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Old 08 January 2002, 09:16   #1
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Country: UK - Scotland
Town: Falkirk
Make: Northcraft
Length: 5.8
Engine: Mariner 125hp outboard
Join Date: Jan 2002
Posts: 19
A question to you all

Hello to you all

I have recently purchased a Northcraft 5.8m RIB and have put a Mariner 125HP outboard on the back, there isn't much else in the boat except for single seated console, fuel tank etc.

When under way all is fine up to about half thottle, after that the boat starts to lean to the left, going further over the faster you go, this can get a bit unnerving to say the least.

What could be causing this.

Any ideas would be helpful.

Cheers

IanA
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Old 08 January 2002, 11:45   #2
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Country: UK - Isle of Man
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Boat name: Jane L
Make: Scorpion
Length: 8m +
Engine: 315 Yanmar
MMSI: 235077935
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Posts: 200
Ian,

Although I am far from being an expert on such matters, I would say that your engine is incorrectly mounted.

It may be that it is not vertical, or that the offset to starboard is insufficient.

I think that there is an accepted figure for offset (something like half an inch to starboard for every 100hp).
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Old 08 January 2002, 12:13   #3
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Country: UK - England
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Engine: 40hp yam
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Dear Ian,

Suggest that next time you go out boating, take your girlfriend, (and her mate, if the problem is severe enough) and get her to sit on the oposite sied to the heel of the RIB, problem solved.

Toby
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Old 08 January 2002, 16:37   #4
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Country: Ireland
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I would agree that your engine is probably not hung correctly.

As the hull leaves the water on the plane , the Paddle wheel effect of a right handed prop will tend to walk / force the leg of the engine to starboard and this will obviously become more pronounced the faster you go and the higher out of the water the hull is .

I would go back to whoever bolted on the engine for you and see if they know their stuff or not .

Many inexperienced people think that you just position it centrally.

This is fine on low powered engines but you definitely won't get away with it from 30 hp up let alone 125 hp !

Best wishes ,

Stuart
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Old 09 January 2002, 07:01   #5
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I'd agree............

with Powerboat!

A subject that is dear to my heart at the moment. We have had tremendous difficulties with set up of the new engine we had fitted last year. Now the supplier of this engine is a decent chap and is in the process of solving all the problems. He is also an extremely experienced RIB cruiser and racer. Our new engine was installed dead on the centreline - which is theoretically the most efficient place - rather than offset like the original. Reasons given for this were that its the best place for it and that the difference in hole pattern between this engine (Merc) and the previous (Johnson) would have meant that new holes would have had to been drilled in the transom too close to the previous ones for comfort.

Can anyone confirm/deny that Merc/OMC outboards have different mounting configuration?

This set up means that the boat falls on its port tubes when it comes off a sea - due to the torque effect as explained by Powerboat. So we've eventually got around to having it moved, along with the neccessary work on the transom to sort our the holes issue.

The other issue we've had some discussion about is that I feel the steering is heavy in one direction - despite the fact that its brand new hynautic hydraulic steering. I believe the cause of this is that there is no trim fin at the bottom of the skeg (facing downwards) as we had on the previous engine (& all other outboards Ive seen.) The previous engine also had a hydraulic steering system albeit a cheaper and nastier one! The trim fin enables you to further offset the torque effect of the engine. My engine supplier says that new Mercs dont come with them - and none of the other boats he has on site have them fitted. I don't neccessarily buy this and we have agreed to fit one anyway.

Again, anyone able to confirm/deny that "modern" merc outboards don't have a trim fin fitted at the bottom of the skeg.

Finally we are still experimenting with trim (as in weight distribution on the boat) and prop type & engine height to get best performance. Originally with the new engine the boat bow steered dreadfully but we have overcome this.

Its all a bit of a black art isn't it!

I don't post this to name and shame the supplier involved - as I say I am confident that they will rectify all the issues as they are a decent bunch of people. (& he's a fellow biboa committee member!). Just as an illustration of the black arts involved in engine/boat set up and to see whether anyone can shed any light on my two questions!

Cheers,
Alan
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Old 09 January 2002, 12:27   #6
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If it is any help, it took the best part of 3 years and 15,000 miles to get the Spirit of Cardiff right and although it was very good from it's first outing,(Around Britain 99) we knew that we could get it better. There is only the" rule of thumb" to work with any new boat which is what ever the builder tells you. As a rule the owners of the boat building company know very little about setting boats up as they do not spend much time out on the water. A well known boat builder was known to have said that "his boats where impossible to stuff" and was very upset when I told him that in that case he had never been in a big sea. As it turned out, this particular manufactor has had more tubes come off than anyone else!The customer always perfects the boat.
The other problem is that depending on how busy the builder is and the time the boat came out of the mould is very important. It is not uncommon for the hull to change shape and twist slightly as it cures, especialy if it is taken out of the mould early so as the builder can get another one in.
The moral of this story is........suck it and see, you will end up setting the engine in a better position than the builder.

Alan P
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Old 09 January 2002, 15:23   #7
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Country: UK - Isle of Man
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Boat name: Jane L
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Re: I'd agree............

Quote:
Originally posted by Alan
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The other issue we've had some discussion about is that I feel the steering is heavy in one direction -


Alan,

Two questions come to mind in trying to assist -

When you say that the steering is heavy in one direction, does that direction change as you trim the engine in and out ? - say heavy to port when trimmed in and heavy to starboard when trimmed out, with a narrow band of trim somewhere in the middle where the steering is equally light in both directions?

Secondly, I don't think that I'm familiar with the BWM hull. Is it a "tubes-clear-the-water" design, something like a Scorpion ?

Allen
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Old 09 January 2002, 18:11   #8
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Very true.......

Alan P...

You are absolutely right. I think a lot of our issue is about sorting out the overal balance of boat trim, propeller, engine height etc etc. Its all about fine tuning and experimentation. Since the manufacturer of our boat is no longer trading, OEM advice is in rather short supply so the onus is very much on us to sort it out to the best of our ability with the help of our engine supplier. We'll get it sorted in the end I'm sure.

Allen C...

IIRC (and its about 4 months since I've been out on the boat!) the steering stiffness varies with engine trim. At high trim/speeds it is very stiff to port but very loose - probably dangerously so to starboard. The DS21 is very different from a Scorpion - the tubes are set much lower and the bottoms of them still brush the water (albeit lightly) when on the plane.

Perhaps something to discuss further at the boatshow on Sat if you are still planning to be there? (Will mail you my mobile details.)

Cheers,
Alan
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Old 09 January 2002, 18:16   #9
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The lean may well go as you trim out. Our merc 90 four strke/humber ocean pro will dive off to the left if trimmed fully in at speed. The phenomena is specificallywarned about in the engine handbook and it suggests fitting a tilt pin (as with manual tilt engines ) to limit the travel in.
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Old 09 January 2002, 19:46   #10
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The lean should indeed moderate as you trim out .

It is important to note that the lean is caused not so much by torque effect ( which is much over rated in outboards - but not in inboards ) but by paddle wheel effect which is quite a different phenomenon caused by the rolling effect of your prop(s) side ways through the water.

Obviously a leaning paddle wheel caused by a trimmed up engine will have a lesser paddle wheel effect .

Steering can also be affected .With right hand props ( especially twins ) prop effect on a badly hung boat can be considerable causing the boat to veer to starboard and making port turns very stiff. Before you go drilling new holes in your transom , a lot of good can be achieved by adjusting the trim anodes .

This may knock a knot off of your top end speed ( more drag ) but it is far better than mangling your transom

( I spent today sea trialing a new mass produced well known RIB make with the steering prob with twin engines )

Best wishes ,

Stuart
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