Semi-displacement boats don't generate lift?

the cool thing about electrons in a sailboat (we just made the leap) is it’s just like sailing, except more so….. range anxiety becomes your best friend, as in the idea of the power of limits sort of way….

 

I'm an enthusiastic electric sceptic. But the question I was alluding to was relative mileage for diesel vs gasoline systems, all other things being equal. I stopped myself before a long discussion about why it's possible to get more power from a diesel engine than a gasoline engine of the same displacement.

 

Hull vane taken wooden wedges on the transom to a new level. Saving 57,000 litres of fuel a year is no small thing.

 

This simple scheme I think might be useful

(1) Surfing: Max: Maximizing a Hull Attitude Bow Up, Stern Down

(2) Planing: Overcome the bow wave and produce a large amount of High Pressure (H) with an Angle of Attack (AoA) of say 4-7 degrees

(3) Semi-Displacement: Min: Minimize Hull Attitude bow up, stern down

Resistance in 0.4-0.6 Froude and 0.6-0.8 Froude is

+ (a) Friction
+ (b) Pushing away, Displacing the water (what we could call the 'core or main part of Michell')
+ (c) Hull Attitude

 

For example in the case of my prototype WindSurf 5.99 @ 0.80 Froude: 1969 N

(1972 N, Savitsky 1964)

1969 Newtons =
+ Water friction: 620 N
+ Water Pushing Away: 391 N
+ Hull Attitude 5 degrees: 958 Newtons (!)

For those looking to consume less fuel the logical path is to level the boat by raising the stern

 

0.80 Froude = 2.66 SLR

2.66 x 39^1/2 = 16.6

 

But according to BMcF, Sep 11, 2023 - and I hear others' testimony as well - actively suppressing the stern wave and keeping weight aft works better than moving the LCG forward, even if the trim is not changed much (flaps literature - different from trim tabs).

 

(1) Surfing Logic
(2) Planing Logic

(3) Semi-Displacement Logic:

For those looking to consume less fuel (0.40-0.80 Froude) the logical path is to level the boat by raising the stern ...

... with High Pressure (H) or with a HydroFoil

 

(a) sharp bow
(b) avoid vertical High pressure at the bow
(c) move the center of gravity aft
(d) completely destroy the low pressure at the stern and instead create High pressure and even lift the stern thanks to a Hydrofoil

 

The reason I said (in this thread on Sept 9) that I'd love if Cusanelli & Karafiath of the David Taylor Model Ship Basin would join the discussion is that their papers and their article in Professional BoatBuilder clearly state that the principle of stern flaps (as opposed to trim tabs) is NOT to change the trim, but to alter the pressure field under the stern.
Stern Flap Design https://www.boatdesign.net/threads/stern-flap-design.52417/

Edited my previous post - my reference was to BMcF, Sep 11, 2023

So my question, CarlosK2, is whether the state-of-the-art has changed since I participated in the design of the Krogen Express 49 (now 52), or since my friend Bob Turcotte designed his most recent lobster boat? Which improvements have resulted in significant gains?

On this subject, many years ago I spoke with FastShip Atlantic designer David Giles who said I was free to copy the lines from his patent, rescale them (differentially, perhaps) and apply them to a yacht or use as I wished so long as my boat didn't exceed the length specified in the patent as the lower limit of what the patent claimed, which is 200 feet. Looked FastShip Atlantic up just now and found David Giles sued the US Navy for violating his patent in the design of the Freedom Class LCS.
https://ecf.cofc.uscourts.gov/cgi-bin/show_public_doc?2012cv0484-181-0
It looks like the sections of both hulls are fully published here.
Jurgen (HJS) note: discussion of interceptors starting p.23

I'd welcome David Giles to this thread, as well as Bruno Bouckaert, the Hull Vane guy.

 

My ignorance tells me a boat that is given more lift at the stern, may need a modification at the bow? Does the bow angle become more negative with a Hull Vane activated? As mentioned, its not a tab, its a lift device......on one end of a lever with the other being pointy and less buoyant. Maybe on 200ft ships my concern is not valid?

 

This is 0.60 Froude

In the 0.40-0.80 Froude Zone, if you want to reduce the Resistance the question is to raise the Stern

Sinking the bow does not work because then the resistance increases by moving the water away.

There are three sources of Resistance: (1) Friction (2) Moving the water away and (3) the Stern sinking into the hull wave.

 

...

(1) Friction
(2) Michell

Residual Resistance of Displacement Vessels https://www.mdpi.com/2077-1312/8/6/400

(3) Bow Up, Stern Down

 

Semi-displacement idea:

(a) reduce the vertical component of the High Pressure at the bow

(b) completely destroy the Suction at the stern and instead create High Pressure and even a HydroFoil

 

Both longitudinal stability and transverse stability matter. If one realizes this and designs full ends, and is designing with a certain Cp in mind this could result in the double-concave rocker of the Sun Fast 3300 discussed at The new SUN FAST 3300 with a double concave line https://www.boatdesign.net/threads/the-new-sun-fast-3300-with-a-double-concave-line.61651

Early in this thread I tried to reference a Daniel Savitsky paper only to find my old link to it no longer works. In that paper he shows that at a length Froude number of 0.9 a non-planing hull (LCG placement correct for semi-displacement) will start to bury its bow and misbehave to the point of being difficult or impossible to control.

I mostly agree with CarlosK2, who has posted some great information. Concerning the hydro-vanes, though, I think most are fixed (the model for full planing hulls being the exception), so there's no 'activation' per se. I don't think the hulls they're mounted on are displacement hulls with excessive stern squat; I think the wake cancellation in their video (last post on page 5) at 4 1/2 minutes and 12 minutes is exactly what it appears to be; wake cancellation.

Of course, one situation in which you want your bow to stay up is when surfing down the face of a big wave. There the wave is doing the work of creating 'pressure' ('lift' if you like) under the stern of the boat. Sailboat designers must be attentive to balance here, too, as well as upwind. If the feel of the helm changes dramatically as a boat heals, this could cause loss of control when surfing down a wave.