Help with SIB handling

[JNewton]

I have an Avon 4.6 SuperSport SIB with a 30hp Nissan tiller steer so most weight is in the back. Top speed about 25-30mph. Handling is very tempremental. Sometimes handles well, other times it is bucks all over the place. Took some kids out this weekend on a relatively calm day, and thought I was going to throw them out of the boat. Put a 200lb guy in the bow and didn't help. Let some air out of the Keel and it seemed to settle down some. It almost feels like air builds up under the hull slowing the boat down and then all of sudden it will take off run flat and smooth for a few seconds and then start over again. This is not "porpoising" but a fairly violent bucking. With a light load you can sometimes "push through it" getting enough speed to keep it on plane. I can't believe this boat won't handle well with a 30hp motor and 3-4 people on board. Any suggestions from any experienced SIB owners: More HP, trim tabs, remote steering, Inflate/deflate Tubes /Keel? Any help is appreciated. Thanks

[codprawn]

I have a much smaller boat - a Quicksilver 3.10 with a 15hp. it is great and no probs. What I have noticed though is that it is sometimes possible to assemble the boat with the keel way off centre so the hull shape is totally wrong.

Also I assume you have experimented with trim? It could also be down to prop choice - some give too much stern or bow lift.

[JNewton]

Thanks CodPrawn, keel is inflatable but fixed to the bottom, so I'm not sure how it could get out of line. Prop choices are limited with a small Nissan, although I'm looking. Will start playing with the trim. I do think trim is an issue because when it's running fast and light you can usually push through it. It seems you need to be running fast enough overcome trapping air under the hull. But then you only have 2 speeds: slow and fast nothing in between. That is one reason I wonder if trim tabs like Nauticus would help. Lower the planing speed and keep the bow down.

If I'm missing something on keel adjustment, please let me know. Any advice/suggestion is appreciated.

[J S]

I have a zodiac 340 and i have had the same experience where a bulge builds up in the floor trapping water and air. I discovered that i had only been pumping the floor up to 3/4 pressure when i pumped it up full i have never had the experience again. So check your floor pressure, i also found it did the same thing when i bought the zodiac trim tabs that are non ajustable they seemed to trap the water and air and then let it out when a high enought pressure built up. For trim settings i have a honda 15hp and i have the pin in the 2nd hole out from the transom i find that works best for all loads and conditions.

James

[SteveB]

I've got a 3.70 mt Mc2c Zodiac (13yrs old)with a 25hp Yam on a tiller and have the same problem if the tubes, keel and speed tubes are not to the correct pressure.

Do you have a pressure gauge to check all is correct?

Have to say Zodiac is fool proof in that pressure gauge has a "operating pressure zone" that is easy pressurise to.

I also get a similar problem if side to side weight is not equalised.

Steve.

[Tideway]

Are you on a long or short shaft engine? A longer shaft and playing with the trim may help. I have also experienced this in a SIB, owner didn't want to get long shaft, so cut the transom down. That didn't work either!

I'd guess that the keel is critical and should be inflated fully.

It seems to be quite trial and error with these craft.

I'd love to know a definitive cure...

Not much help I know, sorry.

t

[JNewton]

Thanks all for response. I do have a long shaft, do not have pressure gauge. Right now estimating probably need to go ahead and get a gague. I've wondered if it's pressure and load related and need to play around with the proper setup. Also figured others probably have the same problem and give some direction. Thanks again.

[russhenchman]

Hello JNewton, I am having the exact same problem with my 1990 Avon S4.00 Typhoon. Have you had any luck figuring it out???

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[Peter_C]

Originally Posted by russhenchman

Hello JNewton, I am having the exact same problem with my 1990 Avon S4.00 Typhoon. Have you had any luck figuring it out???

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I doubt JNewton is going to reply. Last Activity: 23 April 2009 07:06

My comment would be do you have a pressure gauge? If not get one, then report back after driving it at the proper pressure. It is hard to guesstimate how much pressure to put in, and I myself ran under inflated.

[willk]

Originally Posted by Peter_C

I doubt JNewton is going to reply.

No, no, he's there, watching us. He's glad that we're asking him for advice. He's safe and with family. He's telling me something, I think, no... yes! He says:

Originally Posted by JNewton

to go ahead and get a gague.

HTH.

[russhenchman]

Thanks but I have the Avon pressure gauge and have tried under, over and proper inflation pressures according the original owners manual.

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[m chappelow]

if you can run the boat about for 5 or 10 mins and then go around with the pump and top up see if that makes any difference starting with the bow working back and the keel last .

[BumbleAbout]

Originally Posted by russhenchman

Thanks but I have the Avon pressure gauge and have tried under, over and proper inflation pressures according the original owners manual.

1) It's important to get the correct pressure whilst the SIB is IN the water.
2) It's important to get the balance of the boat right (if solo, then a tiller extension may be required to move weight forward etc).

[martini]

Sounds to me like you might have too much weight forward, the bow could be getting stuck into the water causing the boat to steer by the bow. I've experienced something similar to what you describe.

[Delta 15]

Originally Posted by martini

Sounds to me like you might have too much weight forward, the bow could be getting stuck into the water causing the boat to steer by the bow. I've experienced something similar to what you describe.

That's the issue with my boat. If there's too much weight near the bow, the boat bucks and the stern rises, causing the prop to aerate. I took the hydrofoil off the outboard to reduce stern lift but it can still be an issue if there isn't enough weight aft.

Another thing to try is adjusting the trim on the motor. Moving the prop closer to the transom keeps the bow down and can increase stability.

Lastly, there's a weird thing I've noticed about my boat is with regard to the surface conditions. When the water is flat calm, the boat will porpoise quite a bit and I have to throttle back. If there's a moderate ripple on the water, I can run at WOT without any problem at all. (Running fast with too much wind chop will knock your fillings out, of course )

[mister p]

Dude, sounds like your hull is dragging. That's a weight transfer issue.
Be prepared to shift your weight in the boat as you're changing your boat's state.
From standstill your crew will have to move their weight from the stern to the bow as you build speed to get you planing and then come back a bit once you're there.
It's an organic thing.
But from the off you have to make sure that the standards of your boat are just that.
Let your SIB assume the temp of the water and get it right once the boat has met the water temp, so take the pump with you and fill once you've been on the water for about ten minutes. It's not just the tubes that meet the temp but the air inside them, and generally they will cool and make your boat bendy.
Imagine the weight transfer a motocross rider imparts when on the bike, over the bars when turning, over the rear when under power, hanging off to initiate a turn etc., same with the boat. Especially a sib. The longer the sib the less rigid it is.......tubes may be larger but the material is still the same gauge. And so it is that it will need more input (weight transfer) once underway.
A 5m sib is bendier than a 3m one, but this can be overcome with crew weight transfer.
You are already noticing the reaction of the boat to the conditions. Your crews' reactions need to counter those very conditions.
If it means getting your passengers (crew) onto the bow to get you out of displacement and onto plane the get them to move forward and bring them back a bit to balance your vessel once planing. It's all good fun, involving and effective. Essentially it's an organic process using your feel for what's required by allowing your "mobile" ballast to trim the vessel. Transient ballast is a very expensive commodity in the most expensive of craft and you have it here for free ( unless your mother-in-law refuses to budge).
Main thing is to get everything that's standard, correct. Pressure ( and do it right, on the water), engine (weight,prop,shaft and trim)..........the rest is moveable. Find a medium, fix what you can, tanks etc., and let the crew know that they are part of the boat and not just passengers. You'll feel the difference and so will they.
And if you're singlehanded then get a tiller ext and be prepared to get yourself up to the bow when you're WOT. but be ready to scramble back when you've gone over the top.
Weight transfer. Pressure. Rigid vessel.

[russhenchman]

Guys, thank you all for your thoughts...much appreciated.

The boat has no issue getting on plane with the 30hp (2-3 seconds with 3 people).
The bucking will happen with only a gas tank and myself in the stern and nothing up forward, although it is more pronounced with crew on board.

From everyone's input I'm thinking:
1. Ditch the hydrofoil. No need for it.
2. The Avon gauge, although feels heavy duty, is 30 yrs old. Maybe it's not calibrated correctly causing under inflation. This would make the most sense.

I will report back this spring...hopefully with success!