Navigate at Low RPM

[HPLou]

Good day, I have a 29 feet Cruiser Boat with 2 Merc 4.3LX & Alpha Ones. My Boat weight is 9000 lbs.
Most of the time we cruiser at low RPM (1400 to 1800) and Enjoy. At that RPM the Bow is UP a bit but confortable to navigate.
I realized that at that same RPM if I put Full Trim Down the boat hull is Flat on the water and the Bow is Full Down.
I can control the boat easyer.
My question is to ask if I should always navigate at low RPM with Full Trim Down ?
And will it affect Fuel Economy because of more drag ?
Thank You

 

[roffey]

It was my feeling you need to trim down when not on plane. I suspect there is less drag when trimmed down, When trimmed up you are pushing water and not cutting or riding on top of it, feel free to correct me.

 

[Stumpalump]

Set the throttle first then adjust trim for max RPM. That will be your most efficient trim angle.

 

[H20Rat]

Set the throttle first then adjust trim for max RPM. That will be your most efficient trim angle.

I keep ending up with the prop out of the water!

Seriously though, that works well for some boats, but not for others. A bad trim angle can cause your prop slip percentage to go up, and thereby also increase RPM's. The BEST way is to set throttle and then look at the GPS to adjust trim. (need something that shows tenths of mph)

 

[roffey]

Set the throttle first then adjust trim for max RPM. That will be your most efficient trim angle.

In my boat if I were to trim up before the throttle I just put the bow in the air and can not go on plane and will normally generate a lot of chatter from the passengers on board. What I do is just play with the trim settings until things 'feel right", lol. It's all experience and trial by failure.

 

[agallant80]

At that speed your trim should be all the way down. There is this twilight area in terms of fuel efficiency between displacement speed and on plain. When you are in that area you are pushing through the water really hard and using as much fuel or more than if you were on plain going really fast.

For example using boattest.com numbers for my boat
2,000 RPM is 1.8 Nautical Mile Per Gallon at 7.4 Knots
4,000 RPM is 2.2 Nautical Mile Per Gallon at 26.7 Knots

Going slow with the trim up pushed the rear of the boat down which is the widest flattest part of the hull. If your trim were down you would be pushing the bow down which is a nice V and cuts through the water rather well.

Run through these videos for more information on trimming.

http://www.boattest.com/Resources/view_news.aspx?NewsID=4054

 

[smokeonthewater]

personally I think you need to speed up or slow down... that is the worst possible speed you can operate... You are getting the worst mpg you can and making the biggest possible wake... keep it under 6-7 mph or get on plane and go....

Yes trim all the way down at low speed

 

[midcarolina]

personally I think you need to speed up or slow down... that is the worst possible speed you can operate... You are getting the worst mpg you can and making the biggest possible wake... keep it under 6-7 mph or get on plane and go....

Yes trim all the way down at low speed

Bingo!!!! you are plowing a large boat through the water, and in doing so taking the real risk of swamping a small craft and having a REALLY mad boater who may want to cause you real bodily harm...........

 

[bekosh]

When I'm going slow like that I keep the drives at neutral trim, 0 to -2 on my trim gauges and the motors at about 1000-1200 rpms. That gets me about 7.5-8 mph which is real close to the natural hull speed for her.

 

[tpenfield]

I add my vote to the pile of those saying you are running at the least efficient speed range. big wake and lots of fuel expended. Maybe re-think your speed choices.

 

[HPLou]

Thank You All for your comments.
I also thought my RPM was to high for Low Speed Cruising.
I'd like to comment on waves witch several brought up. I can confirm that at 1800 RPM with Full Trim Down I
make less Waves then with no Trim. I create Turbulance that does not seem to bother other boats around.
Maybe I am wrong !!

 

[BigDfromTN]

Thank You All for your comments.
I also thought my RPM was to high for Low Speed Cruising.
I'd like to comment on waves witch several brought up. I can confirm that at 1800 RPM with Full Trim Down I
make less Waves then with no Trim. I create Turbulance that does not seem to bother other boats around.
Maybe I am wrong !!

Just because they tolerate it, does not mean it don't bother them.

Add me to the clan of speed up or slow down for best efficiency, ride, and courtesy to others.

 

[gtochris]

You sound like your operating in the trailing mode as no one should be able to be bow up at such low RPM... Trailer level type mode is hell on the gimble and universal joints. Unless you are in REALLY shallow water- Trim ALL the way down until you get on plane and then trim accordingly.

 

[HPLou]

I think I was not clear. Sorry
I am not taking about my Stearn Drives Trimmed Up,
I met (Trim Tabs) Full Down at Low RPM & Speed to keep the Bow Down.

 

[Scott Danforth]

in my boat, I can plow along at 2000 RPM making a huge wake and burning fuel at 8.2mph per the GPS, or I can back it off to 1100 RPM, barely make a wake, and burn very little fuel at 7.8mph. if your bow is pointed skyward, your plowing. Was recently on a friends boat and he was going along plowing like crazy with both his diesels laboring and black smoking. I asked him if he liked wasting fuel. he said no. then I showed him his exhaust, wake, GPS speed and RPM's. had him back off RPM, tab down and he was moving nearly same speed.


if you like to go under planing speed, then back off your RPM, adjust your trim and tabs and make it enjoyable for you, your wallet, your passengers, and your fellow boaters.

 

[HPLou]

Thanks Scott for the advise.
This is what I will experment on my next Boat Ride this weekend.

 

[rbh]

Our boat is about the same size as yours, we have trim tabs but very rarely do we use them.
For us take the boat up to plain with the drives all the way down, back off on the RPMs till we almost fall off, trim out till we are perfectly plained out, then back off the RPMs till you almost fall off plain.

I am thinking that's optimized.