Bulbous bow sizing

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Bulbous bow sizing

Postby slabaugh » Fri Jul 15, 2011 11:42 am

My father being a do-it-yourself boat builder has recently launched his most recent 10 year project. In early runs he has felt that the wave making at cruise (approx 11.5 knots) was more than he expected. He asked me to use my familiarity with the internet to look into bulbous bow specifics and that is how I have come across this forum. I have included a couple of photos that have unfortunately obscured views of the hull. The basic dimensions are length 21 m at waterline, beam 7 m, draft 2 m, displacement around 85,000 lbs. A couple of questions, does a bulbous bow make sense with this hull shape? And more specifically, the dimensions that I have seen indicate the starting point for BB cross-sectional area of approximately 5-6% of midships displacement area (I calculated this at around 8 square meters but could get more precise if needed). Does this make sense as a reasonable starting point? I recognize the shortcomings of this guesstimate but would appreciate any advice or recommendations. Thanks.
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Re: Bulbous bow sizing

Postby JollyJack » Fri Jul 15, 2011 6:43 pm

You craft you are showing has a hard chine hull with a flare bow. This hull configuration is designed to plane. At the speed you mentioned, 11.5 knots, the hull will be deeper than design draught, bow up, stern down, and will generate heavy wake waves. Get her up over 15 knots, you'll find the arse end rises up, speed will pick up for less power requirement and the heavy bow wave will flatten out. You may need trim tabs on the transom to give the initial lift to the stern. If you can't get up to the plane, either you have too much weight in the superstructure, or you need more power.

A bulbous bow is designed to reduce the first standing wave on a submerged bow. A flared bow and hard chine hull is designed to lift the craft out of the water to some extent, the bow is not submerged, so a bulbous bow would have no effect.
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Re: Bulbous bow sizing

Postby JK » Sat Jul 16, 2011 1:25 am

also if the engine and propeller are undersized and not matched the boat will not get to speed.
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Re: Bulbous bow sizing

Postby Big Pete » Wed Jul 27, 2011 1:10 pm

As the other contributors say, I think a bulbous bow is the wrong solution for your problem.
Bulbous bows make use of a phenomenon first observed by early experimenters in hydrodynamics who experimented with towing floating and totally submerged objects in test tanks. They observed that a submerged cylinder being towed through water crated suction pulling down the water level immediatly above the cylinder, creating a wave. They also observed that the "bow" of an object being towed through the water raised the water level at the bow, creating a wave. By attaching the cylinder to the bow of the ship, underwater the two waves cancel each other other out, minimising the power used to create the waves, effectively reducing the drag, and increasing the speed for the same power.
This only works within a narrow range of drafts and trims. Most commercial vessels only benefit from a bulbous bow when fully loaded at the designed trim. At other conditions speeed and fuel consumption are usually slightly worse than a comparable vessel without the bulbous bow.
The answer to reducing the drag on this type of boat is to get up on the plane. Remove any added weight that has accumulated, so that the boat sits at its designed draft and trim when stopped. Make sure the engines are producing their rated power and correctly matched to the propellers, if this doesn't work investigate trim tabs.

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