Inflatable Tunnel Cat Speed wobbles

Jeff Salinas · 21 October 2008, 21:53

Hello all,

I have just upgraded my outboard motor on my DUX Hammerhead from a 15 hp to a 30 HP. I just took it out yesterday with my new outboard for some test runs. I installed the Suzuki DF30 30HP 4stroke. I did my test runs in Oceanside, CA in the harbor and out in the open ocean. A comfortable and easy handling of the boat likes to go at about 15mph. When I did run OWT I got up to 24mph and climbing pretty quickly, but the boat also liked to skip around the surface of the water and the tiller was moving left and right really quickly. I would compare it to speed wobbles going down a hill really fast on a skateboard or bicycle. Now, it was only myself in the boat. All the weight was in the back; the 200+ lb outboard motor, a 6 gallon tank that was half filled, and myself. I would like to ask anyone outhere with experience in these boats at highspeed. Can you please share setups, aftermarket accessories to tame down the handling; steering dampeners and such. Here are a few pics of the setup. Thanks for your help in advance.

codprawn · 21 October 2008, 22:13

Get a longer tiller arm - you will sit further forward and have more leverage.

I think there is an adjustment to tighten up the steering swivel. Also removing the prop guard may help.

Easyrider · 22 October 2008, 00:57

Is that a long shaft engine??? Its hard to tell from your pics - you need a short shaft one for that boat- it certanly needs to be raised up on the transom by a fair bit. Yes an extended tiller will help but it wont stop the speed wobbles- you really need a steering damper like this used on the thundercats http://www.leisureshopdirect.com/mar...al_Extras.aspx

and a bracket to mount one end onto the top of the transom and mount the other end on the tiller near to the cowling- you will get a super smooth speed run then.
I use this setup on my boat with a 50hp-without it it is a real handfull keeping it in a straight line especially when going over 40mph.

Jeff Salinas · 22 October 2008, 03:40

Quote:

Originally Posted by Easyrider

Is that a long shaft engine??? Its hard to tell from your pics - you need a short shaft one for that boat- it certanly needs to be raised up on the transom by a fair bit. Yes an extended tiller will help but it wont stop the speed wobbles- you really need a steering damper like this used on the thundercats http://www.leisureshopdirect.com/mar...al_Extras.aspx

and a bracket to mount one end onto the top of the transom and mount the other end on the tiller near to the cowling- you will get a super smooth speed run then.
I use this setup on my boat with a 50hp-without it it is a real handfull keeping it in a straight line especially when going over 40mph.

Thanks for the responses. Yes, my motor is the long shaft.
do you have any pics of your 50 with the dampener installed?

thanks,

Jeff

Andy JC · 22 October 2008, 10:23

Quote:

Originally Posted by Easyrider

Is that a long shaft engine??? Its hard to tell from your pics - you need a short shaft one for that boat- it certanly needs to be raised up on the transom by a fair bit. Yes an extended tiller will help but it wont stop the speed wobbles- you really need a steering damper like this used on the thundercats http://www.leisureshopdirect.com/mar...al_Extras.aspx

and a bracket to mount one end onto the top of the transom and mount the other end on the tiller near to the cowling- you will get a super smooth speed run then.
I use this setup on my boat with a 50hp-without it it is a real handfull keeping it in a straight line especially when going over 40mph.

I thought that engine looked very low. The Dux website states a 15"/20" shaft for that model which as far as I can tell would mean a short or long shaft

Leapy · 22 October 2008, 13:05

Why not ask the guy sat on the pontoon in the first picture. He looks like he might have an opinion

Easyrider · 22 October 2008, 16:04

Quote:

Originally Posted by Leapy

Why not ask the guy sat on the pontoon in the first picture. He looks like he might have an opinion

He does look a bit long in the face doesnt he

Jeff-As soon as ive worked out how to post pics ill put them on here

Easyrider · 22 October 2008, 16:39

Here we go-not sure if its going to work + pic quality isnt great.
The last pic is to show you what level my prop/gearbox is in relation to the hijackers.

Jeff Salinas · 22 October 2008, 17:08

Thank you so much for posting the pics. That helps a great deal.

Very nice setup you have.

JS

Locozodiac · 22 October 2008, 17:28

Quote:

Originally Posted by Easyrider

Here we go-not sure if its going to work + pic quality isnt great.
The last pic is to show you what level my prop/gearbox is in relation to the hijackers.

Hola Easy

Just an issue as I'm only a sibber, what is the purpose of the steering dampeners for, to put more friction on the tiller, not to be so light moving side ways ? Work with Tohatsu engines, most of them are factory delivered with steering friction a bit light, adjust all of them by means of a + 5º CW adjustment on the steering friction nut. Now much better steering performance than before. Can drop the steering handle with no prob. Why don't you just adjust the nut a bit and test.

Happy Sibbing

Easyrider · 22 October 2008, 19:30

Hola Loco.

Yes they do exactly that-the problem is,without the damper even with the friction tightened up they still snatch a lot, especially with a semi cleaver/cleaver prop-reason being that with the tunnel hull sibs the prop is generally piercing the surface of the water most of the time so even in a very small chop it can be in and out of the water alot even at moderate speeds so you get torque then no torque all the time even though the ride on the boat is super smooth.
Another thing that exaggerates it is that because they handle like they are on rails the slightest movment on the tiller will change the direction of the boat, so once it starts to twitch its hard to keep it in a straight line(without the damper) as youre allways correcting.
V hull sibs have the prop/keel lower in the water so you dont get it snatching so much-my previous Bombard aerotec380 with a 30 Tohatsu never had any steering issues even with the friction screw slackened right off.
I regulary drive another sib with a 50 Mariner(rebadged Tohatsu) no need for a damper on that either.
Hope that explains it.

Jeff Salinas · 27 October 2008, 15:44

Hi Everyone,

Thank you all for chiming in and sharing your tips and experience. Since my first test ride with the motor, I have then did another test drive without the prop gaurd and this time with another person to help ballast it out. I have also tightened the friction clamp on the tiller. My results are much better but can also be improved more. It it was much more easier to control and handle the boat this time. I can go WOT now in open water. And I got a top speed of 26mph via gps. But I did get water over the transom. My motor is too deep. With the trim set to the 3rd notch up the water over transom does lesson a bit but I get some serious bow rise before planing out. I think next what I'm going to do is raise the motor. The most I can raise it up using the holes on the outboard to bolt on would be 2". I would still need a shim to fill in the gap. I'm hoping I can find a 2"x2" piece of oak at Home depot and use that. This would be short term. Ultimately I think this transom elevator would be the solution.

http://www.iboats.com/Transom_Elevat...-view_id.40157

But for now, I'm going to try to raise it by 2" and a piece of wood. If anyone has any other solutions or experience with raising their outboards, please share.

Thank you,

Jeff

Locozodiac · 27 October 2008, 16:26

Is your transom a straigh horizontal one, ----------------------

or has a middle cut to place the engine

Jeff Salinas · 27 October 2008, 16:33

It's a straight horizontal one -----------------------------

no cut.

Locozodiac · 27 October 2008, 17:10

Quote:

Originally Posted by Jeff Salinas

It's a straight horizontal one -----------------------------

no cut.

What is the total distance from upper transom to lower transom base in straight line 38 Cm, 40 Cm ?

Had this same prob with a splashing sib engine configuration, but my sib had a 38 Cm, middle cut transom that was raised to 40.5 C, with fiberglass. The ideal would be to try the metal engine suuport. Keep in mind that rising a transom is a time demanding job, has to be done by a skilled boat tecnician, a 30 HP engine is a powerful one. Would advise stay away from experimenting rising transoms with wood, seen people down here lost their engines because of poorly modofied transoms. To have an idea of rising a cut transom.

ian parkes · 27 October 2008, 21:03

Jeff I have a tiller steer motor on a small rib and as easyrider says its ok without any damper . However some sort of damper makes driving much more relaxed and faster in rougher water .
I have rigged two 8mm thick bungees on adjustable hooks atatched from the tiller to the tube . They work really well

that steering damper is a mercedes van steering damper , a vw beetle steering damper is also very similar but smaller which may work better on the 30hp . They all cost around £12 from a car parts store

Jeff Salinas · 28 October 2008, 06:15

So I raised my motor by 2" using the variable thru holes of the outboard. I drilled through the transom and bolted it with stainless steel 1/2" bolts. It seems sturdy, but my main concern is the empty space as seen in the pics. I haven't rode it like this, I just wanted to get peoples' opinions on what they think if this will work.

Thanks,

Jeff

dim. · 28 October 2008, 13:50

Quote:

Originally Posted by Jeff Salinas

So I raised my motor by 2" using the variable thru holes of the outboard.

Not really an expert but I would say 1. Add washers you used on transom side to a motor side also. 2. Lover motor to second hole. That way motor clamps would be fully engaged. At least to the test run 3. Place peace of wood between motor and transom.

Locozodiac · 28 October 2008, 13:51

[QUOTE=Jeff Salinas;269454] So I raised my motor by 2" using the variable thru holes of the outboard. I drilled through the transom and bolted it with stainless steel 1/2" bolts. It seems sturdy, but my main concern is the empty space as seen in the pics. I haven't rode it like this, I just wanted to get peoples' opinions on what they think if this will work.

Thanks,

Jeff

Hi Jeff

If this is the best sib/engine position to perform best with no splash problems, you will need to fill all the blank space under the engine as the engine must always be seated against something, transom, extention, side plate, whatever. According to your pics you have a solid metal transom in which case a fiberglass fill in would not be the way to go.

You could place under the engine a metal/hard wood bar the same transom width with sufficient height to fill empty space and to adjust clamps ties against transom/bar, fasten the metal/wooden bar against the transom lateral bars with bolts/nuts or u shape metal bars, the ones that have end nuts. Something like this to have an idea. Personally would have opted for a proper transom elevator from T & R Marine as the first issue is too much work, or just lucky enough to find the perfect metal/wood bar size you need. When you have obtained the best engine heigh position performance. then fill accordingly.

Happy Boating

Locozodiac · 29 October 2008, 16:06

Once you have finished adjusting best engine height peformance setting, will ned to adjust trim angle. The best position for most sibs is the second hole from transom, trim engine so anticavitation plate/engine is 90º, once on wot/plane with mates middle to bow weight distributed will have a near instant bow down, a perfect horizontal displacement.

If your fuel line is long enough, tie your gas tank to bow, when completely filled will give you about 25 extra kilos in bow, will help distribute better overall weight in sib performing superb, with a 30 HP, you'll make your day, test and see.

Happy Boating