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Old 13 July 2014, 14:52   #1
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Country: UK - England
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Honwave 3.8 engine compatibility?

Hi, new to the forum so hello to everyone.

I've just started using a Honwave air floor 3.8m and put my short shaft Johnson 15hp on thinking it would be a good fit.

I trimmed the motor to be vertical when the boat is empty, floating horizontal, as it might be on the plane.

I measured the transom height and found that the johnson 'leg' was 17.5 inches from top of transom to cavitation plate - looked like it was 2 inches too long, but I tried it anyway.

At low revs it worked very well, but as the throttle opened more and speed increased, the prop started to cavitate. This was at around 4500 revs and just about when the bows dropped going onto the plane. The water was deflecting off the upper plate very nicely but the cavitation plate was just breaking the surface. Its better with more weight on board but with around 300kg on, it cavitates. (9.5 x 10 prop)

I was wondering what length the reccomended Honda motors are for this inflatable?

The tubes and sponsons are so effective there's not much of the inflatable in the water and as a 2 stroke the engine isn't very heavy - around 45kg.

Any ideas? Didn't want to start modifying the transom just yet!
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Old 13 July 2014, 15:32   #2
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Over to Loco!

AV Plate > Water Flow
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Old 13 July 2014, 16:09   #3
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Thanks, I've read some of the articles written (I think by 'Loco' ) on another forum and tried to follow the guidelines - the floor on the Honwave seems to scoop up before the transom - its not a flat run for the water and I'm thinking this lifts the transom a bit more than on other models. - its also got the trim tab extensions beyond the transom.

While it's stationary, the anti cavitation plate looks to be very deep in the water, but it lifts quite quickly as the boat picks up speed. I would have thought it would have been too long so to find it loosing grip and over revving was a bit of a surprise.
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Old 13 July 2014, 16:21   #4
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The accepted lengths for engines are:

Short/Standard Shaft 15"
Long Shaft 20"
Extra long 25"

But on larger short shaft engines, most of the manufactures seem to produce 17" shafts (15-40hp). Your engine is the correct shaft length for the boat.

The Honwave V airfloor is pretty good, but it needs to be at exactly the right pressure or the floor will develop a concave section just in front of the transom.

This has the effect of dumping huge quantities of air into the water right in front of the prop and causing cavitation. Are you topping the floor up once its been in the water for 15mins ? It makes a big difference. However even with the floor correctly inflated, my Honwave would still have this problem when used with the maximum engine option and fully loaded.
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Old 13 July 2014, 17:02   #5
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Yes, as above the 3.8 Honwave does seem to have this reported issue, all airfloor SIB's even at correct HP (notably bar the 3.8 Aerotec) suffer to some degree with the 'undulating air bubble water bed floor syndrome' but from previous topics IIRC the 3.8 Honwave was just a step too far in length in retaining rigidity in an airfloor without the Aerotec design.
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Old 13 July 2014, 17:29   #6
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Thanks for that - I think it sounds pretty close to what I was experiencing.
I temporarily put a packing piece on top of the transom to try a different height and the cavitation happened at lower revs. The water deflection off the leg seemed better but the cavitation plate was very high in the water.

I did inflate the floor with a high pressure (0.8 bar) pump on shore - near to the car - but not again once it was in the water - I can see that the cold water would have an effect. I had noticed the concave section when it was less than fully inflated on shore.

It sounds like I need to persevere with floor pressure and height. We were using it on an estuary for the last week and a bit of chop and swell on the water didn't help.
At least if i know the leg length is correct i can discard that as a problem.

If I were to replace the Honwave, are there any other more stable air floors around with better performance? I like the room in the 3.8 and the large tubes. It seems pretty bullet proof in construction.
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Old 13 July 2014, 17:42   #7
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The bench mark is the Aerotec 380, completely rigid floor and great performance, but it feels a lot smaller inside than the Honwave.

Try inflating the floor once its cooled down in the water, you will find it a lot better.
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Old 13 July 2014, 17:46   #8
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Quote:
Originally Posted by Sailpower View Post

If I were to replace the Honwave, are there any other more stable air floors around with better performance? I like the room in the 3.8 and the large tubes. It seems pretty bullet proof in construction.
It always comes down to the same old SIB and forum favourite, the Aerotec.
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Old 13 July 2014, 18:19   #9
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Thanks, so is it feasible to top up the floors on the water with a stirrup pump for example? its too far away from an electricity supply.

I'll look out for an Aerotech 380 and see what they go for - may be worth thinking about as a replacement. Just seen one on ebay actually!
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Old 13 July 2014, 18:26   #10
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I owned a Honwave T38 and had exactly the same problem, we had a Mercury 15hp 2 stroke. As you have described its as if the tubes and the air floor are “too” buoyant, lifting the prop out of the water.
I felt like I’d made a big mistake by upgrading from a T32 to a T38, I think they’re designed for a big heavy 20hp Honda 4 stroke.
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