Keel question
 

I am building a new keel for an Aquacraft Paradise. The original keel has about .7 taper ratio. It seems to me that having even more of the area up near the hull will tend to give better "lift" with less "roll" (sorry, I am an airplane guy, but don't hold it against me, I am trying!) Does anyone have some input on ideal keel shapes? Is most of the lateral resistance from the hull, and the keel need only be structurally sound? Or is it's shape important?

I have a 1 lb. torpedo shape fishing weight for the ballast, and the keel will have a core of 1/4" 5-ply spruce with balsa for shape fore and aft. Glass covering, nice fillets of Bondo, and an epoxy finish.

I will post photos as I go, but need to find some 4mm Stainless rod for the keel attachment and the new rudder (per the other Paradise thread, probably 2x the area of the original).

It would be helpful if anyone can point me in the right direction for the 4mm diameter rod material. I expect it would have to be from a "Metric" country, as all I can find are inch sizes.

Thanks, Larry

RE: Keel question
 

<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; background-image: initial; background-attachment: initial; background-origin: initial; background-clip: initial; background-color: rgb(255, 255, 255); font-family: Arial; font-size: 12px; ">Bulb shape is at least as vital as everything else under the water.  Various trick shapes have been tried, but the winners all gravitate to an elongated teardrop/torpedo shape.  To maximise the righting moment, the CoG of the weight needs to be as low as possible so some have tried a flat bottomed triangular cross-section, but at the same time drag has to be minimised, so the circular section prevails.  Full size yachts have a crew, who do their level best to ensure that their boat stays as upright as possible, so strangely shaped ballast weights that benefit a boat sitting vertically in the water work for them.  Most model yachts don't, either because of class rules or because the gains are outweighed by the losses,.</div>

RE: Keel question
 

Thanks for the input, but I am asking about the shape of the keel, not the ballast. My ballast is a torpedo shaped, 16 ounce fishing weight, so that is fixed. I want to know about the area distribution on the flat "flying" surface of the keel. I agree that more vertical position of the boat is good, and that is what I am trying to achieve. It seems to me that more area higher and less below yields less rolling moment.

There have to be boat design books and studies out there, people have only been doing this for a thousand years or so, but where to find them? Anybody?

RE: Keel question
 

Look here for details of fin theory under 'appendages'

http://onemetre.net/Design/Design.htm

Wider at the top is better, but a straight fin is easier to make,
getting the shape right is important, IMO.

Edward

RE: Keel question
 

<div>Sorry about that, "keel" has so many uses under a boat.</div>There have been very successful designs where the fin is broader at the bottom than the top from the side view.  This increases resistance to roll because of the increased side area further out along the lever, but at the expense of the strength of the fin which needs strength at the top.  It was noted that unexpected gusts could lead to a recovery operation, one for the boat now lying on one side, and another for the fin and weight.  Since the main idea is to keep the boat as upright as possible, lift is the last thing you want below the waterline when the boat is heeled over, as it just heels all the more.  What is wanted is minimal drag, with just enough side area to give the power from the sails something to push against, and enough strength to survive.  Its all about compromise, and the conventional answer is "If it ain't broke, don't fix it".  The number of times I have made a boat worse by "improving" it..........:)<div>An important thing to remember when applying aeronautical knowledge to boat design is that planes work fully in a compressible medium, boats have water which doesn't compress, but gets displaced.  Statement of the obvious, but it does get forgotten on a regular basis.</div>

RE: Keel question
 

Again, thanks for the input, guys! Yes indeed, water ain't air, and that is why I am asking questions. If it was an airplane, I would already know the answers. The dynamics and hydrodynamics of boats is new to me, and probably very different from what I know.

And that website mentioned above is a goldmine! Thanks!

RE: Keel question
 

I haven't found 4mm Stainless rod, but I did score some 5/32" which is within .002" of 4mm! I have 6 feet of it coming (about $15, so not too bad). If anyone needs some, I will sell it for $3 a foot including shipping. That should be about a break-even for me.

Has anyone ever tried letting the bulb pivot to drive a trim tab at the bottom of the keel to help keep the boat upright? I suppose spring centering and some dampening would be needed to keep the system stable.

RE: Keel question
 

OK, I have the Stainless rod in hand if anyone wants some. I tried threading it today, but the cheezy Chinese die turned out not to even handle soft Aluminum. I will go get the real McCoy of a die this week. I can provide appropriate lengths of rod for the rudder, and plan to be able to provide the correct length keel rod, tapped in the future. I will post more as the project goes on. I have 5.6' of 5/32", 306, stainless steel, rod (.002" under 4mm) available in whatever length you need, left.

Only have one boat, and only plan to make one new keel and rudder.

Larry

RE: Keel question
 

Perhaps a little late now, but trying to find metric rod for a keel or rudder can be tougher for us here in the US than standard SAE... Simpler to convert.
Simply divide (in this case) 4mm by 25.4 and you'll come up with .1574". If you then go to a Tap & Die chart, and look up clearance hole sizes for various thread bolts, you'll find that a #6 bolt requires a .1440" hole for a tight clearance, and a #8 bolt requires a .1695" hole for a tight clearance. Since 4mm/.1574 falls between the two, you could choose the larger of the two for threaded stock- and #8 stainless threaded stock can be procured from your local marine supply- or McMaster-Carr. This also gives you a diameter to go to your local hobby shop and purchase a piece of music wire (stiff steel wire) in whatever diameter you choose.
You don't have to buy metric.
Wes

RE: Keel question
 

I think you didn't read my entry carefully enough. I HAVE 5/32" stainless rod which is .156 compared to .1575 for 4mm. It fits perfectly with no noticeable slop, and I have successfully tapped it for 4mm x .7 to match the retaining nut. Problem solved. I will happily make up keel and rudder sets for those who want them. I figure the two pieces with shipping included should go for $5. I guess I have enough rod for 10 sets, after that, you are on your own. :)