Battling the Bulge, with a Trim?

[Explore Cowal]

Sure I've read something about this before, but searched and can't find it.

I've managed to put a few shifts in with the Honwave Airdeck including a nice 36 mile run on Loch Fyne and latterly a family trip on Loch Eck.

Yesterday I had the engine singing in glass type conditions.





However once the boat was shifting there seemed to be a bulge in the floor that I could feel moving from front to back....bow to stern for the pro's. It seemed to impede progress at it settled at the rear of the boat till a dropped the revs.
Now, should I adjust the trim so the front is lower in the water, although it feels and moves as if it's sitting nice? It is set at the middle of 5 settings.
Relating to trim and getting the boat on the plane, I need to be exerting some power to get the bow down, this is fine when it's calm, but in the chop, speed isn't always the most comfortable.....should the trim be set as above, to keep the bow down and break the surface?

Cruising.

https://youtu.be/aNxN3hUz0Sw

Loch Fyne....hope links to blogs are ok, could see anything saying not. I'll delete if need be, just to lazy to upload and type again.

https://explorecowal.com/2016/08/06/...o-otter-ferry/



Thanks

[Fenlander]

The bulge you are experiencing and the way you describe it is typical of most air floor SIBs although the Honwave 3.8 with its V shape to the floor suffers this less than types with a flat air floor and separate keel. Annoyingly it is often at its most noticeable in totally flat conditions when otherwise you would be going perfectly. I have described it in the past as feeling you have run along a whale's back.

There may be some improvements for fiddling with the OB trim and how you spread your load/people. Most important it's worth checking the pressures... floor in particular... when it's been in the water for a few minutes. Low floor pressure is the most likely thing to make it worse but even when right it may be a character trait you will never totally lose.

Regarding the boat's attitude at different speeds. Are you talking about the problem of the bow rising as you fall off the plane down towards displacement speed when it gets rough?

This is always an issue in smaller craft when rough water makes you slow down and in such conditions it's not really possible to stop and fiddle with the OB trim setting which would in any case make little difference to the wall of water you end up pushing in this transition phase.

You can move weight forward and... much to her annoyance... I use teen daughter as moveable ballast if needed! Depending on conditions she can be anywhere from opp me on a tube near to the transom... to sitting on the floor in the bow.

Nice read and pictures BTW.

[jeffstevens763@g]

we had that problem on our SIT 5 m inflatable air floor at work we got it down to wrong pressure in the keel tube the floor was ok but what we had was a bubble forming in the slack floor as it built up and increased in pressure it then traveled to the stern and was that bad the engine cavitated.

[Explore Cowal]

The bulge you are experiencing and the way you describe it is typical of most air floor SIBs although the Honwave 3.8 with its V shape to the floor suffers this less than types with a flat air floor and separate keel. Annoyingly it is often at its most noticeable in totally flat conditions when otherwise you would be going perfectly. I have described it in the past as feeling you have run along a whale's back.

There may be some improvements for fiddling with the OB trim and how you spread your load/people. Most important it's worth checking the pressures... floor in particular... when it's been in the water for a few minutes. Low floor pressure is the most likely thing to make it worse but even when right it may be a character trait you will never totally lose.

I've only checked the pressure prior to leaving the house, never once launched.................so i'll check next time


Regarding the boat's attitude at different speeds. Are you talking about the problem of the bow rising as you fall off the plane down towards displacement speed when it gets rough?

Yeah,If i'm going to slow in the rough the bow wants to rise, making progress difficult. If i need to get it on the plane, that involves speed which makes it a rough journey. I was thinking if I adjusted the trim to lower the bow at lower speed, hopefully the V will do the cutting of waves rather than the underside.

This is always an issue in smaller craft when rough water makes you slow down and in such conditions it's not really possible to stop and fiddle with the OB trim setting which would in any case make little difference to the wall of water you end up pushing in this transition phase.

You can move weight forward and... much to her annoyance... I use teen daughter as moveable ballast if needed! Depending on conditions she can be anywhere from opp me on a tube near to the transom... to sitting on the floor in the bow.

I'll need to wait another 9 years to have a disgruntled teen, maybe a couple of sandbags would suffice.


Nice read and pictures BTW.

Thanks

[R18628]

Likewise, our T38 does exactly the same. Need to check floor pressures once in the water.

[Fenlander]

>>>If i'm going too slow in the rough the bow wants to rise, making progress difficult. If i need to get it on the plane, that involves speed which makes it a rough journey.


This is a bit of a smallcraft issue you have to accept.

We've been out in gusty conditions the last couple of days. At low displacement speed... say 5kts and less... it takes ages to get anywhere and from time to time a wave or spray will slop into the boat. On the plane at 13kts+ it's very bumpy into the waves and wind. Our choice has been a semi-displacement speed of around 8-10kts where having the bow slightly high keeps the water mostly out of the boat. Very bad for fuel economy though.

[Explore Cowal]

I'd be happy with 5 knots if my back was still in one piece, plus coming from a kayak....that'd do. I'm going to order a tiller extension for 1up weight transfer. Trial and error.....and have fun finding out.

[Stigomery]

I think the "bulge" is an inherent trait of the T38. I've not been able to totally eliminate it but it is less noticeable in a well balanced, loaded boat.

1-up, in a flat clam over 16 knots and it is very noticeable when it appears. Sometimes as low as 16 knots and sometimes not until about 20 knots. The feeling is horrible, like the engine and boat want to go 5 knots faster and there's this big anchor trailing behind you holding you back!

[slate1234]

I had the same problem with a 3.2m sunsport I was thinking it was becasue it was a copy so to speak, No matter what I did that bloody bulge would be there doing my head in, I cut the transome down 40mm that helped a lot but when I ripped the floor I was acutely very happy as I had a good excuse to buy a small 3m rib, the best thing I have done,

[jeffstevens763@g]

For my interest is this bulge a design problem as I have never come across it other than under inflation in all the boats Ive owned or specific to certain makes.

Cheers

[Fenlander]

Jeff the potential for this bulge effect has been in pretty well every air flor SIB I've owned. Its severity depends on speed, outboard power, load carried, floor length, wave conditions etc. It's worse the flatter the floor shape and with under inflation. In all cases where I've experienced it the floor pressure has been correct.

The amount it is commented on by any particular owner also depends on their preconceptions of how travelling in a SIB feels... some hate it and others barely notice.

[jeffstevens763@g]

thanks david i did have a small honwave for a while but only used at displacement speed but i was amazed on how solid the air floor was.

cheers

[Stigomery]

Unless I'm going for all-out speed I never really notice the bulge, but of course that's when it's most frustrating and as Fenlander says, so many things affect it including the difference between following waves and cutting through them... If I was simply running a 10HP I'd never notice, but I am at the limit of what the boat is rated to carry from an OBM perspective so perhaps little niggles are to be expected under certain conditions?

Undoubtedly the longer the hull the more scope for it to appear/be a problem but, aside from Aerotecs I think every inflatable floor's SIB has the potential to suffer this affliction... I dare say even a 4m Aerotec might be pushed into this with the right (wrong) combination of set-up/conditions.

[simonafloat]

Before I realised that plonking the boat in cold water knocked a little off the air pressure, my aerotec 420 suffered from this.

I now inflate the floor to 12psi so that the cold water effect knocks it down to the correct 11.2 pressure or thereabouts. No problems since.

[simonafloat]

To be honest I'm fairly confident that the airfloor and tubes can survive being a little overinflated so that's what I tend to do. 3.6 psi in the tubes, 12 in the floor.

[jeffstevens763@g]

I bet their tested twice the pressure before they burst carnt imagine the working pressure is too close to failure. Good question if anyone knows?

[Stigomery]

Originally Posted by Stigomery

Unless I'm going for all-out speed I never really notice the bulge, but of course that's when it's most frustrating

Well, I've had the Honwave out the last 3 days and it was beautiful, however I suffered "the bulge" worse than ever before. It drove me to distraction. I'm not sure what it was, badly balanced boat, trim, conditions... but it was insufferable.

I also carved up a chunk of the sea bed with my prop, those Eastern sandbanks are a killer.

I tried to take a picture of the bulge but it's impossible to get the right perspective.

I've often wondered if it would be possible to put down a hinged wooden floor that wedges the V down but not yet experimented.

You can only just make out in this photo how the front, round the fuel tank, is being sucked down into the water and how the rear, by the engine, is being forced upwards...

[Fenlander]

Stig this is super techy but a research paper on the performance and design of inflatable boat hulls part funded by the RNLI.

http://eprints.soton.ac.uk/183099/2/...r_Halswell.pdf

In section 5 it explains exactly how this bulge develops and moves under the boat floor.

If you look at the area where the bulge appears on yours the V is so shallow it would be quite hard to make any sort of inner wooden floor that would help to any extent.

If you notice in the report it says rough water tends to largely prevent this effect and I've said before our most extreme experience has been on millpond flat lochs with a Zodiac air floor.

Trust me if you find the Honwave annoying you would tear your hair out with a conventional flat air floor and blow up keel. The only way to move away from this is a GRP hull or an Aerotec where this effect is unable to develop in the same way as... 1) the floor is as stiff as it could ever be with the near constant V right to the transom... 2) if a pressure bulge does try to form it "spills" up the V harmlessly away without getting trapped under the floor. Very very rarely this does happen and can be felt as a slight steering effect as the pressure spills to one side.

[jeffstevens763@g]

it seams that your all in the middle off the boat 4 people/fuel/kit box to some degree point loading problem?,at the top of the picture tube line to floor seems as if its pushing the floor down going towards the bow as if the bow is trying to fold backward and the tubes moving outwards exposing more of the floor which is hidden at the stern?
i suppose the big difference between the air floor is its got a lot of rigidity over its length were as the ali/wood floor has an hinging effect at the bow and each floor panel allowing it to snake along as it were.

[Stigomery]

Originally Posted by jeffstevens763@g

it seams that your all in the middle off the boat 4 people/fuel/kit box to some degree point loading problem?.

In that photo not quite and we did play around with the loading quite a bit (see sketch for rough loading in that photo).

When I was on the Scottish SIB trip I hardly suffered the bulge, but the seas were far from glass like.