Is 320EX w/ Twin 4.5L Under-Powered?

JimGuy341

What do you guys think?  Is a 2017 320EX with twin Mercury 4.5L 250HP B3 under-powered?  Boat is also equipped with a hard top and a generator.

Ian

There are folk here that suggest the 5.0 MPI with B3 in a 2003 310 are too under powered yet I am quite happy with ours. It's relative to how you expect to pilot your vessel; I can get up on plane pretty well quick enough for the 2 of us; even with 4 on board it gets up and cruises nicely without having to use trim tabs to pop her up. If you have F1 launch expectations that's a different story.

LaRea

Take all answers with a grain of salt ... some people will call any boat underpowered unless it will do 50 mph. 

Look at horsepower per 1000 pounds.  Here are two combinations that have worked well for me:
342 w/5.7:  600 hp / 16 kip = 37.5 hp/kip
370 w/496:  750 hp / 19 kip = 39.5 hp/kip

Here's what you're asking:
320 w/4.5:  500 hp / 13 kip = 38.5 hp/kip

By those numbers, it should be fine.  

Liberty44140

agree with @icoultha that you are going to get different opinions. I had the same boat as his and feel that his 5.0's in a 310 is plenty of power but some disagree. I do think that 4.5's in a 320 sounds pretty low though, just my opinion. I think I would want to go out on it with 4 adults and see how she planes before I write a check. You'll see on this forum that those with trouble planing have been able to make large improvements spending $1k/drive on 4 blade props and making some drive adjustments. 

Ian

Good way to work it out. Makes mine (when new and dry) 41.6hp/kip.

As I said, it's all relative to how you expect to cruise, pilot or launch...

earl1z19

You also need to consider what the torque difference is between 5.0's and the 4.3's. Hp is one thing but torque is the other. Merc doesn't list the torque for any of their engines but torque is what gives you the push to get up on plane. The 4.3's would be working very hard the entire time on a boat that size.

Cableguy Greg

One thing to keep in mind is the 4.5 is the new V-6 from Merc since they don't offer the 4.3 anymore. There is no longer a 5.0, it has been replaced by the new 6.2, which comes in 2 different flavors, a 300HP model and a 350HP model.

212rowboat

Earl is all over it...

where in the band is that 250hp present?  what's the TQ curve and where does it peak? 

it's like folks who drop a LS6 in an SUV and then try to tow away a 5500lbs boat- they have no clue why the 450HP/435TQ engine can't match the LC9 5.3 325HP/335#TQ for towing and why it falls on it's face.... the LS6 is mapped above 5000RPM while the LC9 is off 1500RPM.... 

Point being: numbers mean little.  they're ball park at best.  

no matter how you slice it, a v6 has fewer power strokes per shaft revolution than a v8 has- which means less torque, period.  unless the stroke on that engine is increased allowing distance of push during the power stroke, it isn't going to catch up to a v8.  you don't need HP to push a boat, you need TQ.  

yeah- even with twin v6's in a boat that size, it's gonna be lethargic.  

you can address it through the magic of leverage/gears and in the form of overall drive ratio equiv on the water, which is your prop(s).  Drop a few pitches and you'll pick up 'out of the hole' and increase stability on plane- better described as the ability to stay on plane at lessor engine load/RPM and fuel consumption- but it will cost your top end the same measure it improves your bottom end in most cases... it's like cutting the top third of a towel with intent to sew it to the bottom for purpose of making it longer.  It would be better to start with the right towel in the first place....