View Full Version : Ian, those weights..

erbus - n/a
4-Oct-08, 09:14 PM

have managed to weigh the craft as best as possible and found a few other specs if required:

Craft without fuel and no extras = 381Kg

(using the bathroom scales in each corner method)

+ 25L of fuel = 18Kg

+ mats = 7Kg

+ bits and pieces =2Kg

Total craft weight = 408Kg

+ 40yr old bloke = 86Kg brings it upto 494Kg

Length 3.3m

Width 1.8m (although narrows at front)

Duct 840mm

Splitter distance 25 - 35mm

Tip 10 - 15mm

Gearbox ratio 1:4

Engine Cuyuna 430H/P (46hp @6800rpm)

Hope thats enough to give some sort of lift figures.

I would be most gratefull if you could let me know what the 'safe' max payload weight would be not including my weight.

Also, is this craft a fat old lady compared to todays standards?

many thanks


Ian Brooks - n/a
5-Oct-08, 11:15 AM

I've been looking at this and the first thing that jumps out is the 1:4 ratio... I don't think this can be right, unless the engine is running very badly indeed. Cuyuna gearboxes seem to come in the following ratios: 2.04, 2.48, 2.65 and 3.04 to one. Of these, the sums suggest that you're using a 2.04:1, ie with this ratio & 37 degrees it should top out at about 6k rpm which is what you see, and at 35 deg it should make about 6700 rpm.

So I've assumed 2.04 for the sums so far. The first thing to come out is the cushion pressure - this is high at about 21 lbf/sq ft, normally we would aim for about 10-15lbf/sqft. This is the reason for the high levels of spray, and a good part of the reason why the lift seems poor. Heres the details:

This is the "thrust-lift characteristic", which is a bit busy but tells us all we need to know about the set-up. Here's how it works:

Light Fan-shaped lines: These are the fan lines, one for each angle from 25 degrees to 50 degrees. Just work out which is yours (37.5deg) and follow it up.

Heavy red line: This is the engine line, and is the max speed the engine will make for any given fan angle. Follow your chosen fan line up 'till it meets the engine line, and thats the max revs you will get.

Heavy blue dashed line: This is the "first lift" line. Follow your fan line up until it meets this line, this is the engine speed at which you will see the fist sign of lift. The nose will rise, the rear will show signs of coming up but it will be dragging the skirt heavily.

Heavy green dashed line: This is the "design lift" line. Follow your fan line up until it meets this line, this is the engine speed at which the craft is fully on-cushion on a good surface, ie smooth hard sand.

Heavy magenta line: This is the reserve lift line, representing a reserve capacity for poor ground, gulleys, etc. Note that this appears as a straight line - this is wrong, and tells us that the fan simply cannot deliver this condition, no matter how much power you have! Because the program can't find an answer, it just outputs a constant "default" value.

For a good set-up, the first lift should be around 40-50% of the max engine speed, whereas your craft is making about 65%, and this represents the key issue I think. You will also note that the design lift line is on the right of the engine line - this means that you never achieve design lift! Hence your report of poor lift.


This is shown in tabular form here:


So the question is whats wrong? In short, the craft is somewhat heavier than it ought to be.

A more typical weight for a craft of this size would be 200kg, if it were of this weight then the picture would be as shown here:


Note that the first lift is now down to about 3500rpm, or about 50% max revs, and that the design lift line has moved to the left of the engine line. This is still not quite "state of the art" but would be quite usable.

You have already gone to 9 blades, which is the right thing to do, so my advice is to go through the craft and reduce weight wherever possible, although I fear that the hull is the main source of the excess which is probably not great news.



erbus - n/a
5-Oct-08, 01:09 PM
Ian many thanks for working all that out..

you given me plenty to digest there.

you are right about ratio being wrong i foolishly took engine rotation from the cooling fan drive which itself is reduced from crank speed.

after looking briefly and just trying to get my head around things:

Is this statement true? - the craft is too heavy and producing too much lift air (hence blowing) but because of the small lift area with respect to excessive weight its struggling to get comfortable lift.

When you say similar craft weigh about 200 is this without fuel?

i can reduce some weight, but you are correct in saying it is difficult to reduce weight substantially.

i will try and see what my rpm for lift is the next time i am out as i have recently changed blade angle.

I take it from the results because the design lift is outside of rpm and dont have a payload figure at all as i'm already exceeding it?

i will try and look at the gearbox ratio again if it helps, once again many thanks for your help and time ian.


erbus - n/a
5-Oct-08, 02:40 PM
Sorry ian, messed up on the weight.

we re-weighed it after thinking something had to be amiss if todays 'similar craft' are about the 200 mark.

i messed up on the front corners (it narrows at the front and my 2 front readings were really one in the same)

the weight without fuel and extras = 277Kg

+ 25L fuel = 18Kg

+mats =7KG

+ extras =2KG

Total weight of craft without me = 304Kg

Also may be of note the length = 330cm and width = 180cm

but the front narrows to 77cm wide reaching the full width of 180cm approx 79cm back (the photo in my original post will show what i mean - the front corners are cut off..)

Sorry about the wrong reading Ian. Hopefully with the 'new' weight' it may lift now!

Are your calculations based on my weight factored into the total weight?

Also witgh this revised weight figure can an additional payload be calculated?

many thanks


Ian Brooks - n/a
5-Oct-08, 04:25 PM

I'm glad you reworked the weight - I didn't want to say, but at 381kg it was little obese!


The new thrust-lift graph is shown:


This shows first lift occurring at about 4000 rpm, still a little high but probably workable, and now you can see that design lift is achieved. In an ideal world you'd be getting somewhere near to max reserve lift to give some reserve - you can see that the fan still can't manage that - it just might be possible to do something about that.

The cushion pressure is still high at about 16 lbf/sqft, which may explain the spray (more on that later).

We're going to need some more data!

1 - A check on the gearbox ratio to see if my guess is right.

2 - We need to measure & count the holes that feed air from the plenum into the cushion. There will typically be one hole per segment, and I need to know the diameter, and how many. Usually the ones at the back are smaller than the sides; sometimes the front are larger than the sides, sometimes the same.

In some designs these holes are a significant restriction and reduce the capacity of the lift system to deliver the high "reserve" lift air. In the calcs so far, I've assumed "typical" values. If you send the measurements I'll see if there is likely to be any benefit in opening them out.

What is the safe payload?

This isn't a straightforward question to answer, but you will be able to work out the answer given a little thought. Here's the issues:

1 - Floatation: The craft should have enough floatation in the form of foam or air-tanks bonded in to the hull structure to float the craft AND its occupants + a 10% margin. So in your case about 304kg + payload (ie about 330 litres +1 liter per kg of payload). You need to find out how much bouyancy is built-in, often in the plenum or in the floor. More can be added if needed.

There is often confusion regarding buoyancy, which comes from the regulations (HC133) ... Section 2.4A calls out buoyancy of 110% of the maximum design operating weight (this includes passengers and kit), which is what I recommend. However, Section 2.4E requires that the buoyancy be capable of supporting the occupants who "are in the the water", leading to an assumption that the buoyancy is only required to support the craft dry weight. My advice is clear - go for the higher value - if you are in the water your life expectancy is measured in minutes due to exposure.

2 - Hump performance: This is the more immediate concern. There is a maximum in the wave drag as you get over hump (about 7-8mph), and you need to have enough thrust to overcome this, otherwise, if you stop on water, your maximum speed will be about 4-5mph on full power, leaving you dangerously exposed if you are more a mile or two from safety. It turns out that the more weight in the craft, the harder it is to get over hump. You'll have to test this - if you need advice on starting from a stop, please ask. Obviously do this near to shore just in case!

Bear in mind that the ability to get over hump is affected by waves (you need to be able to get over hump in a good chop), wind, and water depth (deep water is easiest, shallow water worst until it gets less than about a foot when it gets easier again).

So - in summary, find out how much bouyancy you have, subtract the craft weight & your safety margin, and that is your safe payload. Then check that you can get over hump in a chop/breeze with that payload, in order to confirm that figure.

Air leaks 1/3 down the craft:

Major blowing of spray about 1/3-1/2 way down the craft usually happens as you accelerate over hump speed. This is because of the wave dynamics as you accelerate; at a speed of about 4-5mph you will have a wave crest at the bow and another at the stern, with a trough in the middle. Therefore there is a major air leak in the middle zone, blowing spray with it, and often excess wave drag at the bow and stern preventing you getting over hump. This effect is lessened by using less stiff skirt material. So if your spray occurs at this speed, then a new, lighter skirt may help because you'll be over hump and away before you get drenched!

Having said all that, higher cushion pressures do lead to more spray, and without seeing yours operating its hard to assess whether this is the case. If it is, then an anti-spray curtain could be tried, these are often seen on commercial craft.



Ian Brooks - n/a
5-Oct-08, 04:27 PM
PS - on the subject of bouyancy, in older craft it is sometime found that the bouyancy foam has absorbed water, adding significantly to the craft weight. Might be worth checking. If so, you could take a lot of weight out.


erbus - n/a
5-Oct-08, 05:30 PM

thanks! that was quick..

gearbox ratio is a tad over 2:1

each segment has its own hole

there are 60 in total

52 holes are 75mm

and i reduced the rear 8 to 30mm

what did you mean by how to start from a standstill?

the spray at a third ways down is really what i'm getting, it probably seems a lot of the time to me as its rarely flat calm and i often come off the hump when turning or messing about.

max payload - when i split the hulls i fitted a 60mm thick piece of closed cell foam board over the floor. there is also a large block of expanding foam in the tunnel roundabout the fuel tank and 5L BDH container integral to the hull. I was always worried about putting too much in and causing plenum problems.

I really just want to be confident that i have enough lift (with a margin) when taking out the wee ones. i'm pretty picky going on the sea anyway and always take appropriate safety gear. Hence, the question as to what payload in Kg just so i have a rough idea.

I understand it is probably a very difficult question to answer as you last post explains.

many thanks for the wealth of information, its also nice to know that consumate with the crafts age, the phrase middle age spread comes to mind, if you have any observations feel free to be frank - I never mind trying to put it right ...