We were tied up at the Jarrett Bay boat yard complex yesterday, doing a bit of preliminary planning with Corey McMahon who will be handling our seven year itch list, when a couple of interesting photo ops passed by. There is a pod of dolphins hanging out off the docks and occasionally riding bow waves (it is hard to concentrate on boat project dialog when the pod is playing in the background).
What was interesting is the boats whose bow waves they chose to ride. There appears to be direct correlation between hull efficiency, or lack there of, and the dolphin’s favorites.
We know from our own experience that the bigger the pressure wave, the longer the dolphins will stay. On earlier, less efficient boats, we’d have them with us for long periods. With Beowulf and Wind Horse they come for a quick hello, and leave us after a few seconds.
In the lead photo the yacht was only moving at about six knots, yet the pod stayed with her for as long as we could watch with a 400mm lens, perhaps two minutes.
We were talking about hull forms and wave trains a few days ago. Here is a great example of why a hull designed to plane is not particularly efficient at slow speed (OK,, they are horrible slow). Three of these sport fishers passed in succession, slowed down so as to theoretically not create havoc with their wakes. Slow in this case meaning maybe eight to ten knots. We are told that a little further south folks on shore take exception to these wakes, and particularly in duck hunting season, salon windows tend to attract buckshot.
A “semi-displacement” hull form, theoretically designed to operate fast and slow, does neither well.
We have a detailed discussion on the subject of bow and stern waves, with lots of video, here.
Posted by Steve Dashew (October 26, 2011)
October 26th, 2011 at 10:36 am
Why did you choose Jarrett Bay, of all places?
October 26th, 2011 at 2:42 pm
Hi Chris:
A cruising friend had done some work by Corey McMahon (Triton Marine) and gave him high marks, so we went by to meet and discuss on our way north. We liked the marine park set up with a variety of vendors surrounding the area. Theoretically this should be very efficient. We have a variable length list, depending on how things go, and we shall report in more detail over the next four to s ix weeks (or longer).
October 26th, 2011 at 6:48 pm
“A “semi-displacement” hull form, theoretically designed to operate fast and slow, does neither well.”
What is the definition of well? Nigel Irens has some hull forms that do well for coastal cruisers, not passage makers.
October 26th, 2011 at 8:35 pm
Hi JR:
“Well” to us means efficiently. The motor yachts we have seen that are the so-called semi–displacement configuration are touted as planing and displacement hulls. They are poor performers in both regimes compared to designs which are optimized for a specific SLR. We are speaking of conventional beamy, relatively heavy designs, in other words, the norm. If you go on a beam and displacement diet, you can get a moderate displacement hull to operate at speed length ratios in the range considered planing, with shapes that are also efficient at displacement SLRs. Nigel Irens work is a good example of this. But he is in the minority.
October 27th, 2011 at 6:51 am
Check out this “race” and the wakes
http://www.youtube.com/watch?v=JoghpmoizUk&feature=youtu.be
Possible duplicate
October 27th, 2011 at 3:16 pm
Under towing tests
http://www.youtube.com/watch?v=31lX6wThxWo&feature=related