# bulbous bow

Discussion in 'Powerboats' started by ROGE WAVE, Dec 25, 2016.

1. Joined: Dec 2016
Posts: 8
Likes: 0, Points: 0, Legacy Rep: 10
Location: Milton, Mass

### ROGE WAVEJunior Member

Hello All:
New to this forum but very impressed by it.
What is the opinion of a bulbous bow on a semi-displacement boat 30 feet in length??? Cost not much of a issue, just looking to improve efficiency.
Also, reading about bulbous in the stern to improve fluid flow to the propellers. ????? Any one do this???? I have some of the math calculation to determine size etc. Thanks

Bill

2. Joined: Oct 2010
Posts: 10,401
Likes: 1,036, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

I'd say some basic research would indicate it won't be worthwhile in that size/style of boat. It could even slow you down.

3. Joined: Nov 2003
Posts: 19,133
Likes: 496, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

### PARYacht Designer/Builder

If your boat moves at the same speed most of the time (90%+) then a bulbous bow would be beneficial, otherwise, not so much. They're used on ships and do improve efficiency, but once these vessels leave port, they're running at a specific speed, continuously or nearly so, until their next port, so it makes sense.

4. Joined: Dec 2016
Posts: 8
Likes: 0, Points: 0, Legacy Rep: 10
Location: Milton, Mass

### ROGE WAVEJunior Member

Well, when researcher do test, it's carried out in a water tank and models are two to three feet long. Proving it will work and use it on larger ships or just scale it up.......
Bill

5. Joined: Dec 2016
Posts: 8
Likes: 0, Points: 0, Legacy Rep: 10
Location: Milton, Mass

### ROGE WAVEJunior Member

Well, when researcher do test, it's carried out in a water tank and models are two to three feet long. Proving it will work and use it on larger ships or just scale it up.......
Bill

6. Joined: Oct 2010
Posts: 10,401
Likes: 1,036, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

They are for displacement hulls.

7. Joined: Oct 2010
Posts: 10,401
Likes: 1,036, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

Scale models of displacement hulls. Ships. They work by modifying the bow wave. It could turn into a handicap more than a help for your boat. If it translated to your boat type, it would be well used, It isn't.

8. Joined: Dec 2016
Posts: 8
Likes: 0, Points: 0, Legacy Rep: 10
Location: Milton, Mass

### ROGE WAVEJunior Member

Just happen to find a RC Model of my style boat on youtube
https://www.youtube.com/watch?v=SW1h6wEnoig

good test on this video OK Mate

9. Joined: Oct 2010
Posts: 10,401
Likes: 1,036, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

Ok, your bulbous bow would be getting dry in the air on that model.

10. Joined: Dec 2016
Posts: 8
Likes: 0, Points: 0, Legacy Rep: 10
Location: Milton, Mass

### ROGE WAVEJunior Member

I do not know if my bow will do the same. Big Question. my boat weights about 7 tons. Thinking will have to put it(bulbous bow) about a foot below the water line. seen some other designs like that. WELL, when the boat is in the water will pay special attention how high the bow rises, I think it's 4 to 6 inches.

11. Joined: Oct 2010
Posts: 10,401
Likes: 1,036, Points: 113, Legacy Rep: 702
Location: Australia

### Mr EfficiencySenior Member

It's a speculator to be attempting this on a boat that normally would not be considered for it, whatever you spend, is unlikely to be returned in fuel savings. Just my guess though.

12. Joined: Nov 2003
Posts: 19,133
Likes: 496, Points: 93, Legacy Rep: 3967
Location: Eustis, FL

### PARYacht Designer/Builder

If your boat is semi displacement, you can forget about any real benefits from the bulbous bow. I've seen some attempts with this idea on downeasters and in spite of some owners attempts to justify their efficiency claims, it's just not realistic, nor particularly sound hydrodynamically. Again, unless your boat operates within a very narrow speed range, modifying the bow wave isn't going to do much. A semi displacement hull would just gain a lot of drag starting around 8 MPH. Below this speed, you could see some improvement, but I suspect your hull is capable of much better than this, so . . .

Simply put, a fairly intense study of your boat and its operating environment, would be necessary to get a bulb that's effective. It's not a backyard kind of thing.

13. Joined: Jul 2012
Posts: 360
Likes: 114, Points: 53, Legacy Rep: 14
Location: s.e. england

### alan craigSenior Member

I've never seen it written, but I've always thought that a bulbous bow allows maximum waterline length without increasing overall length, while still allowing a hull shape which will deflect waves. If the ship had a plumb bow for max waterline it would have no flare to deflect waves. And I have never seen a bulb extended beyond the furthest forward point of the bow.

Does anyone else think this is a reasonable description?

14. Joined: Dec 2001
Posts: 2,474
Likes: 117, Points: 63, Legacy Rep: 1728
Location: Oriental, NC

### tom28571Senior Member

A bulbous bow is designed to partially cancel the normal bow wave of that particular vessel in a narrow speed range. It can work for models and small boats IF the boat only runs at that speed in favorable conditions for the size of the boat or model. A 30 foot boat running in most normal sea conditions where wave action has considerable affect on pitch is not favorable. I've see it tried but its a waste of effort on such small boats. Paul Elvstrom, the best sailor of the past century tried it on a sailboat but quickly abandoned it for the reasons given above.

As always, try it if you wish and report back the results.

15. Joined: Dec 2016
Posts: 8
Likes: 0, Points: 0, Legacy Rep: 10
Location: Milton, Mass

### ROGE WAVEJunior Member

Thanks for all the good replies. I had concerns about this design but the semi-displacement hull is a wild card. That's why I came to this forum to get answers.

NEXT QUESTION:

bulbous bow, in the stern, to improve fluid flow to the propeller. I will call this a bulbous stern. seen some info on this design but not much. my hull had a bulge ( semi-circle slightly elongated ) near the prop to improve water flow. I had to remove it to repair a section of the fiberglass hull. Now looking to add a design back on. What's the opinion on this????

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.