Submerged transoms give higher speeds?

Buoyancy

Hi, Phil - I have put a lot of flam into the bows, as well. I realize that there are those who think this turns them into 'brakes', ie. can cause pitchpoling by increasing resistance forward, (though you don't seem to be one of them,) but we are talking about a cruising cat, not a racing cat. In the kind of situation where this is a danger, this boat would have a sea anchor out. -Tim Dunn

 

What do you base this comment about Oram boats on?

 

Ohil
I agree with your take on the distribution of bouyancy, not too much aft, but enough to carry the load, and lots of reserve up front, which I have, also the increase at the bows is much greater than the stern.


Anton du Toit the NA on the boat, gave me some preliminary results of the CFD.
First issue is Prismatic coefficient. Originally we had it at around 0.58, but from what I have come to understand, it might be advantageous to go higher up. The results of the CFD showed lower friction for higher cp's all the way up to 0.7 !!! (with non immersed transom) This with a Lwl/hull beam of around 12:1

Optimmising for 6-8 knots boatspeed and factoring in not having too high a cp for tacking ability, we expect to end up around 0.63/0.65.

He will then do the numbers with the transom just kissing and finally with the transoms slightly immersed.

I can't wait to see what the results are. Lowering the transoms will of course increase the Lwl/Hb ratio, as the Lwl increases. It's a question of juggling a whole bunch of parameters and deciding where you want to compromise.

I know this is probably a bit over the top for a cruising cat, but I would like to feel that we have made some educated choices and done out utmost (within reason and funds) to get a well performing boat.

As we are looking at speeds way below the theoretical hullspeed, we are seeing friction as the primary resistance, as wavemaking resistance first gets serious higher up.

I hope to have some more information by the end of next week, as we have to finalise the shape and get the files off to the frame cutting guys. But it is alot of work, as each change in hull shape requires new "fairing".

When we're finished I expect to be able to publish our results, as my impression is that not many do this. At least I haven't been able to find much.

Alan

 

Orams and transoms

I looked at photos of a number of them, and I looked at their designed displacements. Their designed displacement is very light, and many had their transoms partly in the water at dock. Nothing against Oram-he just seems to have designed a number of boats with the intention that the owners would exercise a great deal of weight discipline-more than quite of few of his owners did in fact exercise.

 

Overloading a cat is a "rare" thing

We probably all do it to some extent if we're cruising

Alan

 

Should be: Higher speeds need submerged transoms?

For power catamarans submerged transoms are ok, they trim less. Sailing catamarans have to operate on a large speed range and they need to tack well too, so transoms up of water.

 

Submerged transoms Do create less drag!

I have just received the preliminary results. At 2.5 knots the immersed transom adds 2.5 kgs of drag from aroun 4 knots the immersed transom is superior.
At 10 knots boats speed (this is a 49 footer) there is 20% more drag on the non-immersed transom!!!

This transforms to around 2 knots higher boat speeds (hulls only) so 12 knots instead of 10 for the same drag.
Optimum point for these hulls is at around 11 knots where the difference is largest.

We have also been playing with the Prismatic coefficient, and preliminary results show that increasing this from around 0.58 to 0.61/0.62 decreases the drag from around 4 knots. At 10 knots there is a decrease in drag of around 10 kgs or 8% roughly.

We are going to fair the lines to have the transoms immersed 50 mm at full load, and end up with a Cp between 0.61 and 0.62, then we can calculate the drag reduction with a higher certainty.

All this work is being done by Anton du Toit Yacht Design. www.dtyd.co.za who are doing a terrific job using Michlet for the predictions.

I hope we can publish a proper report when we are finished.

These predictions have suprised me a bit by the magnitude of difference in the drag, I did not expect to gain up to over 20% drop in drag at typical cruising speeds!!

regards

Alan

 

Increasing Prismatic to over .62 is the way to go. All my designs have at least that high a Cp and have done so for over 25 years

Don't forget that, unlike a powerboat, a sailing catamaran heels, especially at speed, and thus the lee transom tends to be immersed when underway even if it is clear at rest. Furthermore, Cp increases as more displacement comes on the lee hull, so the faster you go, the faster you go.

Having said that, the hull is a 3D shape and so you cannot look at any one feature in isolation. For example a canoe stern hull (like a Wharram) will always have a transom immersed. In other words the width of the transom is relevant, you cannot just say: Immersed transom "good", or "bad"

Hope this helps (Wifi working for once at anchor here in Spa Creek Annapolis)

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Many of them? Where did you find these photo's of many of them? I've checked Bob Oram's website, and there are some photo's of a few boats, and only two, Drumbeat and Outahia, appears to be slightly overloaded. In fact I know these boats, and on Outahia the waterline was incorrectly marked at first, which gave the illusion that the boat was floating deep.

The one boat I know for sure is much heavier than it was initially intended to be is Drumbeat, which was largely due to a misunderstanding the owners had of the idea of a lightweight boat. They added a flybridge, bigger engines, huge batteries, huge fridge/freezers, a large genset and much more to their specifications after the displacement had been agreed on.

That boat does float much deeper than intended. It's worth noting though, it still performs extremely well, and the owners are very happy with it.

Due to this misunderstanding Bob Oram now offers a heavier displacement version of his 60 footer.

And from Nordic cat's research it would appear that slightly immersed transom are an advantage, so load those cats up!

 

Hi Alan,
I'm not questioning your remark. I did an experiment int the pool (yes darn nearest water here ) a few days ago. If you slide a thin flat material through the water it slits though without resistance. I was actually surprised how easily it it went and I couldn't feel any resistance.

A wider subject however had significantly more resistance, higher speeds actually needed some force to be accellerated.

In the case of a hull I would think that the displacement would be the first force to overcome ? Even below hull speed there is displacement that must take place before you can move. For a hull to move it's own length, the amount of water that gets displaced is whatever is below the waterline.

If a theoretical hull with 0 draft lies on the water surface then you would have friction as the primary resistance, right ? Or have I the cat by the tail here ?

 

As for the submerged transoms, a power boat needs an initial huge power to overcome it's weight to water pressure on the hull to plane. The water that is against the transom initially has to be dragged along untill the water speed under the hull becomes faster than the water can flow to the transom. It is like dragging along the weight of that amount of water without adding to the length of the boat, and I think this is what prevents a boat from geting on the plane faster than it does. Once it sheds the extra weight it goes on the plane easier.

If a sailboat's stern is under water the same is going to happen, it is going to drag that extra amount of weight at no benefit, and I agree it is going to make a difference in the sailing speed - taking into account the hull speed may not go so fast that it would shed the water. Keep in mind also that water cannot be compressed, the under water stern part is going to act like a suction cup pulling back on the hull all the time.

He he... you could of course move more weight foreward....

 

Hi, Alan - To some extent, you and your designer seem to be reinventing the wheel.

Planing boats have submerged transoms, no rocker, and the stern is as wide as the widest part of the boat, as they are optimized for a speed of 2 x the square root of the dwl, or more, the dwl being expressed in feet and the speed expressed in knots. Cruising sailboats that operate under all conditions are usually optimized for a speed of about .8 x the square root of the dwl, and they have considerable rocker and no immersion of the transom. Their sterns have traditionally been considerably less wide than the widest part of the boat, though in the last 20 years or so, transom width has been creeping up.

And, of course, to keep the prismatic low you need rocker, as a low prismatic requires that there be less under body at the ends of the boat, and rocker gives you that.

As others have stated, the question of immersion of the transom has to be considered in terms of how deeply it is immersed, and how wide it is compared to the hulls greatest beam.

I think Fannie's remarks about how quickly the water flows in to fill the space behind the transom gives a basis for an intuitive understanding of why submerged transoms work best at considerable speed. In effect, you must outrun the suction of the water flowing in behind the transom.

Tim Dunn, http://www.dunnanddunnrealtors.com/Catamaran.html

 

Thanks for the comments Guys.

Fanie and Big Cat:

I know that what we are doing is nothing revolutionary, just what you would expect all designers to do as part of the design fase.

As other threads and posts indicate, there is alot of knowledge out there, but I needed to quantify the different influences to be able to make an informed decision.

N.B. The figures I am quoting are of course only valid for these hulls, with a couple of empirical factors thrown in as well, so the certainty of the calculations is of course not at 100%.

As Richard Woods says: "the hull is a 3D shape and so you cannot look at any one feature in isolation"

This is the next step we are looking at, to quantify the influence of increasing the prismatic and submerging the transom at the same time. The software splits the drag up into all its multiple subcomponents of wavemaking, and we will of course be taking a serious look at that as well.

If we can achieve a drop in hull drag of 20% at cruising speeds, compared to the conventional design (Cp of 0.58 and transoms free, hull draught 50 cms) then I personally feel that this excercise has not been futile.

The negative effects claimed for submerging the transoms and increasing Cp seem to be difficulty in tacking.

Maybe Richard or others would care to comment on this? I know other factors also influence this....

Alan

 

G'day,

If you really want a surprise, ask Anton to do the numbers on narrow, rockerless double ended hulls with 2 rudders in one hull, no daggerboards.

I use prismatics around 0.8 with no rocker.



Works exceptionally well and is also easier to build.

regards,

Rob

 

Hi Rob,

We have done the numbers up to 0.7, so I can imagine where 0.8 will go to.
But the low speed drag really starts increasing with this hull shape when we increase Cp, so not a viable route.

It could be interesting to see the numbers for one of your Proa hulls. Have you ever had them done Rob?

I am in no doubt that the proa configuration has advantages over the cat in a number of areas, and I don't think I'm overly conservative, going with a bi-rig, 3 helm stations etc. BUT, at this time in my life, I'm not ready to go that far out on a limb

What we need to keep in mind with the numbers I posted earlier, is that the Lwl increases by nearly a meter when moving to immersed transoms. This in itself has a beneficial effect on "hull speed"

When looking at the preliminary curves for drag, it is clear that there is an increase in drag at the speeds traditionally designated hullspeed. (1.34*SQRT Lwl) in fact a bit before, but it is not as sharp an increase as it would be on say a monohull, and seems to stay pretty linear up to the 20 knot mark.

Once we get the final numbers we can see what kind of wave drag and sizes are involved.

Alan