26x with 8hp engine... 26XC??

[restless]

We're forwarding our plans by a year and I'm taking Peaches & Restless to the Aegean for a 2 month tour.
As an extra precaution I've made a further mod whereby I can hang our 8hp tender engine on a rudder bracket should something turdgid ever hit the fan.
It then occured to me that we could actually plod along on this engine all day if we wanted to, probly saving quite alot of fuel in the process.

For those that don't know these waters, there's about 9000 odd islands, generally not much more than 20km apart. There also tends to be no wind from sunset to midday, and it is prevailing n / n-w. So it would make sense to head into 'wind' on motor in the mornings, overall I guess we may well do 400m of passage. There are not so many petrol pumps too, so range and economy are important.

I'm just wondering what the difference would be in terms of fuel economy between the different engines when plodding along at 6-7 kts. We will be pretty loaded with kit, including scuba gear so I dread to think what our actual weight will be. The concept of planing is now the exclusive domain of our tender (inflatable V-air deck... planes with 8hp even well loaded )

[Sumner]

Not a X, but our S is really loaded down. I don't think the weight though makes much difference. On a car the weight kills economy accelerating it up to speed and I would think for the most part that is the same on a boat. Once you are at speed most of the HP goes into overcoming air drag with a car and water drag with the boat.

With the S we usually motor at about 4.5 to 5.5 knots at partial throttle. Our Honda 8 HP 4 stroke got about 10 to 12 mpg. Our new Tohatsu 9.8 gets around 9 to 11 mpg. It has a higher thrust prop on it vs. the Honda and I think that might be the difference.

I'll bet with the right prop motoring at about 9 knots you would also get close to 10 mpg, I'll let you converter to metric . Anyway I'll bet you get a lot better mpg percentage wise with the smaller outboard and it sounds like a great trip. I'll be anxious to read about it,

Sum

Our Endeavour 37

Our Trips to Utah, Idaho, Canada, Florida

Our MacGregor S Pages

Mac-Venture Links

[mastreb]

It's a good question that I think you'll just have to empirically test. Put a gallon of gas on each motor, fix the speed at 5 knots, and see how long it lasts. Motors are usually not their most efficient at WOT, but larger motors have more inertial losses, so my guess is it'll be a wash. In that case, you're better off running the big motor since it won't be stressed.

[restless]

aye.... the dreaded exponential suction of fluid dynamics.
I was brought up on mpg, so no worries on that. I was talking to a powerboat racer earlier today who suggested a 4bladed prop. I can't justify buying more props... got far to much other stuff, like charts etc
good figures though... so I should easily do 20m on a 10L tank. great info for planning My 8 is a yamaha/mariner 2stroke.

Anyone any ideas what a tohatsu 50 does mpg wise at 6kts?

what's a WOT? I'd rather get an idea before getting there and finding out. I may yet do some extra over-enginering to the bracket as hanging an 8 is quite a weight. Worth the effort if it'll make a substantial saving. No tthat I'm mean.. would rather spend on wine than extra carbon

[mastreb]

WOT=Wide Open Throttle, sorry. My engine manual says that running at WOT is not efficient, one should go to WOT and then back down by 1 knot to run at the fastest reasonably efficient speed.

[restless]

ah.. yea, full bore fun is an expensive game! I reckon the 8 should shove us along in calm water at a useful speed less than WOT. Not that I'm worried about stressing the engine, I think the buzz will stress the Mrs more!
She's hassling me for a yam 50 4stroke since hearing my friends one on the slower M version...

[Sumner]

aye............... My 8 is a yamaha/mariner 2stroke....

2 Stroke = not good on the mileage vs. a 4 stroke. If it was a 4 stroke I would feel quite confident thinking you might get at least 25% better mileage on the 'little' outboard vs. the 'big' one. Being that the 'little' one is a 2 stroke I wouldn't put money on that at all Now if the big one is a 2 stroke also then I might put some money back in the pot .

I think Mike posted some good numbers on cruising at 5-7 knots with a larger outboard on his Bahama trip, but can't remember the numbers for sure. I was thinking about 7 mpg. If that is right then the 25% better figure with a small '4 stroke' vs. the larger outboard might hold . If my memory failed me then I'm wrong . Mike what where those numbers?

Sum

Our Endeavour 37

Our Trips to Utah, Idaho, Canada, Florida

Our MacGregor S Pages

Mac-Venture Links

[Craig LaForce]

my old Tohatsu 50 2 stroke gets around 3-4 mpg at 5 to 6 mph, although I never really measured it too much. The new modern 2 strokes are about the same efficiency as a 4 stroke.

A small 4 stroke is the way to go for fuel savings. I would also expect it to be around 10 mpg. Madd mike outfitted his mac with 2 small outboards to gain ful economy as well as redundancy.

[restless]

2 smaller outboards would be ideal, say a couple of 30s?... I've toyed with that idea.
3-4 mpg? that's grim!
the good news is that the engine was virtually free and mechanicaly as-new, and it is for the tender, so will be along for the ride.

[vizwhiz]

Being 2-strokes, you'll probably be doing some research for all of us here...

I'd love to see pics, and underwater ones also!
Yer gonna be near Greece, essentially, right? I'd make sure you have a lock for everything...economy's kinda shaky over there right now, isn't it?

I kinda like the idea of two 30's...probably turn that boat into quite a scooter!
And mounted where the rudders are now...leaves the middle open for boarding!
You'd have to come up with a new rudder idea, though.
Hmmm...how about making them retractable like the daggerboard, mounted along the two back corners of the boat? I've seen something like that before HERE.

[restless]

If 2 30hps fell into my lap I would mount them in the middle.. I just made a river mod to see if I could run off a 4hp.
In this photo I knocked up a rudder bracket into an outboard hanger. It sort of works, and I'll take it with me.

An the port rudder I added same ally that allows me to plod along with a rudder half way down that could still flip out flat should the admiral steer us aground ( )
So I made a QD outboard transom that drops ontop of the port bracket, so I could put the 4hp on and over the port rudder in seconds. Now I have an 8hp, and I'll have to beef up my brackets a little more. I reckon I may end up doing a few hundred miles motoring, so with 5mpg, and $2 per liter fuel, that's about $2/mile on the 50hp. So if I could halve that I would be quite happy!


This was for the 4hp. Thankfully I over engineered it to the point where it is happy with the 8. I use the backstay set up to take the weight of the engine. Where it goes round the bimini, it comes down almost inline with where I want to move it, ie stowed, on dinghy, on bracket. I'll take some more photos later, raining now



here is the very non standard port hanger, rudder off for the moment whilst under development. There is a 'z' system so I can do the ballast valve without boathook, plus it is easy to see if open or not.( Photo above, white ball on the black carbon fiber rod)

[restless]

you can just see the drop nose 10mm pin in there... pull it out and the whole lot just comes away.
Got another thing like this to do for the rear anchor on the other side. Got 2 1/2 weeks before we go... got to weight the cb and shim it so it doesn't clunk when down at night... as well as more canvas stuff.....