From winged keel to bulb keel

I own a 36' sailboat built with an iron winged keel. As I don't have de keel drawings I have scanned the surface and calculated the volume and surface using a CAD software. Now I am planning to cut the keel just above the wings and add a lead torpedo with the same weight but lower volume and area. Besides nicer the new keel has significant less area so I expect to lower the drag at lower speeds. I have the means to cast the bulb but the main difficulty seems to be how to fasten the bolts though the bulb to the bottom of the original keel ... I want to be sure it worths but due to the shape complexities I still could not find a way to calculate the performance differences. Any suggestion as how to do it?

 

Can't help you there but I like the idea. The bulb would be less likely to hang up on rocks/coral in a grounding than the winged keel would have I suppose...

 

Did you consult with the original designer about these modifications? Everything in design is to some degree a compromise-and changing one thing can have unintended consequences elsewhere....

 

The boat is a Tony Castro design from 1992 but the original keel is full lead and much deeper. I met once a boat with this original keel but the rigging and configuration was different so I cannot use her performance to compare. My boat is 5600 kg displacement with 70 sqm sailplan. It's fast but has a poor performance with light winds. I'd like to submit both digital models to CFD tool but is eather too expensive or complex to me. Would be great to find someone able to do it to me ...

 

The wings have the effect of making the keel act like it is higher aspect ratio, if you remove them you will have the same effect as going to a low aspect ratio keel. This means it would not quite point as high, but as you surmised would also have less drag. If the righting moment is the same there should be no safety issues.

I would think if you just bolt the new bulb the to the bottom of the keel with 2 or 3 large bolts (the heads recessed into the bulb). This requires drilling and taping or threading the holes, but it should work fine except for the dissimilar metal issues. So put several coats of sealant in the holes,and on all mating surfaces before assembly. install the bolts "wet" into the sealant. And inspect it regularly until you have some experience with it.

Good luck

 

The thing with a bulb is you have to justify the drag with the ability to carry more area to over come it. You can shape a bulb well, but still have this huge frontal and surface issue. Unless the bulb places the CG substantially lower, so you can carry more area, you'll be slower.

 

I have been working hard on the new keel shape and have succed to reduce the wet are by 25% yet preserving the original stability parameters. The key point is how to quntify the gains? Hoaw faster will be the boat on light wind? How much will I loose upwind? Is it worthwhile to invest so much time and money changing the current keel? If I don´t find a simple way of virtual simulation I am strongly considering to build a scale boat with the current and new keel and drag it around trying to evaluate my concept. HHow do you loke the idea?

 

There's no simple way of doing either that "virtual simulation" as you call it. You have three options here:
1) CFD analysis
2) Scale model tests
3) Modify the keel of your boat and see how it behaves.
Each one of them has their pros and cons, but the only the solution n.3 will give you the correct answer - the other two are just tools with a limited precision and reliability. If not done properly, they could even be misleading. If it was my boat, I would be pretty wary of the results given by either the option 1 or 2.
By the way, what are your boat's dimensions and target speed?
Cheers

 

Spring 36
Designer Tony Castro, 1991
LOA 11,33 m
LWL 9,91 m
Beam 3,4 m
Draft 1,50 m
Displacement 5600 kg
Winged keel 2,36 Ton
Sail area Upwind 120% 70 sqm
I 13,92m
J 3,85m
P 13,98m
E 4,97m

 

How much lower is your CG with the bulb? Even though you've shaped the bulb to have less wetted surface, the wings have good "penetration" into flow, the bulb, not so much if typically shaped. Can you post some images of the two configurations?

 

sure! here it goes ...
if you need I can send IGS files as well!

 

Have you considered lateral resistance? When the boat heels the wings start contributing lateral resistance...

 

It is what Petros described as higher aspect ratio ...

 

I could run a basic CFD comparison for you if you send me the IGES-files, just for the sake of general interest.

 

Don't believe so... yes, the wings act as end plates for the keel and reduce tip losses and induced drag, giving the same effect as a higher aspect keel, which is what I understand Petros as saying, but if you consider the boat heeled 45 degrees then both the keel and the wing surfaces are contributing to lateral resistance. Quite how much depends on the effective angles of incidence of the surfaces. If you consider the boat almost knocked flat then the keel will generate very little lateral resistance but the wings (well the lower one anyway [grin]) will be generating a lot.