[Nick Flint Posts: 5010 Status: Online]

Yes Al, agreed on RPM.
My early morning brain says-
1. RPM - Thrust-lift all maximal /optimumal. - noise less so!!!!
2 Its known that this crafts design (finger skirt, width/length ratio, nose up trim at cruise) create "arse dragging" and poor WET slow speed water operations, IE no better or worse than your average Marlin (ie not good).
Marlins are NOT all bad for goodness sakes, BUT this zone is a well known poor aspect,
3 Soooo ---> Just "perhaps" Al you should
A) revisit the marinations on your craft to ensure reliability in wet conditions (which shouldn't be occurring). 
B) Then I feel you may have the confidence to set off from floating close in, facing away at RIGHT angles to shore and
C) wind up throttle to full (two secs max) heading away from land sitting right up front (as you ARE doing already!).
D) It is my "gut feeling" that you will then have a constantly increasing sea bed depth so that instead of the hump wave reflection gradually building and working in resonance, the feed back reflection will gradually change
E) and then within seconds you will be operating in deeper water  which we all know then helps get over hump.
F) The final things to perhaps consider are


By setting off at RIGHT angles you MAXimise the chances of getting over the hump, and hey "go for a buzz" above hump.
Your present concerns over  marinisations  are limiting you to shallow water, where it is WORST!
Dreaded question Al------------Does it still float if the engine stops? IF it does , arrange to test during an onshore wind and drift in or
groinal river fishing wellies perhaps?
Finally we get to my 76 year old operations---->
1 Arrange onto water transitions around 10-12 mph, because this is above hump- there can be NO problem.
2 When coming in to land on a beach, i make SURE i TRY TO come off the water around a decaying 12 mph approach
so that the beach angulation further slows the craft, being mindful of skirt wear IF I use my brake.
3 And finally when floating (perhaps moored up to Ians pontoon on the River Severn for a long time), then I boat away some distance, allow nose up on throttle up, to allow skirt water to collect at the rear, then slowly purge said water increasing rpm/ lift pressure, and allow craft to also slowly spin around, which really helps fling the water towards the skirt rear central expulsion flap.
Then its simply a matter of FULL power pull out occasionally swinging the craft about to help water purge from any other areas!
Clearly all of this No 3 only applies to sevs / otters / and the like, but I mention it here as ive found this out 9 years? of using the same craft!
AS EVER All IMHO 


Enjoy the rest of your day!

[Al Posts: 931 Status: Offline WWW]

I removed the wire extension that I added and tachometer seems to be reading the rpm about right



https://youtube.com/shorts/-qxBrMlOwxI?si=6iMLzuyg5Vyhv6Kc

[Al Posts: 931 Status: Offline WWW]

The induction tach arrived this morning,  I had a quick play to see what RPM the engine is producing.  At the moment I have no idea, as the tach keeps jumping from 3000 to 3500, down to about 2500rpm.  I've checked to make sure it is set to 2 cylinder 4 stroke.  Maybe something to do with me extending the pick up wire.  I'm going to try with just the wire it came fitted with, to see if that makes a difference.  On the amazon advert for it, it does state that the feed wire can be shortened or extended up to 5 metres , I haven't gone that long with it.


I think somewhere I have a tacho gun so may try that as well to see how they compare.

[Nick Flint Posts: 5010 Status: Online]

This is exactly where sevs and Otters out shine anything else especially finger craft. I know it IS tiresome listening to it but Gary Palmer of Sevtec design was a genius on hovercraft, especially this achilles heal of shallow water wave reflection. Ive even tried to create hump difficulties in shallow water! YES even Otters can be affected IF you maximise ALL the bad bits including slow increase in throttle to build up a big undercraft concavity but Otters Still lift up and go.
ONE of my areas of messing about with MY skirt system has involved altering the stance of the craft to encourage a more than usual nose up attitude which Ive found helps the craft "to carry on skimming".
I really like exploring how craft perform at various stages of cushion decay when coming into land on a beach. I aim to hit the beach JUST before the cushion decays creating this concavity and consequent exaggerated spray. The trick is to avoid falling off cushion leaving that annoying 10 MORE feet so you need to power up again to get up the shore- resulting in your pleasant shower for yourself and everyone around! Practise does indeed get perfect results EVENTUALLY (AKA - sometimes!) but it IS all part of the fun IMHO.
Hovercraft are like cars!
Easy to drive
but HARD to drive well. 
Let's hope you decide to join us on YOUR craft as well as jumping aboard our  craft.

[Ian Brooks Posts: 3581 Status: Online]

I just looked at the video - the water is much too shallow to get the Marlin over hump. You’ll need to be in at least 2ft of water (preferably 3ft) to get it over. Marlins are a bit tenuous on huimp performance at the best of times - Paul Hazel's one will get over fine in the river outside here (10ft plus) but struggled in less than 2ft in the Wye. Segment skirts are intrinsically more difficult for hump.

[Al Posts: 931 Status: Offline WWW]

It could be me not giving it enough throttle, I hate taking Briggs engines past 3300rpm, but I think if I remember rightly FF used to set them so they would rev to nearly 4000 rpm.  Hopefully someone on here is familiar with the Marlin, so I can get some advice from them.  I have a funny feeling that a Sev build will be in the pipeline for me shortly as currently not impressed with these.


I haven't decided if I'm bringing it to Loch Fyne

[Nick Flint Posts: 5010 Status: Online]

Hy Al, will you be bringing your craft to Loch Fyne?
Just from far off visuals, it looks as if the craft may not be achieving peak rpm /power/thrust / lift values?
I can't understand that as presumably, it was constructed to known working set ups?
The difficulties the craft encountered when on some sea weed strewn rocks being notable towards the village on the left.
Loch Carron?
Looks great BTW including weather. I remember it well. 

[Al Posts: 931 Status: Offline WWW]

Does anyone know roughly how many lbs of thrust I should be getting from a Marlin and at how many RPM?


Also does anyone know where the balance point is on a marlin, it should be correct as standard from factory, but would be good just to make sure and eliminate that from possible cause.     


I checked the rear has 15 closed segments  across the back and rear corners, so not that :-(


I have 3 x 2" holes drilled along the bottom side of the rear of the plenum, so any water in the plenum, can drain out even if the bilge pump packs up.
 
There isn't a rev counter fitted on the craft, so have ordered one, so I can see how much RPM the motor is running at.


My next test will be with the Marlin tethered on the beach with a set of spring balance scales attached to see how much thrust into and down wind the craft is producing.

[John Robertson Posts: 5240 Status: Offline WWW]

1. Shallow water (12-30") - almost all hovercraft will struggle to get over hump speed at this depth due to the wave reflection from the seabed
2.  It was over hump and running nicely for the 1st 30secs until you slowed down below hump speed
3. Check the rear segments are closed (pretty obvious).
4. Don't try to sneak up to hump speed - accelerate as rapidly as possible.
5. Craft is either dragging badly on the engine/fan isn't setup properly as it seems under-powered.

[Al Posts: 931 Status: Offline WWW]

I finally got around to test flying the Marlin.  I'm still not impressed with it and honestly at the moment, I can't see why so many people love these things.  Last time out, I discovered that it hadn't been marinized and with all the spray it generated from me trying to get it over the hump it started missing and spluttering.  At least I seemed to have sorted that issue as both me and the engine got well wet today, but the engine didn't miss a beat


I purposely kept it close to shore, so I if had any issues I could walk back to dry land.  I had several attempts at trying to get the craft over the hump, in water that was between 9" & 24" deep.  I tried to slowly increase the RPM to try and gradually build up a little speed so it should pop over the hump, but that didn't work.  So I also tried giving it full throttle from almost a still position, that didn't work either, but the craft and I got soaked from all the spray it generated on every occasion that I tried to get over the hump on the water.


The video that I uploaded has not been edited, but gives you an idea how the Marlin is performing.
It maybe my technique or something else is going on that I'm missing.  I have plenty of other videos of me getting wet, but will save them for another day 


I had a quick check at the rear once I got home and pretty sure the rear skirt segments are open, so I'm thinking at the moment this maybe the issue.


https://youtu.be/XP4tHNlPNQA