Buoyancy & Handling

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mcgruff
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Buoyancy & Handling

Post by mcgruff » Wed Dec 20, 2017 11:15 pm

Let's say the "buoyancy ratio" is the ratio of the volume enclosed by the hull in litres to its displacement in kg (the mass of the boat plus paddler plus cargo).

A 1:1 ratio - neutral buoyancy - is obviously way too low. Big waves would roll right over the paddler. The kayak would rise back to the surface but not in any great hurry. That's a sub-optimal situation for an air-breathing mammal.

The greater the buoyancy ratio, the faster you rise to a wave. A high ratio means a boat would tend to bob around on top of the water like a cork. Lowering the ratio means a boat will sink further into a steep wave before the buoyancy kicks in and the hull heaves itself up and over.

Buoyancy ratios probably won't be noticeably different in calmer conditions so this is all about handling in rough, steep seas. Is there an ideal buoyancy ratio for a sea kayak? It's tempting to think more buoyancy is always a good thing - especially if a compartment springs a leak - but perhaps a boat handles better with just the right amount of inertia and a wetter but less "skittish" paddle?

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Douglas Wilcox
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Re: Buoyancy & Handling

Post by Douglas Wilcox » Wed Dec 20, 2017 11:42 pm

Yes to the above and also a smaller volume boat will often be easier to manoeuvre on both flat water and in wind and waves so don’t be in a rush to buy a big volume expedition boat. Windsurfing nearly died because beginners were directed towards long high volume boards. They have a much easier time now starting off on shorter lower volume boards. Sea kayaking coaches make a good living teaching people how to handle big boats :o)

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Re: Buoyancy & Handling

Post by mcgruff » Thu Dec 21, 2017 12:50 am

Don't suppose there are any rules of thumb for this? 2:1 maybe?

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Douglas Wilcox
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Re: Buoyancy & Handling

Post by Douglas Wilcox » Thu Dec 21, 2017 8:23 am

Hi McGruff, sea kayaking does not really lend itself to doing it by numbers, there are too many variables in design of the hull, physical characteristics of the paddler, sea and weather condition. Finding the right boat it is much more a seat of the pants thing. It is much better to just go out and paddle in as many different boats as you can.

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Chris Bolton
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Re: Buoyancy & Handling

Post by Chris Bolton » Thu Dec 21, 2017 8:42 am

You can work out typical ratios. Look at http://rockpoolkayaks.com/our_kayaks.php (other manufacturers are available) for volumes. My GT has a quoted volume of 380 litres and weighs about 22kg. With gear I'm about 85kg, and at the start of a trip I probably have about 40kg food and equipment and 15kg water (or beer), total 162kg, so that's about 2.3. At the end of a trip it's probably over 2.8, and on a day trip somewhere around 3.1 - the GT isn't really designed as day boat although it works. If I had an Alaw Bach for day trips that would be about 2.6.

Generally, the storage volume of a boat in relation to the total volume prevents loading it any further (There's an [in]famous photo of one of the regular contributors to the site with a lot of deck cargo, which somebody might post). I may have loaded mine to about 2 at the start of one trip where I wasn't expecting to find fresh water, which left the cockpit rim only a few cm above the water. Based on that I wouldn't want to go below 2, and on the other hand my previous boat, without much rocker and no skeg, would have had a ratio of 3 on a day trip and became hard to control in wind.

But as Douglas says, there are so many other variables that any rule will have exceptions.

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Re: Buoyancy & Handling

Post by mcgruff » Thu Dec 21, 2017 4:51 pm

Thanks for the advice. I take the point that it's not a hard and fast rule but it does help to get an idea of the range.

I guess different shapes of hull will distribute the volume in different ways. More volume in the bow/stern sections would create a bigger pitching moment and so an extra few litres of air here would affect the handling much more than the same volume in the middle.

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Re: Buoyancy & Handling

Post by PeterG » Thu Dec 21, 2017 7:35 pm

The squirt boaters can probably teach us a thing or two

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Re: Buoyancy & Handling

Post by Jim » Thu Dec 21, 2017 9:23 pm

It is not about how much volume, but where the designer puts it.

Traditional sea kayak with highly flared upturned ends are designed with plenty of reserve buoyancy in the ends to make sure they lift to waves even when well loaded.

Race style sea kayaks may have more volume overall, but with little or no flare and no upturns they are much slower to rise even when paddled empty.

If you don't like the way traditional boats pitch when unladen, try something with a finer bow. If you are having problems with a fully laden boat plunging through waves and water running up to the cockpit, try something with more flare at the bow.

I haven't paddled my Taran fully laden very much, and not for a while, it runs pretty flat lifting more parallel than pitching, and I don't recall ever hitting a wave pattern that caused waves to run up as far as the cockpit - the bow has always risen before it gets that bad. It is very noticable if you paddled with a group made of traditional boats and Tarans how much difference there is in the way the boats pitch in the waves. The boat pitching least is probably the one with the most reserve buoyancy because of the way it is distributed.

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Re: Buoyancy & Handling

Post by Robert Craig » Fri Dec 22, 2017 9:28 pm

The bit of the boat not in the water - the "reserve buoyancy" - is in the wind. So it's importance matters a lot when it's windy, and less when it's not.

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