Semi-displacement boats don't generate lift?

I tested a half size model at 13ft, of a box keel motorsailer, to either confirm or dispel some myths. I had read that Gerr had suggested a SL of 1.8 was only possible with a MAXIMUM 133 disp/length ratio. My box keel boat with a 164 DLR, maxed out with SL of 1.9, using an old worn 4hp outboard. It would confirm that having a large flat area as a keel base certainly helps giving lift.

 

Stephen D:
I'm very interested in the topic of improving hull performance at semi-displacement speeds. Should we get into that in this thread?

If you are going to start a new thread, call it semiplaning. I think it shows better what it is about. Semidisplacement is a strange expression.
JS

 

Except its been used to define motor yachts that exceed "displacement speed" for decades. Its become the industry definition for "semi-planing".

 

Semi = partly, half ...
JS

 

About the bottom line and the sections, I developed some thoughts for a sailing dinghy which I think can be also relevant for a motor boat to a certain extent :
** For a full planing motorboat, the bottom line is typically a pronounced curvature from the bow end to about the first 1/3 of the length followed by a flat run for the aft 2/3 of the length. For semi planing, this is not optimal, a better line is, from the bow forefoot itself, a regular decrease of the curvature vanishing to zero at aft transom, instead of a straight flat run.
** About the sections : it is typically a V for a full planing motorboat, with more or less deadrise angle at the transom, say from 7 to 22 deg depending the balance performance / sea keeping you want. For semi planing, still ok for V sections for say the bow 1/4 of the length but then developing progressively a flat central zone around the bottom line for the remaining 3/4 length, i.e. « flat and shoulders » shape sections, able to provide more lift despite a low hp motor.
More on these ideas here :
About the hull shape of sailing dinghies https://www.boatdesign.net/threads/about-the-hull-shape-of-sailing-dinghies.67577/

 

0.60 Fn

"provoking a sucking (i.e. a negative Dynamic Lift and a bow-up moment" (Jean-Francois, November 2022)

E x a c t l y

(1) Ludwig 1922: 1000 Newtons
(2) Savitsky 1964: 2500 Newtons
(5) Ludwig 1922: Low Pressure, Sucking: 1000 Newtons

some ... for semi-Displacement and Planing ... seek to destroy Low pressure at the stern

and ... others ... for wave Surfing ... want low pressure at the stern to trim the hull bow-up

some are looking for speed, and speed on more or less flat terrain so to speak

and others are looking for Comfort, Control and Safety in big waves in small sailboats

Interest guides knowledge

 

Semi-planing boats https://www.boatdesign.net/threads/semi-planing-boats.65025/
Semi-planing boats https://www.boatdesign.net/threads/semi-planing-boats.65025/page-3
http://sassdesign.net/Keyhaven Skiff, a semiplaning lightweight skiff.pdf
JS

 

I'm a fan of Mr. Sass's powerboat ideas - very nice!

Sailboat designers have also made contributions in this area. Two who have brought their skills & insights to power launch design are Paul Bieker and Nigel Irens. The hydrofoil rudders Bieker brought to the I-14 class were designed to suppress their stern wave, interestingly. A hook in the run or a stern flap can have a similar effect, as discussed here.

Bieker argues that, at semi-displacement speeds, optimizing longitudinal distribution of volume remains critical, and that doing away with rocker entirely may be a mistake. Perhaps this is what you're implying as well, CarlosK2?

I recently came across this article & boat, a nice implementation of a box keel. Box keels have been featured in Phil Bolger's work and the latest America's Cuppers. The Atkin design in the article might also bear some resemblance to the Vripack Radical Slide Hull. Would someone like to beat me to posting the lines of that hull here, perhaps from their patent? I'd be interested in seeing the Moxham Hull patent while you're at it.

I see some related discussion at this thread and elsewhere:
Stern taper on semi-displacement hulls https://www.boatdesign.net/threads/stern-taper-on-semi-displacement-hulls.60336/

 

I think this is the Vripack Slide Hull patent:
NL2012973B1 - Vessel. - Google Patents https://patents.google.com/patent/NL2012973B1/en
I don't see drawings, though. Your assistance requested.

Moxham Hull patent:
EP4043330A1 - Marine vessel comprising a planing hull - Google Patents https://patents.google.com/patent/EP4043330A1/en
https://patentimages.storage.googleapis.com/5b/63/63/2a92d7896bacd1/imgf0003.png

Anyone have experience with the Shannon powerboat hulls? What's their resistance curve like?

 

A bottom view of the hull at photo 7 :
https://www.boatinternational.com/yachts/news/yacht-creators-unveils-new-27m-leven-yacht--41435

 

S. Ditmore: Vripack and Moxham
These boats you refer to are not optimized for semi planing speeds.
JS

 

Then Jurgen, would you join Bieker and Irens in favoring a gently rockered bottom for "semi-planing" speeds?
What about a hook, flap, or foil at the stern?
What do you think of the box keel of the Atkin hull?
The Atkin does share with the Vripack a reverse angled surface - Vripack as very broad spray chines that carry forward, Atkin as reverse deadrise at the stern.

Vripack Slide Hull
https://cdn.boatinternational.com/convert/bi_prd/bi/library_images/mduaFc9RCyCu7CHZZDZ8_Leven-yachts-hull-Design.jpg/r[width]=1366/mduaFc9RCyCu7CHZZDZ8_Leven-yachts-hull-Design.webp

https://www.boatinternational.com/yachts/news/yacht-creators-unveils-new-27m-leven-yacht--41435

 

I looked at the PT Skiff of Paul Bieker : from the photos, one can note the lower chines very flat on about 3/4 of the length (except the stem zone), with a deadrise of 3° (at transom).
I am designing myself a sailing dinghy supposed to be optimal at such semi-displacement speed, and I shape a central zone where the deadrise is less than 3° (the green line here below). Here the area of this flat zone is 49% of the waterline area at rest (the red line), its center is at 34 % Lw to compare with the one of the waterline at 41% Lw. I think that a motor boat with same purpose can be designed with a similar flat central zone.

 

Interesting, Dolfiman. Also interesting is the thread you started about the Sun Fast 3300:
Doug Lord pointed out that the amas on Gitana 17 have similar hollows bow & stern. I think this is also true of the 50 Class trimaran Arkema 2 designed by an associate of Verdier's. In that case the sterns are, I believe, near rectangular, creating an effect along the bottom similar to the flaps discussed here. They are the subject of a Professional Boat builder article here.

 

S Ditmore

There is more than just rocker at the bottom that needs to be calculated. The most important thing is to adjust the area and depth of the transom below the waterline.

If the boat needs a hook, flap or foil in the stern, I recommend an interceptor over the entire bottom width instead.

The boxkeel is suitable for a boat with an inboard engine. Its advantage is also that the propeller is protected and that the total draft is small.

The Vripack boat is beyond my competence to judge, to put it kindly. From my perspective, that bottom shape is not optimized for semi-planing speed.

Don't guess, calculate!
JS