Best RPM for Gas Mileage (1995 325 Aft Cabin)

[snowtrax00]

I have twin 350's and I'm trying to find the engine speed for which I can get the best gas mileage. I don't care how long it takes me to get there because I'm never in a hurry. So which rpm is the best if you're going for the highest mileage? 1000rpm, 1500rpm, 2000rpm etc?

Here is a picture of it just for entertainment

 

[pole position]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

Hull speed around 8-9 knots would be most economical.

 

[Scott Danforth]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

its going to vary from hull to hull and motor to motor, however measuring fuel flow is the most accurate. check out boattest.com, they have done just that on many boats

on my 19' searay, it was 3200 RPM / 26mph to get the best fuel economy based on boattest. on my friends Carver 2557 it was about 2500 rpm / 18mph based on his fuel meter.

on yours, my guess is just above planing speed.

 

[Georgesalmon]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

Right. Get it up on plane and cut back till it starts to fall off. Then a little more and your there. Fuel flow is the best.

 

[snowtrax00]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

With this boat you can't really "get it up" and "cut back" because it doesn't really plane under 3500rpm and it redlines at 4200rpm. By the time it gets up on plane it's pushing quite a bit of water which is why I would think it would be more efficient near 2000rpm because like I said, getting it to plane is using quite a bit of throttle.

 

[Scott Danforth]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

I would suspect something is awry. have you had the boat weighed? with twin 350s you should be able to hit 35mph. and the 350's you have, your redline is higher than 4200 RPM. it should be about 4500-4600. are you over prop'd?

 

[snowtrax00]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

I've never run it wide open but I've read about it and people say the redline is 4200. Also, in this video the boat is pinned at 4200 as well.
http://www.youtube.com/watch?v=C2ztJoBCFFM
I don't know if you know what the boat looks like but it isn't exactly aerodynamic. Add bedding, luggage, waste, fuel and water and you have a lot of extra weight. If conditions were perfect I might get it to cruise at 32-3400rpm but I don't really want to get into that topic.
If I stay off plane, what would be the most efficient rpm/speed?

 

[Scott Danforth]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

if your off plane, your most efficient RPM is idle.

have you trimmed it out? again, the redline of the motor is higher than 4200 RPM. if the engine is pegged there, its over-propped. the redline of the same GM 350 in my boat is 4600/4800 RPM with the factory rev limiter set at 5100 RPM

My buddies Carver has a dry weight of 5500#, wet weight about 7200# and will do 36/38mph with a single 350. another buddies 32' searay with twins will hit 37mph when calm. his dry weight is about 10k, wet weight about 12k

 

[snowtrax00]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

The video I posted isn't of my boat so other people with the same boat are getting the same redline. There is no trim but I have the trim tabs positioned properly. The engines in my boat might be tuned differently than the ones you've experienced but I feel that is the correct redline. I emailed someone who has an identical boat to mine and he said his best mileage is at 7mph where he can achieve 1mpg. If he gets on plane the highest mileage he can achieve is .9mpg. While it's not a huge difference, it is substantial.

 

[Scott Danforth]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

the engines come off the same assembly line as other GM 350's. do you have the 230hp versions or the 250hp versions?

 

[snowtrax00]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

So does anyone know the most efficient speed while staying off plane?

 

[Scott Danforth]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

idle

 

[Tommyfmu]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

So does anyone know the most efficient speed while staying off plane?

Your boat weighs about 16000 lbs with normal stowage and passengers, etc. The maximum it should go at WOT is spec'd at about 28 mph at something like 4200-4400 rpms. Normal cruising should be about 18 knots at about 3200-3400 rpms and you should get about .75 miles per gallon.

Ideal fuel efficiency is indeed hull speed around 8 1/2 knots. That should give you around 1 1/2 miles per gallon. It's a heavy boat that probably could have used 454's instead, as the 350's are about 260 hp each, at least on my '96 325AC with Crusaders. My tabs are just about always fully down or I fall off plane and push water because my bow rides way too high.

By the comments above about cruising 30mph+ and efficient fuel at just above plane, it should like they are thinking the boat is an express cruiser, not a salon cabin M/Y.

 

[snowtrax00]

Re: Best RPM for Gas Mileage (1995 325 Aft Cabin)

I looked on the engine cover and the maximum rpm is between 4000rpm and 4400rpm. The engines are actually Crusader 350 XLI's so they're the "top of the line" for when they came out, pushing about 300 horsepower. The optimum speed for gas mileage is about 7mph achieving 1mpg or if I get it up on plane at 22mph I can get .8mpg.