Revisiting keel bulb shape/weight.

[i550sailor@aol.com]

My keel and bulb ended up at class max, now after racing it this season, in what ended up being mostly light air, I am thinking several of you (micah included) are correct about the extra weight is not needed and may in fact be slowing the boat down.

So heres my question, is there a better more efficient keel/bulb shape, with the goal of going to the class minimum in weight, but optimize foil shape, chord etc.

Let's here some ideas!.

[ryderp]

I used the beaver-tail bulb plug that Chad created and then shared with the class. I don't think that anyone can tell you if this shape is more hydrodynamic than just using a cylindrical NACA shape, or any other profile for that matter. I do like the look of it however and I like the beaver-tail instead of just having a point back there that could easily break off (maybe while trailer launching). My foil started off as a NACA0012 shape, but due to my travails with expanding urethane foam (between my keel box and keel sleeve) is now more like a NACA0010.

As far as weight is concerned, my keel is about 10 lbs under class maximum and I honestly don't think that this matters in any way. I've had the boat over on its ear a couple of times, and it definitely self-righted once I got the pressure off the sails.

On a related note, one of my sons (a 420 sailer in his youth) has been bugging me about hanging trap wires one the boat. I think that I'll do that before next summer. Not exactly class legal, but I'm pretty sure it would be a lot of fun.

[Chad]

Yep, extra weight almost always slows a boat down. And building lightweight but strong boats takes skill and cash. If you've built a reasonably tidy hull, the best place to spend the excess weight is in the keel. If OD class racing isn't an issue, then there's a lot of places to make your boat lighter (especially if it isn't built yet!). The stock plans have a lot of extra wood- I'd reduce that as much as possible, before shaving the lead. Like Phil, I've crawled over the high side a few times and seen the bulb in the air. The bulb gives you the confidence to leave the spin up longer, especially shorthanded.

If you have your heart set on min weight, I think the only way is to cast the last +/-2' of your keel as foil-shaped ballast. It'll need to be a thickish section to have the needed volume and no/very little taper.

[Warren Nethercote]

The bulb exists to carry ballast and its only constraint is the 3.5 inch 'included sphere' requirement. So following from a previous post, more of the ballast could migrate to the fin. The vestigial bulb might then be designed on the basis of best end treatment for reduced induced drag (increased effective aspect ratio) - although that may be a fool's errand given the 3.5 inch sphere constraint. I haven't bothered looking for the source, but my memory is that, absent winglets, bulbs (or symmetric wing-tip tanks, for that matter) don't do a lot to reduce induced drag, and might produce increased interference or frictional drag that offsets induced drag reductions. Asymmetric tanks or bulbs offer greater opportunity, but not many of us sail proas. That being said, a smaller bulb should reduce wetted surface, although the scale of the effect might be in the noise.

[micah202]

If you've built a reasonably tidy hull, the best place to spend the excess weight is in the keel.

I'd disagree,, as would starboats., even though they require substantial heel and therefore engage keelweight more effectively than an i550.
The best place for the excess weight is at the centre of the mass. Light ends,,light mast,,,lighterish keel.

.........sail it like a dinghy.

[Chad]

both i550 classes penalize excessive corrector weights- neither let you put much in the middle. In the keel is much preferable than splitting it between f53 and f169.

[micah202]

both i550 classes penalize excessive corrector weights- neither let you put much in the middle. In the keel is much preferable than splitting it between f53 and f169.

...good point!

[Mist]

I just completed a one piece bulb pour. It came out at 161 lbs. I plan to only insert the keel blade enough for strong attachment to the bulb and add more lead in the cavity underneath. Check out the process on my blog mistnotmissed.blogspot.com . I do think my boat will end up light. I plan to put a battery in it which will be a significant weight. Haven't reviewed the rules to see if the battery counts in the hull's weight. -Tim

[admin]

Pretty much same here, Mist. My pour was a lot simpler and as a result, my casting is probably a lot less fair and accurate, but like you, I added a bit more lead in the final transition to the completed, 1-part bulb/foil mash-up. My gorup 27 battery for my electric trolling motor puts me over the class limit and the battery is snug down low against the keel box and fits in a slot that guarantees it doesn't move, even if the boat is inverted. (I slurged and bought the Walmart battery box that has a display for battery charge, an aux plug for cigarette lighter style connectors, and a decent handle, etc...plus it just barely fits into a snug, immobile slot)

In any case, CONGRATS on the successful pour and anytime you get this done with no one suffering a third degree burn somewhere on their body is a victory! All Hail Plumbum and fire!

[Mist]

Mist's bulb is almost complete. Weight came out at 155 lbs, slightly over class minimum.

[jray]

Very nicely done! Looking good Tim.

[admin]

That is one slick looking bulb, great job Tim!

[i550sailor@aol.com]

Looking really good, Spring will soon be here. Happy New Year!

[Mist]

Thanks for the compliments!

I miss quoted the min weight rule in my earlier post. The min weight for the keel is 150lbs. Since I was on track for that I didn't bother adding more lead/epoxy slurry. I'm becoming concerned that I'll need corrector weights once the whole boat is weighed. It'll probably end up light even with a battery. I'll cross that bridge when I get there.

TF, thanks for the headsup on the battery box,switch,and gauge. I'll be looking into that soon.

[Tim Ford]

You can probably get one cheaper than the 60 bux I plunked down for the Minn Kota Trolling Motor Power Center - sounds a little overblown, I think. But having access to the external leads and the battery meter is really nice.

I haven't taken my battery down past 50% yet, which was reassuring, as it's a bit of a hike to get out past the fairway into out creek, especially with a good breeze from the south which it almost always is