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.