Help me get more fuel efficient!

P

Progers

New member
Hi all,

This is my first season with our new to us 22 cruiser, and I’m really enjoying getting to know the boat.

I’m wondering if anyone could offer advice for better fuel efficiency with a heavy cruising load. Different prop? Weight distribution? RPM cruise speed?, etc.

So far I’ve crunched some basic numbers with different circumstances. We’re boating in SE Alaska with a 22 cruiser that has a 90hp Yamaha with a permatrim. This is what I’ve got so far.

- (4.4 mpg) 30 mile run w/ 3-4ft following seas @ 4,000 RPM 12kt average speed. 15 pitch aluminum prop. Loaded for a day trip, 1/2 fuel, 3 ppl.

-(2.1 mpg) 63.5 mile run, mixed conditions, 4,700 rpm 12 kt average speed. Very heavy load. 78 gallons fuel, gear for 3 weeks, 3 ppl, etc. 19 pitch prop. Boat plowed, did not get on plane.

-(2.42 mpg) 60 mile run, mixed conditions, 5,200 rpm 18kt planning, & 4,700 rpm 13kts plowing. 88 gallons fuel, moderate cruising gear (1 week) , 3 ppl. 19 pitch prop.

- (2.84 mpg) 54 mile run, calm conditions, 5,500rpms 20 kt speed, 2/3 fuel moderate gear. 19 pitch prop.

As I write this I’m seeing an obvious correlation with the weight of the boat and planing vs. not.

I’m also still learning and experimenting with props.
The boat came with a 15pitch aluminum. The WOT was 6,800 rpm’s . That didn’t seem right. I switched to a 19pitch aluminum and that got my WOT down to 6,000 rpm’s lightly loaded and 5,500 with a moderate cruising load.

I’m wondering if different prop will help w/ mpg?

I’m also curious if my average cruising rpm is not ideal. I seem to operate the boat most often around 4,600 rpm’s @ 13kts. This seems to be the average comfortable zone in 2 foot chop which is common here. This puts the boat halfway between on step and displacement , kind of plowing more or less.

How do you guys run your boats for fuel efficiency? While still traveling at a reasonable speed?

Is my 90hp underpowered for a super heavy load?

Will trim tabs help me plane at lower rpm’s? How much of a difference does it make with a heavy load?

Thanks in advance!
 
It's all about weight. My boat heavily loaded for Alaska with 90 gal fuel with a DF140 at 15,16 or 18 mph would do 2.0 mpg. At 6 mph I would get 5.1 mpg. I was very heavy the splash well drain was under water. With 45 gal fuel the splash well drain is out of the water by a 1/2". My boat is a 2007 and has the fiber glass interior. My guess is the boat weighs 400 to 500 lbs heavier than a pre fiberglass interior. The published weight or the boat 1925 lbs is the weight of the pre fiberglass boat and is nothing but false advertising by the factory. I wish they would weigh a boat and put the correct weight on there website.
 
Lots of issues going on. What is the gear ratio of the Yamaha 90? I see 2,15 vs 2.33 for the Honda and 2.59 for the Suzuki...Is that 2.15 correct.

Is your tach correct? I believe the rev limiter on the Yamaha 90 is at 6100 RPM. (Measure RPM on flywheel with photo tach-Amazon $20 --don't know how good that is). Fluorescent light at night, or some timing lights have RPM settings).

You want to get the boat fully loaded with the prop you are going tu use for that weight 0n a plane and get the RPM at least to 5,000 RPM ( I feel far more comfortable with at least 5500 RPM)

So you have to know the gear ratio, & correct tach to begin with to start to look at the issues.

You want to avoid lugging the engine (despite wanting to get better fuel economy.)

"Normally" one goes with the 13 x 15 on Honda (and Yamaha), the Suzuki tends to go with 17" or 19" pitch... running loaded (and at higher altitude), we often switched to a 13" pitch prop. (Honda 90 and Evinrude oil injected 2 stroke)

Then there is: not all props are created equal. SS vs Aluminum, how much cup and where, number of blades, and rake. Actually manufacturers also make a difference. For example on my Caracal Cat it is widely accepted that the SS PowerTech PF04, 14 x 19 4 bladed (stern lifting) prop is the correct prop for that boat for the 140 hp Suzuki.

Ideally you want to get the outboard into its stated RPM range (For Yamaha 90 it is 5,000 to 6,000 RPM). I tend to like to be at that upper limit.

I also like to have trim tabs along with the Permatrim. The trim tabs are adjusted to the boat--the Permatrim to the outboard., You want to have the "thrust" of the outboard close to parallel with the surface of the water, not up or down. So you need to adjust "attitude" at same RPM to get most "efficient" running attitude.

Also you want to calculate prop slip.

You are definitely running a heavy load--worst place for economy to be is semi displacement. I put the lower limit of "planing speed" to be in the 15 mph range--12 is too low, even if the boat is starting to lift at that speed.

Best MPG--you already know that. It is 5 knots. (compromise between lowest speed acceptable and economy).

I believe you with the 2' chop--and that is a bad area--because you want to be getting the best ride. This means bow down and throwing some spray.

Generally with the two 22's I owned, I had props at 11, 13 and 15" pitch. My gear ratios were in the 2.3/1 to 2.4/1 range. My "mileage" would vary depending on sea condition, altitude, and load. I tried to get the best trim, where leaving the throttle setting at the same, we got the best speed.
 
You engine manual should say what the proper WOT RPM range is. 6800 seems a little high (based on my Honda 90). My Honda 90 has a 13.5x15 Al prop.

Plowing is the worst possible condition for MPG. You either need to be at low speed (minimum wake, about 4 mph) or fully on plane. The fuel consumption vs. speed curve is a U.

My 22 when loaded for cruising gets about 3.3 mpg at my normal cruising speed of 17-19 mph (~4000 rpm). I figure on 3 mpg when planning routes. If you look at the various published boat tests it seems that most outboard powered boats get the best MPG around 4K rpm. In my experience, once the C-Dory 22 gets fully on plane, the MPG curve is pretty flat.

The speed required to be fully on plane is not a fixed number. It varies with load.
 
Some helpful has already been said.

My take to get the best fuel economy, you have to:
1) Drop the weight. Cannot have both weight and economy.
2) Drop planing speed. Run as a displacement boat; slow. Theoretical max.
displacement hull speed is 1.34 x square root of water line length in feet. See 5).
3) Drop the crud off your hull. Clean it. Dirty bottoms are slow, increase drag.
Apply a slick bottom paint.
4) Drop on a smaller [trolling] engine, maybe 10 - 15 hp; prop it right for your
lightened load, run it at its best operating rpm. Run the bigger one when you
want to get somewhere.
5) Drop the existing boat. Buy a different longer boat for more speed by
increasing your waterline length. See 2). Avoid short and fat. Go long and lean.
6) Drop bucking the seas. Go with the wind, current, waves when planning a
voyage when possible.
7) Buy an electric outboard or eboat when higher power sources are proven.

As a practicality, "the best fuel economy" is relative to the trade offs you are
willing to make in time, money and enjoyment. It differs as do people.

Aye.
 
Another thing to check is the mounting height of your Motor. It is not uncommon for the motors to have been mounted too low.

You want no more than the slightest film of water running over the top of your Permatrim when you are on plane.
 
2 MPG is terrible...just bad. I've been there with you though with a heavy boat. I strongly suspect the prop you took off was the one you want on the back of the boat when it's loaded heavy. Max RPM unloaded and with light fuel is very different than max RPM with 3-4 guys, 80 gallons of fuel, and provisions for a number of days/weeks. I've played around a ton with props in my life - especially when skiing. Like noted by others, you really want it to reach max RPM when loaded for your actual usage. Any lower and you are sacrificing power AND efficiency - probably way more than we realize. Plowing is terrible for efficiency.

Also, there is a point where 90 hp just is not enough...it will only move so much. I suspect the original designers chose that power level because it is more than enough for 2 people, a typical amount of water, fuel, and gear for most users - meaning folks that might overnight here and there and are mostly using it as a day boat. There were not wrong.....it's perfect for most folks. I really like how my boat scoots along when loaded that way. Any faster would be too fast.

With that said.....while they are not active on here, I have seen 2 boats in PWS with the 140 Suzuki and the 150 Honda hanging off the back of the CD 22. I spent time talking with them and they run heavy with fuel drums, generators, wall tents, and lots of weight for hunting and fishing and are very happy with the performance of their boats. They say they are using less fuel, cruising 20-25 mph, and getting decent economy. I don't really doubt that. Faster (to a point) is helpful because it creates more lift, which is needed with a heavy load and keeps the boat from plowing.

Do keep in mind the most the boat was ever rated for was 115 hp and more power and weight puts more stress on the structure of the boat. The limits? Beats me....I doubt it was ever even field tested. The guys with the 140 and 150 on the back are doing just that though. I often run a heavy boat as well, but I don't think I'm brave enough to put more than twin 60s on the back of mine. Sometimes.....I wish I had more boat.
 
I agree with T.R. Bauer's observations about horsepower. I had a slightly different boat (a Cape Cruiser Venture 23), but basically same type and size. I ran mostly with a very heavy load for long range cruising. My fuel efficiency actually improved just a little when I replaced my Yamaha twin 50's with twin 70's. My most efficient planning speed was 20-22 statute mph at around 4200 rpm.
 
You can gauge your boats relative efficiency by watching the wake. As soon as you are pushing water around you are losing efficiency. On my boat, below ~4 mph, there is very little wake. As the speed increases there is more water being raised. In the 5-10 mph range the wake is actually the worst. As the speed increases into the planing regime the wake starts decreasing. Once the boat is fully planing, the wake is much reduced (it is longer, but there is less water going out to the sides).
 
If you have a usual heavy load and know your boat's usual cruise speed, overing
the Mfg maximum hp suggestion, as per above, simply means the larger engine,
with more cu inches or other, is not working as hard as a smaller one would.
This can translate into better economy all else being tuned (props, etc).

Plus, most boats with age, like many people, gain weight. These reasons support
the smile on my face knowing I've never regretted going bigger on boat engines
vs smaller (to save money...?) when given a choice.

Aye.
 
TR Bauer, I did run into one of the owner of a 22 with a suzuki 140 in Whittier. He told me that he has no regret whatsoever and highly recommended going that same rout if/when I need to repower. The ramp was very busy so we unfortunately did not have any more time to chat. His boat did look quite heavy so I'm sure what works great for him might not be best for everyone.
 
When talking about the 140 Suzuki, remember that this is the 115 Suzuki block, and weight is 410 lbs. Honda 150 is 487 lbs. The 90 HP Honda is 359 lbs. These weight in the back of the boat are very significant.

Also The heavier and more HP outboards put a lot more stress on the transom. It is not engineered for that much.

Yes, the Venture 23 is rated for 150 hp, but engineering is different. Be very careful putting more HP on the boat that rated for. I know of one transom which broke when a relative small piece of floating debris was hit with the lower unit.
 
thataway":1jbgfyu0 said:
When talking about the 140 Suzuki, remember that this is the 115 Suzuki block, and weight is 410 lbs. Honda 150 is 487 lbs. The 90 HP Honda is 359 lbs. These weight in the back of the boat are very significant.

Also The heavier and more HP outboards put a lot more stress on the transom. It is not engineered for that much.

Yes, the Venture 23 is rated for 150 hp, but engineering is different. Be very careful putting more HP on the boat that rated for. I know of one transom which broke when a relative small piece of floating debris was hit with the lower unit.
Click to expand...

Good points.
Buyer beware. Do your homework. Know what you are doing. Get the facts vs
armchair engineering..

Aye.
 
thataway":1z84k0y6 said:
When talking about the 140 Suzuki, remember that this is the 115 Suzuki block, and weight is 410 lbs. Honda 150 is 487 lbs. The 90 HP Honda is 359 lbs. These weight in the back of the boat are very significant.

Also The heavier and more HP outboards put a lot more stress on the transom. It is not engineered for that much.

Yes, the Venture 23 is rated for 150 hp, but engineering is different. Be very careful putting more HP on the boat that rated for. I know of one transom which broke when a relative small piece of floating debris was hit with the lower unit.
Click to expand...

The 2022 Df140 weighs 20 lbs lighter than the 2007 DF115. When I first talked to Sportcraft Marine they didn't want to do it without me signing a waver. Then they called and talk to the factory about it. The factory said there was no issues with doing it. The factory had told me in the past that it is Weight not HP that they are concerned with.
 
In my photo album, take a look at a transom core from a 28' high performance
powerboat. It's the thickest core sandwich on the right. The lower wood section
is end grain balsa -what is found in the rest of the balsa cored hull; the upper
marine plywood - in the transom only (well, maybe in the stringers too).
This is "beefy", much more HD construction than any C-Dory transom which
carries the weight and power of the engine(s). This beefy transom does not carry
engine weight, just the weight and power of the outdrives. The inboard engines
are mounted ahead on beefy stringers integral with the hull.

Our C-Brat engineers can please opine on what forces (shear, torque, compression,
etc) act on a powerboat transom that requires heavier construction.

Something tells me, more than weight and power act on the transom.

Aye.
 
An I/O is not hard mounted to the engine. There is a gimbal bearing. Many I/O have differential joints on the shaft. The mounting bolts will transfer some of the forces from the lower unit to the transom... I have seen owners repair the stingers, and ignore the transom, only to have the transom fail because it was damaged or weak. Although the Arneson surface drives have a more direct attachment to the engine, there is minor force transferred to the transom.

If you want to know there forces and design factors for an outboard: This thread on Boat Design addresses those forces from a engineering/design prospect

Torque or twisting effect of the lower unit are important. Differential thrust on the top or bottom of the mount, both forward and reverse are also important. I recently saw a boat where the splash well was only about 1/4" thick, and there was about 3/16 inch between it and the transom--that splash well fiberglass had been crushed and the upper outboard motor mount bolts were loose.

The thickness of the transom is both a convention and design factor. For example an aluminum or steel transom 3/8 of in inch thick has pretty of strength. The convention for many years has been 1 1/2 inch thick transom for outboards of the size which most of. us use. The most common outboard transom is 2 pieces of 3/4" plywood. A few layers of fiberglass over it is the common production boat. For dinghies 3/4" is often plenty; small boats to 50 hp often only 1". As you get over 100 hp it is often to see over 2". The larger motors are. held on by bolts; the smaller ones with the clamps. I would prefer a plywood transom to a balsa core transom.

The way the force is transferred from the transom to the hull is very important. On occasion there are boats built which seem to ignore this factor. The strength of a transom attachment to the hull involves knees, stringers/grid, or the design of the splash well, as well as the tie in to the sides and bottom of the hull.
 
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