planning05.htm

 

Time for another stupid question......

I dont want a racing boat. In fact I dont want to go fast, so whats this thing called hull speed and why should I want one?

Er.....

Hull speed....

Not actually anything to do with racing.

Its to do with how fast water moves.

Belly flop into a swimming pool. It hurts. This is because the water cant get out of the way fast enough. The water actually feels like a solid if you thump it fast enough.

Yet you can walk through water. This is because the water has enough time to move out of the way.

Same thing happens with a boat hull.

If you push it through the water slowly enough, the water moves out of the way and there is no water heaped up at the sides as happened with the sugar.

If you move the hull faster, it reaches a point where the water cant get out of the way fast enough and the boat starts to ride up onto the water - plane (not as in flying, but as in skimming over the water rather than plowing through it). The speed where the hull is going as fast as it can without piling up water is called its hull speed. This is the speed the hull will go through the water without using loads of effort - which is why hull speed is important even if you arnt racing.

If you have a short, fat boat it will take as long for the water to move out and back in as with a long boat of the same width.

So... theoretically...a 20 foot long boat will have a hull speed twice that of a 10 foot boat of the same weight, draft and width.

In this pic, the round bits of water move the same distance as each boat goes past - regardless of the boats length. However, the longer boat goes further in the same time as its...well...er....longer.




The longer boat has a higher hull speed.

So far as I am aware, its not possible to compress water.

Exactly what happens to the water that flows under the hull, Ive No Idea.

My guess is that it is sort of pushed slightly forward, then it streams back along the hull at speed, before filling the hole left as the hull passes over.

Found this water tank test clip that shows the water rising again after a hull passes over it.

Time for another stupid question...

So why dont I want a 100 ft long canoe thats a foot wide and only 1 inch deep?

Er....Rear too wide?

That said, a new term....

Wetted area.

Water is sticky. It hangs onto things including the hull

If you put something under a tap, you can move the flow of water off to one side.

If you put something under a tap that the water can stick to and is the right sort of shape, you can see it ignore gravity to stick until there is too much of it and it starts falling off.

In this clip, the water hits the cup and should slide off to the right, but, being sticky, it stays stuck to the cup and slides round it all the way to the left...

Something else to try...

A kitchen scourer....

Cut a strip out of it, sort of pencil size and shape.....



Have a go at the pencil flicking test with the sponge foam and a piece of pencil of about the same size.

The holes in the sponge make the surface of it much bigger than the surface of the pencil. This makes it slow down much faster than the pencil when you flick it....



The bigger the wetted area, the slower it goes.

Thats why a 100 ft canoe isnt going to go very fast as well as being a bit difficult to steer around rocks in white water.

Next stupid question...

Round bottom or flat - whats more stable?

This one upsets people, so Ive been worrying about it for a week or so.

I think I have the answer. No doubt if Im wrong, someone - or everyone will tell me lol

Decided to go high tech on this one and found some ready made hulls that just needed a little adjusting....



I cut the top third off the round hull...



...and a lot off the flat one...



This is my dehydrated test dummy as Im just a little too heavy for these hulls..



Oh...Im going to do the testing on Lake Sink...

 

Hmm... Rehydration unit....



We are ready to go....

Round hull first. Just an inch of water in the dummy so that its centre of gravity is really low...



Er...Struggling with this already...



It wont balance....



This is how much water I used out of the 1/2 pint...



It will stand up on the beach - just....

Same amount of water in the flat bottom....



Its still unstable, but not impossible...



2 more fluid ounces.....



This one wont stay up now either...



Works fine on the beach though....

Drank some more milk....

New wide hull....



This one is very stable...



Even tried it with the dummy leaning over the side...



I think its width to centre of gravity height that counts on stability, not bottom shape.

 

Ate some ice cream and argued with the wife about hairdriers and had a brainwave....

I dug put my kitchen blowlamp....



...and filled it with gas as one of the kids has been sorting out some paperwork with it, then ovalised the round hull...



It now takes the bottle easily...



Its not even that worried if the dummy isnt quite upright...

 

It took a while to persuade the flat bottomed hull to change shape...



No worries keeping this one afloat now either....





It seems the shape of the bottom isnt anything like as important as the angle of the sides for stationary stability.