Submerged transoms give higher speeds?

Alan,

What is your optimizing speed range?

Rob,

There is no transom area as well.

 

The PC increase to 0.61 shows a drop in drag from 5.5 knots with a maximum at 10 knots of around 10kgs.

The submerged transom show a 2 kg increase in drag at 2.5 knots, back to zero difference at 3.3, and a drop from 4 knots and up. Maximum drag decrease is around 45 kgs at 11 knots.


The speed range where drag is most important for me is the range where I expect to sail most, so from 4/5 knots and up to 12-14 knots.

Later on I will be doing a windage resistance and compare the unstayed bi-rig to a traditional rig, which will be interesting.

Please note that the figures I am using are without underwater appendages, but the differential values will of course remain roughly the same.

regards

Alan

 

Remember that it is easy to overcome drag at low speeds, when, by implication, you are sailing in light winds as you can simply add more sail (eg a masthead screecher). It is when it is windy that it is important to have a low drag hull because then you won't have the sail carrying power to overcome excess drag. So I wouldn't worry about drag at 4-5 knots, but concentrate on reducing drag at speeds of 12knots+

Of course, another advantage of high Cp boats is that they pitch less.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Does anyone have an idea of what the speed is one begins to outrun the transom water ? It may not be very fast. I suspect it may be a constant speed value for any submerged transom.

 

I get the impression that hull shapes and their performance is like V8's the guys custom build. Some goes well, others are just un be lievable. I once saw an article on a V8 built with very near standard parts, the carburettor for one was a single barrel holley and was in theory much too small to make performance. The dyno proofed otherwise.

So regarding the hull shapes, if one could change them in real life while sailing it would be easier to chop and change and then get to something optimum at different speeds.

I always ask myself if the shapes we assume to be efficient really as efficient as they can be or is there something not so obvious that is supposed to be obvious... like the earth is flat. There was a time boats were believed to sail downwind only. Are we missing someting else maybe ?

 

The initial results show a drag of around 150 kg at 10 knots and 400 kg at 20 knots. The drag increase from around 12 knots is very linear.

The the increase in drag from 10 to 20 knots boat speed is only 2.67 times up, which sounds relatively low to me.

(For large ships a rule of thumb is to cube the power needed to double the speed)

But I don't know how much we can actually improve this, given the actual hull shape. Going with a Cp of 0.7 will reduce the total drag only around 2.5%.

Of course longer and thinner hulls would help as always.

For comfort reasons I doubt that speeds much over 15 knots will be typical, but should be quite easy to achieve on a reach, as the VP at full load predicts around 90% of TWS.

Any suggestions Richard?

regards

Alan

 

G'day,

I prefer real life results, as per the video. A couple of people have run tubes with pointed ends (not far off the hull shapes I use) on Michlet and the results are excellent. Makes me doubt if tacking ability is worth all the extra drag of conventional hull shapes.

I was not suggesting you build a proa (although it would be a smart thhing to do ;-)), just that, if performance is your goal, there may be better solutions than standard catamaran hull shapes.

Thanks for the heads up on T O F. Looks great, hopefully more tolerance of robust debate.

Regards,

Rob

 

Fanie, flow behind transom sterns is still very poorly understood.

A useful fundamental quantity is the transom depth based Froude number, F:

F = U/sqrt(g * T)

where U is the speed in metres/sec, g is gravitational acceleration (9.81 m/sec) and T is the depth of the transom in metres.

Saunders suggests that on full size ships, the transom becomes fully dry when F > 4.0.

The experiments of Beck, Maki, Doctors and Troesch showed that the transom became fully dry between about F=2.5 to F=3.5 for some small model hulls.

Tuck and vanden Broeck found (mathematically) that steady flow cannot exist for F < 2.23 for a 2D transom stern (i.e. infinitely wide).

There have been many attempts at modelling the flow behind transoms. Most of the simple approaches use a fictitious extension behind the stern.

Couser and others have tried a flow model based on the classical "backward-facing step" problem. (This is the basis of one method used in Maxsurf). Their approach works reasonably well for some hulls, (e.g. the NPL series on which they based their empirical constants) but is not always consistent for all types of stern shapes.

Doctors and Day have tried similar models, but they too are inconsistent.

There are some serious problems with flow models that use a virtual extension (e.g. an air pocket, or hollow) behind the stern. An open hollow behind the transom cannot sustain a pressure from the water, and so it cannot have any effect on wave-making or wave resistance. These types of flow models can give reasonable estimates in some circumstances, but they should be considered as empirical attempts, not as gospel.

Beck, Maki, Doctors and Troesch have tried CFD on the problem. They found that CFD was not quite up to the task. In their work they used small model hulls: the problem is much much more difficult at realistic speeds and hull sizes.

The rate at which the transom becomes dry from its fully wet condition (i.e. at zero speed) is also extremely difficult and poorly understood. Doctors and Day, and many others have tried a variety of "unwetting" functions, but the results based on one set of experiments do not always do well on hulls of a different shape.

Michlet uses a transom model similar to that in Maxsurf. I also use an unwetting function based on the work of Doctors and Day, but I've augmented it with hull wave effects as well. For example, at some speeds, there will be a hollow in the wave profile at the stern, which causes the stern to unwet more quickly than when the hull wave has a crest at the stern.

But I'm not at all happy with Michlet's mish-mash of theory and hand-waving when it comes to transom sterns. Firstly, large transom sterns violate the small slope assumption in Michell's thin-ship theory. Secondly, as I mentioned before, there is the physical impossibility of a finite hollow generating waves.

Sorry that I couldn't give you a more precise, helpful answer, but there isn't one. If you are really keen to find out more, Google for the names I gave above, e.g. Maki transom, Doctors and Day, etc.

All the best,
Leo.

 

Thanks for the explanation Leo, it just proves that CFD alone is not enough if you really want to optimise the hull shape.

All in all probably a good thing, otherwise alot of people would be out of a job if you can do it all on a computer and get it right.


When observing the flow around the transom of my present cat, I hav often wondered if the "hole" just aft of the transom, and the way the water flows together, could not be "helped" to do the filling in a more orderly way, as the resulting wave is wasted energy.

Has anyone experimented with small asymmetric vortex generators? I could imagine that instead of 3 primarily large flow directions, (i.e. the 2 sides and bottoms) a number of small "shedder bars" could generate small counterrotating vortices the neutralise each other?


I have seen some sketches showing a downward facing "lip" at the bottom of the transom on a sailing boat, would that be solely to create a bigger upward force, or does it have any effect on the total drag as well?

Regards

Alan

 

Hi Leo,

Phew...

I'm aware that many theoretical things can be calculated and very close to the real thing too. Personally I would get on a boat and gradually accellerate until you can see the water being left behind. If it is 8kn then at 8kn you should suddenly experience an accelleration to where your hull again reaches the hump.

The depth a transom is below waterline is going to determine the speed and more power is going to be required to get away from it, the larger the volume of water, asume the width is more than the depth the more power will be needed.

If the depth of a transom goes to the bottom of the hull depth, then that hull should be faster at higer speeds and only displacement from the bow has to be overcome, as well as surface friction.

I looked at the racing multihulls like graupama and SodebO's amas, and they don't have a lot of curvature. In fact they look like streight tubes and very blunt transoms. When the boat heels and at speed the hulls act more like planing hulls than displacement hulls, which I suspect is the intent.

If you know at what speed a hull will leave the transom water behind, you will know of you could go for a deeper transom or a shalower one, depending on what the craft type is intended for, as well as if the craft does have the power to overcome the initial drag and that depends on the size blanket you have.

So in my opinion if you have a casual cruiser and you would 90% of the time do 3 to 6kn then have the dransom end above the waterline.

If you want to race or want a fast workboat drop the transom. Once the initial drag is overcome the speed should be higher. If the sail pops open in a brisk breeze you're only going to hear 'plop' as the transom's are plucked free from the following water and you'll be on speed

I have a video here af Graupama put foot. The thing is planing, not displacing. It even leaves the windsurfers behind.

 

Fanie,

some people argue about what planing is. I don't want to get into that discussion here

The big tris you mention and all sailing cats,do not plane in my opinion, rather they run in displacement mode. The can of course "surf" on a wave.

If we look at the trend in cruising power cats, then we see a clear trend towards keeping them in displacement mode for even higher speeds, as this is more efficient than planing if the hulls are right. This trend is led primarily by the AUS/NZ designers(as are alot of other multihull related things) like Tennant.

When sailing, i have a couple of times on a cruising cat experienced the "climbing out of the hole" you mention when exceeding the "hull speed" by more than a factor of 2. But I think it was more because the lee bow gets depressed more and lifts the transom, than coming up on a plane.
Still a great feeling to go over 15 knots on a cruiser...

Alan

 

G'day,

Leo,
Thanks for the explanation.

Fanie,
The tris have curved centreboards which lift the bows of the lee hull. Looks like they are planing, and by some definitions, they are, but it is due to the foils, not the hull shapes. Tubes cannot plane.

regards,

Rob

 

Cats are complex

I am not trying to burst anyones balloons but when it comes to drag and cats the hard part is knowing at which point to measure drag.

Do you measure the drag of an immersed hull, a lightened hull, a pitched down hull or a stern down hull. Unlike monos which do not vary in displacement a cats hulls are constantly shifting weight form one to another so any drag prediction based on them sitting on their lines is only a rough guide.

When my cat is going to windward it goes slower and the lee hull gets deeply immersed so I like a clear transom . Downhill the lee hull is lightly loaded and the speeds are higher so I would like more immersed transoms. I hope the NA in this process is not being bamboozled by numbers without a thorough understanding of the hugely varying displacements and is at least putting the hull through different displacement conditions and pitch attitudes to cater for motoring as well. Then you have to optimise the design for the sailing the boat is going to do. Computers give great numbers but one must be careful that the computers tests the design as the real world will do.

Although Richard has far more miles under his belt I don't agree with his ideas on drag. When it came time to build my 38 footer I asked the designer -Robin Chamberlin, consumate seaman - to design me a boat that gets up to 8 knots quickly and I don't care if it goes any faster. I want to reduce drag in the light stuff so I can sail and not motor. When the wind comes up I am reducing sail cause if I go as fast as the boat can go the kids and my wife ( and me too) get tired form excessive accelerations. Speed is fine but only if the autopilot can steer and you can still get on with shipboard life. I have cruised a fast racer and it was darn hard work.

Don't get too hooked on high speed. Get going first and make sure you can tack well too.

cheers

Phil

 

All design is a compromise, and I agree about getting up to a good cruising speed fast, and not bothering too much with the high end speed.

That is why we are looking at limiting the prismatic.

A good compromise to get 10-14 knots anything above is just for a fun day with the guys....


Alan