Calculating fuel consumption - 5hp Outboard

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Any idea on how to get a rough theoretical caclulation of fuel consumption for a 5hp Mariner 2 stroke pushing a 22ft yacht with c.1000kg displacement?

Half throttle = 4 to 5kts boat speed in reasonable conditions

Cheers
 
Rule of thumb (mine) says 0,5 liter per hp per hour for a two stroke, 0,25 for a four stroke. You have to estimate the number of hp's you are using. But at half throttle, it will be about half the max power.
Hope this helps,
Theo.
 
rule of thumb I am aware of for 2 stroke OB is 1gph per 10hp engine rating at wot/full revs.
This would give a maximum consumption of 0.5gph/2.3lph flat out.
Estimating the relationship between the throttle setting and revs when under load is somewhat difficult on small OBs pushing loads such as yours and I would be loath to work on anything below 1.75 lph without the benefit of practical experience as you are unlikely to be correstly propped let alone have a rev counter!
Empty the tank - add 1 litre - set off (carrying plenty spare!)at your 'cruising speed' and time the burn.
This rule works well for larger (carb based) OBs but they are also intrinsically more efficient than very small units too.
 
2 stroke uses lots when on full power, but with my Jhonson 6hp which in spring i use to go shopping pushing a 50kg?? ridgid tender, at least twice a week,and the town with the big village--rio marina is just over 5 miles from where i moore, a retern journy uses 8 to 11 liters depending on conditions, im useing about 75% power as more dosent make any difference.In very bad conditions it just takes longer to get back then i use 11 liters.
The same journy in a boston whaler with a 25hp Jhonson at full power uses more than 20 liters, and i have to row the last mile or so!!
 
Perhaps a safer way of measuring consumption:
Fill the tank
Run for 1/4 hour (1/2 or 1 hr would give more accurate results if you and the motor have the endurance)
Stop the motor
Refill, measuring the amount you put in, or if easier, use a full can, then measure how much it takes to refill it.
 
My MERC 5HP 4 stroke used 12 litres to get back from Cherbourg in a flat calm at about 5 knots (14 hours, 60 miles) on a Parker 21. I usually reckon about 5 miles/litre. A 2 stroke might use more.

Geoff
 
It seems to me that speed in calm conditions is the way to get a uniform sort of power standard. So at a constant speed of say 4.5 knots you could measure consumption as described. Then try again at 5.5 knots to give you an idea of practical consumption. Obviously as you approach 5 knots more throttle equals more fuel not much more speed. For my 21 fter with 6hP Johnson I count on 1.5 Nmiles per litre but it probabl;y does much better than that at around 4 knots. regards will
 
Your post makes depressing reading! Ive been thinking of changing my very old but VERY reliable Jhonson 6 for a Honda 4 stroke,i dident realise they were that fule efficent!
My big problem,is that i dont think i can afford a 4 stroke. perhaps theres a smaller one made by another,less expensive than Honda manufacturer??
I really must look at price to buy verses maintanence, fule ecomomy, total running cost per season. Got me thinking again--why do i never have enough money?
 
I've motored all the way back from St-Vaast to Christchurch in fairly calm conditions with an old 8hp Honda 4 Stroke on my 26 foot boat and used less than 2 litres per hour at 5 knots. I've just changed the engine for a brand new Tohatsu 9.8 4 stroke which isn't yet fully run-in, but I used it for the first time for about 6 hours at 5 knots on the way back from Cherbourg and still got under 2.5l per hour. I think you'll find the 4 stroke outboards are nearly as fuel efficient as the diesels, plus you don't have to mess around with 2-stroke oil. Less smelly, quieter and smoother as well.

Dave
 
My bank account thinks the jhonsons not bad for another season! A 4hp is over£1000
a secondhand 6hp 2 stroke £50. Tohatsu used to rename other engins and were first class,and less expensive than Honda.I wonder how much i have----
 
All good advice, my view is to take plenty of fuel with you, after a few outings in different conditions I ended up with a fairly good grasp of what can be expected. Rough conditions really push up consumption, but there is usually wind as well so you'll probably be sailing.
 
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