In “Of Yachts and Men”, Atkin describes Grampus Jr. as a private commission. If this was the case then proprietary rights to the design would belong to the customer who paid for them, and Atkin would not have been free to publish them.
However, it cannot be a coincidence that in the same year (1930), Atkin did publish plans for a Tunnel Stern runabout of exactly the same length with the same horsepower and speed specifications as “Grampus Jr.”. That boat was called “Heron” and it is almost certainly the same boat. Plans for “Heron” were published in the Ideal Series book, Volume 16 (1934) and I can scan them for you if you’re interested.
“Rescue Minor” was published in November 1942 and there were four other Tunnel Stern designs released in the intervening twelve years. (Nanuk III, Islamorada, Everhope & River Belle) So, it is reasonable to suppose that the design changed over that time too, if for no other reason that “Rescue Minor” was specifically designed for plywood construction.
I also suspect that Atkin’s understanding of the Tunnel Stern changed a bit in that period too. For example, when discussing the draft of “Heron” in 1930 he says that the stern will “squat” once underway, whilst for “Rescue Minor” in 1942 he says it will rise. Given the designs are so similar, one of those must be wrong. Interesting......
would anybody have the offsets for Grampus Jr.
sure would like to compare them to Rescue Minor
just to see what changes were made
- Thanks Alan, I was thinking that over these many years Grampus
might have become public.
You are right, taking another look at Heron I do see the similarities
though to topsides are quite different.
I mainly wanted to see the changes in the tunnel that eliminated the squat.
I was thinking a bit more about the apparent difference between “Heron” and “Rescue Minor” in terms of “squat” when underway.
Given that the Station spacings are about the same, you can directly compare the Offset measurements for the Tunnels from the two hulls. Working aft from Station 7 you get the following:
Heron:- 3.25”, 4.75”, 6.75”, 9”, 11”, 10.75”
Rescue:- 3.25”, 4.5”, 6.875”, 10”, 11.75”, 10.5”
In front of the propeller, the tunnel on Rescue Minor is a little more pronounced than Heron, but the killer is the downward curve after the propeller at Station 11. The difference between the tunnel’s highest point and the height when it leaves the transom on Heron is only 0.25”, whilst Rescue Minor is 1.25”. So, the wash leaving the tunnel in Rescue Minor is being pointed downward at a much greater angle than Heron, which presumably accounts for the extra lift.
- Alan you have answered my questions and confirmed my thoughts
sounds like you've done a lot of research along these lines
many thanks Gary
Atkin did publish another Tunnel Stern design after “Rescue Minor”. In 1947 came “Nibble”, (Ideal Series, Volume 38). “Nibble” is 17’ long, but surprisingly draws 13.5” compared with “Rescue Minor” at 6”, which seems a backward step if you’re trying to design for very shallow water. I wonder if perhaps there were stability issues with “Rescue Minor”; not that I’ve read a bad word about it anywhere.
Do any of the members “out there” have any ideas ?
I don’t have the Offsets for “Nibble”; if anyone has access to them, I’d be very keen to have a look!!
I’m building a “River Belle”, so I’m very interested in Tunnel Sterns. It’s not in the water yet, but I’m really keen to see if it performs as well as Atkins suggests. Check www.RhapsodyInGlue.com if you’re interested.