Boat "snakes" at low speed

[explorer500]

Hi all,

I have just purchased a 2005 Cruise Craft Explorer 500 with 2-stroke Yamaha 115hp (same as pictured). My first cruise through southern Moreton Bay last weekend found an interesting behaviour in the boat.

It "snakes" at low speed (say 6 knots). Yup, that's right - a lovely sinusoidal weaving through the water, about 1.5-2m off course on each peak, with about a 15 second period, with the steering held firm.

To correct the weave, I need about half a turn of lock each time. Obviously my forearms will look like Popeye's after a few hours of trolling...

I am after any suggestions of what may cause this behaviour. Boat appears stock, there is no significant weight distribution apart from that designed (fuel tank, seating positions etc), the motor is the same as that pictured in the brochure etc etc. Trim appears to have a slight effect on the weave - trimmed up, the weave is less obvious than trimmed right down.

At speed, the boat is rock steady and doesn't move off course.

I am leaning towards a misalignment of the motor - my theory is if the vertical axis alignment was slightly off, the position of the propellor on one side of the centreline will have a tendency to push the boat in the opposite direction. This push will cause a lean in the hull, which will increase the water resistance and cause an opposing change in direction, repeated ad infinitum.

Any comments or relevant experience would be appreciated.

Thanks,
Mark.

http://www.cruisecraft.com.au/images/showroom/explorer500/explorer500a.jpg

[seatime]

G'day

Quite a few boats will do that, it's not uncommon. Usually at a particular speed, slower or faster, and they steer better. Can be one of, or a combination of transverse thrust of right handed prop, underwater hull shape, motor trim and type of steering. It's more obvious in calm water than chop. Try trimming out and see if it helps, might be something you'll have to learn to live with at that speed.

cheers
Steve

[Noelm]

sometimes all weight in the stern and motor up a tad higher (trim that is) will help eliminate this issue, it is usually not a fault, just a fact of life! you should try to steer a jetboat at low speed, now thats a 'treat" try to slow down a bit more, or speed up a bit more and see if it goes away.

[Kiktz]

I would be going just a lil faster or slower, our Carrabean used to
be prown to this at very low speed. just a tad faster sorted it out.

AJ

[Roughasguts]

My boat will go side to side in a small chanell, heading out, never comming back in. But I have figured it out to be a slight cross wind pushing the bow out, and then over compensate in the steering, by laying of some drift, when the wind stops same problem.

You sure it ain't a cross wind.

[INTIMIDATOR]

Hey mate!
Nothing wrong with your boat.
Cas't your mind back to when you learn't to row a boat!(you did learn i hope)
Do you remember how hard it was to go straight when your passenger sat at the front instead of the back?
Get as much weight down the back as you can when trolling.
What happens when you increase speed it lift's the nose and give's you more controll.
I hope this help's a little and gives you something to think about!
Cheer's Payney!

[insideout]

put ya stubbie down and drive with 2 hands!!

[death_ship]

its normal, if your arms hurt i think you shoulda went the hydraulic steering option!

[Blueroo]

Never come across it before. Very very surprising.
I had a late model 550 explorer and it had no vices. Driving it in the bay was an absolute pleasure particularly in a following sea.
When your 500 snakes is it trimmed too low maybe.
Was it snaking going into the wind or in a following sea.
Sounds like a set up problem. I think you need to have a bit of a yarn with the dealer.
Hope this helps
Stu

[Roughasguts]

I tend to agree with 4Reel i'm on my forth boat and never encounted it.
Mind you out boards, with no keel and a rearward centre of gravity, do tend to sugest that it's very likely, With the wind in the right direction, tending to make it weather cock.

[Tinn]

Looks like you just got to go everywhere fast. Sounds like an alignment problem get an expert to take a look or post up some pics so we can see your motor setup

[Cheech]

Happens with my 5.5m mustang. Very annoying.

[Roughasguts]

Hmmm any one here experiance it with a run about or C.Console.

Think it has a lot to do with a cab boat and canopy, therfore thinking wind is the big factor.

[Chimo]

Hi RAG

Hey stranger, where you bin?

The idea about front weight, trim as suggested by Payney / Acolade does make a lot of sense. If you load up the front of any little boat you get the extreme example like the little row boat or tinny with people up front who are not balanced by comparable weight in the stern. Weight on one side or the other will tend to make the boat steer that way. Try it in a little boat and see.

Explorer and others with the issue probably do have extra weight up front and its only as the boat speed goes up and the bow rises that weight transfer to the stern and water flow around the hull changes so the weight inbalance / distribution then tends to work to help the boat knife thru chop and give the softer ride we all love. Lifes just a bundle of compromises eh?

MTB'sW, for what it worth

Cheers

Chimo

era

[explorer500]

Thanks for everyone's advice and recommendations.

I'm not convinced it is a weight distribution or wind issue for the following reasons:

Weight: 2 adults in boat sitting in the seats (factory position). 2 kids (5 & 7), lucky to be 20kg each, wringing wet. They changed position between rear seat and cabin with no noticeable change in trim. Everything else in the boat was in the factory fitted place (eg fuel tank under floor), and a small bag of personal gear was stowed in the cabin.

Wind: Snaking happened regardless of location - right off boat ramp in the channel, and in the open water. I also had various amounts of wave slop that didn't really affect it.

I will go out this weekend again and spend a bit more time at different speeds and motor trims.

Cheech, what effect does different speeds have on your weave?

I will also take some photos of the motor setup - just in case.

Thanks,
Mark.

[Roughasguts]

Cool the plot thickens, so now i'm wondering if you should look at the trim tab,(zinc anode) on the leg of your outboard.
Cause the leg is just a Symetrical aerofoil wing shape, so once it gets some sort of deflection, (maybe trim tab out a tad) then the leg skeg Etc will try and corect the mis aligment and in doing so will over compensate until the leg trys to compensate in the other direction. (kind of like a dutch Roll the boeing 727 Gets with the ailerons)
So maybe when your out next, take your spanner and adjust the trim tab and see what differance you can make, even let it flap around to start with.

I know this is only part of it, as were not taking gyroscopic, blade, and torque effect in to consideration yet.

[banshee]

My 500 explorer/115 Merc does it between 4-6 knots,it doesn't worry me at all as the only time I pick up on it is first thing of a morning in the go slow areas before I head to sea,I think it can be exagerated greatly by over correcting in an attempt to keep the boat dead straight.If it is a design fault or the like it is something I'm quite happy to live with and a small price to pay for a boat that has so many attributes.

[death_ship]

It is normal at low speeds ( 6knots) my haines and the 680 i go in often both do it at 6 knots, even after replacing motor, if its doing it when on the plane there is definitely something wrong, maybe steering problem.

[Roughasguts]

Geeez, i'm really curious about this. So what does it exactly do.
does the boat turn (yaw) or does it roll first, get to a certain roll angle then drop as if to fall of the Vee, then continue to go the other way.

[banshee]

Mine just changes direction ever so gently,the boat remains stable and rock solid,no lurching rolling etc,then the slightest of corrections with the wheel(and I mean moving my hand about a quarter of an inch with the wheel) will have it wander off in the other direction while the boat remains perfectly level (relative to the trim),the whole thing is so subtle in mine that no one else is aware off it,earlier on when I was over correcting it was noticed by a few of the deckies that used to come with me.

[Cheech]

Only when going through 6kn restrictions. Otherwise I am up on the plane and do not have it happen.

[Owen]

Sounds like you're using the steering system out of my old FC holden ;D

It'd be interesting to see a pic from the side of the boat(s) at this speed.
I was out at the dam the other day and saw a 4-5m tinnie trolling around with a very pronounced nose down attitude. Couldn't see what was in the boat, but there was two blokes in it in the rear half, which made me wonder how much weight must be down the front to offset the motor plus the two guys :o
I suppose a long boat (relative to width) would be somewhat less bouyant at the front?

[Roughasguts]

Thanks cheech & Banshee, bloody hell, so you guy's got foils on your motor or anything like that. I guess you both have 3 bladed props, I wonder if that and the slow idling power setting has any thing to do with it.

Something like the power stroke working better on the down going blade, then a few seconds later the power stroke working better on the up going blade, and that somewhat altering the thrust line when going slow.

Wonder if a 4 bladed prop would do it as much or at all.

[explorer500]

Hi again,

Thanks a lot to everyone for their replies. I had the boat out again on Saturday and experimented with the motor trim and boat speed.

Banshee, I also find that a small pre-correction once you get the feel of the weaving seems to keep it more or less on course.

I did find that if I trimmed my motor up to 6 or 7 on the trim-o-meter (courtesy of Yamaha!) the weaving was noticeably less. I just need to remember to trim down before taking off.

RAG - no foils on the boat - not sure if a 4-blader would be different. I also tried experimenting with the fin anode, with no results.

When I get some time, I'll take some photos of the motor setup from back and side - just to see whether others think it is in the right spot.

Cheers,
Mark.

[MyEscape]

Mark,

I have an identical boat, except a 115 Merc on it, brand new still under 10 hours on it. I haven't had a problem similar to your yet however where we launch from in Gladstone the under 6 knots area is not a long area to travel so perhaps I just haven't noticed it yet.

Nice boat though aren't they.

Steve

[explorer500]

Steve, you are right - they are a great boat!

Makes me want the 6.25 for the extra room, though. Oh well, maybe next year...

Mark

[Daisy]

Hi Mark
Sounds like a classic case of nose walking, goes good on the hop and then gets pushed around at lower speeds weaving back and forth as you compensate as you should. NoelM is on the moneywith more weight astern. As for Roughasguts I reckon less fuel on the way home might be the answer (wild stab in the dark) I hope this helps or I could be full of crap. Let us Know how you go.
Cheers Mick Day

[kingtin]

Never had it happen to me with a rig with an outboard, and I've had 6.

The moment I got the Whittley with the Mercruiser it was a bloody nightmare, snaking all over the place at 6 knots. I figured that stern drives were just oversensitive, I was "oversteering" and I would have put up with it. Now, 18 months later, I manage to steer ok although all the kids and deckie have problems. I figure that 'cause I'm the one who does most of the driving then perhaps I have just subconsciously learned a new skill and that some engines/boats simply behave differently. If it was a fault, or un-natural, then I reckon one wouldn't be able to compensate, unless my brain's as un-natural as my rig ;D

kev