On 5-Jun-2004, (William R. Watt) wrote:

I think you'd better take another look at what I wrote. Hard chined boats
do have a bit more wetted surface but the turbulence at the hard chine has
a bigger effect, moreso as speed increases. (Lapped strake boats have the
same increase in resistance.)

The difference between hard and rounded chine in practice is negligible - at
least for kayaks. You'd have a hard time finding two kayaks that have a
difference that you could attribute to the chines and could also feel the
difference.

Interestingly, and contrary to what yoru
write above, a spherical hull does nto have the minimum wetted surface.
That's because only part of the shpere is immersed, ie. a chord of the
circle. John Winters (www.greenval.com/winters.html) has some diagrams to
illustrate this. I thought as you did until I saw his examples.

You'll have to be more specific. I looked at all the pages and figures and
can't see anything that specifies the characteristics of a spherical hull.
He has circular cross sections, but not spherical hulls.

If the spherical hull does not have the least surface to volume ratio,
please tell us what shape does.

Mike

"Michael Daly" ) writes:

The difference between hard and rounded chine in practice is negligible - at
least for kayaks. You'd have a hard time finding two kayaks that have a
difference that you could attribute to the chines and could also feel the
difference.

TF Jones disagrees with you. So do I based on what you wrote earlier about
the effect of hull scratches and gouges.
You'll have to be more specific. I looked at all the pages and figures and
can't see anything that specifies the characteristics of a spherical hull.
He has circular cross sections, but not spherical hulls.

If the spherical hull does not have the least surface to volume ratio,
please tell us what shape does.

Could you explain the significance of "spherical" hulls, because only the arc
of the circle below the waterline gets wetted?

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On 6-Jun-2004, (William R. Watt) wrote:

The difference between hard and rounded chine in practice is negligible - at
least for kayaks. You'd have a hard time finding two kayaks that have a
difference that you could attribute to the chines and could also feel the
difference.

TF Jones disagrees with you.

Reference please. My information comes from John Winters himself.


So do I based on what you wrote earlier about
the effect of hull scratches and gouges.

??? When did I ever write that gouges and scratches have a significant
effect on drag?

If the spherical hull does not have the least surface to volume ratio,
please tell us what shape does.

Could you explain the significance of "spherical" hulls, because only the arc
of the circle below the waterline gets wetted?

They are a starting point in looking at the effects of hull shape on resistance.

Mike

"Michael Daly" ) writes:
On 6-Jun-2004, (William R. Watt) wrote:

So do I based on what you wrote earlier about
the effect of hull scratches and gouges.

??? When did I ever write that gouges and scratches have a significant
effect on drag?

sorry? you claimed the opposite. that was the difference on which this
discussion is founded.


If the spherical hull does not have the least surface to volume ratio,
please tell us what shape does.

Could you explain the significance of "spherical" hulls, because only the arc
of the circle below the waterline gets wetted?

They are a starting point in looking at the effects of hull shape on resistance.

I see. I thought you were writing about an actual hull shape. I don't see
how anyone can disagree that the circumference of a cirle encloses the
largest area for the least perimeter, but actual kayak hulls aren't built
that way due to other considerations such as stability, draft, and
tracking. I build a perfectly circular hull once out of 55 gal plastic
drums cut in half. Photos on my website. I was informed of someone in
Mayalsia who built a boat out of large diameter plastic pipe. I had to put
sponsons on the boat to keep it from rolling over. Log drivers used to
have the same problem. The carried a long pole. My boat was 2 feet across
and 1 foot deep. Because of the perfectly round shpe it sat deep in the
water, 6" of draft. I guess it could be agruged that kayaks are
traditionally ocean-going craft and given the average depths of the
oceans, kayak draft is not important as it is in canoes.
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William R. Watt ) writes:

I see. I thought you were writing about an actual hull shape. I don't see
how anyone can disagree that the circumference of a cirle encloses the
largest area for the least perimeter, but actual kayak hulls aren't built
that way due to other considerations such as stability, draft, and
tracking.

Sorry, that was not clearly worded. If the waterline is the circumference
of the circle then the circular hull has the least girth for the area
enclosed (equvalent to the least wetted surface for the largest voume of
water displaced, or displacement). However, as is shown on Winters'
website, if the waterline is shorter than the diameter of the circle, ie
an arc of a the circular section, then flattening the sides reduces the
girth and wetted surface, an intersting and counterintuitive phenomenum.



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On 8-Jun-2004, (William R. Watt) wrote:

sorry? you claimed the opposite. that was the difference on which this
discussion is founded.

Then what are you saying? I paddle both hard and soft chine kayaks and
can't find any noticable difference that I could attribute to the
chines. The effects of chine shape on kayak performance are negligible
for most kayaks and are highly overstated by folks like you.

how anyone can disagree that the circumference of a cirle encloses the
largest area for the least perimeter, but actual kayak hulls aren't built
that way due to other considerations such as stability, draft, and
tracking.

Most recreational hulls are built for stability, but advanced hulls are
made with rounded, or nearly rounded, bottoms.

My Ellesmere has a nearly rounded hull section. It feels fairly tippy
and most beginners describe it as very tippy. However, at larger angles
of heel, the hull is extremely stable. In calm water it feels tender,
but in rough water it is very solid.

Racing kayaks and canoes are built with very tippy hulls - they cannot
sit upright when empty. They have a negative righting moment at zero
degrees of heel. Yet the paddler can relatively easily keep the craft
upright when paddling. The LOA and LWL are almost equal and they
track quite stiffly.

There are good reasons for making a rounded hull. There are many
examples of hulls with rounded sections that work well. Your examples
of tubular hulls are irrelevant, since that's not the shape given
to canoes and kayaks. The shape above the water line is not round
and the secondary stability can be significant. If you talk to advanced
paddlers, you'll quickly find that they discount the primary stability
as a factor in design. As long as you have good secondary stability,
you can paddle the vessel just fine.

Mike

"Michael Daly" ) writes:

Most recreational hulls are built for stability, but advanced hulls are
made with rounded, or nearly rounded, bottoms.

Yes, I'm going to have to backtrack on the subject of circular cross
sections for kayaks. Yesterday at the river I saw a short cheap bright
yellow plastic kayak with a very round cross section, tapering toward the
ends. I was ignoring how low is the centre of gravity when a paddler sits
on the bottom of a round hull with its deeper draft. Although it's not
easy to get into such a boat, once the paddler's butt is firmly planted on
the bottom the boat is more stable. Also, the manner of paddling a kayak,
compared to paddling a canoe or using a small sail, keeps the weight
centred so the lack of reserve bouyancy is not much of a problem. It was
unstable getting into my plastic barrel canoe, and it needed sponsons to
carry sail. In addition, rolling a kayak over is not supposed to be a
problem. It's a feature.

When sailing my narrow sail-and-paddle boats I have to lie on the bottom
of the boat to lower the center of gravity and counter the heeling force
of the sail, even though both boats have a flat bottom and reserve
bouyancy. The smallest one has to have sponsons to carry sail. I've had to
make backrests for both of them to lie back on when sailing.




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