twin engine mount

[timo]

I have an RIB with twin 40hp yam's. Due to the length of the tie rod, the engines are placed farther apart than they need to be. I think the engines were placed farther apart in anticipation of cavitation, however I don't believe the boat is well ballanced. Can the distance apart of the engines cause chine walk? Is it worthwhile to buy a std size (shorter) tie rod and re-mount the engines closer together - or will this have little affect?

[gonzo]

I don't think the engine spread can cause chine walk. That is usually the result of dynamic forces on the hull. Engine spread and cavitation are not related. Cavitation is caused by too much load on the propeller blades. Ventilation, what you may be referring to, is caused by an engine installed too high or other ways air can be sucked by the propeller.

[pistnbroke]

If one engines prop lifts out the water when truning at high speed then they could be too far apart (or too high on the transom..) I dont see how the spacing could cause other problems

[Jimboat]

timo - you didn't say if your RIB was a tunnel or vee hull, but engine setup can be a contributor to the onset of chine walking on some hulls. Dynamic balance of the hull is often inherent to the hull design, but there are several things you can do to improve the chine walking situation of a hull/setup.

Here is another thread on Chine Walking, that may be of interest.

Also, here is an article in RIB magazine on "Chine Walking", that has some info.

[daiquiri]

My 2 cents worth... If the distance between props is such that the props are working closer to the water surface than usual, then at some point one of the props could start ventilating with the other still submerged. That would produce an unbalanced thrust force which indeed could onset an oscilatory heel motion and chine walking.

[timo]

Thanks everyone for the input . I have not actually been experiencing chine walk (sorry - the chine walk question was a tangent thought, but Im glad I asked anyway), I was merely wandering if it could happen due to the engine spacing. the boat is second hand and the tie rod was modified - cut and spliced to lengthen - so the engines could be placed farther apart. Im not sure why. The boat is a vee hull, but it's not especially deep. It feels like the boat wants to lean to the port side ever so slightly (in fact one may not even notice it although I feel aware of it) - but as far as I can tell they are placed symmetrically. I thought it would be better if the weight of the engines were closer to the boat centre. The other thing that bothers me is that if for instance I use just say the starboard engine, the engine wants to steer the hull to the port side. Obviously this will happen with a twin set-up, but maybe the effect could be reduced (I do a bit of crayfishing with nets and it's not economical to run both engines all the time.). I also thought that under quick turns the boat would respond better with the engines closer (due to reduced inertia/angular momentum)? Im no physicist - my understanding of these things is from a layman point of view. My main objective here is to find out if it's worth buying a new tie rod and drilling extra mounting holes in my transom? Maybe it's a trial and error thing and I will only find out after i've done it - but money is a bit tight and even such trivial things are felt in my current budget.

[pistnbroke]

Maybe the original owner moved them apart because he had a problem ...BUT you imply there are no original holes closer together so perhaps he was advised to put them apart ...if you have no problem leave it alone ...are the props counter rotating ? ...all bopats lean to port due to engine torque which is why the driving seat is on the starboard side on most boats ...I guess yours is in the middle ...move the fuel tanks ????

[timo]

There are other holes in the transom - but from a single engine - which is another reason that i'm loathed to add more holes. The props are not counter rotating so the engine torque makes sense. However when a I take her out the water she also leans on the trailer - always to the left. It is a centre console with the tanks inside the console.

[Fanie]

If the motors are too far apart the one may run too shallow if the hull bank.

The other thing to look at is how the cavitation plate sits wrt the hull. I've seen guys mount the motors at an angle so the cavitation plate and hull interacts properly.

The long tie rod is a poor excuse for incorrect motors mounting

Instead of two same motors you will do better using one larger motor and have a smaller one as a backup, limp home or trawling motor.

A single 30Hp is lighter on fuel, faster, more powerfull and weigh less and cost less than 2 x 15hp's.
Goes for all outboards, twin vs single.

[timo]

"The long tie rod is a poor excuse for incorrect motors mounting" What excuse would you prefer? I was asking a question to eliminate possible causes to investigate since I did not really know.

Im not really gonna go through the trouble of trying to sell my engines, buy a Second hand (cause that's what I also can't afford) single engine and trawling motor.
With a single motor you may only go 1NM out. (Cat. E)
At 5 NM I don't want to rely on an electric trawling motor or a 5hp.
Two single engines the equivalent of one single is not that much slower - and I hardly ever get to go full throttle anyway, so speed is no issue.
Yes, they are heavier, but it does not really matter that much and if my 'balance issue' has nothing to do with the engines it matters less.
A single is not necessarily more fuel efficient - particularly if the two smaller engines offer more efficient combustion than a single large engine.
My engines are single carb - a 80HP engine may have 2 or three carbs making it less fuel efficient (possibly 1 carb more).
Fanie, I don't know if you are familiar with Kommetjie? Easily you may end up going through a bed of Kelp and the engines stall. While both engines can stall I find that twins better my chances by 50%. Kommetjie is not a place to be hanging around- the swell can get huge and I can't think of another place in the Cape that sees more drama.
And single engines are for Joburgers, Twins are much cooler.

By the way, I checked the boat out and it seems that the starboard flange of the hull supporting the pontoon is 20mm lower than the other side. I'm not sure if this is my gremlin but it does not account for the bias to lean to the left while putting the boat on the trailer.

[pistnbroke]

In South africa you have to have two engine each capable of planing the boat if you want to go I think its over 2 miles off shore ...this is because they have no coastguard

Seems like we are trying to fix a problem that does not exist other than moving some weight about or fitting a trim tab to one side of the boat to level it at speed

[timo]

I think so. thanks.

[Fanie]

The requirement is that you can help yourself get back to launch, it doesn't matter what size motors you have. If you go far offshore you will want singles that can both plane the boat or it will take you forever to get back to shore.

One big motor and a small motor qualifies equally as the twins, it was only a suggestion, but if you have two motors you are happy with then it's fine.

The right thing for the kelp would be a jet drive, but if you have trim and tilt you can work your way trough the kelp if the props chops the surface water. It is slow but at least the props doesn't get wound up or stall.

I wonder if some sort of a shield could keep the kelp off the props. You can always clip it off once you're through it.

On a cat the motors are far apart, on a mono -depending on the hull shape - you may want them as close as possible together without interfering with each other. A single motor is more ideal as it will sit in the center, the twin motors would be either side of the center. If the cavitation plates are in the the right position and interface with the hull correctly it makes a huge difference in drag, fuel economy and general performance you may experience as to some misallignment, even if you don't have speed in mind !

Trim tabs are unusual on such small boats. I think you should make sure your hull hasn't taken on any water causing the lobsided floating, if there is a problem it could begin to cause other problems including flotation issues, poor performance etc.

Perhaps post a few pictures so one can better see what it is you have there.

What excuse would you prefer?

If you will be patient, so will we. We are trying to help you resolve the issues, if any. We don't know what the boat looks like other than the description you gave - which isn't much to go on.

Is it worthwhile to buy a std size (shorter) tie rod and re-mount the engines closer together - or will this have little affect?

Let's see what the hull looks like as to the engine mounting positions. The long tie rod is a poor excuse for incorrect motors mounting - there may be a reason they were mounted where they are.

[Wynand N]

Originally Posted by timo

With a single motor you may only go 1NM out. (Cat. E)
At 5 NM I don't want to rely on an electric trawling motor or a 5hp.

Originally Posted by pistnbroke

In South africa you have to have two engine each capable of planing the boat if you want to go I think its over 2 miles off shore ...this is because they have no coastguard

Wrong on both counts....

Only category A, B, C requires twin engine installations for offshore use. Cat E (1 nautical mile offshore) & Cat D (5 nautical miles offshore) only requires one engine.

BTW, I am an accredited boat inspector/surveyor with SASCA (South African Small Boat Association) and do COF's almost on a daily basis.

As for the single / twin engine issue, Fanie is correct. One single engine will be more economical, faster and lighter than the equivalent combined HP of twin engines that equals the single engine HP. It is all about pitch of the propeller. An 80hp for instance swings a much larger pitch prop than a 40hp on twins, and larger pitch equals more speed, period. Another drawback with twins are the added drag of the second motor.

My 4.5m bassboat (inland use) has a 30hp Suzuki (tiller) and as a backup I carry a 2hp Suzuki to limp home with should the big engine fail.

[Fanie]

Cat E (1 nautical mile offshore) & Cat D (5 nautical miles offshore) only requires one engine.

I would still urgently suggest more than one motor. The closer to shore the quicker you can end up in the surf or on the rocks if you have no other means of propulsion.

Wynand, perhaps you can offer an opinion on Timo's boat that seems a bit lower in the water on the one side... ?

If he post a picture we can see what the possible problem is re the motors installarion.