Outboard power delivery

[Nick_H]

I've never quite understood how outboard manufacturers create a number of different power ratings from the same displacement engine, with the same RPM at WOT. Intuitively you'd think that if you squirt the same amount of fuel and air into the same size space, and set fire to it at the same frequency, you'd end up with the same amount of power each time ?

For my own curiosity, I'm quite interested to understand how that works, but what i'm really keen to find out is whether higher ratings of the same block provide more torque and power throughout the range, or just higher peak power.

Specific to my case, will a boat with a 250 HP 3-litre Merc Optimax get on the plane quicker than the same boat with a 200 or 225 HP version of the same 3-litre block, or will it just have a faster top end speed?

I can't find power and/or torque curves for these engines on the internet, anyone have them, or know where I can find them?

 

[rafiki_]

Nick, the different power/torque outputs will be down to fuelling and engine timing. Even for the same throttle opening and revs, you can vary the fuelling and spark timing. With modern controllers, you can have multiple sparks per firing cycle too. The injectors will vary, as with the controller maps. I expect ease of planing will be more down to torque than power, with power giving you the top end performance.

 

[Nick_H]

Nick, the different power/torque outputs will be down to fuelling and engine timing. Even for the same throttle opening and revs, you can vary the fuelling and spark timing. With modern controllers, you can have multiple sparks per firing cycle too. The injectors will vary, as with the controller maps. I expect ease of planing will be more down to torque than power, with power giving you the top end performance.

Thanks Raf, you mean they squirt less fuel in each time, so same number of explosions, just smaller ones? It's a shame I cant find torque curves for them. The absolute numbers wouldn't mean that much to me, but the comparisons between them would be really useful.

 

[rafiki_]

Thanks Raf, you mean they squirt less fuel in each time, so same number of explosions, just smaller ones? It's a shame I cant find torque curves for them. The absolute numbers wouldn't mean that much to me, but the comparisons between them would be really useful.

Well, you are right in principle, but the engine controller map determines when and how much fuel to squirt in. This controls the burn rate, so there are not separate explosions per cycle, but a much better controlled burn, where the right amount of fuel is burnt at the right time. The same old suck, squeeze, bang, puff 4 stroke cycle exists, but the bang bit is a lot better controlled for power, torque, emissions and economy.

I have a tiddler Yammie F6 on my tender. Exactly the same engine as the 4 and 5 hp versions, but with a different carb and timing. So no clever engine controller, and I guess easy to uprate the 4 and 5 hp versions with carb tuning.

 

[vas]

200-250bhp is a 20%+ difference.
I'd expect differences is injectors, timing, camshafts and even possibly intake/outlet design.
I'd agree with Raf that the lower down torque should be what you're after, top end is probably good for impressing ppl when they see the decals and that's all

Arranged the shipping yet Nick? When is she arriving up there?

cheers

V.

 

[Nick_H]

Yes, agreed, that's what i'm trying to work out. Will a bigger HP version of the same block have more low down torque and mid range power, which is important to me, or just more top end power, which isn't. I'm still not clear on that to be honest, and I can't find any comparative data to give me the answer.

Current plan is for seller to put the boat on a ferry to Trieste, and I arrange collection from there. Just finalising logistics of that at the moment, whilst they de-register the boat and prepare the paperwork.

 

[spannerman]

Two strokes and low end torque are not usually found in the same sentence, for torque you really need a 4 stroke, as the power delivery on a two stroke is peaky, i,e, occurring at a much higher rpm the a 4 stroke.
The Optimax will in the larger version be able to swing a larger pitch prop which translates into more speed, but to get on the plane you need some grunt i.e.hp low down, if the boat is under powered you will struggle or have to go to a smaller pitch which will hit top end speed, if the boat has a slight excess of horsepower you can use this to your advantage to get on the plane while still running the optimum prop for top end performance, and you don't have to use all the horsepower all of the time whereas as with smaller engine you will have to.
All of this is down to boat type, weight, intended use and most important of all that the engine can achieve the correct WOT rpm or you risk damaging the engine, especially if its over propped and bogs down.
In your case a 250hp with the correct prop will plane faster/more easily than with the 200-225hp, for a given load, but its important to get the right prop, the best is a stainless prop as these translate more thrust under hard acceleration than an aluminium prop, but then you have the choice of 3/4/5/ blades depending on the boat type/use. A bit more info on boat type would help.

 

[Nick_H]

Thanks Spannerman

I recognise that as the conventional wisdom from my motocross days, but I thought the newer direct injection 2 strokes were supposed to have more low down torque than equivalent 4 strokes. Certainly ETEC claim that as a selling point, and seems to be borne out by this graph:

I've also noticed, and others have commented the same, that 4 stroke outboards seem to have far less punch than even the older 2 strokes, and don't really get going until they hit higher RPM.

Then again, this graph seems to say the opposite at low RPM. In fact based on just this one graph, it looks like the Verado is the way to go

I remain confused!

 

[Nick_H]

This is a different size range, and 4 stroke not 2, but it demonstrates the question i'm asking.

The Honda 135 and 150 seem to have almost exactly the same power curve until 5,000 rpm, when the 135 tails off. In every day use these outboards would probably feel identical, because most people don't go above 5,000 rpm.

 

[Nick_H]

A bit more info on boat type would help.

It's a modified V planing hull, Boston Whaler type, 23' long, about 2 tonnes all up with engine and crew, and with a reputedly very efficient hull.

 

[MapisM]

Nick, I don't have the numbers you are after, but I'm wondering if they are really so important for your application.
I mean, of course you don't want to fit an engine which struggles in getting the boat on the plane, but you aren't thinking to use her as a slalom skiboat either, I guess?
The Optimax 250 performance just can't be significantly worse in the holeshot, compared to the 225 or 200. No way.
That could only happen if the 250 would be specifically tuned for higher rpm power, i.e. with lighter pistons, smaller flywheel, and all the other stuff which normally are used for racing engines.
Afaik (though I can't be 100% positive) there isn't any "mechanical" difference like that, between the 3 versions of the 3 litres block, and the different power ratios are only achieved electronically.
In other words, they are only meant to squeeze some more bucks from those who are willing to pay more for 25/50 more ponies.
As long as their weight is the same, and I think it is, the choice is a no brainer, imho.

And then, IF you still will be looking for a better holeshot, fit some good trim tabs to the boat.
You would enjoy them also in other conditions btw, like keeping the boat from leaning in crowsswind or with unbalanced load, for instance.

 

[Nick_H]

Thanks MapisM

I'm trying to decide if it's worth spending the extra £2000 for the 250, if I gain nothing in everyday use.

I'm now leaning away from the Optimax to the ETEC, as it's supposed to be a lot quieter, and I like the 3 year service intervals. The big block 200 is widely used by commercial operators, so should be durable. The boat has trim tabs, in fact it has auto tabs, but I didn't like this function on the Windy so i'll probably turn it off.

 

[Scarron]

Not used one but I belive the ETEC has an excellent reputation and the torque curve says it all - bear in mind that the only comparable fourstroke the Verado is supercharged with all the added complexity and reduced fuel economy that brings; How about the 250HO - http://www.boattest.com/engine-review/Evinrude/410016_E-TEC-250-H-O- note 195 fewer parts than a comparable fourstroke let alone a Verado.

 

[Nick_H]

Not used one but I belive the ETEC has an excellent reputation and the torque curve says it all - bear in mind that the only comparable fourstroke the Verado is supercharged with all the added complexity and reduced fuel economy that brings; How about the 250HO - http://www.boattest.com/engine-review/Evinrude/410016_E-TEC-250-H-O- note 195 fewer parts than a comparable fourstroke let alone a Verado.

Yes, a 250 HO would be very nice, but i'm convinced a 200 HO will be fine, and there are some good deals around on them. The reccomended outboard for the boat is a Yamaha 225-250 (because builder has a tie in with Yamaha), although it will take up to a 300. It's widely reported that the ETEC 200 HO dyno's up at well above 200 (close to 220), and has more torque and power than a 4 stroke in the mid range, so should still give high performance. They're not the best on fuel, but that's the least important factor for me as we'll mostly just hop around the Solent, so wont be big fuel users anyway.

 

[rwilson]

Hi Nick, here's my take having owned an Optimax 150, and currently have a twin 250 Yamaha setup.

The 2 stroke Optimax, great hole shot, spins up quickly, snappy throttle response, perfect for RIB but being a 2 stroke quite noisy. 2 bangs per cycle vs the 4 strokes 1 helps.

4.2 ltr V6 4 stroke Yamahas on the other hand are quieter, not as snappy on the throttle, but are very well put together and having had a Yamaha prior to my Optimax, am very happy to be reunited with the brand.

As for Etec's, does seem to be a marmite kind of thing. Lots on views on rib.net/forum if none on here.

I would go for Yamaha any day of the week but they aren't the cheapest option.

Hope this helps!