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Old 12-25-2016, 09:47 PM
ROGE WAVE ROGE WAVE is offline
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bulbous bow

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
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  #2  
Old 12-25-2016, 10:06 PM
Mr Efficiency Mr Efficiency is offline
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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.
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Old 12-25-2016, 11:17 PM
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PAR PAR is offline
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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.
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Old 12-25-2016, 11:18 PM
ROGE WAVE ROGE WAVE is offline
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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
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Old 12-25-2016, 11:23 PM
ROGE WAVE ROGE WAVE is offline
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Quote:
Originally Posted by Mr Efficiency View Post
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.
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
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  #6  
Old 12-26-2016, 12:37 AM
Mr Efficiency Mr Efficiency is offline
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They are for displacement hulls.
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Old 12-26-2016, 12:47 AM
Mr Efficiency Mr Efficiency is offline
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Quote:
Originally Posted by ROGE WAVE View Post
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
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.
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Old 12-26-2016, 03:09 AM
ROGE WAVE ROGE WAVE is offline
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Quote:
Originally Posted by Mr Efficiency View Post
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.
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
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Old 12-26-2016, 03:19 AM
Mr Efficiency Mr Efficiency is offline
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Ok, your bulbous bow would be getting dry in the air on that model.
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Old 12-26-2016, 03:33 AM
ROGE WAVE ROGE WAVE is offline
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Originally Posted by Mr Efficiency View Post
Ok, your bulbous bow would be getting dry in the air on that model.
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.
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Old 12-26-2016, 03:52 AM
Mr Efficiency Mr Efficiency is offline
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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.
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Old 12-26-2016, 04:49 AM
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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.
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Old 12-26-2016, 07:45 AM
alan craig alan craig is offline
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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?
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  #14  
Old 12-26-2016, 09:03 AM
tom28571 tom28571 is offline
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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.
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  #15  
Old 12-26-2016, 01:53 PM
ROGE WAVE ROGE WAVE is offline
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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????

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