Engine selection for fuel economy

[Carver121]

may have been covered before but I can't find anything.

I'm getting closer to biting the bullet on a new motor, and wondering about fuel consumption between sizes. In theory would a 9.9 use the same amount of fuel to go say 10km at 20kmh as a 20hp to do the same? Or would the 20 be running lower rpms and use less? Or would it actually burn more being larger displacement?

I know its hard to account for everything, but just in general terms how would they compare typically?

 

[jimmbo]

If the 9.9 was at WOT and the 20 was at part throttle, the 20 would likely have better economy. At very low speeds the smaller engine would have better Economy. In reality, even a 20 hp is not a gas guzzler. It will have more torque, get the boat on plane quicker and can be throttled back, while still having more power when you need it.

 

[Carver121]

Thanks. Thats what I kind of figured. For slow trolling I'll likely be running electric anyway. I realize neither are big or gas guzzlers but it all adds up. The price difference in engines isn't massive, more power would be nice but most cruising will be at not a massive speeds.

Now that I think about it, would the 20hp also be the better option so I won't be runni g wot as much and therefore easier on it/last longer?

 

[achris]

If you are concerned about fuel economy, don't buy a boat.

When cars get anything up to 7 to 10 litres per 100 km, a boat's fuel usage is measure in litres per hour, and the number isn't small. My best 'economy' speed is around the 27 knot mark, and I'm burning 31 litres per hour! In 'car' terms, that's 60 litres per 100km....

Chris.....

 

[dingbat]

Motor selection is based on a boat’s max. HP rating.

Having said that, a bigger motor is nice, but also substantially more weight.

What is the max, HP for your boat?

 

[Scott Danforth]

if you want fuel economy in a boat, get a sail or get slaves to row oars like the vikings did.

 

[racerone]

Think this----On a level road you put your vehicle in neutral and you coast and coast some more.------On a boat when you go to neutral the boat slows down quickly.-----Water causes huge drag on a hull.-----Fuel economy is difficult to figure out with boats.

 

[Carver121]

Yeah, i know the whole 'if you care about economy don't get a boat' spiel. But yet there's plenty of topics here about fuel economy. All things being otherwise equal who wouldn't try to save a bit over the lifetime of an engine/boat. And the other reason is on a small fishing boat fuel space becomes an issue, so the more economical engine allows longer days running around to fishing spots without returning for a fill up. Its a 14' long, 60" beam modified v hull.

dingbat t's rated for 25. Its possible I'd want to use it on smaller lakes with 10 hp restrictions but unlikely really. It's also a big jump in price between the 20hp and the 25hp in the Hondas I'm looking at and being an older boat I figured it'd be safer not maxing it out using a new four stroke.

 

[achris]

.. It's rated for 25. Its possible I'd want to use it on smaller lakes with 10 hp restrictions but unlikely really. It's also a big jump in price between the 20hp and the 25hp in the Hondas I'm looking at and being an older boat I figured it'd be safer not maxing it out using a new four stroke.

Buy 2 engines. A 20 or 25 for your normal runs, then have 10 for lakes that limit your HP... Takes 15 minutes to swap small outboards.

When my brother and I were (much) younger we used to take our father's 14' ali boat out for the night on the local river. That had a 20hp Merc, and a 20 litre fuel can was plenty for the trip, and didn't get in the way at all. When we took the same boat fishing at a local offshore island, Rottnest, we ran with 2 of those 20 litre tanks. Again, never had a problem with space. We also removed the engine after every trip and tipped the boat over onto it's side so we could hose it out and clean it properly... We were 15 and 17 years old....

And we used to sleep on that boat, in the middle of winter!

Chris......

Circa 1977.

 

[Carver121]

achris, no way I can justify the budget for 2 motors, if it could I'd be looking at a bigger boat.

like I said the 10hp limit isn't a big deal and not a significant factor. I still have an old 4.5 and an old 9.8 if I can get a couple things fixed and keep it going. I'm motor shopping though as I'm hoping to use this time off work to fish, not tinker and wait on parts and do that again each year. Also realistically the small lakes I'd be using my 12 foot and an electric anyway.

 

[Sea Rider]

Whichever HP portable/medium range motor delivered with an installed prop you intend to buy for rcreational purposes must be tached to install a lower prop pitch for motor to run as loaded towards its max wot rpm range factory stated. Once on plane can reduce throttle to a position to maintain excellent plane with much less fuel consumption. Applies for all 9.9-10-20-25 and higher HP motors as well.

Happy Boating