Hi Ian
If it isn’t slipping, why all of a sudden is it losing lift.
It’s has never ever done it before.

Please tell me if I’m being thick here…..
In this system there are 4 pulleys, three of them have bearings, one is the crankshaft.
On the crankshaft and the driven pulley on the lift fan, the belt has a full 180 degrees of pulley contact.which is as good as it  gets.
It is also known from the plans, that a single belt can transmit the amount of power required to drive the lift fan.
It is also known that on this craft, this system has operated faultlessly for some considerable time.
Looking at this from the prospective of a simple car mechanic.
Check all the bearings and belt contact surfaces. If the idler pulleys or contact surfaces are suspect, there are some in the boxes in the craft.
Put the belt on tighten it.
If it gets warm ……it’s slipping….tighten it…We know a single belt can absorb the power of the lift fan.
It doesn’t need any modifications, it just needs to be tight enough.
I can’t believe it can be so hard.

My input (bearing in mind i operate a Hovtec OTTER)) is that reducing vibration (Simple Harmonic Motion) is essential to ensure ideal power transmission, with subsequent decrease in operating noise, (ie you reduce hull vibrations -AKA- noise)


To THIS end, ive incorporated an antivibration damper in the V belt drive up to the OTTER thrust with very considerable effect, reducing vibration and any feeling of "harshness" through out the rev range.
 
On my previous Hovercraft that I designed, (KINGFISHER 1) I have taken phone video of belts operating at their harmonic state which show that the pulleys can exhibit 50% reduction in engagement, ie 50% of the driving/driven V surface was not touching! On that craft I positioned damper blocks at 35 % and 60 % distances from the ends, ie driven and driving sides were PREVENTED from harmonic vibration and the improvement was considerable.
 
On YOUR craft- IS there noticeable increase or higher vibration at your cruise revs?  IMHO only by videoing the belts at those revs 3000rpm? will your vibration video be instructive, not as much at 1200rpm IMHO. Can you fasten a video camera and power up on shingle 3 inch pebbles (to absorb all lift) and check to see if there is measurable vibration / power transmission loss?.


As Ive said before (I think) Perhaps remove the lift area cover to check IF that is strangling the lift input area. It will only take 5 mins to check this out, and let us know. As Ive also said before, I did exactly this 30 years ago on a craft to encompass the noisy lift, only to find that the lift actually produced was strangled, although noise was reduced a bit. Upon removal of said cover - the lift was SO improved, that REVS could be reduced again! In short, my noise reducing cover reduced LIFT rather than noise!!!! History sometimes helps and this piece of hovercraft history is passed onto you FOC! 

1. What is the belt tension (force/distance)?  Hard to tell which side is the driven one as it looks really slack!  Is engine at idle RPM?
2.  Driven and/or driver pulleys not fixed to shaft adequately.
3. Pulley grooves contaminated
4. Bad belt (what type/part#?).
Has to be one (0r more) of those.

 Trailer:

1. Push craft (on full hover) so the rear of the hull is 2ft behind the end of the ramp.
2. reduce power until craft rear settles on surface (front on full lift)
3. Drive trailer out fro under craft.

Absolutely no need to push the craft off the trailer!

Went to take the Surveyor out again today and it was a pain getting it off the trailer to start with.  I'm thinking of building a new trailer, as this one is awkward.  Once off the trailer I flew it down to the beach and it started bottoming out like it did before     So I stopped the motor and checked the lift belt.  It looked OK but was quite warm.  The two prop belts were cold.  So I tightened the belt slightly and flew the Surveyor out on to the loch to see what lift was like.  It didn't seem too bad, so Becky stepped onboard and we set off reasonably close to shore and  headed towards the top of the loch.  I had planned on going to Plockton today, but wanted to check it was flying properly first.  Anyway travelling around 24mph at 2500rpm and the nose dropped again as before, so I turned around and went and parked it on the beach, then loaded it back on the trailer 


Today there was next to no wind, probably under 5mph.  XC weather said 2 to 3mph wind.


It was a new belt and probably has done less than 2 hours, three at most.


Something is causing the belt to slip, so will be removing the belt and checking all the bearings on the lift system to make sure none of them have a tight spot or feel rough.   I did notice that there is a lot of movement on the belt which' I'll try and upload a video to show.  I'm wondering if maybe the belt needs something like a guide block to stop it vibrating so much