Re: Bob Perry's take on Wolfenzee's dream boat
There are a couple of issues which I would like to touch on in more detail that may prove useful at this point. The first that I would like to start with the issue that Brent Swain raised in Post #312. In that post Brent said, “The so called "Modernized" version, with wide, flat aft lines and super lean bows, is the kind of hull which goes down by the bow, and the stern lifts up, when she heels, the centre of buoyancy moving well aft as she heels. This is the kind of hull shape which gets extremely hard to control downwind, a boat with zero directional stability.” Brent and I have had this conversation on other occasions on other forums around the internet, but for those coming in late this is something which you see people say but which is not necessarily true.
While it can be true that a poorly designed boat can and will change trim with heel angle, it does not have to be true, and it is not true for most well designed modern boats. I know that this may seem counter intuitive on a boat with a pinpoint for a bow and with a stern so wide that if asked to, ‘Haul ass’, would need to make at least two trips.
To understand why these ‘triangular’ boats do not change fore and aft trim with heel, I will give this example. Visualize a sailboat that was the shape of a cone, that floats with the point of the cone being the bow and located at or above the surface of the water and flat end as a transom, with a keel and rudder mounted.
You can visualize that as that cone rotates around its axis, in effect heeling, the cone would simply spin around its center axis without changing trim. The reason the boat of that shape does not change trim is that the shape of the boat is such that the fore and aft distribution of the buoyancy does not change as the boat heels, and assuming that the contents of the boat do not shift fore and aft with heel angle, the center of gravity remains the same as well. No matter how blunt a cone you can visualize, (effectively no matter how big the transom is) that cone would not change trim with heel angle.
Of course boats are not cone shaped. But the same principle applies if the boat is to remain in trim at a range of heel angles. What happens as a boat heels is that some of the bottom comes out of the water, and some of the topsides becomes immersed. Simplified by ignoring dynamic forces acting to change trim, the key to maintaining the desired trim is to make sure that the distribution of buoyancy does not move fore and aft with heel angle. This means that at each point along the boat, that the distribution of the volume contained within the submerged topsides and portion of the bottom still in the water needs to remain in proportion to the static upright buoyancy.
Before computers, this could be wildly difficult to calculate, but modern computers will comparatively easily calculate trim angle with heel, allowing the designer to tweak the topsides and bottom shape to allow the boat to maintain the desired trim over the range of heel angles at which the boat will typically be sailed.
There are secondary issues involved with big transom boats and maintaining control with heel. When you heel a boat with a fine boat and broad stern, the whole windward side of the boat, rotates upward (rolls up). This also raises the vertical center of gravity. For comparison, traditional narrow boats, typically rotated without rising and in some cases, actually ‘rolled down’ so that the vertical center of gravity actually moved downwards.
As the stern rolls up, to one degree or another, the centerline of the stern of the boat is being lifted out of water. I think this is probably the reason that many people are under the impression that the boat is changing trim with heel.
With a conventional centerline rudder, as the center of the transom rises, the rudder is being withdrawn from the water, moving closer to the surface or even out of the water. This allows the rudder to aerate, which means that the low pressure side of this partially extracted rudder blade is sucking air down the blade of the rudder. That air mixes with the water flow and makes the flow less effective as a steering medium, and therefore makes steering harder and in the worse case, impossible so that the boat wipes out.
This phenomena is the main reason that designers have begun using twin rudders on these broad transom boats. By offsetting the pair of rudders from the centerline at least one rudder does not extract from the water and remains fully effective. And once you commit to dual rudders there are a range of tactics that designers can employ to take fullest advantage of having two rudders instead of one, such as canting the rudder vertically so that the rudder is close to vertical at the normal heel angle that the boat is designed to sail at max speed. The rudders may also be oriented so their leading edges are slightly toed in when vertical, but which puts them on axis with the centerline of the immersed hull when heeled. That needs to be designed very carefully since there is some trade off in increased drag when sailing flat.
These boats have the possibility of tracking reasonably well if the shape of the immersed area is narrow enough and generate counter-rotational forces symmetrical with the heeled rotational of the rig. It is claimed (I have not sailed one) that these boats in effect behave like a skinny boat, which will track simply on the polar resistance of the long narrow hull form in water. Looking at video of these boats sailing in waves, that appears to be the case in that when heeled, there appears to be very small movements of the helm to keep these boats on course.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 05-20-2013 at 10:00 AM.