Cruising Speed

R

Rick D

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Hello all and Merry Christmas or Happy Hanukkah... which ever you prefer.

If you don't have a flow meter installed on your boat, how do you find the correct cruising speed? My manual makes no mention of top speed or cruising speed. I've read all sorts of numbers for my boat... but want to really know how to find the max speed/fuel conservation combination.

2 5.7L 2 barrel carbs 210h each
1997 Larson Cabrio 310

--Rick
 
I go by RPMS, 2300 which is 300 off my WOT. Boat runs all day at that speed.
 
quote:
Originally posted by HOGAN

I go by RPMS, 2300 which is 300 off my WOT. Boat runs all day at that speed.

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Thanks Hogan. But how do you know you are getting optimal performance for the fuel being used? Is it a just a feeling sort of thing you develop overtime after owning for a while or do you have some sort of way of determining best performance/gas ratio?

--Rick
 
If I want "optimal performance" I would use idle speed - but it would take me forever to get where I'm going. As for cruising, I could slow down to 2000 RPMs and burn less fuel, but I would lose at least 5 knots in speed. Since I don't have flo-scan, it's really a seat-of-the-pants thing... So long as the engines don't feel/sound like they are stressing out, I'm comfortable with my cruise.
 
You keep accurate records of all trips and amount of fuel consumed, then back track the records to find the best operations speed...

Or actually do test runs, starting w/ topped off fuel tanks at the beginning and measuring the amount re-filled at the end. After repeated tests runs you can develop a speed ( or RPM ) chart against nmpg.

Be sure to keep in your records full course data, passenger carried, etc. All of this is part of the measurement.

---

I don't think that there is any argument that direct, real-time measurement is best, but you can, over time, develop a very useful and rather accurate profile by using trip data alone.
 
Do not be concerned with max anything other than Mfg recommended WOT RPM for engines. Everything else will be a function of everything else...wind, tides, bottom condition, weight on board....etc......

Your WOT should be no lower than the low end of the range and no higher than 150-200 higher than the high end. Hitting mfg recommended WOT spot on with a clean hull, full fuel, full water you and a couple of folks plus provisions for an overnight trip is a dream come true.

Everything you see in the literature from boat makers about speed of anykind are under optimum conditions with no load.
 
Use an outboard tank connected into your boats fuel line, downstream of any shut off valve. Fill with gas and run at 2000 rpm till empty. See how far it got you. When it runs out, swap to boats gas tank.
Repeat at different RPM's, going higher and higher, till you find the right RPM.
It is time consuming, but can get you in the ball park. The more tanks you have, the easier it will be.
You will have to run on boats tank till you are in open water before going to small tank. Start at same point and go in same direction each time. Be sure to repicate conditions on every run.
 
quote:
Originally posted by HOGAN

If I want "optimal performance" I would use idle speed - but it would take me forever to get where I'm going. As for cruising, I could slow down to 2000 RPMs and burn less fuel, but I would lose at least 5 knots in speed. Since I don't have flo-scan, it's really a seat-of-the-pants thing... So long as the engines don't feel/sound like they are stressing out, I'm comfortable with my cruise.

Click to expand...


I’m with Mike on this one, "optimal performance" varies with moods & needs.
If it’s the highest cruise speed with the lowest fuel use, than on our boat that would be at 18knots with a 30+GPH burn.
As flow meters are standard with my engines I can squeeze out the ‘best’ MPG but I choose to let my other ‘needs’ be the drivers of my choice of speed.

So, like Mike I don’t like running at "optimal performance" and run ‘normal’ cruise at 22-26knots with a 40- 48GPH burn depending of wind, wind direction, wave height yada, yada.

There have been times I’ve cruised at 27/28 knots and to He!! with fuel burn, I just wanted to get home to have that first drink at the dock!
 
quote:
Originally posted by Rick D

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Your engine manufacturer may have provided published nominal fuel flow curves... to at least give you a starting point.

-Chris
 
Keep it simple. I preferred to get up on plane, adjust the trim tabs and then bring down the RPM to a point where the boat still stayed completely on plane. I pretty much ignored the speed gauge which was notoriously wrong. I never ran full throttle except for getting on plane and diagnostic maneuvers. If my cruising RPM was less than typical, the conditions were in my favor. I found that over time, I burned 6 gal/hour and used that for planning purposes. And do NOT trust your fuel meter!!! They lie 99% of the time......

If you really want to get precise, install a Flo-meter.
 
All the above are good ways to estimate fuel consumption. Seems like even when making long trips, simple fuel math gets muckered up with some kind of issue. Don't ask me why.

Whenever you can pull the throttles back, and still maintain plane (no 4 barrel or secondaries open yet) you are getting fuel economy as good as it gets for a 4 barrel non fuel injected gas engine. Sometimes you can do this, sometimes you can't, depends on current and load . For sure with a 4 barrel engine, once the secondaries begin to open, fuel economy goes down the tubes.
 
It's exactly as Dave said. Another way is to be able to measure the lightest load for throttle opening.

This is DIRECTLY measurable by the absolute (intake) manifold vacuum. Set your throttle to the highest vacuum for the speed you're looking for.

There are also a number of factors that affect load...which directly transfers to wetted surface (drag) of your hull.
 
There lots of factors full tank of fuel or 1/2 tank? Fully provisioned for one crew might be 2 days for another. Are the water tanks topped off? For those with watermakers maybe only 1/2 full no watermaker probably full depending on tank sizes and days you expect to be on the hook. Also it's a real good idea to keep track of all the changes made to the boat and add or subtract wieght. You should know the boats wieght when you bought it if the travel lift had a scale on haul out inspection. Keeping track of all this stuff is real importnnt maybe more important if you have diesels. Still important in a gas boat but maybe not as bad? Then it comes down to the number of people at any given time and balancing the load depending on your boats beam and size.
Did you add a hardtop or do you run with a Bimini? Are the upper helm windows or enclosure open or closed, up or down? Is the waste tank full or not?
Even gassers are they modern California Emissions approved engines using an ECM and fuel injection or throttle body fuel injection perhaps even carb'ed? I think it's a matter of knowing your engines, overall gear ratio, hull style, total wieght and weather & tide conditions.
Also salt or freshwater use has an impact. Too many variables for hard numbers. Having the ability to watch the speed, RPM's and fuel flow certainly help optimize fuel burned while staying in the safe zone for your engines. I think the key word is operating within "safe" limits for the power plant you have. Mine is most efficiant running around hull speed but that doesn't get you anywhere fast. If I want to get a move on it I can burn 60+ gph.
Bill
 
Since you are talking about most economical cruising speed, I would suggest just attaining full planing attitude then backing off to the lowest 100 RPM increment the boat will retain plane with at least average onboard load of POB, liquids and gear.

Then, fine tune the trim tabs to maximum GPS SOG at that RPM. Note that RPM & speed on a sheet of paper (or computer device).

Throttle up 100 RPM , fine tune tabs and record that engine & GPS speed.

Repeat for each 100 RPM increment until at least 80+% of WOT RPM is recorded.

Then find the difference ("delta") in speed attained between each successive 100RPM increment.

You should be able to find the 100RPM range where incremental speed delta has maxed just before it drops lower again. That is the most economical cruise speed for those conditions of load and sea. if you keep a table and/or graph of those observations, it could help you confirm loss ( or improvement) of efficiency later due to ignition, prop, hull fouling , etc., issues.

Besides the accurate fuel used totalizer function , the beauty of a fuel flow management system interfaced to GPS speed input is dynamic NMPG data display available full time for all speeds, load, trim tab & sea conditions, even favorable or adverse current.

I'd also note Hogan's observation about running 300 RPM off WOT throttle indicates his engine(s) is diesel, so that definitely would not apply to your gassers.

Several gas engine mfgrs. like Crusader recommend about 75% of WOT for planing cruising speed , but most planing boat operators find actual observed most economical RPM to be lower or substantially lower than that
 
Rick,
What drives are you running? Charlie has it right when he describes manifold vacuum(old school). The seat of the pants is dependent on how well you know your boat. I always ran my boat at 3300-3500rpm depending on the variables(wind,load and temp). I asked the same question your asking here and listened to what the guys were saying and started running the boat 3600-3800rpms. The boat gets better fuel economy but doesn't ride as nice so I run it where it feels right no matter what the fuel economy is.
The reason I asked what drives your running is if your running Alpha drives your a perfect canidate to switch to four bladed props. If your running BravoII's a different pitch might do the trick or BravoIII now offers a four bladed prop in the front. I'm also taking for granted that your presently running props that allow you to reach recommended WOT.
Rob
 
"Several gas engine mfgrs. like Crusader recommend about 75% of WOT for planing cruising speed"
The reason for this recommendation is Crusader feels for longevity of the motor this applies. With my boat this is spot on for best speed (boat feels good) and fuel burn.
Bottom line is there is no way for you to know what speed will be the most economical without fuel flow monitors. Reason being is every time you use your boat the variables are different as Dave mentioned above (tide, wind, weight, currents, etc)
 
you should be able to do it by feel man. you will know when you hit it. just feel the vessel just feel the vessels nature
 
bobalong and johnnyreno +1, the thoughts they put out are good..and the rest you'll learn after you get more feel of pants info on your boat.
 
My work boat had a GM 327/QJet.
I brought it up to Cruise speed slowly and when the Four Bbl's opened up, I backed off a bit till they closed again.
That was my Cruise Rpm and the Best fuel mileage/cruise speed. The Rpm's were a bit different all the time becuse the load and water conditions were different.
The only way you'd ever know anything about your fuel flow without a meter is to run for awhile and fill up!
I had three tanks for 70 Gal's and it wasn't easy to make sure I filled to the same spot every time.

I never actually did this:
To make a Fuel Flow gauge, all you need is a Paddle wheel (like the first electronic Boat speedometer) in the fuel line and an Inductor sitting outside, reacting the the blades on the paddle wheel.
You don't have to calibrate it just watch the Volt-meter and Tach at the same time.

I saw one that featured a little steel ball being driven around in a closed loop by the passing fuel.
It's spurt for a second and sit Idle for several seconds till the Needle valve in the carb would accept more fuel.
It must have worked. Some people bought it, but I think my idea is better.
 
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