After 22 years of weighing the benefits of bulb or no bulb there is still debate.
By JIM LEISHMAN
Nordhavn saw the benefits of a bulbous bow first hand during seven exhaustive test sessions at the University of British Columbia Ocean Engineering Center within its test tank facility. I went with Jeff Leishman, my brother, who is Nordhavn’s in-house naval architect,
The results were favorable, to say the least. The bulbous bow achieved reductions in resistance on all boats tested. On the 40 we noted a 6.5 percent reduction in power required to run at cruising speed, with 8.6 percent reduction on the 50, 10.2 percent on the 57 and an impressive 12 percent on the 62.
We did not test every model for sea-keeping, but when we did, we noted reductions of up to 20 percent in pitch amplitude while running into head seas. It was interesting to see the resistance increase on the bulb fitted hull as the sea state increased which illustrates that power is being consumed to reduce the pitch motion. In other words while running into head seas the resistance benefit of the bulb fitted hull diminishes and is replaced by pitch reduction.
From the first testing of the Nordhavn 62, we also noticed the characteristic slapping created while running into the steepest of head seas. As the bulbous bow dropped into the test tanks artificial waves, the water could be seen and filmed breaking away from the bulb sides and impacting the advancing and dropping topsides of the bow, well above the waterline. Counterintuitively, it’s not the flat bottom of the bulb slapping the water that makes the noise.
It is clear that 62-foot boats benefit more from the bulbs than a 40-foot boat but the main reason for not including bulbs in the smaller Nordhavns is the noise created by the bulb. Very few people will notice a 6.5 percent reduction in fuel burn but they will notice a bow that pounds. The thought process is similar to that used during propeller selection.
We proved that a Nordhavn 40 will run the same speed with 10 percent less fuel burn by using a three bladed propeller – but the four bladed prop is much smoother. We chose comfort over efficiency even where 10 percent greater range during our record setting Nordhavn 40 Circumnavigation (the ATW) would have added a very nice reserve.
There have been two cases where bulbous bows have been added to existing Nordhavns. Two 46s received the retrofit and one N75. We were never able to get reliable post installation performance observations on the 46s and any changes to sea-keeping were not quantifiable so no real conclusions were drawn. On one of our 75s we did add an elliptical bulb and very carefully recorded performance both before and after the installation.
Unfortunately we could not really see improvement, however this particular bulb was slightly larger relative to the vessel than bulbs previously used. We always used the recommendations provided by bulb pioneer Gerald Stensgaard of the Ocean Engineering Center and Jeff Leishman designed our bulbs with the largest cross section generally being about 20 percent of the largest cross section of the hull.
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