Bulbous bow - A good thing or bad?

[rawlitwn]

Hi All,

Is having a bulbous bow on a 50_60' trawler a good thing? I've read that having one in calm waters is ok, but in rougher waters, makes the bow slap.... any opinions?

Thanks,

T.G.
CT

[Comodave]

I think it would depend on how well it was designed. It should enhance fuel economy.

[Sabre602]

It's not a simple answer. Properly designed, it may improve fuel economy. But...yeah, it's not a simple answer. Here are what a couple of naval architects have to say on the matter:

Bray Yacht Design & Research Ltd - Bulbous Bows

George Buehler's thoughts on bulb bows

[markpierce]

Less than a ship, doubt a boat can ever recover the investment (cost of new bow) compared to any possible fuel savings. Leastwise, it has never been demonstrated to me. As a "fashion statement" it could be a worthwhile investment for someone.

[makobuilders]

Good articles above.

I doubt the cost of retrofitting a small boat would ever be recaptured. However for a new construction it would be different. Two shipyards that I have been discussing with have bulbs in their designs. In one case the boat is very front-heavy and the added buoyancy is a benefit and pitching is slightly reduced. In both cases the waterline length is extended about 4-5 feet, so LWL is almost the same as LOA. The naval architect told me he estimates about 5% efficiency increase, which coincides with the longer LWL.

Regarding to negative motion, articles indicate bulbs can act like a deep forefoot, and when running downwind in heavy seas there may be a slight tendency to drag, but how significant this risk is to you is debatable.

In general T.G. I would say that if the 40-60 ft boat you're buying comes with a bulb, then great. And if it doesn't, then great. But if you want to learn more from actual naval architects, and some experts, then head on over to boatdesign.net/forums

Oh BTW you need to be careful about anchor roller/hawsepipe placement.

[dirtdoc1]

I would think that if you spend much time on the hook a bulb could be problematic. Personally I would look in to that potential problem with great interest.

There was a Nordhavn 46 for sale recently that had a bulb. It was painted like an Orca. It looked great!

[DavidM]

Nordhavn tried one on an early boat and it didn't do any good.


David

[O C Diver]

While there is nothing wrong with a little more efficiency, I was under the impression that on smaller boats, the purpose was to improve wave entry going into seas. Obviously beyond a certain size wave, they would loose their effectiveness. Just curious if anyone has cruised on a <80' boat and can comment on the effectiveness of it improving the ride.

Ted

[Insequent]

I understand bulbous bows are tricky to get right to gain efficiency. Usually it involves testing of a model in a towing tank facility to get size and position correct. More often they are likely to be there to offset bow-down trim.

[FF]

On ships it was found the bulbs were built for long term operation at one cruise speed.

Change the cruise speed and you need to remove and redesign the bulb.

Big bucks unless you measure fuel burn in tons per day.

[twistedtree]

Originally Posted by Sabre602

It's not a simple answer. Properly designed, it may improve fuel economy. But...yeah, it's not a simple answer. Here are what a couple of naval architects have to say on the matter:

Bray Yacht Design & Research Ltd - Bulbous Bows

George Buehler's thoughts on bulb bows

I wish these articles (all articles) had dates. We are always learning and evolving. My guess is that both are pretty old.


I also suspect there is much more understanding of the exact dynamics of a bow and hull form. Good computer modeling should have removed all the mystery from this, but who knows. Certainly not me.

[BIG CAT]

I talked to a engineer that I used to work with. He basically said most pleasure boat are too small to get any benefit from them.

[nwboater]

My Selene 47 has a bulbous bow that was added by the previous owner. I suspect he did this because the boat lived in Oregon and was cruised up the coast each summer to B.C. and Alaska. Thus, the bulb might have reduced pitching when running against our prevailing NW winds and currents. Since owning the boat, I’ve not noticed any particular advantage or problems.

[Woodland Hills]

I’ve always been curious how a bulbous bow affects anchoring. Does it interfere with the anchor as it comes up?

[Flybull]

Don't know about a sub-80' vessel... but I had over 4 years on a guided missile cruiser with a large bulb covering the sonar dome, and can tell you in heavy seas, as the bow is coming down hard, the bulb would add a side-side lateral shimmy motion at the bottom of the downstroke (for lack of a better term).

As our berthing (sleeping) quarters were in the bow at the waterline level, I can tell you that it is one thing to be pitching vertically displacing your body 40-50' in a matter of seconds... but that shimmy at the bottom was enough to toss you out of your rack if you weren't smart enough to wedge yourself in...

Of course that boat was 533' long...

[Civilitas]

Originally Posted by FF

On ships it was found the bulbs were built for long term operation at one cruise speed.

Change the cruise speed and you need to remove and redesign the bulb.

Big bucks unless you measure fuel burn in tons per day.

I have read this in relation to smaller yachts, too.

Particularly, I was reading about the bulb design on the Nordhavn 62. The bulb offered advantages in fuel burn at full cruise. But if one throttled back slightly to extend range for a long passage, the savings evaporated.

[Codger2]

Originally Posted by Sabre602

Here are what a couple of naval architects have to say on the matter:

Bray Yacht Design & Research Ltd - Bulbous Bows

George Buehler's thoughts on bulb bows

Thanks for the links! Although they are two guy's opinions....their credentials have more weight than TFers with no design experience weighing in.

[twistedtree]

This is all anecdotal and/or second hand, but this is what I know from Nordhavns.


- The fastest and most efficient hulls they have made are the 50 and 57, both of which have bulbs. How much of that comes from the bulb I don't know.



- A common complaint was/is the slapping when in heavy head seas.


- The slapping is not the bulb hitting the water surface, but rather the water coming together over the top of the bulb as it submerges. The water coming back together "claps" as you would clap your hands.


- Bulbs are now only offered on the 76 and larger boats, and have a flattened top surface that reduces the water clapping.

[leeman]

Originally Posted by FF

On ships it was found the bulbs were built for long term operation at one cruise speed.

Change the cruise speed and you need to remove and redesign the bulb.

Big bucks unless you measure fuel burn in tons per day.

Fleming initially offered a bulb option on their new 58 but that has disappeared from their literature. If I remember correctly they stated that it was designed for a steady cruising speed. There didn't appear to be that much of a fuel savings though.

[rawlitwn]

Thanks everyone, a lot of great feedback....seems the biggest concern is when anchoring or when docking bow in.....