4 stroke outboard fuel consumption???

[Jammer]

Originally Posted by muskeg

I have this idea displacement boats wont mind an additional 12 gallon jerry for safety. Do you think this is overkill.
My mother and father in law, the sailors responsible for our safety, are considering bringing a backup 20hp four stroke but it is a short shaft. What would you guys do? I want to trust the nearly new Merc and use sails as backup.

That's a different question -- we've gone from a narrow question on fuel consumption to general voyage planning advice.


I would not recommend trying to swap outboards at sea. It's hard to do in a harbor without outside assistance unless you have davits or a stern arch to use for lifting; even then it's awkward. So if you bring a spare it's because you think you might have a marina install it after you get back to shore. Generally, also, short shaft motors rarely work well on sailboats; whether they work at all depends on how the bracket is set up. The short shaft gives little possibility for extra depth to compensate for heel or for heavy seas, and places the motor where it is difficult to reach and more likely to take on water.



Checking the route from Vancouver to Prince Rupert, Navionics calculates 795 nm which is over twice the distance you estimated, generally following an inside route.


https://webapp.navionics.com/#boatin...y=ehzyHhg%7D_W


Someone who knows the area can comment on whether it is best to take an inside route, or go outside on the ocean, and on how much of the route can realistically be sailed rather than motored. With an outboard motor, you will not be able to tolerate as heavy seas as you could with an inboard, so that should be a factor in your planning.



I wouldn't recommend relying on sails for emergency propulsion unless you are comfortable using them and are certain they are in seaworthy condition.


I have a 26' Hunter, and use an 8 hp extra-long-shaft Yamaha high thrust outboard on it. I've had the motor for three years having previously used it on another boat of similar size -- the Hunter is new to me this year. I've motored quite a lot up and down the Mississippi river.


In addition to fuel, I think the key points to consider for your journey are:


1) The hazard posed by a following sea breaking over the motor, causing it to fail


2) The fact that outboard motors of this size typically do not have any useful electrical generating capacity, leading to questions on how you will power your lights and any other equipment you plan to use


3) The fuel logistics problems you have noted.


Be safe

[waterman46]

It's very difficult to predict fuel consumption based on just the info given. My advice is to do a test run, it won't take long, then you'll know the answer much more accurately than anyone can calculate or guess.

[wingssail]

Originally Posted by muskeg

Needless to say You guys are awesome.
That is exactly the type of math i was hoping for. The story is oil change gone wrong but i wont pry until I meet the prev owner in person. And the shop has agreed to let me take away what remains of the Universal m-18. I hope the gear box, prop and shaft are still there.
So Jammer's 50 to 75 gallons with 19 gallon tank on board and my 12 gallon jerry is 1.6 to 2.4 fills.
Leaving loaded that means 1 or 2 fills on route, fuel stations permitting, should do it.
I have this idea displacement boats wont mind an additional 12 gallon jerry for safety. Do you think this is overkill.
My mother and father in law, the sailors responsible for our safety, are considering bringing a backup 20hp four stroke but it is a short shaft. What would you guys do? I want to trust the nearly new Merc and use sails as backup.
Does anybody else want to come, the boats not quite full yet??

That is a great trip, but since you are not planning on sailing much I'd not be interested. The Gulf 29 is a nice boat, it will sail well. Your route, however, this time of year, will include a lot of northerly winds. This will be against you. Some places, like along Texada Island and Johnstone Straits will be wind on the nose and choppy, if transited during the day. Then there is the crossing of Queen Charlotte Sound, a bit of open ocean. Going inside will get you out of the open but it will be a longer distance. Currents will be an issue but will cancel out, they may be against you on one tide but will be with you on the next.

I am skeptical of the 5-6 knot planned speed. I believe that 4.5 knots will be more likely. Against chop and a headwind it may be 1/2 that. The distance is probably close to 500 nautical miles and if you average 4 knots that is 125 hours. A conservative estimate of 1 gal/hour gives you 125 gallons. Treat your motor gently and it should last.

The 20hp short shaft motor will be of doubtful value. With the longer overhangs of the Gulf I'd expect that any wave action will take that propeller completely out of the water. I would test this before leaving port. If you do decide to take it and want to change motors at some point I would recommend using the main halyard to support the each of the motors during the lifting stage. It will still be very awkward but less likely to injure someone.

I know it sounds unsafe and some here will loudly object but the technique used by locals all over the world is to run the engine directly off of a 40 or 60 liter plastic jug. They simply insert an extended fuel line into the jug through a hole in the cap and no transferring of fuel is needed. I really don't like transferring fuel at sea.

Practice sailing, including reefing. You may need it.

BTW, your motor has an alternator, 12 amp, you can recharge your batteries.

[thomm225]

Originally Posted by barnakiel

Hey Thomm,


You will find small adjustments to speed will make big adjustments to fuel consumption.


Going 10% slower, saves more than 10% of the fuel. It is a non-linear relationship.



Playing at 5 vs 5.5 vs 6 knot will show big fuel savings, biggest (per mile sailed) as you slow down.



How happy are you with 5 hp? Money no object, would you use a more powerful unit, if so, what hp would you elect?


And do you have a long, or an extra long leg?


Thank you in advance! Our boat also displaces about 6-7t and given the age of our inboard I often mentally rehearse moving to an outboard.


Cheers,
barnakiel

Hi B,

For the boat I have now and the type sailing I'm doing, I'd stay with my 58 lb 5 Hp 4 stroke Mercury outboard. (btw the 10 hp Bukh diesel I pulled out of this boat weighed 352 lbs)

When I start cruising long term, I might change my mind or add another tank. Right now, I have the 3 gallon external tank with hose and connections and a 2.5 liter plastic jug to add to the integral tank.

My outboard has a switch for integral or external and it usually stays on the integral tank because i rarely use the external unless i'm motoring those 18 miles back home on a Sunday am after a few days short cruise North

My 5 hp can push my boat at about 5 knots in still water.

A good example would be this.

I used to race against a friend of mine. He on his Alberg 30 with Atomic 4 engine and me on my Bristol 27 with my 5 hp.

After the racing, we would motor in almost side by side for a couple miles. I believe the A4 is near 30 hp.

Also, my first test sail for long term cruising will be ICW to Morehead City, NC then back up to Cape Lookout for a few test sails out to the Gulf Stream then back North to the Chesapeake Bay on the outside.

I'm thinking that trip will tell me lots about my boat's setup.

Btw, James Baldwin who has circumnavigated twice has a 6 hp Tohatsu in the well of his 8,000# Pearson Triton 28

One picture of the Triton and two of Bristol 27 Outboards and placement

In the videos, it had been near flat calm for 15 miles then this cloudline passed over bringing in some breeze so I rolled out the jib but left the engine running since I was only about 5 miles from my marina

https://www.youtube.com/watch?v=yEw7pYeO0Oc

https://www.youtube.com/watch?v=Jz9x5oRthfk

[thomm225]

ps. My other 6-7 outboards have been mostly 1960's/70's 2 stroke (Johnson/Evinrude/Mercury) with 1 other 4 stroke a 1960's Bearcat 55 hp and it was great on gas!

One of my Johnson 40's was so old it said on the pop down lid to add 1 quart of SAE 30 to 6 gallons on gas!

https://www.youtube.com/watch?v=ccIDT7B3jHY

[Captain Puget]

I suggest you spend a day and test fuel consumption on the actual boat and conditions you plan to run, Lots of good guesses provided, but real data with the exact variables seems the best approach.

[muskeg]

Volvo Penta md2b 25 hp
What do you guys think about jamming that in the Gulf 29 Pilothouse? Other than the fact that it appears to be raw water cooled and I would have to put the head back on (broke a push rod/all parts purchased ready for reassembly) , and volvo parts seem expensive, the price is low and i love a project. It is out of the host boat and ready for pickup.

[thomm225]

This chart is showing about 1.5 gallons/hr for the 15 hp 4 stroke at max throttle of about 5500 rpm.

For the 5 hp 4 stroke, it's .5 gallons/hr at max throttle but I run mine at about 2500-3000 rpm which is why I can get by with .4 gallons per hour or a little less

So if you run the 15 hp 4 stroke at less rpm that will help.

Also, the reason I don't max it out is because the boat speed does not increase much at all after a certain point.


https://tohatsuoutboards.net/resourc...oard-fuel-faq/


Tohatsu Outboard Fuel Consumption
Home Resources Fuel Consumption
We've put together the chart below to help you determine what type of fuel consumption you can expect from your Tohatsu outboard. Please remember though, all figures are approximate.

There are a great deal of factors that affect the amount of fuel consumption and therefore the figures listed below will not necessarily be what you will obtain. Factors such as weight, hull shape, hull condition, water conditions, etc. may drastically affect the consumption figures shown below.


The following chart shows the approximate fuel consumption of each model:

Approximate Fuel Consumption Chart

Model/HP RPM Gallons/Hour Liters/Hour


5 (4-str) 5,500 0.5 1.7

9.8 (2-str) 5,500 1.3 4.9

15B (4-str) 5,500 1.5 5.5
15C (4-str) 6,000 1.3 5.0

[mainer]

If you can motor-sail you will do much better on MPG.

[MicHughV]

outboard motors suck a prodigious amount of fuel....I have owned outboard engines from 2 hp all the way to twin 150's....the 150's are burning 7 gallons each per hour at "cruise" of around 4,300 rpm...go up to 5,300 rpm...and fuel consumption doubles to 15 gallons per hour per engine for an extra 5 mph....at WOT....around 6,000 rpm, fuel burn is around 45 gallons per hour for both engines.....I know as my fishing boat has a fuel burn gauge....insane !!!

probably the biggest reason is that most outboards generally operate at high rpm....4,000 rpm plus...
think of you car engine turning that speed....must cars "cruise" at 2,300-2,500 rpm....an out board engine is " cruising" at 4,000-5,000 rpm...
if your car was turning 4,000 rpm plus on the highway, you fuel consumption would be dismal...
your exotics....Ferrarri's etc, have miserable fuel consumption...sure...big engines...many cylinders...but they operate in the 6,000 rpm plus range...

small outboards typically operate at high rpm as that is where they develop any meaningful power...

it's a complicated science though.....engine size...rpm...prop size....boat size...etc....

interestingly, I've noted on my fishing boat, that fuel consumption is not better going slow...as the boat is not on plane then...and the engines are working hard trying to push the boat " thru'" the water...

a small sailboat with an outboard usually has a " happy" place with boat speed and engine rpm both being at " acceptable" levels....takes a bit of juggling to find it....

[Compass790]

Originally Posted by muskeg

Volvo Penta md2b 25 hp
What do you guys think about jamming that in the Gulf 29 Pilothouse? Other than the fact that it appears to be raw water cooled and I would have to put the head back on (broke a push rod/all parts purchased ready for reassembly) , and volvo parts seem expensive, the price is low and i love a project. It is out of the host boat and ready for pickup.

I run scared of volvos as the parts prices are horrendous but you might get more info about that engine if you start another thread or PM DougR as he knows volvos.
We have a 40 year old raw water cooled engine & I dont think raw water cooling is as bad as its made out to be if you keep up with the anode checks.
Block will easily do another 40 years AFAIK.
Would love to crew on that trip if you had a diesel inboard, I wasn't downunder & you weren't in a hurry.
When I said probably less than a gallon an hour I was thinking of pushing her at 5 knts, should have explained that. As others said it makes quite a bit of difference if you want to push it at hull speed. Doubt you want or need to run at WOT as it would get noisy.

[MicHughV]

I'm very familiar with the raw water (seawater) cooled Volvo engines....reliable...sure...but a pain in the rear end. You need to do a "mandatory" head removal each year to clean out the salt encrusted water passages....
the other issue is the exhaust manifold...it too has raw water cooling passages which will rust thru'.....keep that in mind....

[n5ama]

Originally Posted by thomm225

That's very surprising that a 9.8 hp outboard if it's a 4 stroke would burn that much gas.

I rarely have more than 1/2 tank (3 gallon tank) of gas and regularly motor home 4 hours or so back across the lower Chesapeake Bay. I've been doing this the past 8 years .......

My engine is a 5 hp 4 stroke but it's pushing a 6600 lb Disp Bristol 27. The Catalina 25 only displaces 4550.

I rarely go above 1/2 - 2/3's though on the throttle

It is a 4 stroke. I'm not sure what the "official Tohatsu" gas consumption is for this motor but it's in good shape with very reliable low speed operation.

I had a 300 mile trip through the ICW and didn't want to be out there any longer than necessary so the 80% throttle was a given. Never missed a beat.

[thomm225]

Originally Posted by n5ama

It is a 4 stroke. I'm not sure what the "official Tohatsu" gas consumption is for this motor but it's in good shape with very reliable low speed operation.

I had a 300 mile trip through the ICW and didn't want to be out there any longer than necessary so the 80% throttle was a given. Never missed a beat.

Your number was correct according to the chart .......I just found it this am

https://tohatsuoutboards.net/resourc...oard-fuel-faq/

9.8 hp 4 stroke 1 gallon/hr

5 or 6 hp 4 stroke .5 gallons/hr

the above at 5500 rpm or so.

I cross the open 20 -30 mile wide Chesapeake Bay at 2500-3000 rpm and the outboard only burns about 1.5 gallons in around 4 hours so a little under .4 gallons/hr

[MicHughV]

considering the rpm only does not give the full story....different outboards use different lower end gear ratios....prop diameter..prop pitch..etc....

some years ago I had a 8 hp yamaha 2 stroke on an 11' dinghy...this I switched out for 15 hp 4 stroke.....the difference in fuel consumption was mind blowing.....ok, got more speed than I needed, but it came at a whoppin' price...