How to Determine MPH - Silverton Express 310

[wakerider09]

Morning all,

I am looking to step it up into a 310 Silverton Express Cruiser, 1999 with twin EFI 5.7l Merc's, boat currently has just under 400 hrs on the engines if that helps. I understand that speed, RPM and weight all play a role in fuel consumption, but with 6 adults on board, at the most optimal cruising speed - on plane (whatever speed that is) what kind of MPH can I expect to see from a boat of this size/style?

 

[steelespike]

Re: How to Determine MPH - Silverton Express 310

Did you mean mph or mpg.
If your 5.7s are 260 hp at wot combined they will use "about" 52 gph. "maybe" you could reduce that to about 35 gph.
Mpg would depend on your speed at optimal cruising speed.

 

[Silvertip]

Re: How to Determine MPH - Silverton Express 310

MPG (average) = distance traveled divided by gallons used.

Fuel consumption on most engines = 10% of engine HP. 260 HP = 26 gallons per hour at WOT. Two engines = 56 GPH. Too many variables to predict what MPG is any other speed between idle and WOT. Only a fuel flow monitor can show you everything you want to know about fuel consumption.

 

[wakerider09]

Re: How to Determine MPH - Silverton Express 310

Sorry, yes I was looking for MPG, not MPH. What are your thoughts on twin 5.7l vs. 5.0l engines in a 30' express... would 5.0's be under powered? Secondly, you said I may be able to get the MPG consumption down to 35mpg... do you mean by finding the most economical plane/cruise speed?

 

[Radarman68]

Re: How to Determine MPH - Silverton Express 310

Generally speaking, it will take a specific amount of power/fuel to push your boat through the water at any given speed regardless of engine size or displacement. Displacement/HP just increases your maximum speed. thus, at a particular speed, assuming both 5.7 and 5.0 can both do it, they should use roughly the same amount of fuel. The sweet spot MPG is usually going to fall just over planing speed, but not enough throttle to be utilizing the secondaries (if you have 4 barrel carbs). Carbs are most efficient using the primary circuits only, and when you dip into the secondaries, the efficiency usually falls a bit.

So, if a 5.7 will run 20MPH at 50% throttle, you can probably assume a 5.0 will require 60% throttle to achieve the same 20MPH. I would hazard a guess that the mileage will be virtually identical. And the 5.7 will last longer due to it is not working as hard.

 

[Howard Sterndrive]

Re: How to Determine MPH - Silverton Express 310

Only a fuel flow monitor can show you everything you want to know about fuel consumption.

silly to not have one on a boat with 16 cylinders. Could pay for itself in 1 outing in a boat like that.

 

[JoLin]

Re: How to Determine MPH - Silverton Express 310

Sorry, yes I was looking for MPG, not MPH. What are your thoughts on twin 5.7l vs. 5.0l engines in a 30' express... would 5.0's be under powered? Secondly, you said I may be able to get the MPG consumption down to 35mpg... do you mean by finding the most economical plane/cruise speed?

Not 35 mpg, 35 GPH (gallons per hour). Okay, here's my guess based on my own experience and several friends with exspress cruisers from 25-30 feet, some single engine, some dual.

1. Fuel injected 5.0's or 5.7's would both be okay. If the boat you like is carbed, you'll want 5.7's

2. The most economincal plane/cruise speed is the one that keeps your boat on plane. On my boat it's about 21 mph. That's the slowest speed I can maintain easily without constantly fiddling with the throttles to keep the boat on plane when the wind and chop work against me.

3. My own research over the years (boat test reports) and conversations with other express cruiser owners tell me that, depending on the specific boat and load, the 'sweet spot' for these engines tends to be in the 3200-3500 rpm range.

In my boat with normal load, 3200 rpm gives me 24-25 mph and consumption of 15-16 gph, or 1.6-1.7 mpg. In the boat you're talking about, 3200 rpm will translate to comparable speed and fuel consumption in the neighborhood of 1.5 mpg. This is all really rough, mind you, and highly variable according to load, wind and waves.