Any mechanical engineers, metallurgists, or metal fabricators on NCKA?

I'm about to try and build my own design for a kayak specific downrigger, but I am a bit hung up trying to figure out what kind of metal to use for the downrigger arm. The arm will be made from a 2' length of 5/16" metal rod. The metal types I'm considering are:

316 Marine Grade Stainless Steel, yield strength: 60,200 psi
15-5 PH Stainless Steel, yield strength: 140,000 psi
17-4 PH Stainless Steel, yield strength: 145,000 psi
7075 T6 aluminum, yield strength: 73,000 psi

My concerns are that I don't want the rod to bend and I don't want it to be so brittle that it snaps if I were in rough water. From the tiny bit I understand about metal design, the yield strength determines the resistance to bending. I don't know how to check for brittleness.

My other concerns are cost. 316 steel is the cheapest, 7075 aluminum is mid-range, and the 17-4 steel is the most expensive. I'd like to go with the cheapest that will do the job without worry.

Does anybody know much about these types of things?

Well I'm none of the above but I do have some experience running downriggers on boats kayaks and trolling lbs of lead from the yak minus the downrigger.


IMO....and worth exactly what you paid in consulting fees  ....

I have big scotty downriggers on my boat with adjustable length stainless booms.  I usually run em out to about 4'-4.6' feet to keep em out of the prop but they extend to 5' I believe.  I usually run em with 15-18lbs of weight and have had them in some pretty sloppy conditions, I think the cable would snap before the boom...probably different with spectra....anyway I've never felt that they were close to snapping although they do flex a bit. 

In yak trolling there's unique issues that require a different approach.  Longer booms get in the way of the paddle stroke and in general, the further your weight is from the centerline of the yak is the more drag your gonna get on that side resulting in alot of one sided paddling......it sucks.  I think a short composite or nylon boom would be plenty strong, something adjustable to allow for different mounting options and still allows you to run it as close to the gunwhale as possible. 

I think the biggest area for improvement in yak specific downriggers is in deployment....you know cause of the whole paddle...drop...paddle...drop scenario (at least with deeper fishing, in sloppy conditions or if you don't have a hobie  ).  I think if you came up with a user friendly mechanical drag that allowed you to deploy to a certain depth without having to do any fumbling you'd be golden. 

Something to consider, is that you're probably not going to find 17-4 and 15-5 stainless in their heat treated state, they'll be "normalized". This means that their yield strengths are going to be much lower than those shown by you. I don't have my Machinery's Handbook with me, but figure in the 60k-70k range, if that. You'd spend more getting it properly precipitation heat treated than on materials cost.

On the other hand, you likely will be getting 7075 in a heat treated state, with a yield near that shown by you. Aluminum has a modulus of elasticity only one third that of steel though, so it's going to do a lot more bending than a comparably sized steel rod. Watch out for corrosion.

Not shown, and it would be my first choice, is 4130 Chrome-Moly. It too comes normalized at a yield of about 63k and is (was, it's been a while) than less expensive than 7075, is cheap. And with a suitably sized container of brine, a good torch, and an oven that reaches 425 degrees, can be heat treated to have a yield of about 165kpsi. There's that rust thing you'd have to worry about though...

Any reason you're choosing 5/16" diameter? I think that's smaller than the rod used by the little Scotty Down rigger, and their arm is more like 18" long, if memory serves.

Scott

Hey Jay -

Why not use graphite? Lighter, stiff and easy to work with. I have been using my spectra downrigger for 3 years and love it. By backing the drag off, you can have a hands free slow controlled drop so your rig doesn't get tangled. http://www.norcalkayakanglers.com/index.php/topic,12008.0.htm

scott

Thanks for that Scott,

The yield strengths I am showing are what the web site I was going to buy from report. http://www.onlinemetals.com/
Do you think I could expect to get the strengths they are reporting, or are they being optimistic?

As far as the length, the way I plan on building this, the arm will end up being around 16", once a handle and crank are installed. I also looked up the 4130 and it is significantly cheaper. I may go with that and increase the diameter up to 3/8. I plan on using a PacBay Roller Guide tip and I think the largest diameter I can get is 3/8".

Do you feel like  yield strength around 60,000 psi or more should be enough not to bend?

Hey Jay -

Why not use graphite? Lighter, stiff and easy to work with. I have been using my spectra downrigger for 3 years and love it. By backing the drag off, you can have a hands free slow controlled drop so your rig doesn't get tangled. http://www.norcalkayakanglers.com/index.php/topic,12008.0.htm

scott

Since I don't have any spare rods or rod blanks lying around I think it would be cheaper to buy a metal rod than it is to buy a stubby blank. Although it may be time to hit the local pawn shops to see what they have lying around. But, I hadn't realized that you built one of these. Your design looks a lot like what I am envisioning. Great minds eh? 

Here's my two cent's. I just went to my material rack and bent a 5/16 316sst rod about 2 ft long. And it did flex about one inch when I put about 50 lbs of force on it. Why don't you move to 3\8 dia. I don't know what it will take to bent it. But it would be better. Just how much force are you going to apply to this rod? Anyway 316 sst will not need to have any plating done to it which will save some money. If you do use the aluminum it should be nickle plated. I made some fish finder mounts for a friend that fishes salt water has had no corrosion problems in the last 6 years.

Here's my two cent's. I just went to my material rack and bent a 5/16 316sst rod about 2 ft long. And it did flex about one inch when I put about 50 lbs of force on it. Why don't you move to 3\8 dia. I don't know what it will take to bent it. But it would be better. Just how much force are you going to apply to this rod? Anyway 316 sst will not need to have any plating done to it which will save some money. If you do use the aluminum it should be nickle plated. I made some fish finder mounts for a friend that fishes salt water has had no corrosion problems in the last 6 years.

That's good to know! I likely will go up to 3/8. Now I just need to see if I can find someplace local (Coeur d'Alene, ID) that sells SS rod.

For my application I may end up running the DR as deep as ~90', so I may use heavier lead, maybe up to 6 lbs depending on how comfortable that is while paddling. It's all an experiment for me right now, since there really is nobody else up here fishing from a kayak. But, the rewards can be great. 15lb king salmon are not uncommon in Lake Coeur d'Alene, and I've seen pictures of 24 lb fish. The only thing keeping me from going after them right now is the lack of a downrigger.

Try Jeffrm20, I believe he has skills

Jay if you cant find anyone close try Graingers or McMaster Carr both carry the rod and will ship it.

Check us out at polcraft.com We got skills.

Jay-

This is an easy calculation I can do for you... how much weight are you planning to mount on the rod?

~Greg

Jay-

This is an easy calculation I can do for you... how much weight are you planning to mount on the rod?

~Greg

Cool, thanks Greg,

I'm looking at a miximum of a 5lb downrigger weight. I'm pretty sure they all would support 5lbs at the end of 2' of rod static weight, but I also know that if I am in a bit of chop the dynamic forces of 5lbs hanging at the end of the rod are much greater. If you have some math that can give me a range of options, that would be awesome.

OK, assuming a final unsupported downrigger length of 16", a 3/8" rod diameter, and 5 lbs of mass, I get the following:

Max bending stress: 15450 psi
Deflection at the end of the downrigger, 7075-T6: .67"
Deflection, stainless steels: .25"

The max stress of 15450 psi is well under the yield stress of the materials. Say you put 10lbs on the thing, here are your numbers:

Max bending stress: 30900 psi
Deflection at the end of the downrigger, 7075-T6: 1.4"
Deflection, stainless steels: .5"

Again, still well below the yield stress of the metals you mentioned.

Have you considered using tube stock instead of solid rod?

That's awesome Greg. Thank you for calculating that for me! It's nice to know for sure that any of those metals will work for my application. 

Unless your time is worth very little why not just  buy the scotty mini downrigger made for small aluminum boats or canoes and adapt it to the yak?

I think they are maybe about $ 75  bucks or something like that. They come with a stainless cable and a stainless boom as well as a working drag and crank to retrive the weight.

-Eliot