Bulbous bow - A good thing or bad?

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

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

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

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.

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.

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!

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


David

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

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.

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.

Sabre602 said:

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

Click to expand...

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.

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.

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.

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

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...

FF said:

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.

Click to expand...

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.

Sabre602 said:

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

Click to expand...

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

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.

FF said:

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.

Click to expand...

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.

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

From my experience as an engineer on a container ship, I agree with FF and MarkPierce. In a 24 hour period, our ship (with a bulbous bow) would burn roughly 64 tons of fuel oil at 95 rpm. So on that ship, it may have helped fuel economy but by how much, nobody I bet could tell you? Because if we were fully loaded, that bulb would be completely under the water line, however, if we were not fully loaded, the bulb would be sticking halfway above the water line or somewhere in between. So, there may be more factors into it than just having one or not. Also, I dare anyone to find a bulbous bow on a ship that does not have anchor chain scratches across it. End the end, I think they give a “small boat” that big ship look and it does look cool but not sure it is worth the money. However, I wouldn’t cut one off a boat either if it had one.

Hi


In this video, a new type of bulb designed for a fast pilot boat on a heavy track, the end of the video is an interesting comparison where the bow's bow motion is measured and the new bulbi reduces jumping quite well (can of course be due to other design


https://youtu.be/pqGVSn6saM8


An interesting article about bulbs...
http://www.brayyachtdesign.bc.ca/Super Fuel Efficient LRC.pdf


NBs

I had a boat with a bulbous and had no problems with anchoring. It all depends on the overall design and the relationship of the bulbous bow and anchor placement.

I knew of one guy on a 76’ that added the bulbous bow for buoyancy. He cruised Alaska and wanted to add extra chain and an up-sized anchor.

Some bulbous bows where designed to hold water which you could fill or empty depending on sea conditions and whether you were in a following sea or heading into the seas.

PacNW said:

Some bulbous bows where designed to hold water which you could fill or empty depending on sea conditions and whether you were in a following sea or heading into the seas.

Click to expand...

Very cool idea. A big bulb could hold several hundred gallons. That’s a lot of fresh water for local cruising (probably wouldn’t want to fill it during ocean passages).

Bad for yachts/trawlers

Good explanation here" https://nordhavnonly.com/nordhavn-models/construction-details/bulbous/

PacNW said:

I had a boat with a bulbous and had no problems with anchoring. It all depends on the overall design and the relationship of the bulbous bow and anchor placement.

I knew of one guy on a 76’ that added the bulbous bow for buoyancy. He cruised Alaska and wanted to add extra chain and an up-sized anchor.

Some bulbous bows where designed to hold water which you could fill or empty depending on sea conditions and whether you were in a following sea or heading into the seas.

Click to expand...

Interesting stuff, thanks!

I was looking to put one on a past commercial fishing boat I owned. My objectives were to reduce my wake (which it seems to do on other vessels I saw retro-fitted), reduce the vertical leap while bucking, straighter tracking in heavy seas, and of course better fuel economy.

The marine engineer I spoke with on the project talked me out of doing it. He felt that a well designed hull does not benefit much if at all from retro-fitting a bulb?

Marlow considered itand even built a boat with one, it must not have worked as I can’t seem to find it offered any longer.

PacNW said:

I had a boat with a bulbous and had no problems with anchoring. It all depends on the overall design and the relationship of the bulbous bow and anchor placement.

I knew of one guy on a 76’ that added the bulbous bow for buoyancy. He cruised Alaska and wanted to add extra chain and an up-sized anchor.

Some bulbous bows where designed to hold water which you could fill or empty depending on sea conditions and whether you were in a following sea or heading into the seas.

Click to expand...

The bulb on our Selene has an open "hole" located on the underside that fills with sea water, thus, it adds weight and reduces buoyancy at the bow. We will be visiting the west side of Vancouver Island this summer, which is open to the Pacific swells, so it will be interesting to see how the Kika runs in both following and head seas. Btw, though the bulbous bow protrudes quite far forward, it has yet to interfere with either deploying or weighing the anchor.