Spinnaker lifts the bow?.... a tricky question for sure.... and I don't profess to having enough engineering prowess to calculate it all and give a definate yeah or nay, but here's my 2c.
There sure are a lot of forces running around a symetrical spinnaker rigged on pole - Forces are transferred to the boat via, the pole downhaul, the backstay, the lead block for the guy, the pole, the mast, the winches and also the tweakers.
Now, I just can't see any of these exerting a downward force on the bow - because nothing attached to the bow is in compression
. What I can see, is that some of these force attachment points will cause the stern to lift (the sheet/ spin block and the head/backstay perhaps) and that this lifting of the stern causes rotation.
However, I don't believe that these forces will exceed those which are lifting either the centre of the boat or the bow - so the overall effect is not to lift the stern, but lift the bow. I say this not because I've calculated and resolved the forces, but because I've raced on many boats and have observed that usually the stern squats under spinnaker and usually the bow lifts (on lighter boats, it comes clear of the water).
Now what we are missing is the effect of the main. The main exerts a lifting force on the stern (via the backstay mostly). In lighter winds this force is not significant, as the boat will be moving only a few knots slower than the apparent wind
(depending on the boat of course). However, as windspeed increases above hull speed
, this force has an exponentially increasing effect on pushing the bow down (by lifting the stern).
Originally Posted by barnakiel
But then, if we do lift the bow, how much driving force from the kite do we lose? Maybe it is better to use the force on the sail to go a bit faster so that there is more hydrodynamic lift. Water is thicker than airs - we go a bit faster and we get much more lift ...
Yes - we definately loose by lifting the bow - and so we should alter the trim of the boat accordingly - by moving crew weight. Look at the water comming out from the transom as a guide.
The only time we want the bow out of the water is when we are over powered and we don't want to trip the boat.