Pressure wave under a non-planing huill

[dylanwinter]

I wondered if anyone on here could throw some light for me onto what happens to the pressure wave under a non-planing hull

say something 26 feet long and weighing 3 tonnes

I assume that there is a pressure wave of some sort around the bow as the boat starts moving forwards

is the rise in pressure related to the size of the bow wave - say you are producing a six inch bow wave - does that mean that is a pressure head equivalent to six inches of water - or only 3 inches - or is there no relationship at all

then as the boat speeds up that bow wave moves back along the hull a bit

I do know from my Enterprise racing days that when you got up on a plane then you could get a lovely spout of water coming up the plate case if the rubber strips were worn or torn

so what happens further back down the boat on a displacement hull as you approach say six knots?

I do recall when reaching with everything up on the eboat and she started overtaking waves it would start surfing and the rudder would start doing all sorts of weird stuff - the tiller would go as light as a feather



D

 

[aquaplane]

I was under the impression that the water which shoots up the centerboard casing on my Mirror when I forget to put the short blanking one in, is due to water hiting the back edge of the casing and being deflected up hill, nothing to do with a pressure wave.

I have heard of a 26' dispacement hull which has a quoted displacement of 3Te actually weighing 4Te, but it's a 35 year old boat, probably nothing like the one you are thinking of.

 

[dylanwinter]

I was under the impression that the water which shoots up the centerboard casing on my Mirror when I forget to put the short blanking one in, is due to water hiting the back edge of the casing and being deflected up hill, nothing to do with a pressure wave.

I have heard of a 26' dispacement hull which has a quoted displacement of 3Te actually weighing 4Te, but it's a 35 year old boat, probably nothing like the one you are thinking of.

wot wot wot - 4 tonnes - where did you get that?

D

 

[JumbleDuck]

I have heard of a 26' dispacement hull which has a quoted displacement of 3Te actually weighing 4Te, but it's a 35 year old boat, probably nothing like the one you are thinking of.

My 26' sailing boat is listed as 3 tonnes displacement, but according to the load cell on the crane which hoiked her out for the winter she weighed 4 tonnes then. That's a lot of kit, even discounting the 250kg of fresh water aboard.

 

[aquaplane]

wot wot wot - 4 tonnes - where did you get that?

D

What JD wrote, different boat same answer.

 

[dylanwinter]

that is okay then

My 26' sailing boat is listed as 3 tonnes displacement, but according to the load cell on the crane which hoiked her out for the winter she weighed 4 tonnes then. That's a lot of kit, even discounting the 250kg of fresh water aboard.

I was worried that my carefully worked through maths was all going to go awry

of course this particular 26 footer should be around 250 kg lighter than the design displacement

add all the junk plus four adults and a dog and we should be close close to four tonnes

I was worried that the whole shebang was going to weigh five tonnes

D

 

[prv]

Some boats, especially older ones, simply weigh more than their design weight because the yard got a bit enthusiastic with the GRP.

Production is better-controlled these days.

Pete

 

[TSB240]

Dylan

Are you are seriously worried about the pressure wave moving aft to the well?

It has done on our TS240 as it has a very slimline keel and bulb and a flat run aft. We start to plane at about 8 knots.
At this speed the well gurgles a bit more but no dramatic fountain or cockpit filling.

However at about 10 knots the leg of the outboard and the prop give enough drag to cause the well to overflow dramatically into the cockpit.

Simple solution was to use the ob as a pump! or to take it off and put the plug in.

We found this out twice once entering Castletown Harbour as we came over the overfalls and once as we surfed off a standing wave in the Menai straits

If you think an old centaur is going to get anywhere near going over the hump you are having a larf!

 

[JumbleDuck]

so what happens further back down the boat on a displacement hull as you approach say six knots?

This is where Uncle Bernouill and his fun packed formula come in handy. The static pressure increase by density x depth x gravity as you go down and by 1/2 x density x speed^2 as you go faster. Divide by (density x gravity) to put it in head terms: the head increases linearly and obviously as you go down and decreases by 1/2 x speed^2 / gravity as you go faster. Since gravity = 10 m/s^2 (suck it up, physicists, I'm an engineer) that gives a head reduction in metres of speed^2 / 5 or, since speed in m/s is half the speed in knots, head reduction is speed in knots^2 / 20.

At your six knots, therefore, the head would be reduced by 6^2/20 = 1.8m. However, this is what a small pressure tapping would see. You might get a scoop effect with the well and there will also be a correction for the flow round the hull, which I'd expect to be a bit faster than boat speed as water has to get out of the way.

However, the Reynolds number based on a speed of 6 kt (3 m/s), a distance from the leading edge of 7m and a kinematic viscosity of 1.5 x 10^-6 m^2/s is 1.4 x 10^7, which is way above the value normally needed for turbulence. If the flow is turbulent I'd expect the head reduction to be rather a lot less.

 

[dylanwinter]

Dylan

Are you are seriously worried about the pressure wave moving aft to the well?

It has done on our TS240 as it has a very slimline keel and bulb and a flat run aft. We start to plane at about 8 knots.
At this speed the well gurgles a bit more but no dramatic fountain or cockpit filling.

However at about 10 knots the leg of the outboard and the prop give enough drag to cause the well to overflow dramatically into the cockpit.

Simple solution was to use the ob as a pump! or to take it off and put the plug in.

We found this out twice once entering Castletown Harbour as we came over the overfalls and once as we surfed off a standing wave in the Menai straits

If you think an old centaur is going to get anywhere near going over the hump you are having a larf!

thank you

I consider most of the objections to the Welly Centaur project with deep thought

just crossing a few Ts and dotting a few Is on the well thang

still looking for a few more candidate boats

dylan.winter@virgin.net

 

[rob2]

I do know from my Enterprise racing days that when you got up on a plane then you could get a lovely spout of water coming up the plate case if the rubber strips were worn or torn so what happens further back down the boat on a displacement hull as you approach say six knots?

Somehow when racing my Ent I always found myself crewing on the enema reach! Sitting astride a CB case on the plane is an excellent test of the "waterproofs" you've just paid an arm and a leg for.

Don't worry - a Centaur will not plane, the energy is all absorbed by the slamming from the flattish section between the keels!

Rob.

 

[alahol2]

Don't worry - a Centaur will not plane, the energy is all absorbed by the slamming from the flattish section between the keels!

A Centaur may not plane but with a powerful outboard pushing it along it will squat. There is also the possibility of 'canal effect' when motoring in the shallow waters that Dylan favours. So make sure you build up the sides of your well above the lowest possible squat depth.

 

[dylanwinter]

A Centaur may not plane but with a powerful outboard pushing it along it will squat. There is also the possibility of 'canal effect' when motoring in the shallow waters that Dylan favours. So make sure you build up the sides of your well above the lowest possible squat depth.

I wonder what the maximum dig in is

and I assume that I will not be able to sail that shallow

D

 

[onesea]

I think Allot will come down to basic design. make it so it is a scoop to lift water and it will fill your cockpit. Design it so that it is more like a dinghy self bailer and it will empty.

How you do it is a different question.

From my experience a tight fitting plug around the outboard will certainly make a difference to the efficiency of the set up when sailing and motoring.

 

[dylanwinter]

A Centaur may not plane but with a powerful outboard pushing it along it will squat. There is also the possibility of 'canal effect' when motoring in the shallow waters that Dylan favours. So make sure you build up the sides of your well above the lowest possible squat depth.

I wonder what the maximum dig in is

and I assume that I will not be able to sail that shallow

D