not over the hump

This week, Paul Larsen and his team of Sail-Rocketeers in Namibia, undertook their third session of tow testing of the bare platform. Lia Ditton has the excerpts from Paul’s blog:

“I must admit that I am still a little concerned about our ‘hump’ drag! The ‘hump’ is the amount of drag that builds up before the boat makes the transition into ‘planing’ mode. You often see this in fast powerboats where a lot of power is needed to get started, but once up and planing, less power is needed to maintain the same speed. This is because they have overcome the transition from ‘low speed displacement mode’ to ‘high speed planing mode.’ Some shapes do it better than others. We are discovering how big our ‘hump’ is right now and will try and use a few tricks to bend it to our will!

During last weeks tow test session, we trialled the addition of a ‘step’ on the front float and saw that it made a big difference to the float’s ‘hump transition’ – it’s ability to release from the water and step up into planing mode.

A new second ‘step’ on the rear underside of the hull quickly caused ventilation at speeds as low as 3-4 knots. This enabled the up forces on the front of the float to win the battle and start lifting the hull upwards as speed increased. This in turn allowed the first step to gain access to the air and also ventilate. E Voilà! The hull kept lifting and we were planning!

While a third step on the back float was a further improvement, it seemed that the aft float still gets caught in the wave pattern generated from the front float at low speed. There is every chance that this might be a phenomenon caused by tow testing and that the addition of the side force of the wing could break the cycle.


Note that the foils on VSR2 are actually pulling the boat down into the water. On most hydro-foil’ers the foils are there to lift the boat up and out of the water. On the Sailrocket designs, the angled wing does all the lifting while the foil keeps it down. When we start off, the foil is fully submerged. As we accelerate and the wing powers up, it gradually begins to lift the boat up with it. This means that the foil rises and the top section comes out of the water only leaving the angled lower part in the water to do all the ‘down’ work. When the boat is hauling along at full steam, the back of the boat will naturally ride at the height where the up and down forces balance. If it gets ‘bumped’ too high then the balance will try and pull it down and visa versa. This means we can only do so much towing the boat around without the addition of the actual sailing loads.

It sure was a beautiful evening! It was a perfect opportunity to let every team member go for a ride! See the video here. I really enjoyed seeing Ben (Quemener) in the cockpit. It always surprises me to see boat builders spend so much time building boats only to see them go out of the shed and the next set of plans comes in. Surely the joy has to be in actually seeing what you have spent so long building in its element? This way you can truly appreciate what it is you are creating. It has to make the whole experience richer.

Helena (Darvelid) was all concentration. She knows that she will have the option to pilot VSR2 for a record attempt at some stage… and she knows that that day may well be approaching. It’s not talk anymore. What she does and how hard she pushes it is solely up to her. There will be no restrictions and if she wants to go for an outright attempt… then that will be her call.


We will do one more session of tow testing where we will actually measure some of these loads… and then we will roll straight into the wing trials.


You will also be glad to know that we ran a successful live video feed from off the boat a couple of days ago. It basically involves an iphone 4… so looks like I have to get one whether I want to or not! Then you can all come along for the ride.”