HOLLOW ENTRY January 30, 2011

When being a practical person, it is sometimes useful to bring around people with a bit more theoretical approach to challenges in life, to have their opinion on my thoughts and ideas.
So when boat designer Fredrik Graven popped up during our visit in Rognan we could not resist challenging him on the subject of a sailpowered fishing boat – for the moment being materialized in the first prototype called the Hybrid. Fredrik took part during some of the talks between Kai the boatbuilder and myself during our first visit to Rognan.
One of the things of concern is the question of what happens with the wet surface when the boat is healing over, particularly during up wind sailing. Does the asymmetry of the wet surface create a constant wish in the boat to turn away from it course? And If so, does the helmsman need to compensate this by using the rudder to turn a bit the other way ? And if all this is the case — how much drag will this actions by the boat and the helmsman cause?
Drag is a drag – that’s for sure.
So basically we wish to dwell around this question a bit and see if we can play around with the shape of the hull, particularly the forward part, to hopefully compensate or ideally avoid this effect.
On key factor in this respect is the so called – hult innløp – or “hollow entry” of the hull which is quite characteristic on a Nordlandsbåt but not at all evident on a modern racing hull shape, which is normally more beefy in the water entry forward on each side behind the bow.

There are so many factors involved in this matter that I always think that at the end of the day I have to make a compromise choice based on some kind of gut feeling – which again of course is also a matter of digesting all comments and opinions being thrown at me during the process of consideration.
This is part of the fun and of course I never exclude from the process of decision making sure that the end result also will balance well visually – before I can decide to go for it.
Anyway these are some of the thoughts that are flying in the air these days – among a lot of other things happening.

Drawing on the left by Eilert Sundt shows well the hollow entry on the old nordlandsbåt, and the picture on the right illustrates the more bulky entry on modern racing hulls that gives a good volume and lift, but which again might create an aggressive angle of attack for the entry of the water flow along the hull. It all comes down to a compromise and personal preferences.

Covering the sketch model with gladpack gives a good surface for playing around, drawing lines to illustrate the hull shape and the underwater wet surface while heeling over. The asymmetric wet surface shape can easily be detected.

Boatdesigner Fredrik Gravem came up with this interesting 3D drawing illustrating an attempt to integrate a classic hollow entry to a modern shape hull. Thank you Fredrik.



So much for theory. Let us wait and see untill we start playing in full size. Kai is busy with some other building and repairs these days
so I need to be a bit patient before we can dig into 1:1 thinking.
But at least we have agreed that the Hybrid will happen, and that is good news for the spring.
Thats all for now – stay tuned. —-jan