Foil Drive vs Duotone Foil Assist — Weight, Power and Runtime

The question we get asked about foil assist systems is almost always "which one is the most powerful". It is the wrong question, and it leads people to the wrong kit.

The right question is what you are carrying for the power you get. A foil is a balance machine. Every kilo you bolt to it changes how it turns, how it pumps, how it feels underfoot and how quickly you get tired. Power that arrives with too much weight attached is not a bargain.

So with Foil Drive's Gen 2.5 range just landing, we have run the whole line-up on weight-to-power — and then put it alongside the Duotone Foil Assist, which solves the same problem in a completely different way. We are a Duotone specialist and a Foil Drive dealer, so we have no particular axe to grind.

2.6kg FM+ 430 battery weight

17% Lighter than MAX High Power

+5 min More runtime, not less

+20% MAX+ thrust vs the MAX

The number that made us read the spec sheet twice

Most Foil Drive riders in the UK are on an Assist MAX with the MAX High Power battery. It has been the default for a couple of seasons and it is a good setup.

Now put the new FM+ 430 battery into the new Assist MAX+ and compare the two:

The FM+ 430 holds 430Wh against 594Wh. That is 28% less capacity. It weighs 2.6kg against 3.15kg — 17% lighter. The complete install drops from 5.91kg to 5.33kg, and gets 54mm shorter on the board.

And the runtime goes up. Fifty minutes light use against forty-five. Thirty-five minutes heavy use against thirty. Five minutes more, both ways, on a battery holding a quarter less energy.

Meanwhile the unit it is sitting in makes 35.5kg of thrust against 29.5kg, and 1750W sustained against 1455W. Both of those are 20% up. Lighter, longer, and a fifth more powerful, all at once. That is not how this normally works.

So how does less capacity give more runtime?

Voltage. The Gen 2 system runs at 40V and the Gen 2.5 system at 43.2V, and that difference does more than the numbers suggest.

As any battery drains, its voltage sags. The higher you start, the longer you stay above the threshold where the motor can still deliver properly — so you get a deeper usable discharge before performance falls away. Foil Drive quote full power down to roughly 10% on the Gen 2.5 batteries.

Higher voltage also means lower current for the same power, which means less energy lost as heat in the cables and the motor. You are simply wasting less of what you carry.

The practical upshot: rated capacity in watt-hours is not a runtime figure. It is the size of the tank, not the range of the car. Comparing a 43V battery to a 40V battery on Wh alone will mislead you every time.

Weight-to-power across the whole Foil Drive range

Here is every combination, with two ratios we have calculated from Foil Drive's published figures: thrust per kilo of installed weight, and watts per kilo. Higher is livelier.

Two things jump out.

First, the MAX+ with the FM+ 430 has the best thrust-per-kilo of any full-runtime setup Foil Drive make — 6.7, against 5.0 for the MAX High Power setup most people are riding today. A third livelier for the same kind of session length.

Second, and this is the honest caveat: the Slim tops the table and should not be read as the winner. A ratio that ignores runtime always flatters the smallest battery. The Slim is brilliant at what it does — short bursts of power to get on foil — and useless for a session where you need to motor. Use the ratio to compare things you would actually ride for the same length of time.

The same trick works on a Fusion

This is the part we think most people will miss, and it is the best news in the launch if you have bought a Fusion recently.

The FM+ batteries drop into a Fusion using the FM+ Battery Adaptor. The unit does not lose anything — 37.5kg thrust, 1850W sustained and 2368W peak boost are identical across all three Fusion batteries. The only thing that changes is what you are carrying.

Fusion with the 860: 8.08kg, and a thrust ratio of 4.6. Fusion with the FM+ 430: 5.78kg, and a ratio of 6.5. Same unit, same power, 28% less weight on the board and 65mm shorter.

Look at what that does to the comparison between units. A Fusion with the FM+ 430 weighs 5.78kg and makes 37.5kg of thrust. A MAX+ with the FM+ 650 weighs 6.71kg and makes 35.5kg. The Fusion setup is nearly a kilo lighter and more powerful.

The trade-offs are real and you should know them before you get excited. You give up runtime — 35–50 minutes against 45–65. And the Fusion install is longer on the board: 593mm against the MAX+'s 540mm, which matters on a short board with a tight mast track.

But it means a Fusion is no longer just the big, heavy, long-session option. With two batteries in the boot it is a long-session machine or a sport setup, and you pick on the way out the door. That flexibility genuinely did not exist before this launch.

How does this compare to the Duotone Foil Assist?

Before any numbers, understand that these two systems answer the weight problem in fundamentally different ways. This is not a spec war, it is a design philosophy difference, and which one suits you depends more on how you ride than on any single figure.

Duotone move the weight off the board and onto you. The batteries sit in an eHarness belt around your waist — two 159.6Wh packs, about 3.76kg for the harness and batteries together. The board itself carries only the eMast and pod. That keeps the foil feeling unloaded, which is exactly the point.

Duotone also replace your mast. The Foil Assist uses an 84cm alloy eMast with the motor pod 17cm below the top plate, and a fuselage adaptor is sold separately. Foil Drive go the other way: the unit retrofits onto the mast you already own, whatever brand it is, and comes off again when you do not want it.

Neither is objectively right. Weight on your waist is weight you are still carrying — it is just not on the foil. Weight on the board is weight the foil has to lift, but you swim and walk unencumbered. People feel very differently about this, and it is the single biggest thing to try before you buy.

The published figures, side by side

All figures as published by each manufacturer at the time of writing. Runtime quotes use different test bases and are not directly equivalent — treat them as each brand's own guide, not a like-for-like measurement.

What we could not compare, and why

We wanted to give you a straight power-to-weight figure for both systems. We could not, because Duotone do not publish a thrust or a sustained wattage for the Foil Assist. They publish energy, weight, runtime and the gear minimums — which is more than some brands — but without a thrust figure there is no honest way to put the two on the same axis.

We have asked, and we will update this post if we get figures. We are not going to estimate them, because a made-up number in a comparison table is worse than an empty cell.

One genuine Duotone advantage is worth calling out while we are being even-handed: at 159.6Wh each, the Foil Assist batteries sit under the 160Wh airline limit. No Foil Drive Gen 2.5 battery does. If you fly to your foiling, that is not a detail — that is the whole decision.

The questions actually worth asking

If you take one thing from all of the above, make it this: pick what you are optimising for first, then look at the kit. Foil Drive put it well in their own materials — every decision you make costs you something somewhere else. Runtime, power and ride feel pull against each other, and there is no setup that maxes all three.

So, in order:

How long is your actual session? Not the session you imagine — the one you really do. Most people motor far less than they think, and buy far more battery than they need.

What do you weigh, and what are you riding? Over 90kg, or on a small board and a small foil, and the extra thrust of a Fusion stops being a luxury.

How much does ride feel matter to you? If you are a surf foiler chasing turns, every kilo counts and you should be looking at the smaller batteries. If you are doing downwind runs or learning, runtime wins.

Do you fly with your kit? This one question can settle the whole decision on its own.

Do you want it off the board sometimes? A retrofit system comes off. A system built into your mast does not.