Staggered engines

[ribsweden]

How much in speed does staggered engines? Offcause depense on boat...

Compare a fountain with staggered and simular at same size and classic engine installation.

[verytricky]

Sorry,

But I dont quite understand the question? Could you ask it in a different way?

[Matt]

You can't isolate the staggered configuration as the primary difference.
The biggest difference will be 1 stepped, padded hull vs 1 traditional V (based on the rib.net thread)
There is also probably a weight difference (not checked though).

[Jon Fuller]

Staggered motors are actually likely to cause a slight reduction in absolute top speed attainable in a regular non stepped 'V'. It's generally considered the way to go for a offshore race boat, as it affords some important improvements as far as weight distribution, and allowing the drives to be installed much closer together, so effectively deeper in the drink with the same 'X' dimension and also pushing (thrust line) closer to the centreline of the boat.

Unless you're talking megga speeds, where having the drives at 30+ inches appart is an issue because it places the drives much farther up the deadrise, ie, if we're talking fast pleasure boats, side by side, and the rearward weight bias it brings, is, I reckon, better for outright, flat water top end. again, this is when talking regular unstepped V hulls

For racing though, handling, and the good average speeds it can give, is everything.

my next boat will most likely be a staggered setup.

all just MHO and understanding of things, codprawn probably has a mate who knows better though.

[cfun]

Not really thought about it too hard but when I looked at Cowes into the engine bay of King of Shaves, I noticed the much longer propshaft on one side as it is staggered, I said to the engineer who was in attendance how much power is lost on the engine with the longer propshaft, he looked a bit blank and said mmm err not really measured it, after collage I worked with MB for 4 years and did factory things, they new exactly how much power was lost through a propshaft if it was lengthened by a few inches or thickened/strengthened they new no guessing for them

[Jon Fuller]

I'm only guessing here, but would have thought that assuming the propshaft is perfectly ballanced (as you'd want it anyway) and in reasonable alignment, correct bearing control, efficient flexi joints etc, the losses would be insignificant in the scheme of things. The average hook up between a motor and stern drive are far from high tech as it is, in fact, they're decidedly crude, and probably pretty wastefull, at least on the pleasure type drives (bravo etc)

[cfun]

Ahh that may explain it

[Boatless-Again]

Drive lines do not eat any measurable power. Now they do add to the torsional vibration of a boat if not set up properly.

To answer your question about speed though; I asked Donzi this same question about their 38 ZR.

I was told, same power (Merc 525) that the stagger engine installation was only 1 mph faster than the same boat with side by side installation.

However, in the rough water the stagger felt better.

Now back to speed difference.

As a V bottom goes faster and faster, it raises farther up out of the water onto the strakes.

As it does this things change for the two setups.

The side by side setup actually starts to suffer from submergence/depth of the propellers and at high enough speeds can lead to the boat not wanting to track straight and can induce chine walking as each propeller fights to bite.

A stagger setup with the propellers much deeper in relation to the vertical height will keep the propellers in the water better as the speed increases. The CG is now lower in the boat and more towards the center of the keel. Combine this with the propellers not wanting to fight to bite and cause chine walking makes for a better package for high speed.

Mid range speed, pleasure boating will actually favor a side by side as the CG is farther aft in a side by side and this allows the boat to run a bit free'er and thus ride better. If one is not going fast (95+) then they really do not see the benefits of a stagger.

Docking: A side by side will out do any stagger as there is far more mechanical leverage on a side by side. A stagger is difficult to dock as the propellers as so close that it becomes difficult to for/aft each and get the boat to react when both are pushing so close to the keel centerline.

What will be best for you will depend upon how you see your performance in relation to your overall ease of use.

1 mph and a lot more difficult to drive is a large trade off. But if you are only in large seas it might be worth it and if you are going big in power and speed it would be worth it.

[Jon Fuller]

And presumebly, with a stepped setup, the advantage of a rear weight bias for top end doesn't exist, or is very small as you're not attempting to 'carry the bows' at the higher speeds to reduce wetted area. (I'm talking flat water now)

[verytricky]

A big heavy prop shaft will not reduce Hp by much, but it will change the acceleration properties of the boat due to the extra inertia contained in the bigger propshaft, so on-off acceleration action will lag in the staggered engine compared with the short shaft.

[cfun]

interesting posts boatless sounds like a very knowledgeable person perhaps a boatbuilder, good to see tricky back posting again things had gone a little quiet

[Matt]

Enough that it's noticeable? Could you just fit a lighter flywheel to the lagging motor?

Originally Posted by verytricky

A big heavy prop shaft will not reduce Hp by much, but it will change the acceleration properties of the boat due to the extra inertia contained in the bigger propshaft, so on-off acceleration action will lag in the staggered engine compared with the short shaft.

[verytricky]

Originally Posted by Matt

Enough that it's noticeable? Could you just fit a lighter flywheel to the lagging motor?

Enough that you can notice. As an example, V8 has a very light, balanced flywheel, and it can accelerate faster than my boat, 'popping' out onto the plane faster. You can feel the difference if you drive it. It has about the same top speed ( using the same propellors ) so I assume it has the same power. It makes my boat feel a little 'sluggish'. The differance in the flywheel weight is only about 1kg, but the flywheel weight is obviously spread further out from the centre of motion, so the resistance to change of motion is higher than you would get on a propshaft, but it should be noticable.

To answer the question part two: Yes, if you calculate the additional resistance due to the propshaft, and lighten the flywheel appropriately, you would negate the effect, and render it unnoticeable.


Just out of interest, with a lightened flywheel, returning into the water after hitting a wave or wash, you feel a definate 'tug' on the boat with a lightened flywheel, in comparison to a standard flywheel. It affects the way you 'land' into the water, and requires a small adjustment on where you would expect the boat to be. A lightened flywheel makes the 'trim' of the boat different when you re-enter the water, dipping the nose slightly more than you would have expected.

[Jon Fuller]

Originally Posted by verytricky

Enough that you can notice. As an example, V8 has a very light, balanced flywheel, and it can accelerate faster than my boat, 'popping' out onto the plane faster. You can feel the difference if you drive it. It has about the same top speed ( using the same propellors ) so I assume it has the same power. It makes my boat feel a little 'sluggish'. The differance in the flywheel weight is only about 1kg, but the flywheel weight is obviously spread further out from the centre of motion, so the resistance to change of motion is higher than you would get on a propshaft, but it should be noticable.

To be honest, I rather doubt that to be the reason. much more likely that the motor is tuned/setup differently.

A light flywheel will let the motor pick up more quickly when it barks (boat flies), or you rev it in neautral, but I honestly don't think youd feel the difference in accelaration. the work (KW) required to spin the flywheel up with the extra KG, is nothing compared to whats required to get you on plane.

You'd need to try the light flywheel on the two different boats with their own engines to really quantify it affect.

I've played a bit with lightened stuff, and other than being a good thing for other reasons, doing that alone has never made any niticable difference in performance with the 'clutch out' so to speak.

Just MHO

[verytricky]

Originally Posted by Jon Fuller

To be honest, I rather doubt that to be the reason. much more likely that the motor is tuned/setup differently.

OK, - Simple answer is I dont know, I have just experianced two very similar boats and the only known differance to account for their different characteristics was a lightened and balanced flywheel. So from that experiance I have made my suposition.

There is definately a measurable differance if you have a bigger flywheel when you dynotune an engine - not on the end result Hp, but on the take up time to get to the next measurement point. The graph when overlayed show the 'lag' with the 'standard' flywheel.

[Matt]

I read an article on it with some sums in C&CC a few years back that calculated the "effective" loss of hp as the motor accelerates based on different flywheel weights.

[Matt]

Lovingly pasted from http://www.gibbs111.fsnet.co.uk/pintotun.htm

LIGHTENING FLYWHEELS - AN EXERCISE IN ROTATIONAL DYNAMICS

When the flywheel of a car is lightened it can have a great effect on acceleration - much more than just the weight saving as a proportion of the total vehicle weight would account for. This is because rotating components store rotational energy as well as having to be accelerated in a linear direction along with the rest of the car's mass. The faster a component rotates, the greater the amount of rotational kinetic energy that ends up being stored in it. The engine turns potential energy from fuel into kinetic energy of motion when it accelerates a vehicle. Any energy that ends up being stored in rotating components is not available to accelerate the car in a linear direction - so reducing the mass (or more properly the "moment of inertia") of these components leaves more of the engine's output to accelerate the car. It can be useful to know how much weight we would need to remove from the chassis to equate to removing a given amount of weight from the flywheel (or any other rotating component). There is more than one way of solving this equation - we can work out the torque and forces acting on the various components and hence calculate the accelerations involved - also we can solve it by considering the kinetic energy of the system. The latter approach is simpler to explain so this is the one shown below.
Let's imagine we take two identical cars - to car A we add 1 Kg of mass to the circumference of the flywheel at radius "r" from the centre. To car B we add exactly the right amount of mass to the chassis so that both cars continue to accelerate at the same rate. If we accelerate both cars for the same amount of time they will end up at the same speed and will have absorbed the same amount of kinetic energy from the engine. In other words, the additional 1 Kg in the flywheel of car A will have stored the same amount of kinetic energy as the additional M Kg of mass in the chassis of car B. To solve the problem of the size of M we need to use the following definitions:
V - the speed of either car after the period of acceleration
R - the tyre radius
G - the total gearing (i.e. the number of engine revolutions for each tyre revolution)
r - the flywheel radius (i.e. the radius at which the extra mass has been added to car A)
M - the amount of mass added to the chassis of car B
Kinetic energy is proportional to ½mv² - the kinetic energy stored in the extra chassis mass in car B is therefore ½MV².
The extra 1 Kg of flywheel mass in car A stores linear kinetic energy in the same way as if it were just part of the chassis. After all, every part of the car is travelling at V m/s - so it stores linear kinetic energy of ½ x 1 x V² = ½V².
To find out how much rotational kinetic energy the 1 Kg stores, we need to know the speed the flywheel circumference is travelling at. The car is travelling at the same speed as the circumference of the tyre (assuming no tyre slip of course). We know that for every revolution of the tyre, the flywheel makes G revolutions. However the flywheel is a different size to the tyre - so the speed of the circumference of the flywheel is VGr/R. The rotational kinetic energy is therefore ½(VGr/R)².
Now we can put the whole equation together - the extra kinetic energy in the chassis of car B = the sum of the linear and rotational kinetic energies in the 1 Kg of flywheel mass of car A - therefore:

½MV² = ½V² + ½(VGr/R)² =>
½MV² = ½V² + ½V²(Gr/R)² => divide both sides by ½V² to arrive at the final equation:
M = 1 + (Gr/R)²

That wasn't so bad then - we managed to avoid using true rotational dynamics involving radians and moments of inertia by considering the actual speed of the flywheel circumference. This did of course involve assuming that all the mass added or removed from the flywheel was at the same radius from the centre. In the real world that is not going to be the case so we need to use moments of inertia rather than mass to solve the equation. The simple equation above is useful though in getting an idea of the relative effect of lightening components provided we have a good idea of the average radius that the metal is removed from. It can be seen that gearing is an important factor in this equation. The higher the gearing the greater the effect of reducing weight - so for a real car the effect is large in 1st gear and progressively less important in the higher gears. We can also hopefully see that when r is larger, so is the effective chassis weight M. So removing mass from the outside of the flywheel is more effective than removing it from nearer the centre.
It might at first look as though tyre diameter is important but of course it isn't for a real car - if tyre size was to change then so would gearing have to if overall mph per thousand rpm were to stay the same - the two factors would then cancel out again.
To show the sort of numbers that a real car might have, I did some calculations based on a car with average gear ratios and tyre sizes - the table below shows the number of Kg of mass that would have to be removed from the chassis to equate to 1 Kg removed from the O/D of the flywheel at a radius of 5 inches.

GEARMASS KG139212364453

So in first and second gear this is a pretty important effect - I built an engine recently and managed to remove nearly 3 Kg from the outside of the standard flywheel - so that would be equivalent to lightening the car by over 100 Kg in 1st gear - not to be sneezed at in terms of acceleration from rest. With special steel or aluminium flywheels even more "moment of inertia" can be saved. The recent trend in racing engines to using very small and light paddle clutches and flywheels is therefore more effective in terms of the overall performance of the vehicle than it might first appear.
There's a final consequence of the "flywheel effect" being dependent on gearing. Small highly tuned, high revving engines need to run much higher (numerically) gearing than large, low tuned engines. This means that the effect can be very pronounced on them. Bike engines are a good case in point, especially as they are now starting to be used in cars so much. A 100 bhp bike engine might only be 600cc and rev to 12,000 rpm. A 100 bhp car engine might be 2 litres and rev to 5,500 rpm. Put the bike engine in a car and you'll need to run a final drive ratio twice as high as for the car engine. As the flywheel effect is proportional to the square of gearing, it will be 4 times as high for the bike engine. You could therefore be talking about 1kg off the flywheel being equivalent to 160kg off the weight of the car. That's why bike engines have such small multiplate clutches to keep the moment of inertia down. On the other side of the coin, it's not worth spending much money lightening the flywheel of a 7 litre Chevy engine revving to under 5,000 and geared for 60 mph in first as the vehicle will be very insensitive to the reduction in weight.
If you are going to get your standard cast iron road car flywheel lightened then be sure to take it to a proper vehicle engineer and not just your local machine shop. Take off too much material and it might be weakened so much that it explodes in use. Given that flywheels (at least in rear wheel drive cars) tend to be situated about level with your feet, it isn't worth the extra acceleration if you lose both feet when the ring gear comes out through the side of the transmission tunnel like a buzz saw at 7,000 rpm. There are plenty of ex racing drivers hobbling about on crutches who'll tell you that this can and does happen. On FWD cars the effects can even more unpleasant - a flywheel entering the cabin can give you a split personality starting from just below the waist that will put quite a crimp in your day. Also when you remove any weight from the flywheel it will need re-balancing again properly. We'll be happy to do the job for you if you don't know of an experienced engineering shop

[Jon Fuller]

So, taking this bit of info:

"So in first and second gear this is a pretty important effect"

And remember that our boat, is always pretty much in top gear

And this:

It's not worth spending much money lightening the flywheel of a 7 litre Chevy engine revving to under 5,000 and geared for 60 mph in first as the vehicle will be very insensitive to the reduction in weight.

I'd like to see a test, swapping the flywheel on Marcs boat (but using the same engine, obviously) and see if it's actually possible to measure a difference. if there is one, I reckon its tiny, and of no real use. the reduction in energy released into the drive on re-entry, and subsequent damage it might do would be more important (I reckon)

[Boatless-Again]

Aluminum flywheels and boats...

There have been many thoughts on this, but what has "typically" been experienced in a boat in rough water conditions is that the engine will lack inertia that is stored in the heavier flywheel.

When the boat launches off the waves, the throttle person pulls back on the throttle. Then upon re-entry, the throttle person stabs them forward.

The iron flywheel will keep the engine spinning and fight the deceleration from the water whereas the aluminum flywheel with less mass will allow the engine to be slowed down by the water and the engines bog.

This is why one sees a real flywheel on a race engine and a flex plate flywheel on a pleasure engine.

[ribsweden]

Originally Posted by verytricky

Sorry,

But I dont quite understand the question? Could you ask it in a different way?

Its my lovely english....

2 friends have different boats with same engines, simular weight.

Boat one, Baja 342 2x420hp 62-63knots. not stepped hull.

Boat two, fountain 35 2x420hp 72-75knots. Staggered engines and stepped hull.

How much is the hull and how much is staggered engines?