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General / Re: Otter Fish
« Last post by John Robertson on Today at 9:25 PM »
The end objective of skirt trim is actually quite simple - it is to make the craft perfectly level when both the divider and bow skirts are trimmed to the surface - it's that simple!  The various methods used to achieve this are designed to overcome the natural tendency of the cushion/skirt to stabilise itself into some other configuration!  The complexity is down to the fact you have three elements each one affecting the other two (the bow skirt, the divider skirt and the main cushion)

The only thing I'd add is that the divider skirt can be trimmed much higher without any noticeable effect on performance (up to 20-25mm doesn't seem to do much - any more and the brake reaction time suffers).


The "blocks" method specified in the plansets only works if the engine is just above idle - not enough lift is being produced to affect the craft attitude.  It also relies heavily on both the skirt and hull being built mm accurate to the plans. it, however, makes it easy to check the skirt is fitted correctly (no high or low spots).


The more general "Otter" method included in the attached doc will work no matter how badly the hull and skirt have been made and fitted (almost!).  It is, however, a bit more complex because of this - but it does work every time if carefully followed!

Once trimmed, the craft should fly slightly nose high to allow the front compartment enough margin to do it's job of stabilising the craft in pitch.  If you don't get the bow skirt trimmed to correctly level the craft then all sorts of strange things can happen (bouncing, plough in, etc.)!  For that reason I now trim the bow skirt first - by defeating the divider by placing something under it to lift it.  That way, it's just a matter of trimming the divider so it's at the same level or higher than the bow skirt when the craft is level.

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General / Re: Anyone building?
« Last post by orkneymark on Today at 9:19 PM »
Yes. Another sev. 80% or so completed

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General / Anyone building?
« Last post by Philip on Today at 8:41 PM »
Anyone building a craft at the moment?  A few years back we had lots of new builds
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General / Re: Otter Fish
« Last post by Nick Flint on Today at 3:25 PM »
Al--I have all the measurements and have the blocks which were used when building both the new Otters here--- and have spent much time (then and since) musing and checking and the measurements and behaviour were all taken into account, and eventually (recently)  resulted in about 20mms extra cut away as there is a "foolproof system" to trim and adjust before the craft ventures out.
All of this still leaves me where I am now!
I think I will workshoperise the craft for skirt investigations as the front wear patches currently create continuous sprays like a speed boat, (ie as if the front part of the tubes are weighed heavily into the water (feels like this when it grabs) and that could be caused by the forward curtain too short, which was born out by the Oban picture. There is zero excess air (spray) for lubrication we normally see.
After discussions, I removed some of the forward curtain and divider to improve the handling, and reduce this alarming slew action, being most anxious ref taking off ANY of the above curtains, (as its easy to remove and DIFFICULT to replace)
Now its gone, if anything--- its worse, (not better) so see no alternative but to replace the material, then start again trimming.
I realise skirts are HUGELY complex beasts, approaching the complexity of the female mind, perhaps.  ;)
John and Ian know FAR more than me about sev and Otter skirts, and I try to understand all before I act. This does not always result in a pain free exercise.
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General / Re: Otter Fish
« Last post by John Robertson on Yesterday at 9:07 PM »
If the craft is trimmed correctly on a flat, level surface then it's fine (a simple measurement exercise)).  All Sevs (and other craft) do the "sticking" thing on certain unusual surfaces. 

Brake effectiveness is determined by speed, cushion pressure AND cushion volume (rpm).  Effectiveness drops of with increasing speed (just like the brakes on a car) - once you reach a high speed (usually 35-40mph or so) when the oncoming dynamic air pressure, acting on the bow skirt, exceeds the front compartment pressure, then the brake ceases to operate at all (the bow skirt just flaps around doing nothing).  If you wanted a craft to exceed that speed and allow brake control then you'd need to increase front compartment pressure (i.e. increase weight and/or reduce footprint of the compartment).  If you increase cushion volume flow then you'd need to increase the brake vent area to allow more effective venting of the front compartment at speed - at low speed/flow/rpm it would be brutally sensitive!

On the displacement thing the main factor is surface viscosity - density and time are not relevant (e.g. if the craft was operating on concrete moving at 1mph it wouldn't displace anything).

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General / Re: Otter Fish
« Last post by Al on Yesterday at 7:38 PM »
Nick,


Just a thought, I know very little about the Otter skirt set up other than it is based on the Sevtec design.  Why not set the otter on height blocks of the height at which it is supposed to hover like Barry Palmer used to recommend?  It will then be easy to see if your front curtain or divider skirt is the correct length or if there is an area of your skirt that is not sitting correctly when on hover causing the front skirt to look wrong.


My first Surveyor had a problem that I spent months trying to sort out, but it wasn't until I used the height blocks the issue became clear instantly, which was at the rear of the skirt, not the front which was where I was thinking the problem was.  I later used that skirt as a template for the second skirt, which turned out to be wrong again at the rear, which John noticed when he & Steve came up here the first time.  John suggested how to check and correct the problem.   I used the height blocks and chalk marks on the ground and found the problem which was the skirt was too long from the splitter bar, which then made the ground contact points at the rear wrong.
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General / Re: Otter Fish
« Last post by Nick Flint on Yesterday at 4:20 PM »
As posted before---- surely THIS shows a too short frwd apron a few days before in North Scotland- The OBAN run?


Theres NO spray from the front length of the tube areas, and my Starboard brake is poking out a little which I will modify to prevent happening, as THIS cant help. Putting the brake on here would have little effect as the main area of air loss (under the "short front skirt" is already allowing huge lift losses so opening a small air bleed brake will have only slight effect---certainly born out from the weekends runs.
Just trying to understand huge complexities in an effort to improve handling and or performances. I realise the brake isnt supposed to work at 3200 rpm, but even lowering rpm displays a rather minimal effect, ie the nose doesnt drop much on FULL brake.


Im trying to remember if i cut some off the divider apron at the same time, ie is this level incorrect?
Trying to remember is NOT the same as remembering----------- ::) 

There seems to be insufficient air lubrication and it looks very different from the Severn photo.
I believe neither photo is showing any brake usage, with my revs on Cruising around 2800 +- 200rpm
Top speed at the week end was around 36mph (3200rpm) but tends to be a right handful when it starts to slew.


In MY mind, I still believe the craft is held aloft by the weight of water displaced by the craft, and at displacement will BE governed by the cushion area ONLY, ie stern down as per my previous post. However I feel at speed- it is STILL held aloft by the displaced water, its just the length of the displaced water is of longer length / duration.
The upward force on a skimming spinning stone is dependant upon its speed across the water surface, so a fast throw will result in a skim. A slower throw (of same stone) will sink because the upward force at lower speed is insufficient to maintain the airborne skim?  ;)

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General / Re: Otter Fish
« Last post by Nick Flint on Yesterday at 4:17 PM »
New cerebral box maybe?  ::)
I guess its part to do with how a plane flies?
Does it fly because of the huge volume (weight) of air displaced down wards as the plane travels thro the air at speed, or the low pressure above the wing, and or the higher pressure below the wing?
I still find it interesting to think even tho my post was not including all aspects of everything!
Is the craft NOT lifted clear of the water by displacing the water?
If so- why cannot it be lifted from the surface by the weight of displaced water as it passes across?
IE if it was travelling at 35 mph, the craft would achieve and settle more level as the stern lifts due to the displaced water being spread across a long length of water. Similarly aircraft achieve more level flight when travelling very fast thro the air, and conversely need a far greater "angle of attack" whilst travelling slowly to gain sufficient upward force to maintain flight, until the wing stalls like falling off the hump?
There just isnt room on the forum or in my mind.
The rear skirt always looks a tad "rounded at the bottom curvature, where it approaches the water" so the rear WILL look low, and thus the craft is pitched up at the front. The picture "reply 1 ) below,,,proves it IS pitched up at the front at speed on water.
Im NOT saying thats incorrect, just the rear is lower than say on smooth concrete workshop floor, which is my comparison.
My handling issues (over this weekend) were (since ive cut 20 mms approx off the frwd curtain) have been IF ANYTHING worse in that the front corners tend to dig IN more now with a huge slewing moment trying to drag the craft around.
Even My Magnificent Ackermann angled rudders were only sufficient to stop the slew- taking 5 secs to bring her round straight again.
The skirt is regularly looked at and wear patches replaced with single layer of sacrificial skirt.
Having flown craft for a while it looks but more importantly FEELS as if the front corner of the longitudinal tube digs in.
During a straight forward low grade plough in (IE if going downwind with big tail wind) it is quite straight line, and ive been testing the brake on Sunday coming back up the tail wind Severn. The brake (if anything) seems less effective which perhaps suggests the front apron is too short?
The craft tends to grab one side or the other UNLESS the water surface is broken up by say 2 inch ripples which makes her fly less grabby.
The North Scotland picture ive posted looks decidedly more shortened on the forward curtain?
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General / Re: Otter Fish
« Last post by John Robertson on Yesterday at 11:23 AM »
You maybe need a new cereal box!  The depression in the water only occurs in displacement (boat) mode.  When over hump speed the depression reduces to a few mms (water viscosity limits the formation of the hole).  It also depends then on speed - faster makes less of a depression.  Attached is the Otter drag/thrust curve from our calculator (HERE).  It shows the drag reducing markedly as the craft exceeds hump speed - mainly due to the skirt drag component.  As the speed increases aerodynamic and momentum drag increase.  The optimum (most efficient) cruising speed for the craft is 17-20mph (minimum drag).  Note that the 17mph is with a nose heavy setup which effectively decreases the average cushion pressure by loading the front compartment more thus reducing wave drag at both hump and cruise speeds.

In any case, your craft looks perfectly "normal" to me - unless there is some serious performance issue I'd leave well alone (from experience with skirt tinkering, it never ends well!).
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General / Re: Otter Fish
« Last post by Nick Flint on Yesterday at 10:53 AM »

Ref Ians picture below (post No 1 here)

Looking closely at the picture in question, the rear quarters of my craft are clearly lower due (perhaps) to the gradual depression of the cushion area that the craft rides on. In practice I guess the area towards the front of the craft, is not depressed at all due to water momentum maintaining its level, but as the craft passes over the initially level water, it is subject to lift pressure and as such is gradually depressed until the weight of the displaced water equals the weight of the craft travelling over. Mr Archie Medes worked this out a while ago!
The result interestingly (to me anyway) results in the water under the craft being sloped Upwards towards the prow.
The rear of the craft is then set down in the water, with the prow raised.
By my reckoning, on the inside of a Kellogs cornflake packet (other cereals are available) assuming the slope under the craft is an even slope front to back gives me a VERY rough figure of 80mms set down  by the time its displaced my approx 400KGs.
Surprisingerlie, salt water only weighs slightly more than fresh at 1.025Kg/Litre.
Looking back at the craft, the REAR IS sitting around 80mms lower than the water level, so this tilt is bound to slightly lift the prow away from the surface. The picture up in Scotland of my craft however shows my front curtain well away from the surface, but could just be a frozen moment when my curtain caught a breeze to lift it further away than normal?
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