design questions

Hi. I'm new here. How many sailboat design questions will all of you tolerate? I am curious about mast torsion, reverse mast spreaders, hydraulics, twin keels. I guess these things are common place. I just haven’t found the right spot, yet.

Thanks, Jeff

 

You are allowed one design question per week. You are not allowed to piggy-back questions in existing threads.
Failure to follow these rules will simply result in your obtaining more answers, which is not good for you in the long run ;-P

However -
Mast torsion. Usually, if the mast is built strongly enough to overcome the buckling loads, it is way strong enough to deal with any torsional loading from the luff. Usually. Wing masts are a different story.
Reverse spreaders. GReat for boats that get put away at night. you release the backstay, and the whole rig relaxes. The main can to put all the way out for running with _much_ less chafe, but - and it's a big "but" in these days of large roaches - you have to have a backstay.....
Hydraulics. IMPO they have no place on a sailing boat - hydraulic fluid is a pest to clean, impossible to stand on without crampons, and smelly. Stick to mechanical devices. (I did say that was my PERSONAL opinion, didn't I?)
Twin keels. If you live in a severely tidal area, and/or have a mooring that dries out, then they are wonderful. Just get used to tacking slowly.

Any more questions?

Steve "saildesign" Baker

 

Is this a twin bilge v fin/bulb keel question? (insane grin spreads ear to ear) Earlier this year, by which I mean over December and January, I did some work on the different variations of keels. Sadly, it was only a text-based project, so I couldn't get hold hod of much test data. It would appear though, that, in decreasing speed we have wing keels, bulb keels, fin keels, twin keels. In terms of upwind handling, the order is the same. Downwind, despite the reduction of drag on the twin keels, the wing-keeled boat is probably carrying more sail!! I would agree with steve, though, wing, bulb and fin keels don't let the boat stand up very well if the mooring dries up. It depends wheter you want speed, or you need a boat that can sit on the bottom.

Cheers,

Tim B.

 

The main reason for bilge keels traditionally having less performance than single keels is they are inevitably made far shorter. If you gave a single keel no more depth than the rudder, it wouldn't perform that well, either.

As for twin keels in tandem, I suspect there's no real advantage unless the aft keel has a tab that allows you to eliminate the rudder. But even then I would expect the single fin + rudder to be superior to the twin keel. For the same planform area and span, the twin keel has nearly the same wetted area. The only hydrodynamic gain I can see is the reduced thickness, but you could reduce the thickness of the single keel to match, too.

If there is no restriction on depth, you could always design a bulb keel that will outperform a winged keel. Wings only make sense when there is some artificial restriction on depth, such as class rules or a shoal draft requirement. Any improvement in effective span due to the wings can be had with less wetted area by extending the physical span of the bulb keel. The bulb can then be made smaller, too, for the same heeling moment.

 

I hate to say this, but it is commonly beleived that the winged keel is, if designed properly, always superior to bulb keels. In this case, the wings do not change the effective span (much), they are simply there to control vortex roll-up and reduce vortex drag. It has been shown that there are other beneficial effects, but little research has been done on those effects.

Incidentally, by 'twin keels' I was trying to separate the long 'bilge' keels so often found on classic sailing boats from the later one to port and one to starboard 'twin keels'.

Tandem keels, have been shown to be competetive in IACC races, because of the 'two surfaces' rule. In this case, only two moving surfaces are permitted below the waterline with unrestricted are. Therefore, having two fins that could lift made sense, since the bulb could be hung between them. With both keels lifting, the righting force from the fin was effectively doubled. The rear flap was also used as a rudder to good effect. In normal sailing, I agree that the tandem keel posesses little advantage.

 

Nearly all of the keels with winglets have some artificial limit on depth - usually class rules, such as the 12 Metre or IACC rules. Wings will extend the effective span, so they're of benefit when the span is limited. But if you can physically extend the span, you can get the same induced drag reduction with less added wetted area.

When a keel with wings is compared to a keel without, but sized so the two have the same effective span, the winged keel can produce its performance with marginally less heeling moment. However, this is not likely worth the added wetted area when the total performance is estimated with a VPP.

This is why you don't see more winglets on aircraft. When span is limited, like Standard Class sailplanes, winglets make sense. Another case is airliners that have limited space at the gate. But in most other cases, winglets are a matter of style, and more performance could be had with a simple wing extension.

 

Many thanks for the input.
My interest in twin keels has to do with: putting a torpedo like structure on either side of the hull using lead and or water at max beam, no lower than the bottom of the boat. The attempt is to carry the weight low and wide. If anyone has any comments or links all will be appreciated.

 

I think this is an excellent idea as it would reduce keel tip vortices however if your keel is at a 30 degree angle do you put them at the same angle as the world cup winglets and slightly astern or horizontally out and then do you use one on the outside horizontally(which is what I think is right) or do you go with two and have to figure out the interference because of the 30 degree angle of the keel with the inside one. This may cause more turbulence than without. Any ideas from the senior members would be greatly appreciated.
By the way I read it over again and what you are suggesting and for what reason(if you want it easily trailorable) or what other reason(shallow docking area) you have to tell people what your design requirements are. You can make long shallow twin keels at the water line using any naca 00xx shape and fill it with water ballast which is only effective when it actually rises out of the water. In rough water it will pound between the fin and the hull. The added resistance will be high and the top hull speed reduced.
Theoretically the twin keels are supposed to be placed 45% to 65% of actual water line length. If you move them forward by several feet resistance at maximum hull speed can go up by 90%. Also they should be down in the water so you can keel over at least 30 degrees without them coming up out of the water and causing pounding. I believe they have received a bad rap for efficiency because of poor shapes and locations. Symetrical shapes will give you maximum speed with the least resistance.
By the way you can have quite a bit of fun with the water filled ballast keels because if you get stuck somewhere all you have to do is pump out the ballast and you float free. Also empty she would probably plane and go quite fast with an attached outboard.
Well, I had the night to think about it and with the ballast that high up at 90 degrees you will turtle over. Then you will probably stay that way. That is why keels are longer to give you more righting moment. What you are proposing is a triple hull, lets say a v in the middle and a half a v at each side. You may as well make the sides pretty flat with a deeper curve in the middle like a catamaran. If you make the whole thing pretty wide it will give you more stability. If you pump the water from side to side and you have an enclosed high pilot house then by having one side heavier than the other you may right yourself more easily. Some catamrans have escape hatches in the bottom and one idea is to have the mast going right through the hull so that when you are upsidedown you take the mast and stick it on the bottom of the boat so that you can sail home. I guess you would have to set up the rudder like that too.
The other thing I suggest you to do is to make models out of wood or even styrofoam and see what happens.