Honwave 3.8 engine compatibility?

[Sailpower]

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!

[Max...]

Over to Loco!

http://www.rib.net/forum/f50/av-plat...low-62602.html

[Sailpower]

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.

[Landlockedpirate]

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.

[Max...]

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.

[Sailpower]

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.

[Landlockedpirate]

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.

[Max...]

Originally Posted by Sailpower

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.

[Sailpower]

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!

[br1anuk]

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.

[Sailpower]

Yep, that's how it looked to me too.

I'll try floor pressures before I change anything significant - its fine for pottering around at harbour / low wake areas but it would be nice to use the full performance.
The Johnson 15 2st. is an easy weight to lug around and is fast enough for what we want. The 3.8 is about max size to fit in the estate boot, so it would have been a good combo.

[Max...]

Originally Posted by Sailpower

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.

Tricky as you've nothing solid enough to push against to get the near limit of strength effort a stirrup pump requires for an HP floor. A battery driven Bravo is the typical (pricy) solution.

[Sailpower]

Thanks max, that's what I suspected.
I got the car battery clip on version - maybe a slight over inflation would do it - the bravo pump pressure dial seems to vibrate around on its own anyway!

[Fenlander]

>>>>I got the car battery clip on version

Then you have the answer... a £15 generic 12v battery for alarm systems etc from Ebay. My Bravo has a side pocket in the case this will fit into. A full charge from a small battery will only just pump up the boat but is fine for topping up.

[beerbelly]

I use a car battery jump pack will pump the sib up a dozen times on one charge you can get them quite cheaply at Halfords machine mart ect the cheep ones are not much use starting large engine cars but pump boats up fine then just chuck it back in the boot of your car

[Locozodiac]

Originally Posted by Sailpower

Hi, new to the forum so hello to everyone.

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

2-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)

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

4-Any ideas? Didn't want to start modifying the transom just yet!

1-Way to begin with, newbies likes playing too much with trim settings on initial setups, which isn't the way to go in order to maintain best prop thrust.

2-How many mates were on board when tested ? Assume deck was well weight ballanced ?

3-Motors are water devices to push just anything that floats, inflatables are water gadgets to have fun with, not necesarily will perfect match between, in most cases will need a engine/transom state of the art height adequacy.

4-After re inflating sib to its fullest tube and deep V deck working pressure, check water flow again, if the AV plate is still exposed when sib is riding parallel to water level and engine at 90° you definitely need a transom height modification. Ideal is water flow skimming right under upper water defector plate. If you already are achieving aereation problems on straight watercourses imagine at close tight turns at speed.

It's useless buying another sib before any attempt is done to have actual one in opt operational fun state, you need to chop transom down at least 4 cm to lower engine for flow to pass closer to upper plate. Once there combo will perform much better. Test on flat calm no wind sea cond preferably.

PD : Chopping transom down does not mean the whole transom lenght, just engine's width middle portion. Just center well engine on middle transom and chop, make 2 plywood plates, screw engine's clamps tight to wooden plates momentarily.

If still not spot on dialed, chop 1 cm more, once dialed raise both original engine's plates to be even with chopped middle transom border. Engine will have a thicker border to sit and work much better.

Happy Boating

[Sailpower]

Originally Posted by Fenlander

>>>>I got the car battery clip on version

Then you have the answer... a £15 generic 12v battery for alarm systems etc from Ebay. My Bravo has a side pocket in the case this will fit into. A full charge from a small battery will only just pump up the boat but is fine for topping up.

Excellent ideas about the battery power packs thank you - I hadn't really considered inflation on the water but those options sound perfect thanks, and should get the boat to the optimum pressure.

[Sailpower]

Originally Posted by beerbelly

I use a car battery jump pack will pump the sib up a dozen times on one charge you can get them quite cheaply at Halfords machine mart ect the cheep ones are not much use starting large engine cars but pump boats up fine then just chuck it back in the boot of your car

I'll check that out thanks, I've seen them in passing but had,t thought to use them for that, cheers

[Sailpower]

Originally Posted by Locozodiac

1-Way to begin with, newbies likes playing too much with trim settings on initial setups, which isn't the way to go in order to maintain best prop thrust.

2-How many mates were on board when tested ? Assume deck was well weight ballanced ?

3-Motors are water devices to push just anything that floats, inflatables are water gadgets to have fun with, not necesarily will perfect match between, in most cases will need a engine/transom state of the art height adequacy.

4-After re inflating sib to its fullest tube and deep V deck working pressure, check water flow again, if the AV plate is still exposed when sib is riding parallel to water level and engine at 90° you definitely need a transom height modification. Ideal is water flow skimming right under upper water defector plate. If you already are achieving aereation problems on straight watercourses imagine at close tight turns at speed.

It's useless buying another sib before any attempt is done to have actual one in opt operational fun state, you need to chop transom down at least 4 cm to lower engine for flow to pass closer to upper plate. Once there combo will perform much better. Test on flat calm no wind sea cond preferably.

PD : Chopping transom down does not mean the whole transom lenght, just engine's width middle portion. Just center well engine on middle transom and chop, make 2 plywood plates, screw engine's clamps tight to wooden plates momentarily.

If still not spot on dialed, chop 1 cm more, once dialed raise both original engine's plates to be even with chopped middle transom border. Engine will have a thicker border to sit and work much better.

Happy Boating

Hi, thanks for the well considered response.
Q1, I read your set up guides re trim and height so I had some good info to start with.

Q2 we were 200kg of people with around 15kg anchor and chain and 12kg of fuel forward in the boat. Seats, boat hooks, oars, spares etc all rounded up to around 300kg maximum. Driver at the back and passenger in the middle on opposite tubes.

Q3 Looking over the transom under medium throttle, the flow looked fine until the throttle was increased - and the av plate broke the surface leading to the cavitation. This is with the AV plate already 2" lower than the bottom edge of the transom.

Q4 I agree and that's pretty much my conclusion too. The transom is already lower in the centre by design.
I could sink the prop still deeper by refitting the 5" longshaft conversion I removed to make it a short shaft, and building the transom up to be higher in the centre until I find the optimum height
or
Cut down the transom in 1cm stages until the optimum is reached.

My reluctance with the first option is that the motor currently fits across the car boot nicely and the rolled up boat fits in too. With an extra 5" it becomes more difficult to transport.
My reluctance with the second option is simply that its fairly invasive surgery and I assumed that there must be an engine / boat combo that already worked when Honda specified it without doing all the mods. I guess I'm thinking about resale value for the sib too.

If the 15hp 2 stroke is too light on such buoyant tubes and sponsons, it will be less expensive to swap the boat than buy a bigger heavier motor. Maybe I just need a buddy to sit further back with me to sink the transom more!

The input is much appreciated. Maybe more on the water experimentation is needed with some photos.

[beerbelly]

the honwave transom is fibreglass not plywood not sure if cutting it is a sensible option.i think this is a designe flaw in the 3.8 drop down to a 3.2 they apparently don't suffer this problem