america's cup yacht

[kreg]

i'm doing some experimet with hydromax about the stability of an america's cup yacht. I would like to ask you where VCG is located from waterline in a yacht with a draft about 4 meters.
Thanks

[Andrew Mason]

roughly 2 metres is a good starting point.

Also be aware that under the revised AC rule, draft is now 4.1 (not 4.0)metres and displacement max is now 24,000kg (not 25,000)

[kreg]

thank you For your attention.
Could ask you another thing?
24,000 kg is just the weight of the yacht hull except the sail etc.?
So, do I have to test the stability with total weight?
I have read the rules version 5 but i did not understand where freeboard shall be measured.
do you think i have to get the measure from MWL plane or LBG plane?
Thanks very much for your help.
You are very helpfully.

[Andrew Mason]

It depends why you want to measure the stability, if it is to determine righting moment for a VPP then obviously you want to measure the sailing condition with sails on board rather than the measurement condition.

Freeboard are taken from the MWL plane, but at the position of the intersection of the LBG plane with the fore and aft overhangs of the hull. The LBG plane is 200mm above the MWL plane.

If that explanation isn't clear let me know and I will post a diagram.

regards

Andrew

[kreg]

it's very clear.
I would like ask you the last ones.
1) Why are there limits (penality) on freeboard and which are the advantages that you can have with lower freeboards?
2) If i understood well this type of yacht entrusts its stability to the weight indeed to the shape. Is it right?
Do you think that a lower beam is better for wave resistence?
Thanks million

[Andrew Mason]

This is really where you should be asking someone who has been involved as a designer of AC boats, however my uninformed guesses would be -

1. I assume that lower freeboard reduces weight windage and seaworthiness, so my guess is that the freeboard limits were imposed to make sure that the boats did not start trading off seaworthiness and safety for speed.

2. Stability on an AC yacht is dominated by the huge bulb and large draft, and beams have reduced progressively over the past 10 years as a result. I guess most designers would argue that beams have reduced to a practical minimum, both from the point of view of the trade off between wavemaking resistance and form stability, as well as limitations on shroud base and sheeting angles imposed by narrower beams. Now that the AC rule has effectively locked length, displacement, draft and sail area, I think a lot of designers will be looking closely at beam.

[kreg]

ok thanks i understood...
i have the last question....
which methods do i have to use for resistence in hullspeed for this type of yacht?
thanks again for your time.

[Andrew Mason]

Only Holtrop, Delft series, or the Slender Body method are suitable.

[kreg]

ok thanks million

[redcoopers]

Kreg,

About your second question, slender ship (Michell) theory predicts that beam is proportional to wave resistance squared. But this is for upright resistance only, therefore there are many caveats:

First, downwind, a large beam aft will help a boat get on a plane and will dramatically reduce resistance. Slender ship theory cannot predict this (since most slender-ship theories do not account for trim and heave).

Secondly, upwind, the more beam you have, the more form stability you have. This means you can carry more sail area to produce drive. Remember that the moment created by the sails has to be equal to the righting moment from the keel and hull.

Therefore, it's a huge compromise. The initial IACC yachts were quite beamy compared to the last generation. They slowly have been getting more narrow, but I think they have reached a point of negative returns. I would expect the new IACC yachts to have just a little more beam than the last round (but I'm not working for any team, so what do I know!)

-Jon

[kreg]

i have another little question...
Are there limitations on aft and forward hoverongs?

[Mario]

Originally Posted by Andrew Mason

Only Holtrop, Delft series, or the Slender Body method are suitable.

In Hullspeed Help is said that algorithm of Holtrop method is designed for predicting the resistance of tankers, general cargo ships, fishing vessels, tugs, container ships and frigates. The results of Holtrop and Delft series are pretty different for a sample of the AC yacht. Only at cca 11 knt the resistance is the same.
What is the diferent between Delft series I, II and III? Are there exactly shapes of the hull assigned to either Delft series I,II or Delft series III?

[Andrew Mason]

Kreg

My understanding is that the length of the overhangs is not limited by the AC rule.

Mario

The Holtrop work was done on fast ship forms, the Delft series on yachts only, for more details see -
http://www.rina.org.uk/rfiles/HISWA/...resistance.pdf

[kreg]

thanks

[Leo Lazauskas]

Just a couple of minor points:

1. Slender ship theory and Michell's thin-ship theory are not the same.
2. Thin-ship theory can be used to predict trim and sinkage. E.g. see
SWPE: Speed-up and Squat
www.cyberiad.net/library/pdf/tsl01a.pdf

Cheers,
Leo.

Originally Posted by redcoopers

Kreg,

About your second question, slender ship (Michell) theory predicts that beam is proportional to wave resistance squared. But this is for upright resistance only, therefore there are many caveats:

First, downwind, a large beam aft will help a boat get on a plane and will dramatically reduce resistance. Slender ship theory cannot predict this (since most slender-ship theories do not account for trim and heave).

Secondly, upwind, the more beam you have, the more form stability you have. This means you can carry more sail area to produce drive. Remember that the moment created by the sails has to be equal to the righting moment from the keel and hull.

Therefore, it's a huge compromise. The initial IACC yachts were quite beamy compared to the last generation. They slowly have been getting more narrow, but I think they have reached a point of negative returns. I would expect the new IACC yachts to have just a little more beam than the last round (but I'm not working for any team, so what do I know!)

-Jon