Cruising speed/high rpm

[nightvision]

Hi All,<br />What is the typical cruising speed for a 17.5 footer (sea ray 180) with a Mercruiser/Alpha setup 135 hp (19 pitch prop). I read that about 2500 - 3500. I assume this means at this speed one will get good economy. In addition, what would be the high RPM zone that one should avoid. Thanks.

 

[cuzner]

Re: Cruising speed/high rpm

All boats are different. I had a 1991 capri, with a 3l engine. The sweet spot was 3200 rpm, about 28mph. Check out boattest, they may have your boat rated, they list best cruising speeds for different engines and boats. I'm guessing yours should be about the same properly trimmed, the capri I had was simular in size and weight, but I had a 17p prop, your rpm may be lower with a 19p prop.<br /><br /> Jim

 

[mattttt25]

Re: Cruising speed/high rpm

good rule of thumb is 3500 for most 2 stroke outboards and inboards. 4 stroke outboards seem to have a higher optimal point, such as 4500. diesels are a totally different beast. that's just based on the many test reports i read in mags and on the internet.

 

[Richard Petersen]

Re: Cruising speed/high rpm

It can vary wildly with people, gas load, extra gear and most important , how soon you want to be there. Over powered boats ALWAYS get BETTER milage because they run at less than 3500 rpm. 5.0 or a 5.7 are the perfect engines for under 19' boats.

 

[mattttt25]

Re: Cruising speed/high rpm

over powered boats always get better gas mileage? can you please explain that statement?

 

[dingbat]

Re: Cruising speed/high rpm

Every motor has a power curve. To obtain the best possible efficiently from a motor you want to stay as close to the top of the torque that curve as possible. Running the whiz out of a 100 HP motor to maintain the 98 HP required to push your boat is a lot less efficient thus uses a lot more fuel than say a 200HP motor putting out 100 HP down in it's power curve. Give me a high HP setup over an "adequate" one anyway of the week.

 

[inthesticksnow]

Re: Cruising speed/high rpm

Originally posted by nightvision:<br /> Hi All,<br />What is the typical cruising speed for a 17.5 footer (sea ray 180) with a Mercruiser/Alpha setup 135 hp (19 pitch prop). I read that about 2500 - 3500. I assume this means at this speed one will get good economy. In addition, what would be the high RPM zone that one should avoid. Thanks.

Great question! Looking forward to other’s responses.<br /><br />Well, here’s what I do on an 18’ Bayliner, new Merc 3.0, Alpha… on a river (no chop). <br /><br /> “Cruising” to me is moving up or down stream “comfortably” on plane. That is 3000 rpms for my boat with a 21p prop, even 2800, with two people. “Loaded” it might be a few hundred more.<br /><br />Because I am breaking in the motor, I have been experimenting with this a lot. <br /><br />At least, this is what “cruising” means to me… up on plane (least drag, lowest (plus a pinch more)) rpms to stay there, best economy with the fuel and optimum pull on the motor. Properly trimmed, of course (easy to figure out if you watch your tach and listen and watch the speedo). In my case I have learned that I can trim a “pinch” up, the rpms will rise slightly, but NO MPH difference. I trim back down a “pinch” and feel a “pull”. For me this is optimum.<br /><br />Others?

 

[mattttt25]

Re: Cruising speed/high rpm

dingbat- good response, but that's not always the case. sure, in your extreme example (100hp vs 200hp engines) it makes sense. but in a more real life setting, people get railroaded into believing they should buy the bigger engine, throttle back, and save all this gas money. it's not true.<br /><br />if you take a 260hp sterndrive engine and compare it to a 320hp equivilant... get to the point where both engines are putting out roughly 220 hp and pushing the boat along at the same speed. they will burn close to the same amount of fuel.<br /><br />power in, power out.<br /><br />boating mag ran a great article that did this very test. same boat, 3 different size engines. monitored every point possible, even with varying loads. the results proved bigger is not always better. their test compared the 220, 260, and 320. turned out the 260 was the most economical engine and that the 320 didn't even produce better performance until the very high rpms.

 

[trog100]

Re: Cruising speed/high rpm

this one is about volumetric efficiency.. the big engine running lower revs at half throttle isnt running efficiently compared to the small engine running the same rpm and generating the same power at full throttle..<br /><br />basically the bigger engine running at a light throttle isnt filling its cylinders and is running a very poor compression ratio (not making best use of the fuel mixture) and because of this it isnt efficient.. <br /><br />in real life this is offset by the need for the smaller engine to run at higher rpm to generate the same amount of top end power.. higher rpm more friction and mechanical waste so things even out more than they should..<br /><br />one thing kinda works against another.. but the theory that big engines running at a lighter throttle are more fuel efficient isnt really true.. <br /><br />to be efficient an engine needs to be filling its cylinders as best it can.. in other words the max torque and max efficiency point is only true at full throttle.. at light throttle nearly empty cylinder situations it becomes meaningless..<br /><br />the most economical engine would be one geared to run at its max torque rpms and do this at full throttle.. course it would have next to no acceleration and a very poor top end brake horse power rating for its size.. but it would be efficient..<br /><br />okay so high revs waste power/fuel thru mechanical friction and such (smaller engine) but low throttle openings waste power/fuel thru running the engine at poor volumetric efficiency (bigger engine).. but from a fuel economy point of view i recon the smaller engine will/should mostly win out.. mostly.. being the key word here cos i know sometimes in real life the bigger engine sometimes proves the opposite..<br /><br />trog100

 

[Mahoney]

Re: Cruising speed/high rpm

you can usually find your torque curve and cruising rpm while running. I find there is a point where you can increase RPM's a couple hundered at a time with slow increases in speed around 18-23MPH or so. I have a heavy 19 FT Cobalt running a 225 5.0. For me there comes a time where I increase to about 3200-3400 RPM where the MPH increase a lot more than the other increases with RPM. From about 24MPH to close to 30. I feel this is my sweet spot.

 

[inthesticksnow]

Re: Cruising speed/high rpm

How do you guys feel about a slow, 1500 – 1600 rpm putt-putt every now and then, like for sight-seeing along a river?

 

[dingbat]

Re: Cruising speed/high rpm

matt,<br />Now that I see where you’re coming from we’re arguing the same point from a different direction. My complaint is with dealers selling these price point boats that are way under powered and telling the customer that they are saving fuel in doing so. People need to realize is that the motor is only part of the equation in the efficiency of the vessel. Each hull is designed to operate most efficiently with X number of HP and the closer you are able to get to this figure the better your efficiency is as a whole. <br /><br />I agree, buying a motor that is larger than the design rating of the hull is a waste hydrocarbons Then again I see a lot of horsepower in use to make up for other short comings

 

[trog100]

Re: Cruising speed/high rpm

slow put-puts are quite fuel efficient from a mile per gallon point of view as long as u dont exceed the displacement speed of the boat.. 6 mph is about the max displacement speed for a 20ft planing boat.. try and do 7 or 8 mile per hours and u use twice the fuel as at 6 mph.. its quite critical..<br /><br />my 20ft boat returns about the same miles per gallon at 6 mph as it does at 26 mph.. from a fuel efficentcy point of view the speeds to avoid are those between the boats displacement speed and its planing speed.. 10 to 14 mph would be about the worse for me.. miles per gallon would drop by nearly 75% at those speeds..<br /><br />trog100

 

[Solittle]

Re: Cruising speed/high rpm

While it is good to have a basic understanding of all the mechanics and theory - when you get out on the water and up on a plane it is good to just feel where she wants to run best that day, with that load and under that condition of wind and water. You can go faster or slower but there is a point where she almost tells you "This is where I will run best for you today."

 

[inthesticksnow]

Re: Cruising speed/high rpm

Trog,<br /><br />>>> boats displacement speed and its planing speed..<br /><br />What exactly is the difference between the two, please.<br /><br />>>> 10 to 14 mph would be about the worse for me.. miles per gallon would drop by nearly 75% at those speeds..<br /><br />The above quote is serious, and I'd like to understand more.<br /><br />>>> my 20ft boat returns about the same miles per gallon at 6 mph as it does at 26 mph..<br /><br />Again, very serious. Please explain in laypersons terms (if possible).<br /><br />Thank you!

 

[trog100]

Re: Cruising speed/high rpm

all boats have an ideal displacement speed.. its the speed the boats slips thru the water best at with minimum water disturbance.. trying to exceed this speed simply starts moving and disturbing water as opposed to moving the boat.. <br /><br />planing boats are not very good displacement boats.. the hull design is all wrong.. whats good for planing isnt good for displacement or slipping thru the water nicely..<br /><br />basically long thin boats that are rounded or tapered off at both ends make good displacement boats.. they slip thru the water easy and can go faster without too much water disturbance and fuel usage.. but all displacemnet boats have the speed at which it becomes uneconomical to try and go beyond.. this is refered to as the boats displacement speed..<br /><br />at 5 mph my little boat slips thru the water nicley with very little wake behind it.. at 6 mph the wake starts to get bigger.. at 7 or 8 mph the wake starts to get really big i would guess it takes twice the power and fuel just to get that extra 2 mile per hour and at about 12 mph its huge it takes power to create huge wakes..again it probably takes 4 x as much power and gas to move the boat at 12 mpg than it does at 6 mph.. <br /><br />once the boats starts to come up on plane the boats skims on the top of the water as opposed to having to push it all to one side.. different things starts to affect fuel use.. the higher the speed the more wind resistance comes into play just like it does with a car.. <br /><br />there are whole articles about about how to find a boats ideal displacment speed but an easy way of judging things is simply to look behind u..<br /><br />tommrow i am about to set off on a 100 mile trip to the sea.. 100 miles of river and small canals.. no place along the way can i reach the boats planing speed without getting arrested.. sooo i recon it will take about about 25 hours cruising at 5 mph includeing locks at 4 or 5 miles per gallon cost UK gas about one hundred pounds.. or i could do it a bit quicker at double the fuel cost.. he he..<br /><br />the round trip includeing playing about at sea will total about 3 or 4 hundred miles.. the gas cost.. hmmm.. depends on my will power.. can i cruise at 5 mph on the 100 mile river/canal part of the trip.. dunno.. he he he<br /><br />basically all speeds between the boats efficient displacement speed of about 5 or 6 mph and its minimum planing speed of about 20 mph are from a fuel economy and legal point of view a no-go area for me.. i would like to do the trip at about 10 mph but it would push the fuel bill up too high..<br /><br />at a rough guess i would say it takes three times the power to get a boat up to planing speed than it does to keep it there.. so if fuel usage matters.. u only have the two speeds.. very slow (no wake) or planing anything in between is just wasting gas and making waves..<br /><br />trog100<br /><br />ps.. planing speed is just the minimim speed the boat needs to be going at for the hull to generate enough lift (like an airplane wing) to keep the boat up on top of the water as opposed to sinking down into it.. go fast enough and the boat really will fly.. literally.. he he

 

[waterone1@aol.com]

Re: Cruising speed/high rpm

It has been my experience with any boat I have owned(16ft runabout - 36 ft cabin cruiser) that the best miles per gallon occurs just above idle(1000 rpm's). Of course, no-one would want to go that speed all day. The next best miles per hour usually occurs just above planning speed, sure it is drinking more gas, but you are moving at 20-25 miles per hour. If you really want to drink gas, go above 3500 rpm's.

 

[inthesticksnow]

Re: Cruising speed/high rpm

Trog,<br /><br />What a great explanation! Thank you for taking the time to break it down for me. As I sit here, I can imagine the perfect fuel-guzzling scenario… bow half up, boat pushing water and sucking behind like a vacuum!<br /><br />Either a slow putt putt, or at least get up on plane and keep ‘er there.<br /><br />Thanks again!