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Old 15 August 2016, 01:43   #1
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Battling the Bulge, with a Trim?

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
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Old 15 August 2016, 04:19   #2
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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.
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Old 15 August 2016, 04:28   #3
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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.
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Old 15 August 2016, 12:41   #4
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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
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Old 15 August 2016, 13:38   #5
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Likewise, our T38 does exactly the same. Need to check floor pressures once in the water.
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Old 16 August 2016, 15:57   #6
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>>>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.
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Old 16 August 2016, 16:58   #7
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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.
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Old 24 August 2016, 07:31   #8
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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!
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Old 24 August 2016, 11:28   #9
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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,
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Old 24 August 2016, 13:15   #10
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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
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