<> Fast and stable flat-bottomed kayaks | Kayak Forum

Fast and stable flat-bottomed kayaks

Submitted by Rick.T on Wed, 11/21/2018 - 22:10

So, I’m about to start a new skin on frame kayak build, and I’m really fascinated with the Mackenzie River delta kayaks that the Inuvialuit built. I’ve never built one because money has been tight and I wasn’t sure if they would be a particularly useful rec boat. Then, I found a review of the Rockpool Taran 16, which seems to have a similar hull:

https://3.bp.blogspot.com/-pwTxZO3S4Hw/UPG9Bl5Tz2I/AAAAAAAAcuc/O3OwCZlPkE0/s1024/20130108-IMG_4668DEW.jpg

http://traditionalkayaks.com/images/imagesCANADA/82723frbott.jpg

Reviewers say that the Taran is fast, with decent primary and secondary stability. Some also say that it doesn’t really need a rudder. This leaves me scratching my head a bit, because it’s a flat-bottomed hull. Everything that I’ve ever read about flat-bottomed hulls says that they are slower and have crappy secondary stability. I also wouldn’t expect a kayak with no central ridge on the underside to track very well in the ocean, but they say that it does just fine. These are hard-core kayakers praising this hull design. What in the Sam Hill is going on here? Lend me your expertise please.

 

Etienne Muller

Thu, 11/22/2018 - 03:51

it is unlikely that any Taran would be sold commercially without a rudder. It is designed for use with a rudder.

it has a literal flat bottom, but I wouldn't classify it as having a flat hull. It is more of a round hull section with the otherwise substantial rocker cut off. Cutting off the rocker must allow it to sit deeper in the water, engaging the ends more, probably cancelling any lost directional stability. I would think that that flat section must interrupt hydrodynamic flow in some way, though with what effect, I don't know.

They are quite popular over here, and are liked by people doing long distances who want some speed. I've never tried one myself.

Et

Rick.T

Thu, 11/22/2018 - 13:36

In reply to by Etienne Muller

it has a literal flat bottom, but I wouldn't classify it as having a flat hull. It is more of a round hull section with the otherwise substantial rocker cut off.

Like you said though, it's literally a flat bottom. I don't understand your reasoning as not classifying it as such. And if you cut off the rocker, don't you just have less rocker?

Etienne Muller

Thu, 11/22/2018 - 15:01

imagine that boat with a normal curved hull as the original shape. Imagine they discovered they gave it too much rocker, so they covered the workshop floor with a huge sheet of sandpaper and rubbed the boat on the paper until most of the rocker was gone. Then they filled in the resulting hole with a flat sheet. That is how I see that hull. To me it does not look as if it was intentionally designed that way. In any event, I know people who are firm fans of the design, especially for fast downwind runs in big seas.

to me, a "flat bottomed boat" is something with more defined chines, and a flatter, wider, bottom below the chines.

I paddled a more standard shaped kayak some years ago, I forget the name, which had a rounded hull section with a completely flat bottom very similar to this, about ten inches at the widest point. It was a nice boat. I wasn't in the boat for long, but I did not notice any effect of the flat section in regard to primary or secondary stability.

et.

to me, a "flat bottomed boat" is something with more defined chines, and a flatter, wider, bottom below the chines.

Fair enough. I would agree that represents the typical flat-bottomed kayak.

I paddled a more standard shaped kayak some years ago, I forget the name, which had a rounded hull section with a completely flat bottom very similar to this, about ten inches at the widest point. It was a nice boat.

Yeah, I'm really tempted to try this concept in a skin on frame. I don't want a rudder or skeg on my boat though.

When looking at the bottom shape with an eye to judging stability, you really want to consider how the bottom shape interacts with the waterline. Stability is a function of what happens at the waterline as the boat tips. On one side the hull lifts out of the water, and on the other it dips into the water. As a result, more water is displaced on the dipping side while the displacement decreases on the lifting side.

What this means is the bottom shape really doesn't have any direct connection to stability. The submerged part of the hull can be all kinds of weird shapes, as long as those shapes are not breaking the surface they will not effect stability. It is only at the waterline that effects initial stability and the shape immediately above and below the waterline that influence secondary stability.

For a "flat-bottomed" kayak like the Taran, initial stability is due to the shape and width of the waterline, and overall stability will not be influenced by the relatively flat bottom until you lean it sufficiently for the flat part to come out of the water. Until that happens the flare of the hull above and below the waterline are all that really matter. 

There is some indirect influence on stability due to the flat bottom. A Etienne suggested if you started with the rounded hull (before sanding in the flat spot), the additional volume would make the boat float up a little higher. With the flare of the hull, it would be floating at a bit narrower part of the hull shape and as a result would be less stable, so flattening the hull bottom makes it a bit more stable by increasing the waterline width. Note however that the flat spot at the bottom means you can not place the seat as low, so a side effect could be a higher center of gravity that would lower the stability to some degree.

But, the difference between the rounded vs flatten shape is probably negligible with regards to stability. That flat spot may serve some other function, but I'm not sure what it is.

For a more in-depth discussion of kayak stability see my article on kayak stability.

JayBabina

Sat, 11/24/2018 - 12:45

Hull: Length 16 feet 8 inches (508 cm) - Width: 20.25 inches (51.5 cm) - Volume: 369 litres.

The old stability opinion thing is about paddlers and not just boats. With those specs, I would consider it an advanced (or experienced)  paddlers boat.  We can all remember when we started and paddled boats that seem tippy and now they seem like barges.  As far as that flat area on the bottom. I would guess that allows for a flat position for the seat allowing the paddler to sit closer to the bottom (as Nick said) which means less hull below the seat which will tweak out less water resistance going forward and the paddler "feels" more stable sitting lower. The deeper a hull sinks, the more resistance it has going forward. A heavy person has to put out more energy going forward than a light person in the same hull.

JohnAbercrombie

Sat, 11/24/2018 - 16:43

Jay said:

The deeper a hull sinks, the more resistance it has going forward.

 This is true. The more hull surface in the water, the greater the frictional resistance (drag).

However, for a given displacement (paddler + gear+boat weight) doesn't a flat bottom boat have a greater wetted area (and friction) compared to a 'round'- bottom boat ? I thought this was the reason that 'Olympic' racing kayaks have such rounded (and tippy) hull shapes.

Etienne Muller

Sat, 11/24/2018 - 17:29

My understanding is that a cylindrical section has the least wetted area for a given length, just as a sphere has the least surface are for a given volume. Very unstable though, I would imagine.

Et

JayBabina

Sat, 11/24/2018 - 17:50

I think we have to assume that the characteristics of the hull were well analyzed on a computer program and its often said in boat design, everything is trade-offs. I remember years back when I asked Derek Hutchinson why all his boats never went over 17 ft - he said "that's the longest I could fit in my garage".

JohnAbercrombie

Sat, 11/24/2018 - 18:12

Jay-

You did say:

which means less hull below the seat which will tweak out less water resistance going forward 

I don't think this is correct, as a flat section will increase the water resistance, for the same displacement.

Etienne Muller

Sun, 11/25/2018 - 05:21

I remembered the name of the boat with the flattened keel that I tried. It was a SEABIRD SCOTT. It was a generally user friendly boat.

Et

A semicircular cross section will have the least wetted surface, but it is not necessarily an inherently unstable shape. Again, waterline width defines initial stability. In the image below each of the 5 cross-sectional shapes have the same beam at the waterline. As a result they all have almost indistinguishable stability up to about 10° of heal.  

After that the flare of the hull above the waterline largely determines the stability. In this illustration, there are 2 flat bottomed examples, one has the least overall stability, the other has the second highest, only surpassed by the round bottomed example.

Image removed.

People often assume that sharp angles in the sectional shape, such as hard chines or a "V" bottom will contribute to increased stability. The logic apparently being that water doesn't like moving around hard angles, so it is harder to tip a boat with hard angles in the cross sectional shape.

A stable body is:

 a body so placed that if disturbed it returns to its former position, as in the case when the center of gravity is below the point or axis of support

I.e. a stable boat is one that when tipped, will return to an un-tipped position on its own. Hard chines by themselves will not do anything to return a tipped kayak to an upright position. A keel on a sailboat does not make it stable by resisting the motion of water, instead it is a useful way to get a lot of weight well below the axis of support (aka: center of buoyancy). 

Round bottom kayaks tend to be less stable because in the effort to minimize the wetted surface, designers make the boats very narrow. And since stability is related most closely with width, narrow boat are less stable. The primary reason for choosing to make an unstable design is to make it fast, and if that is the goal a rounded bottom also works towards that goal.

 

As Jay says, there may be design criteria that are not immediately obvious. While I don't think through computer analysis is all that common among kayak designers, that doesn't mean that there aren't good reasons for doing something that will decrease the theoretical efficiency of a design.

Adding a flat spot on an otherwise rounded bottom will increase the wetted surface area because the volume lost by making the flat spot will be recovered by the boat sinking deeper. While the flattening lowers wetted surface in the flat spot, it requires more wetted surface around the whole waterline of the boat to replace the volume removed at the bottom.

So what

The difference is negligible. In the example of the Taran, the amount of drag added due to increased wetted surface area is tiny. There are more important things to worry about.

People tend to get too caught up in optimizing the efficiency of a kayak design, as if easy of propelling the boat forward is the only thing that matters. Those that don't care about this tend to over emphasize stability, as if the most stability is all that matters. So, we get discussions that focus on speed and stability. There are other qualities that make a kayak enjoyable to paddle.

Tracking, sea handling, maneuverability, etc also things like: comfort, ease of building, storage, weight, and lets not forget: aesthetics, fashion trends, and marketing points are just some of the aspects of design that a good designer will take into account while developing a new kayak model.

I don't know why the Taran has a flat spot on the bottom, but it will have an effect on tracking, maneuverability, surf riding, as well as the speed and stability aspects already discussed. And don't discount the marketing value of making a distinctive feature that causes people to discuss it. It would have been very easy to blend in the edges of the flat area with potentially very slight improvements in performance in some areas, but making it less obvious would also make it less interesting. If a designer is going to include a feature that they feel is worthwhile, it is also worthwhile to make it stand out.