Hull Design

[Chris Waite]

After all my repetitive rubbish about flat bottom hulls and chines, I was just commenting on rigs as well and I can't currently remember quite how it happened as I have been at the Sauvignon, but I pressed a button and up came this:

http://www.ivorbittle.co.uk/Articles/Bulbous%20bow/The%20bulbous%20bow%20for%20my%20site%20This%20one%20again.htm

Wonderful!  There are pictures of wave patterns made by warships, ducks (Canada Geese actually, but what the hell) and models.  It then goes on to discuss the flow round the bows of 'swimhead' barges.  Because of the Sauvignon I can't possibly appreciate it all tonight.

However it does bring up water birds.  Nature not only abhors a vacuum, but has worked out how to do impossible things like make beetles fly; well look at them, what else keeps its wings under armour plates then flips those up into the airstream ahead of its wings and takes off?  So there has to be something in nature that can really 'move it' across the surface of water; like 'what' we want to do.  Not under water, that's fish; has to be water-birdies, doesn't it?  If they were not dynamically efficient enough to take off, then there wouldn't be any, so survival of the fittest indicates that our hull shapes should imitate this....

That's enough for this Ancient and Marinated tonight

CW

[Jeremy]

Bulb bow shapes are surprisingly effective for hulls where wave-making drag dominates over viscous drag, but do add wetted area, so increase viscous drag a little.

They work for birds (and model boats) because their speed is high in relation to their waterline length.  It's down to the well known (but sometimes erroneous) rule-of-thumb that predicts that the onset of significant wave-making drag is at a speed (in knots) of around 1.34 x square root of the waterline length (in feet).

If you take a Canada goose, for example, it probably has a waterline length of maybe a foot.  It should have a "hull speed" of around 1.34 kts, but in reality they manage to swim at maybe two or three times this speed, by virtue of their "bulbous bow", which reduces the amount of paddle power they need to exceed "hull speed".

Scaling up to the size of boats we play around with shows that the advantage this sort of bow gives is quickly eroded.  For, say, a 10ft waterline length, the normal hull speed would be around 4.2 kts, probably as fast as we need on inland waters.  Adding a bulb to the bow, especially with fins, would increase the viscous drag and probably mean the boat would need more power at speeds below 4 kts than it might without them.

On the other hand, if one wished to build a smaller boat, say around 6ft waterline length, with a cruise speed of 4 kts, then fitting a bulb bow might just pay off and allow you to squeeze that extra knot from the hull without needing too much extra power.

[gmatkin]

I haven't time to study it, but the with and without shots show an impressive effect. I wonder, though, about wetted area etc... It would take a little maths to figure out how much it will add in a small boat.

Are you going to try it?

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From: Chris Waite [via UK HBBR Forum] <[hidden email]>
To: gmatkin <[hidden email]>
Sent: Tuesday, 22 November 2011, 20:43
Subject: Hull Design

After all my repetitive rubbish about flat bottom hulls and chines, I was just commenting on rigs as well and I can't currently remember quite how it happened as I have been at the Sauvignon, but I pressed a button and up came this:

http://www.ivorbittle.co.uk/Articles/Bulbous%20bow/The%20bulbous%20bow%20for%20my%20site%20This%20one%20again.htm

Wonderful!  There are pictures of wave patterns made by warships, ducks (Canada Geese actually, but what the hell) and models.  It then goes on to discuss the flow round the bows of 'swimhead' barges.  Because of the Sauvignon I can't possibly appreciate it all tonight.

However it does bring up water birds.  Nature not only abhors a vacuum, but has worked out how to do impossible things like make beetles fly; well look at them, what else keeps its wings under armour plates then flips those up into the airstream ahead of its wings and takes off?  So there has to be something in nature that can really 'move it' across the surface of water; like 'what' we want to do.  Not under water, that's fish; has to be water-birdies, doesn't it?  If they were not dynamically efficient enough to take off, then there wouldn't be any, so survival of the fittest indicates that our hull shapes should imitate this....

That's enough for this Ancient and Marinated tonight

CW

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[Chris Waite]

Under the influence of the Sauvignon, I hadn't really got as far as bulb-bows

I was just delighted to see the birdies and the swim head barges getting a mention by someone other than myself.  I think building anything that sophisticated onto one of my exterior-grade, cardboard cut-outs is probably jumping the gun a bit; it's also way, way beyond my abilites

You should ask an amateur boat builder

CW  

[Paul H (admin)]

Chris Waite wrote

Under the influence of the Sauvignon, I hadn't really got as far as bulb-bows

I was just delighted to see the birdies and the swim head barges getting a mention by someone other than myself.  I think building anything that sophisticated onto one of my exterior-grade, cardboard cut-outs is probably jumping the gun a bit; it's also way, way beyond my abilites

You should ask an amateur boat builder

Ancient (and hopefully less Marinated when you read this) - if you can get hold of sketches and pictures of the bulb-bows we can probably crunch them in CAD/CAM software to produce bulkhead shapes that you could strip plank with ply.

You will be amazed how software can trace curves around the edges of shapes in a picture, then generate 3D models from those curves, then create bulkhead cross sections through the model.

Just think of the water ballast you can fit in a bulb - tempted?

-Paul

[Paul H (admin)]

Here is the bulb profile generated from a picture in the article - a curve was extracted from the picture and swept through 360 degrees to produce the solid shape. Then bulkhead cross sections were taken and construction lines added - everything we need to build the bulb.

Edit: Updated with an example half-hull shape:





cheers
Paul

[Jeremy]

Sat in my pile of "may come in handy" bits I have the pod from a flexwing microlight.  It's very close to that shape, moulded from GRP.  I'm reasonably sure it could be grafted on to a small boat hull, the question is, do I really want to build a very short boat just to try it out?

I suppose the next question has to be why just stop with a bulb at the bow?  Why not reduce the above water hull down to just a small cockpit-sized boat shape, with a submarine-like underwater body in which you sit?  It'd be a bit like an iceberg, with most of the boat submerged.  Add some side fins to act like the edges on the Paradox et al and give it a strong righting moment (could even make them asymmetrically pitch adjustable, like stabilisers), and it might even sail quite well.............

[Chris Waite]

You go for it Guys

I'll hold your coats

CW

[David Bewick]

Blimey, this is looking more and more like an AYRS forum every day

[Chris Waite]

Well if you think about it

Jeremy might minimize the whole thing by sitting inserted into the back of his bulbous pod....

But then of course he'd need a bulb-bow on it

CW