Section shapes for cruising yachts

There is one question about sailing yachts, designed for cruising, that is puzzling me for a long time now.
If one locates in the lines plan the section with the largest immersed area, then this section has neither the largest beam nor the largest draft. Usually the largest draft is located forward from the largest area and the largest beam is significantly abaft. For high speed planing crafts this makes sense because a wide and flat bottom in the rear supports planing.
For cruising ships that sail most of the time at speeds below the Froude number = 0.4 it makes no sense to me. The largest resistance part at low speeds is the viscous resistance. This resistance is significantly increased if the largest draft and the largest beam are not located at the same section. In such a case large cross flows (secondary flows) are induced in the boundary layer, which increase the skin friction and hence the resistance.
It might be possible to avoid this situation if the bow is drastically trimmed down and the aft part of the hull is clearly out of the water. In reality at low speeds the sail power is small and will not produce a significant bow down trim, especially not when sailing to windward.
May be this design is just a fashion copied from fast ships?
Uli

 

I think the area you are talking about is where the engine and crew (cockpit) are located. The biggest immersed area has to coincide with the biggest loaded area.

 

Check up with:

A CFD Investigation of Sailing Yacht Transom Sterns
by
Jens Allroth & Ting-Hua Wu


Department of Shipping and Marine Technology
Devision of Marine Design
Resaearch Group Hydrodynamics
Chalmers University of technology

Master's thetis 2013:X-13/296
www.chalmers.se

 

Uli

Cruising yacht design often imitates racing yacht design. Is that the type of craft you are refering to ?
I'd suggest that most heavy displacement cruising hullforms don't conform to the paradigm you mention.

 

Thanks Jürgen for pointing out this interesting new paper! Incidentally the hull-variations in this thesis concern only the transom. The position of the section with maximum area and that with maximum draft are very close together in the chosen model. I would expect therefore only minor cross flows in the midship area. Obviously there are cross flows created directly at the transom, because the boxy type with chine-type edges shows a higher resistance than the rounded transom.

Mike, you are right. I was referring to the "sharp looking" sailing yachts that are presented at the current boat shows. And yes, the traditional displacement cruisers do not have the section form that I described. So my question could be: what is the progress in the modern progressive designs?

 

For general reference, there was this older thread - http://www.boatdesign.net/forums/hy...wave-drag-38260.html?highlight=hull asymmetry - was quite lively, but if anything much was nailed down, I don't remember what it was. But more than any other single thing, it inspired me to get serious about learning hydrodynamics.

I've been keeping a list of odds and ends which bias the fore and aft hull shapes. Most exploit the thickened boundary layer aft of midship.

1. Power. The prop can take advantage of the slower flow well aft and mitigate the total wake drag. Fitting the wake to the prop is worth doing. Doesn't scale well.

2. Righting moment. Also size dependent. Increasing RM comes at an inescapable cost. You pay in surface area and also in wave drag due to the form of the waterline. There is a bias towards gaining necessary RM in the aft sections since the local friction coef is lower there. Even Godzilla will spit out asymmetrical sections on small craft with BM MIN requirements if run near hull speed. That, presumably, is purely due to the viscous effect, and not due to the wave effect because Godzilla uses linear wave theory. Ie, the displacement curve is nearly unaffected by the hull asymmetry bias.

3. A general failure of linear wave theory in the near surface, near hull region. This depends on the height and length of the near-field waves. Linear wave theory tosses out a kinetic energy term that causes the actual surface wave to lag a good ways behind the the model wave. Same with pressures on the hull. It would be nice if there was a Glauert-like transform that would take a linear wave optimization and transform the hull into one which actually has the pressure distribution that the linear model specifies. Again, the effect is more pronounced on small boats than big ones, and especially on shallow boats relative to their length and speed.

So you end up with a fairly strong aft bias at waterline both from an RM requirement and also from a nearfield wave model that includes the second order terms. But as the draft increases, you still want a nearly symmetric volume distribution. This leads to the draft forwards and beam aft of the CB.

 

You must not forget that sailing yacht operates in a variety of conditions and configurations. If you just consider two of the basic ones - upright running downwind and heeled beating upwind, you will see that the flow conditions and the aero/hydro forces and moments are very different between them, and hence require very different hull sections. The objective of the design is to find the best overall shape for the given set of modes of navigation, for given weather conditions, and taking into account other constraints (accommodation, construction techniques, eventual class rules, road transportability, dimensions-based taxes and marina fees, etc. etc.).

 

The thesis mentioned in post#3 (thank you for pointing it out Jürgen) adds some interesting information, I have attached 2 figures out of it. The pictures show how an increase in transom size moves the point of largest beam backwards and increases the righting arm when heeled. The diagram shows that the heeled resistance of the boat with the largest transom is 36% higher than the version with the smallest transom. It is really questionable if the increased righting moment allows an increase in driving power high enough to offset the resistance increase.

 

A higher center of gravity in modern sailing yachts would be one trend. This is tied to shape changes, but which is driving the other would seem to depend on the case at hand. If the Cg is below the canoe body, you have one case. If it is somewhere between the canoe body and the waterline, you have a second case, and above the waterline, you have a third case. Very roughly, there is little motivation to make the hull shallower in case one, built down hulls are better suited to low cg. Case 2 is where shallow bodied monohulls rule because there is good reason to try to keep the hull bottom near the VCG. Case three suggests multihulls.

 

Theu are lighter displacemnt hullforms. Sparkling light air performance and able to semi plane in suitable conditions.

 

I can't find this on the Chalmer's website. Do you have a direct link to it, or does one have to order it?

 

Here it is: http://publications.lib.chalmers.se/records/fulltext/203795/203795.pdf

 

True, auxiliaries have gotten so much better in the last 50 years