WaveProp

I have been thinking about a new project: a foil that is submerged below the bow of a boat. the foil is hinged at near the leading edge and its travel is limited by an adjustable stop above and below to define pitch. as the boat pitches, the foil moves to the stop and propels the boat forward and the same wether up or down

That's a genuinely interesting propulsion concept — let me think through the mechanics with you.

what size and type of foil should i use

Great question, but before I can give you a useful answer, I need to understand the application better. A few quick questions:

The boat:

What size and type of boat are you thinking about? (Hull type, weight, length)

What speeds are you targeting?

Is this purely wave-powered propulsion, or supplemental to an engine?

The conditions:

What wave environment? (Offshore ocean swells, coastal chop, lake waves?)

Typical wave period and height you'd be designing for?

The concept mechanics:

I want to make sure I understand your setup correctly — the foil hangs below the bow, hinged near its leading edge, so as the bow pitches up, the foil hits the upper stop and is angled to generate forward thrust, and as the bow pitches down, it hits the lower stop and again generates forward thrust. Essentially converting pitch oscillation into a thrust vector in both directions — like a fish tail working vertically. Is that right?

If so, this is similar in principle to the Sails of Whales / Minesto class of devices, and also to the Wavefoil system used on commercial vessels — so there's some engineering precedent to draw from.

Getting the foil geometry right depends heavily on the pitch frequency (wave period) and the speed of the bow's angular travel, which ties directly back to the boat and wave conditions. What are you working with?

initially im disigning for a 21ft beneteau sail boat as an emergency source of thrust or alternative thrust. for ocean swell and chop. designed to get you home if there is a problem with the sail or engine

Perfect context — that sharpens everything considerably. Let me think through this with you.

The Beneteau 21 gives us good constraints:

Displacement roughly 800-1000 lbs

Bow pitches with reasonable authority in ocean swell

You need maybe 50-100W equivalent thrust to make meaningful headway — even 1-2 knots would get you home

For foil size and type, here's my thinking:

Size — probably 400-600mm span, 150-200mm chord

Too large and the foil overpowers the bow's ability to move it — it just locks up

Too small and you're not capturing the energy available

This gives you roughly 0.06-0.12 m² of area — meaningful but not overwhelming

Profile — a symmetric section is almost certainly right

NACA 0012 or NACA 0015 are the classic choices

Symmetric because the foil needs to generate thrust equally on the upstroke and downstroke

Thicker sections (0015) are more tolerant of varying angle of attack and less prone to stalling — probably better for irregular ocean conditions

The hinge point matters enormously

Placing it at roughly 25-30% chord (near the quarter-chord point) is where the aerodynamic/hydrodynamic center sits — the foil will be nearly moment-neutral there, meaning the water pressure won't try to slam it to one stop or the other. It will move freely and the stops will do the work.

What I'd watch out for:

Kelp and debris — a foil hanging below the bow in open ocean will catch things

Cavitation isn't a concern at these speeds

The attachment strut creates drag even when you're sailing normally — you want this retractable or removable

A rough starting point I'd suggest building toward:

NACA 0015 symmetric foil

500mm span, 180mm chord

Hinge at 28% chord

Adjustable stops giving ±15° to ±25° pitch range

Have you thought about how the strut attaches to the bow, and whether it needs to fold up when not in use? That might actually drive the foil geometry as much as the hydrodynamics do.

yes the foil was originally thought to clamp on to a spinaker pole and the pole attached to a fitting on the anchor roller with two brace rods to the cleats on the gunwales for support. that war the parts are small and can be stowed when not in use. it also puts the foil 6 or 8 feet down

That's a really elegant solution — you've essentially solved three problems at once:

Modular and stowable — spinnaker pole, foil, braces all break down separately

Structural — the two brace rods to the cleats make a proper triangulated mount, the anchor roller fitting is a natural hard point

Depth — 6-8 feet down is excellent, gets you below most surface turbulence and chop, cleaner water flow, and reduces the risk of broaching the foil in wave troughs

The depth is actually a significant advantage over commercial wave foil systems like Wavefoil, which mount much shallower. You're getting into more stable, less aerated water.

A few things this configuration suggests:

The spinnaker pole gives you roughly the right diameter for a strut — but you'll want to think about strut drag. A round pole is not ideal hydrodynamically. Could you sleeve it with a fairing, or is the drag acceptable given this is emergency use only?

The hinge and stop mechanism at the end of the pole becomes the critical engineering piece. It needs to:

Survive real ocean loads

Allow easy adjustment of the stops

Be simple enough to deploy when something has already gone wrong on the boat

The brace rods to the cleats — are you thinking rigid rods or wire/line? Rigid gives you compression resistance which you'll need as the foil generates thrust and tries to push the pole aft.

How were you imagining the foil connects to the end of the pole — is the hinge axis horizontal, running athwartships?

the attachment will be a round fitting split at the front with an overcenter clasp lock. recently I have been thinking of a dedicated pole which has a torque rod with conter rotating threads so that when the rod is turned the pitch stops could be trimmed for best angle...?

Now we're getting somewhere really interesting — that torque rod idea is clever. Let me think through it with you.

The counter-rotating thread mechanism:

So you're thinking one rotation of the rod moves the upper stop one way and the lower stop the other way simultaneously — keeping the stops symmetric around neutral. That means you can trim the pitch angle with a single control input from the cockpit potentially. That's a significant seamanship advantage — you could tune it while underway as conditions change.

This suggests a dedicated pole is the right call over a spinnaker pole for several reasons:

You can integrate the torque rod cleanly inside the pole

Design the pole section for hydrodynamic efficiency, not just round tube

Size it specifically for the loads rather than adapting a spinnaker pole's geometry

The round split clamp fitting at the foil end becomes a designed interface, not an adaptation

The torque rod raises some good engineering questions:

Where does it terminate? Ideally a control at the cockpit end — a simple knob or wheel you can reach while monitoring progress

How do you lock it? You want it to stay put once trimmed — a friction lock or detent

Sealing — the rod runs inside a submerged pole, so you need to think about water ingress at the foil end bearing

What I find most elegant about this system overall is that it has almost no failure modes that make things worse. If the foil jams at an angle, you pull the pole. If the torque rod seizes, you have fixed stops. It degrades gracefully — which is exactly what you want in emergency equipment.

Have you thought about what the optimum trim angle actually is, and whether you'd want asymmetric stops — slightly different upper and lower limits to account for the bow's pitch asymmetry in real swells?

a stainless rod and nylon bushings will not be damaged by sea water. pitch stop asymetry might be good, I havent tried it yet

Good material choice — stainless rod in nylon bushings is basically maintenance-free in saltwater, low friction, and the nylon is self-lubricating. That combination is used everywhere in marine hardware for exactly this reason.

On asymmetric stops — here's the physics case for it:

In real ocean swells the bow's downward pitch is typically faster and more forceful than the upward recovery — gravity assists the plunge. So the foil hitting the lower stop has more angular velocity behind it and potentially more force available. You might want:

Lower stop at a larger angle — say 20-25° — to capture that extra energy

Upper stop at a smaller angle — say 15-18° — where the slower upward pitch is still enough to generate thrust without stalling

This is something you'd dial in empirically on the water, which is exactly why the torque rod trim system is so valuable — you can experiment in real conditions.

For your first prototype I'd suggest:

Start with symmetric stops at around ±18°

Get a feel for how the foil behaves in actual swells

Then introduce asymmetry by adjusting and noting boat speed response

The torque rod with counter-rotating threads gives you a natural way to test asymmetry too — if you add a second independent adjustment for one stop only, you could offset from symmetric. Though that adds complexity. Might be worth keeping simple for version one.

Are you planning to build the foil itself from fibreglass composite, given your experience with the beam challenge? A NACA 0015 foam core with fibreglass skin would be very achievable with your existing skills.

yes. I was also thinking it should have tip fences, and high aspect ratio to reduce slack time

Both of those are exactly right — good instincts on both counts.

Tip fences:

They serve double duty here more than on a conventional foil:

Prevent tip vortex losses — keeps the pressure differential working across the full span rather than leaking around the tips

Act as end plates — effectively increase the aerodynamic aspect ratio beyond the physical aspect ratio

Reduce induced drag — critical when the foil is generating thrust at varying angles through the pitch cycle

Could also act as debris deflectors — kelp and line tend to slide along a foil and off the tip. A fence stops that and could be a problem actually — worth thinking about the fence geometry so it sheds debris rather than catches it

High aspect ratio — absolutely correct reasoning:

The "slack time" insight is sharp. A low aspect ratio foil has a wider, more gradual lift curve — it generates meaningful thrust across a broader range of angles but never peaks strongly. A high aspect ratio foil:

Has a steeper lift curve slope — generates thrust more decisively when it hits the stop

Spends less of the pitch cycle in the transition zone between stops

More efficiently converts angular pitch motion into thrust

Practical target for aspect ratio:

For your 500mm chord starting point, pushing span to 900-1100mm gives you an aspect ratio of roughly 5-6 which is achievable in fibreglass and still manageable in terms of structural loads at the tip.

The foam core layup becomes interesting at high aspect ratio — you'll want a spar running spanwise, even a simple hardwood or carbon tube spar, to handle the bending loads at the root where it meets the hinge fitting.

Have you thought about whether the tip fences are fixed or whether they do anything structural — like reinforcing the tip of the foil itself?