Reducing the Drag Hump - Smooth Planing

I was reading through an article by Frank Bethwaite on the possibility of a 'Tasar 2', which, sadly, the class association turned down. This would have been the same hull (but built of glass, carbon and epoxy), but with 9er style alloy foils, an improved thwart (with no traveller) and an updated rig with better automatic gust response. The all up weight would have been 40-50lbs (20kg) lighter. In the event, this proposal got watered down so that the only thing that changed was the sails, which are now mylar and have slightly more roach in the head. Anyway, one line in the article caught my attention. It read something like "The rig will be sized such that the peak of induced aerodynamic drag is displaced from the hump in hydrodynamic drag - to give a more effortless motion to windward.". Presumably, this is the method used in the 49er and 59er to get their dynamically smooth, humpless planing charecteristic, but I have not seen it explained in this way before. I'm not totally clear what he means, but I interpret it to mean that the rig's areodynamic drag is maximum just before the mast yields at the 'design wind' strength. Therefore the 'design wind' strength must not produce boat speeds that see the hull enter the forced mode, semi-planing regime (S/L = 1.3ish). Either the rig must be big enough that it yields before hull speed is reached, or small enough that it doesn't need to yield until full planing is achieved.

Any thoughts/comments?

There are other threads on dynamicall smooth (humpless) planing here, but seoerating the aerodynamic and hydrodynamic drag peaks was not discussed:

http://www.boatdesign.net/forums/showthread.php?t=10070

By the way, I've also learned that Bethwaite is developing a carbon mast for the Tasar, so maybe the Tasar 2 will happen piece by piece...

 

Don't believe so. The Tasar era boats have a definite planing hump, quite a few later ones don't. I suspect its a lot about bow profile and angle.

 

All the designers I have spoken to (with one exception) about this say that the humpless hull is probably created by having the DLR so low that the boat is planing before it reaches hull speed. The exceptions is Julian, who says that because Frank is now interested in other things, they'll never know why it works!

PS

As one of the leaders in the anti T-2 move (according to FDB), I'm glad the proposal for the Tasar 2 was dumped; we would have had the same old-fashioned hull without the redeeming qualities of big fleets and fully competitive second-hand boats. Before the class brought in minimum weights some boats were in the 59kg region (compared to the current 68kg minimum) and the difference was may 25-50m per run. That's not enough to make a difference in marketing or in feel to the average sailor, but it's enough to make old boats very frustrating to race (we were regularly rounding the top mark 1st out of 76 in a heavy boat and hitting the bottom mark 5th). The fact that the Tasar is steadily gaining popularity against the development class it came from (NS14) shows that development is not necessarily the way to enliven a class. If we Tasar-ers wanted to develop the boat we'd have got N12s or NS14s instead of one designs.

The add-on monofilm sails and carbon masts are fine.

 

Agree the Tasar has a big planing hump. Also agree that low DLR is critical for a smooth transition (I think there are a number of benefits, that result in a higher 'hull' speed (a la Gerr formula) and lower planing speed, so that planing can occur before hull speed is reached). But FB seems to be saying that something else is important too, i.e. ensuring the max aerodynamic drag doesn't occur at the same time as the max hydrodyanic drag. To me, this is new thinking. It makes sense, but I hadn't ever seen it suggested before.

Good points about the T2 Chris. From an English perspective, we don't really have the fleets anyway, and if no-one in the world is buying new ones, I think something needed to be done. But from the existing owners point of view, I guess the new sails are a good compromise as they don't make all the older boats uncompetitive. BTW, do you have the new sails? Is it possible to singlehand one with just the main up?

 

I'm not convinced, because of something rather odd I noticed with my one off singlehander. The boat didn't really generate a bow wave in any meaningful way. Right up into planing conditions there was damn all there. The contrast with an IC, which has a better DLR than the 14footer with me on board, and which kicks up one hell of a bow wave, is really quite striking. And if you're not kicking up much of a two wave system you can't have much of a displacement hump...

 

That's interesting. I think the longer hull for higher hull speed and lighter hull for lower planing speed must help, but there are probably other factors as well. Do you know the Cp and DLR of the two boats? Do you think the depth of forefoot plays any part? What about rocker? Was the bow on your c++ vee'd or U'd?
Notwithstanding that, have you heard of any other designers sizing the rig to avoid the hump?

 

If I knew the anwers I'd be smarter than Bethwaite... As a completely uneducated guess I suspect that its probably something to do with the included angle of the bow. The section shape was partway between V and U with very slab sided topsides. Really its an ideal job for a towtank where you could pull a number of models and see what happens...

 

The Inland Lake Scows have a smooth transition to planing. They have a long, flat run and large overhangs that result in them essentially surfing their own stern wave. Pressure on the aft surface has a forward component that helps to recover part of the wave drag.

For example, the M-16,

has a design waterline that is 13 ft long - 20% less than the 16 ft LOA. The sections througout look like a Group Finot design's stern - flat in the center with rounded chines.

 

I suspect the idea of avoiding matching the high drag regime of the rig to the hull is a distraction.

The drag bump for a hull - even something less humpy like the narrow chine, low rocker, low wetted surface Bethwaite hulls - is going to be a relatively gross effect.

The drag of a rig will be much smoother (rising at the square of the apparent wind velocity) of so much smaller consequence that pumping a sail might have a profoundly greater effect.

I look forward to someone more robustly theoretically based making some comments.

 

I don't really know the M16 - sounds kinda similar to a rounded Fireball? Long overhangs would suggest a high DLR (esp if its a US boat!), so this would seem to be a totally different approach to achieving a smooth transition to planing - trying to plane at the lowest possible speed, rather than displacment sailing to the highest possible speed.

I see what you're saying BoatMik. I guess the thinking is to get aero and hydro components out of phase so that the overall drag varies as smoothly as possible. Every little helps...

 

I was just speculating that it was a nebulous or non existent or far too tiny effect and trying to draw out someone who knows the theory in a more numerical way than I do.

You can go faster if you polish the front of your mast and the shrouds before every race too Or more realistically washing the bottom with detergent.

But for me the time and effort outweighs the result. If I get teh start right I gain a thousand times the time of either.

And there is a good chance I think that such an optimisation will prove a performance disadvantage somewhere else along the line.

Ah - just thought - there is a logical flaw with the idea ... the boat hull will do a wide range of speeds in the same wind strength. So it will do the same speed (for the hull hump) in different windspeeds (for the rig "hump")

So which windspeed are they going to choose?

MIK

 

I think he was talking about upwind boat speed.

 

No new sails as the boat has been sitting around for a couple of seasons without being used. It will get some use over the next year because we are building a fleet at our club.

My information (from the world champ and from the former Tasar champ who makes the sails) is that they aren't all that much quicker than the old. I do like the fact that they are cheaper, although the old mains were competitive for about a million years. The boat looks sexier, it has had a marketing boost, and you can still race with the old sails. Good stuff!

Singlehanding with the main up is slow in a breeze, because you can't get the mast spanner over without hassle. WHen I was competitive and training I'd singlehanded in the light stuff for practise but after about 8 knots it was very slow.

I think the hull is just too beamy and has too much windage for one person. Interestingly the singlehanded version of teh NS14 looks like a lovely boat but isn't anywhere near as quick as I'd imagine; not much quicker than a Laser I'd say. Again, according to Stu Friezer, excessive hull volume (as the singlehanders use the 2-person hull) is the problem......but start shaving the volume and you end up with a Moth/RS600 hull which is too tippy for the target audience. Maybe they need a ++?

 

I race against a Tasar in my Finn in a mixed fleet. 3 Finns, Tasar, 2 MGs and several 29 ers. The Tasar has a new set of plastic sails this season. In a good breeze he is definately faster, light winds downwind he is slower. The boat does look good, unfortunately if he is some distance off on the other side of the course he looks like a 29er.

 

So a NS singlehanded is slower than two up? That's quite interesting, because, it has much better DLR and SA/D figures. Only goes to show, there's more to design and performance than a few ratios.