Semi hydrofoiling

[jakeeeef]

Any thoughts of what would happen with the overall drag of a very fine hullform (e.g., rowing shell, racing kayak), if one placed a small lifting foil below the craft’s centre of gravity, (or perhaps a little behind it to account for hull squat)?

So this foil size and attack angle would not be big enough for full foiling, and full foiling would not work anyway with single foil/ no pitch control. The idea would be to lower the level of the waterline/ slightly lift the hull so the U- shaped hull is running on a progressively smaller and smaller underbody the faster it goes (in that respect a little like a planing hull). But it’s still partially supported by its displacement activities.

Because the boat is not trying to fully fly it will keep close control of the foils AoA- this will always be similar to the horizontal line of the hull. The foil will be able to be optimised and angled for just one flow scenario (just different flow speeds), unlike a fixed foil that has to provide both takeoff and cruising duties on a full hydrofoil.

Of course all displacement hulls I know of suck down with increasing speed, so this foil would additionally counter that strand of increasing drag.

But would the drag of the foil and foil mast/s be more than the drag shed by halving (for sake of argument) the vessel’s effective displacement?

The appeal of the idea is twofold:
1) Easier to do in terms of maths and fabrication than full foiling (roll and pitch stability are still dealt with by the remaining immersed hull above).
2) Lower power requirement than full foiling.

I know there’s no answers without trying it, but what do people…guess?

Jake

 

[Skyak]

A displacement hull that is designed well has better lift/drag than 'good' hydrofoils until wave losses dominate at higher speeds. We covered this on your other HPHF thread.
A foil might be more efficient than the added flat surface of a planing hull at higher speeds for a semi-planing hull.
If the foil is not designed to add stability you will have nothing to replace the stability lost as the mass center rises and the metacenter drops.

This can all be calculated. If you are guessing you have more to learn.

 

[Heimfried]

It has been successfully done, search for HYSUCAT.
For example:
http://www.boatsbycoastal.com/pc/Hysucat-Development- Internal-Report.pdf

 

[Mr Efficiency]

Of course Hysucat is not a displacement vessel.

 

[jakeeeef]

Thanks, yes, I've seen hysucat. Bit different, I'm talking about much slower speed, non planing.
There's a commercial marine system for planing monohulls (RIBs) too ( might be van oosanen/ hull vane, but can't remember. It uses a buoyant bulb/ torpedo with small lift wings under the boat on a mast, so part SWASH, part what I'm thinking. Again though it's for planing, (presumably lower speed attainment of planing condition, and hence fuel savings at usual duty cycle).

Re: roll stability. Yes, it would need surface piercing inclined foils. The test bed I'm thinking of is a trimaran already with a crossbeam just aft of CoG.
Easy winter job to fabricate some inclined surface piercing foils and crossbeam brackets to make them adjustable in all dimensions.

And to touch on Skyak's last point. Yes, sure it is calculable, but this and a lot of discussions on this forum boil down to where people prefer to spend their time... In the workshop and on the river, or in front of a computer screen.

 

[Heimfried]

(Replay to #4)
You are right. So I should change the "It has been successfully done" in "Some information about the idea of boats with hulls partly elevated by hydrofoils", proved in theory and reality.

 

[Heimfried]

[...] Bit different, I'm talking about much slower speed, non planing. [...]

Very hard, if not impossible.
The lifting force of foils is increasing with the square of velocity and only linear with projected area. Increasing area means to increase the drag also.

 

[portacruise]

Jake, there may be other ways to accomplish the end result of what you have written about in your posts, without using hydrofoils.

 

[Skyak]

Thanks, yes, I've seen hysucat. Bit different, I'm talking about much slower speed, non planing.
There's a commercial marine system for planing monohulls (RIBs) too ( might be van oosanen/ hull vane, but can't remember. It uses a buoyant bulb/ torpedo with small lift wings under the boat on a mast, so part SWASH, part what I'm thinking. Again though it's for planing, (presumably lower speed attainment of planing condition, and hence fuel savings at usual duty cycle).

Re: roll stability. Yes, it would need surface piercing inclined foils. The test bed I'm thinking of is a trimaran already with a crossbeam just aft of CoG.
Easy winter job to fabricate some inclined surface piercing foils and crossbeam brackets to make them adjustable in all dimensions.

And to touch on Skyak's last point. Yes, sure it is calculable, but this and a lot of discussions on this forum boil down to where people prefer to spend their time... In the workshop and on the river, or in front of a computer screen.

I think I speak for everyone when I say we love to see people try and document the result (the exception is "garbage is cheap, so I want to build a huge yacht out of it!"). Foils calculate out pretty well and because lift is proportional to V^2 it is easy to get into trouble if you don't do the math first.

There is another advantage possible that we haven't considered/discussed. If you use a prop, directing the higher velocity prop wash over the hydrofoil greatly increases lift. There will be drag but my theory is the total L/D can be improved by the wing harvesting some of the rotational energy from the prop wash. It also gives relatively more lift before takeoff when prop wash is much faster than boat speed.

The hull design you need is also a bit different. You want something that drops lots of drag with lower displacement, but doesn't increase drag much with higher velocity. No rocker, keeps it's waterline length, but drops it's waterline beam at lower displacement.

 

[portacruise]

Found the link to one thing I was thinking about, which might also work in the human power range,

https://www.sspa.se/ship-design-and-hydrodynamics/hull-air-lubrication-future-and-challenges

It bypasses many of the serious detriments of using hydrofoils, including dangers with even minor collisions with debris, entanglement with debris, need for ramp up and minimum for flying, no shallow draft running, etc.

 

[jakeeeef]

Re:
You want something that drops lots of drag with lower displacement, but doesn't increase drag much with higher velocity. No rocker, keeps it's waterline length, but drops it's waterline beam at lower displacement.

Yes, I was thinking if my experiments with a U shaped hull showed any promise, a deep V shape with a similar V foil beneath it might be a better way to rapidly reduce displacement drag as it climbs further out of its trough. Sort of a single chine canoe design of the stitch and glue plywood kind.

Good idea on the prop wash, having the prop around the centre of the hull is achievable with human power. Wonder though if the mix of vortices off the prop would all be at the the angles that the foil would enjoy.

Also, I'll be endeavouring to have a highly efficient high aspect 2 bladed prop, so hopefully there won't be much wash off it.

 

[Skyak]

Re:
You want something that drops lots of drag with lower displacement, but doesn't increase drag much with higher velocity. No rocker, keeps it's waterline length, but drops it's waterline beam at lower displacement.

Yes, I was thinking if my experiments with a U shaped hull showed any promise, a deep V shape with a similar V foil beneath it might be a better way to rapidly reduce displacement drag as it climbs further out of its trough. Sort of a single chine canoe design of the stitch and glue plywood kind.

Good idea on the prop wash, having the prop around the centre of the hull is achievable with human power. Wonder though if the mix of vortices off the prop would all be at the the angles that the foil would enjoy.

Also, I'll be endeavouring to have a highly efficient high aspect 2 bladed prop, so hopefully there won't be much wash off it.

I was thinking my hull would be a three panel -deep V with the point cut off. As displacement is reduced waterline beam drops but length stays.

The greater the prop sweep the lower the velocity add needed for thrust. Prop wash is linear velocity (thrust) and spirals (wasted energy). They aren't all beneficial but over a good turbulent flow section there is power to scavenge.

 

[jakeeeef]

Why would you cut off the point of the V?
Does it reduce the area (skin friction) ?
Or is it to make it possibly behave as if planing? With a flat, vertical transom for release?

I've found lightening a hull's displacement by lifting it by other external means can encourage planing as much as dramatically increasing speed/ power. (A far from unknown phenomenon, but still interesting to experiment with in the wild) For example, for short amas that are far outboard on a slow trimaran, I've found a flat bottom aft and transom stern makes the ama behave like a little RC boat, so planing at something like 3 knots, but that's due to the fact it's probably only supporting 1 or 2 kgs.

I'm going off topic, but I've always enjoyed the fact that a multihull can combine a central displacement hull with planing outrigger hulls all at the same speed. What is so different is the loading on those hulls.

I think a lot of people that build trimarans build a long, slender displacement central hull and then without a huge amount of thought go for similar profile displacement amas, when they might be better with surface skimmers.

I guess that's what I'm getting at, just other ways of somehow 'cheating' that usually unavoidable displacement in the same sort of way. But for the main hull.

It comes from lots of human powered boating and wondering not how can I get more power'. , but ' how can I effectively halve my weight while still putting in the same power.

 

[Skyak]

Why would you cut off the point of the V?
Does it reduce the area (skin friction) ?
Or is it to make it possibly behave as if planing? With a flat, vertical transom for release?

I do it to minimize wetted surface and lower my butt as much as possible for stability so I can keep the waterline narrow.
No planing -too little power, too slow, too narrow, not enough angle.

 

[container]

I have tried something similar on a SUP back in the day. The board (pictured) was 3.6m long, 700mm wide and essentially a double ended hull although not exactly. It was designed for efficient flat water paddling, the design even had a little bulbous bow which worked great when it was calm but would tend to hold the bow down when running downwind in a breeze so I built this board without it.

when downwinding in 1-2 metre seas, big enough to surf and start planing, the board would suck down by the stern, creat a massive rooster tail and become uncontrollabe in roll and yaw. To remedy this i put a small ~300mm span foil on the bottom of the fin- which was glassed on and nearly vertical so could handle the extra stress. the added drag was undetectable at cruising speed however once planing on a wave the squatting disappeared, instead of dragging the stern with a large rooster tail the wake was comparatively flat with flow seperation from the hull occuring as far forward as the fin mount (700mm forward of the stern) it absolutely achieved everything i thought it might

So, not human powered because it was 'surfing' a wave, and not displacement speed because the board was planing, nevertheless it was a foil assisted paddleboard

Just design a more suitable hull in the first place and flag the foil.