NavigationNot much happens without their keels. The engineering involved intimidates: But irrational keel fear is pretty darn dumb, once you know what it does and why. What keeps this lovely keel under Gingerand not lying on the bottom somewhere, is an interesting nautical puzzler, indeed. For starters, t bulb keel matter how much adhesive, basic wood framing, or any sort of rudimentary attachment systems tren 75 might have, nothing easy and simple keeps a keel, a keel. These big heavy underwater wings are too massively loaded, and work too hard to be attached t bulb keel anything basic.
How keel type affects performance - Yachting Monthly
By mrnanney , January 31, in Sailing Anarchy. Supposedly the T shape is more hydronamically effecient. Anyone have a goof list of the trade offs? From what I have read, the T provides the optimum location for the CG of the total bulb-strut package. The L allows for a more "user-friendly" keel for the reasons you have mentioned without much CG trade-off. Mark Mills discussed the reasons, and if i recall, he said it mostly had to do with getting the center of mass of the bulb far enough forward, when other factors constrain the fore-aft position of the keel itself.
IRC doesn't seem to penalize the bulb as much as it helps performance from what i can tell. I remember speaking to one of the Johnstones about the J and I believe it was a while ago that J decided to stick with the L for practical concerns regarding kelp and pots. I think he mentioned that they were able to balance the boat by moving the mast forward as well.
Can't criticize whatever they did, as the J has done rather well in IRC events. John Corby explained in a seahorse article some time a go, that he switched to T-keels to be able to move the mast back while keeping a sensible LCG. The rationale was to get a larger J measurement for bigger spinnakers and bigger non-overlapping genoas.
Having owned boats that have both I find the biggest difference is the L keel if shaped correctly creates lift primarily because the strut and bulb can be more of a proper foil shape and the T not so much L keel less likely to sustain damage from groundings too. J builds great boats but also looks at who their target market is and what they'll buy.
T keels would need a hefty and maintenance free so to speak kelp cutter to be truly marketable on an even plane with L keels. A T-bulb allows a longer thinner lower drag shape at the expense of some extra wetted surface area. Now that IRC penalises the flying bow knuckle that was common in the days of the L-keel the boats are now being trimmed bow-down as this shortens WL and raises the counter.
That needs weight further forward hence the T-keel. However IRC now requires that the keel shape be identified in more detail than before so they could if they wanted apply a penalty to Ts if they think they are in some way undesirable or better performing. And then there is the separate issue - on smaller, trailerable boats that a T shaped keel will 'balance' in it's trunk and an L binds because it wants to 'cock' it's heavier aft-edge down.
That's cool - to you have the pit dude also work the keel? How big of a keel? Is this the same basic operating principle as trim tabs on the keel like some of the AC boats used to do? Entirely passive system based on keeping the boat at a certain angle of heel; no issue downwind as the models usually aren't on their ear. Not sure I would be that keen to do it on a full size boat as might require a bit more quality engineering; however it does work; with a boat that sails best flat though it wouldn't kick in as it requires heel to get the keel to twist.
About then he was quoted in an interview in some NZ magazine talking about how using the L keel to induce twist helps with the stiffness of the foil. That makes sense - at first I was picturing the whole keel pivoting a few degrees on a vertical axis, kind of like having one giant keel bolt Well you can't do that anymore cause now with fast planning hull I rode an article in a French magazine with an interview of JK , and he said on VO 70 , even 1cm of offset betwen CoG and Neutral axis of the fin , would create twisting and then vibration at high speed.
There was a thread over on boatdesign. It's not so much a matter of lining them up , it's a case of making sure they're in the right place according to the engineering dynamic analysis. A common source of vibration is oscillating vortices coming off the trailing edge, or some interaction with a trim tab or it's mechanism.
But offsetting the Cog from the neutral axis will create a bending moment anyway: Well, offsetting the CofG would get you a static twist in smooth water, and yeah, you'd get some torsional 'bounce' when the boat hits waves, but when you say "vibration" I'm thinking of anything over say 10 Hz. The buzz you get off a rudder or centreboard runs along those lines as there's very little mass to dampen out the torsional motion.
On the intentional twist, anytime you increase the lift of the keel non-canting then you increase the heeling moment. That's why fin-keel boats have a definite limit of heel that's fast -- for some reason, older full keels seem to like to put the rail in and go I don't know why they're different -- maybe something to do with the full body dynamics.
You can tailor things using composites that you can't do easily with a cast or fabricated steel fin -- you can make the fin supporting the ballast bulb stiffer in bending but still allow it to be more flexible in torsion, promoting the twist you want to mimic a gybing centreboard.
Seems like T keels are getting a special penalty on IRC this year. Number of revalidations coming through for race boats with T keels are showing hull factors going up a few points, as is the rating. X 41's hit by 4 points. Helps keep the marks from wrapping around the keel as well! Been doing keels that way for a while and they seem to work well, and I did notice that if you let them flex too much the quit working well.
It's all about the balance of the boat and where the centre of effort for the keel is and its most effective positioning. A Young was originally designed with a "Raked" L keel, however as developments to rig and sails have come about so has the necessity to move the Centre of effort aft to compensate, however where the ballast is still needed hasn't moved, hence the need for a T keel.
T keels have some significant advantages over L keels is most areas, however one cannot right off an L keel completely as being significantly slower. L keels if using a Tapered Raked section generally sit better in the fin case when down, that is they tend to jam themselves into position preventing rocking fore and aft in the fin case, as T keels can be prone to do.
Draw a T keel and a L keel and you will see: If the keel is extreme little chord blade and deep bulb under a fast boat like this one I would always choose the T solution, and place the bulb CG aligned with fin CG. It's for obvoius structural reasons, it's bad if you snap your fin twisting it. If the keel is not extreme think about a J or X yacht keel, longer chord relatively slow boat etc HCW wrote about the centre of effort beeing aft on modern boats, but when designing a boat you play with it and with the volumes of the hull LCG so if the COE is aft LCG will be aft too a good exemple of a radical design is a mini or open60 with their big asses, very aft keel because of their very aft rig, they all have T keel because a L would break.
I think J-Boats like their L-keels because they like to have the rig pretty far forward. This does two things- it gives them more room down below between the companion way and the mast, and it gives them some room aft of the boom.
If you look at a the main is pretty big, but the boom does not pass over the helm's head. All I know is that when I retire from my current job and begin my new life as a Lobsterman, I am going to buy a used Club Swan 42 to use as my lobster boat. There is no more efficient lobster pot retrieval system on earth than the T keel on that boat Interesting to hear about other reasons J maybe using the L.
I suppose that also keeps the headsail relatively small which I think has been a big part of their philosophy since coming out with sprit boats. As cited above, as mainsail areas have increased, especially in the form of massive roach, and headsail areas.
The CLR must also move aft. The mass of the keel bulb. Except that it's not. The L-keel has to be stiff enough to keep it from twisting, the T-doesn't. That makes T-keels cheaper for a given area, span, bulb-weight. But within the rather large safety factors that most production boats use, it probably doesn't much matter cost-wise.
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T keel vs L keel? Posted January 31, I notice the trend is to T keels. J still uses the L keel. L keel may have advantages in clearing kelp, etc and possibly better grounding protection??
Share this post Link to post Share on other sites. J decided to stick with the L for practical concerns regarding kelp and pots. I wonder has anyone designed an L keel to twist the foil while heeled? The T probably gives a better end-plate effect to the foil. Posted February 1, As a matter of fact I have, and with excellent results. Posted February 3, Yep, that's what I was referring to. Heel, the keel twists due to the placement of the bulb and the deliberately twistable keel.
Posted February 4, Yes for sure But offsetting the Cog from the neutral axis will create a bending moment anyway: Posted February 5, Yeah yeah yeah But you're still loosing energy Beats me, all I know is it's fast upwind and down, and points super high.
Maybe I'll make another that exactly duplicates that one, but will not twist, and then compare.