strength by bending it against the ground in a way you might string an archery long bow.