The Design of Inboard Flair
Having undertaken many main beam and hull to underwing repairs on what we considered well designed and constructed yachts, but not designed or built by us, we wanted to offer our clients a hull shape that was gentle on the multihull structure.
We focused our design effort on dramatically reducing concentrated loads on the beams, reducing wave pressure in the area around the hull to underwing joint and reducing underwing slamming.
Numerous main beam models were tested to destruction, the strongest beam had a 45 degree gusset from the waterline to the underwing. We had to develop a hull shape that incorporated a 45 deg gusset.
We constructed 2 identical multihull yacht models 1000 mm long 580 mm wide. We also designed and constructed an extremely deep V monohull model, this hull shape being capable of high speed with a smooth motion in rough offshore waters. This mono hull model was split in half down the centre line and the port half was fitted and glued to the starboard inboard multihull model hull and the starboard half was fitted and glued to the port inboard multihull model hull. We now had a multihull model that incorporated an inboard flair.
When the two multihull models, one with the inboard flair and one with a conventional hull shape were placed in waves with a motion comfort meter (a bowl of water), the results were amazing. At all speeds and wave angles the hull with the inboard flair had more water in the bowl after the test, indicating less motion.
The flow of waves under the underwing and around the models were carefully studied. Less waves seemed to impact on the underwing of the model with the inboard flare.
The models were then drag tested. A bridal off the bows was tied to a spreader bar to separate the models. The tow line with a short bridal was attached to the middle of the spreader bar and then towed in waves beside a dinghy. The model with the inboard flare was always ahead indicating less drag. The question is how much less?
The tow line was moved away from the hull with the inboard flair, until both models were in line while being towed, indicating equal drag and a strain gauge (fishing scales) were added to the tow line. The exact percentage less drag on the model with the inboard flair was calculated for different wave angles.
The models were then tested for their ultimate stability, they were thrown into beach surf. The models were then tested for stiffness, then destruction tested. The flared model was stiffer and stronger.
Results
The advantages of the inboard 45 deg flare starting at the bow and ending at the aft beam and on the light ship waterline are:
A stronger lighter beam structure.
A stiffer overall yacht.
Less underwing impact.
Less motion at sea.
Less leeway due to asymmetric hull above static waterline.
The leeward hull has more reserve buoyancy.
Less spray on deck.
The yacht with the inboard flair has more internal volume.
Easier assess to hull from bridge deck and best of all is faster.
Testing undertaken by me in 1985.
Recent computer modelling indicates that if the total mass of the yacht was placed on one hull it would have less drag than if spread evenly on two hulls.
A 12m full bridge deck cruising yacht was designed and built with inboard flares and tested extensively, the results were very impressive. We believe the development of the inboard flare by me is the biggest design development in recent multihull history.