Planing or semidisplacement

I've been looking at a couple Atkins designs. Namely Xlnc and Russell R are these hulls planing or semidisplacment. I'm asking as the have a significant amount of rocker between the last station and transom.

 

Welcome to the Forum Chris.

Here is a link to the Russel R -
Atkin & Co. - Russell R. http://www.atkinboatplans.com/Oar/RussellR.html
I would think that she (he?) is more planing rather than semi-displacement - she has a very flat bottom, and should get up on the plane fairly easily.
From the Atkin webpage the designer notes that "My feeling is that a motor of about 12 h.p. is large enough for Russell R. The speed with this size unit will be a good 17 m.p.h. "
She will definitely be planing at 17 mph!

And here is a link to XLNC -
Atkin & Co. - XLNC http://www.atkinboatplans.com/Utilities/XLNC.html
I would think that she is also more planing than semi-displacement as the designer notes that "The plans show a single cylinder 6 h.p. Baby Huskie Palmer motor. With this or another motor of similar characteristics the speed of this latest of MoToR BoatinG's fleet of useful boats will be 13.5 miles an hour."

However with both boats there is no point in fitting much larger engines than what the designer suggests above - if you have a quest for speed (and consequent fuel burn) you would be better off with a more conventional planing hull form.

 

However you want to describe them, they are certainly slender by modern standards, and would not tolerate too much power or weight. If you have modest load and speed ambitions, you could make lively progress with little fuel used. Not sure what they were supposed to be built from, but being straight sectioned, the topsides at least would not be strictly developable.

 

Looking at the recent constriction pictures of the XLNC, the power plant is a typical modern V type, of about 20-24 horsepower, far far in excess of what the designer specifies. The picture of the boat underway illustrates the folly of overpowering such a hull.

 

I assume the photos Fred refered to are on this page: Photos of XLNC http://www.atkinboatplans.com/Photos/XLNC/index.html
The upward curvature aft probably contributes to the bow up attitude when planing.

 

I'm sure Atkins put the rocker in the bottom for a reason. I'm just wondering why? Less wetted surface on plane? Aids a low power craft to achieve plane? Does anyone know the year of design for these boats?

 

I will hazard a guess that they are intended to be used at displacement speeds a lot of the time, and with the rocker there is then less transom immersed, hence the resistance should be a bit less?

 

I'd really like to see an example of the Atkin Everhope, and test it, a very attractive little boat that supposedly makes good speed with not much engine power. Has to be better than a flat bottom.

 

Robb White (+), boatbuilder of Thomasville, Georgia, has built, tested and written a lot about RESCUE MINOR another Tunnel Stern Seabright Skiff, an
excellent flat water boat but hydrodynamically not so effective (look at the wake). With the help of google you can find a lot of discussion
about this design. rescue minor rob white - Google Search https://www.google.com/search?client=firefox-b-d&q=rescue+minor+rob+white

 

Thanks for this link Manfred re Rescue Minor.
Robb White, Boatbuilder, Thomasville, Georgia https://www.robbwhite.com/rescue.minor.html
She certainly is very economical (28 mpg! Better than many vehicles) - a quote from this page :
"It will run 20 knots in six inches of water and gets about 28.6 nautical miles per gallon of Diesel fuel running at its most economical speed of 10.5 knots".
And a classic quote from the 'Sportboat' page - Robb White, Boatbuilder, Thomasville, Georgia https://www.robbwhite.com/sportboat.html
"I have noticed that the more the horsepower of a boat increases, the less polite the people who operate it are".

 

Robb was known for being a bit generous in his various claims of performance as well as having strong opinions about most things boating. In being aboard a Rescue Minor with equivalent power source, speed was found to be much less and mileage per gallon looks to be similar affected. No doubt Robb had designed and built to get the best out of the boat but!! These are good boats for specific uses. Operation in rough water is not one of them and no amount of encouragement would cause me to take one through one of the east coast inlets.