A question of speed

[thylacene]

As a matter of curiosity I am interested to find out opinions on what sort of speed we should expect to get from our Quintrex 540 115HP EFI 4 Stroke Merc combination.

Took it out on Clyde river last weekend with GPS in hand and speed at WOT 5400RPM varied from 48kmh upstream to 52kmh downstream. 4 Adults and a full tank of fuel on board.

I note that the engine has a plate stating that max revs are 6400, and assume that the boat is slightly over "propped". Even if this is the case I am not intending to run off and change it, the boat is going to be fast enough for anything we are likely to want to get up to, and I figure that engine will last longer if it is not running at its mechanical limits.

So the question to those who have had more experience than myself, is the 50kmh what I should be expecting from this combination?

Specs show Hull @ 595 Kg Motor @ 185 Kg assume 100 Kg for fuel and a casual 360 Kg of bodies.

Your thoughts? Does being over propped have any disadvantages?

[frankgrimes]

Not same boat, nor motor!, but mate has a 549 stacer with 115 merc opti, and he gets mid 60's...I would expext your rig to do better than ~50ks

[Apollo]

Mate

There has been a few excellent threads on propping here and I, like you, originally thought that over propping wasn't a problem, but after it was explained to me, it is damaging to your motor as it loads it up too much. Do a search on this website and a much better explanation will be uncovered.

As far as speed, you are 10-15km short of where you should be at a estimate.

Cheers

[ozscott]

I reckon you are low mate - I have a Seafarer Vagabond - much heavier boat with 21 degrees at the stern with a 1993 115 Yamm V4 2 stroke swinging a stainless prop and with 4 adults I get 55-60kph depending on conditions at WOT 5,500rpm. It also gets out of the hole quickly too.

Cheers

[gofishin]

...Does being over propped have any disadvantages? Yes, considerable!

From those figures you may not be actually over-propped, as the speed is still way down. Did you have the motor trimmed properly/efficiently? Regardless, I would put money on your motor being too low.

With the cav plate parallel to the keel, take a pic from side on and another from behind (both pics down low, ~ level with the cav plate). Do a search on here & you will find heaps of relevant threads etc.
cheers

[The-easyrider]

Just to help with speeds we had a stacer 525 with a 115hp di 2stroke an would top out just under 70ks with 2 onboard and the bimini off

[Skusto]

correct me if im wrong to fellas but being overpropped and not being able to achieve the required WOT RPM will actually do more damage to the engine then being able to achieve the proper WOT Rpm? due to it labouring the engine when it cant get its peak rpm?

[ozscott]

I would agree - but I have a feeling that 2 strokes are more prone to the problems that go with labouring than 4 strokes, but happy to be corrected.

Cheers

[cormorant]

I would agree - but I have a feeling that 2 strokes are more prone to the problems that go with labouring than 4 strokes, but happy to be corrected.

Cheers


Bad for any motor. Old motors couldn't tell and electronic newbys can't compensate enough as it is outside their "fuel map" Will shorten life of any motor ( 2 or4) and probably use more fuel and plugs in the process.

In this example do you normally run this "fat mates or family" load ? If you do check your set up and do same runs unlaoded and see if it will rev out. The numbers will help select correct style and pitch of prop.

Every prop style and pitch is a compromise but regardless you need to prop it to get correct WOT with normal load. With rev limiters on modern motors being slightly under propped may use a little more fuel but you are ulikely to explode a motor off the tops of waves unlike in the past.

Were all teh fat relos at the rear of the boat as some hulls are suseptable to weight in the bum especially with a 4 stroke.

[ozscott]

Cormorant - but is a 4 stroke at those revs labouring? In the example posted above, just say the WOT that he achieves of 5400rpm meant that most of the time he was cruising at 4,000 then a 4 stroke might not be labouring compared to a 2 stroke with 6200 red line pushing load...because the inherent 2 cycles per revolution requires higher revs to stop labouring than a comparable 4 cycle engine - ie the 2 stroke has much less good old fashioned torque compared to a 4 stroke. To give you an example my Land Rover V8 pulls the Vagabond comfortably on the highway at 2500rpm...its not pinging nor labouring...but a comparably POWERFUL (ie comparing shear hp to hp) 2 stroke engine in the LR would not do that and if you tried would burn holes in the tops of the pistons....

Cheers

[cormorant]

With your example a correctly designed and tuned 2 stroke V8 could also do it at those revs. A lot of the big ship motors on bunker oil were 2 strokes slow revers It would have totally different ports and possibly a lot of CC. There was plenty old 2 stroke diesels 20 years ago as an example. 2 strokes have more torque at lower revs than a comparable 4 stroke except for very large cc for hp 4 strokes. No one likes to show those graphs and with better mapping and VVT etc the 4 strokes are getting very close. All motors are a balance between petrol use power, noise , lifecycle and compactness.

Basically your V8 is designed to put out HP and Torque at a certain RPM and the gearbox is matched to that in a compromis eto get decent ecconomy. I guess the question is what HP and Torque is teh V8 putting out at those revs and could another motor design do the same.

In history 2 strokes were seen as buzz boxes and older 4 strokes were too heavy and couldn't rev out as so many mechanical bits of poor tolerance were spinning about. Times have changes and with better tolerances , oild , metallrgy and balance 4 strokes can now rev out 10000rpm.

We all get caught in marketing and keep thinking about history not new technology motors. Many changes have happened specifically for fuel conservation as opposed to outright performance. Just gotta look at a old 2 stroke mercury or OMC with a set of racing injectors and see HP a certain CC or RPM can put out.

The whole thing is getting very blurred in the future with mixed motors getting developed that are 2 stroke but have air operated exhaust valves , air injection , fuel injection and dry sump and being able to fire on every stroke or second stroke depending on power or ecconomy needed.


So in arather long round about way - is teh 4 stroke at those revs labouring. Yes according to the manufacturers desired map for motor longevity assuming a lot of other factors are equal. Without a gearbox with selectable gears outboard manufacturers set a pretty thin band in which their motors last at certain loadings. If a motor isn't reving out to WOT at some or all points in it's RPM range it is overloaded and thet overloading may lead to stress in metal components, carbon build up, heat stress ( top of pistons) , cooling system overload, correct gasflow etc etc.

The basic building block of a outboards design is that it will achieve WOT as that is one less X factor in getting it to last a decent lifespan and perform with standard props avaliable. That is the one premis in the outboards computer map of fuel and ignition timing that the manufacture based all it's testing on. Change that and there is a lot of unknowns. Some hull shapes have done this in the past where they only perform ( plane properly) over a particular speed ( the hump) and a craft slightly underpowered will ineffecient and burn out motors fast but a well pwered boat will perform and be effecient.