10 HP longtail vs. 10 HP conventional outboard performance?

I am planning on building an 18' plywood dory for use on some local lakes, all of which are 10 HP limited. Rather than a conventional outboard, I am thinking of building a longtail from a kit such as this one:

http://www.mud-skipper.com/

What I'm wondering is how a 10HP four stroke longtail will compare to a 10 HP two stroke conventional outboard. For example, one of the designs I'm looking at should plane fully loaded with a 10HP outboard, but what about a longtail? The way I see it there are two factors-since the longtail is direct drive the only losses should be frictional (which should be minimal assuming good lubrication and bearings) rather than from a direction change and turning a gear train as in a conventional outboard. However, the direct drive also means just that, no gear reduction-I'll fully admit that I have zero grasp of how greater torque at the prop via gear reduction affects power and speed.

So basically, with the same boat, same load, same conditions, etc...can I expect a 10HP longtail to provide the same level of performance as the average conventional outboard, a bit less, or perhaps even a bit more? One final question: If a 10HP longtail will not plane my boat, but will move it at, let's say, 6-8 MPH, will that be horribly inefficient in terms of fuel consumption?

 

I don't have figures, but in general, a larger, thinner, slower turning prop will have greater efficiency. Even if you need to add some gears.

 

jonr is correct that a larger slower turning prop generally can provide better propulsion efficiency. The losses in the gearcase are generally only a couple of percent, and in this case are not significant to your question.

The outboard will typically have a maximum engine RPM of about 6000, with about 2:1 gear reduction, so the outboard prop turns 3000 RPM. The mud motor four stroke engine is probably rated for a maximum of 3600 RPM, and with direct drive you get a shaft rotation close to the outboard, more or less the same.

Therefore there is little significant difference between the outboard vs mud motor setup. The significant issue is getting the right prop for your boat, proper matching of prop diameter, RPM, and pitch for your application. Of course the outboards have limited prop diameter, as do the mud motor kits. The other issue is $$ cost, outboard vs your costs and labor to build a mud motor setup.

 

OB vs MM

"So basically, with the same boat, same load, same conditions, etc...can I expect a 10HP longtail to provide the same level of performance as the average conventional outboard, a bit less, or perhaps even a bit more?"

ANS, Yes, performance potential will be about the same since either option provides about the same prop RPM. Selection of the proper propeller is the dominant factor for either setup.

" One final question: If a 10HP longtail will not plane my boat, but will move it at, let's say, 6-8 MPH, will that be horribly inefficient in terms of fuel consumption?"

ANS: If you cannot get up on plane (due to excessive displacement of the loaded boat), then you may be getting poor performance at that speed. 6MPH will use very little power (about 3 horsepower only is needed), 12+MPH planing speed. So, at least at 6MPH, your fuel consumption would be far better (Miles per Gallon) than at full power, planing at 12+ MPH. Between 6 and 12, perhaps somewhat worse.

You mention a 2 stroke outboard, vs a 4 stroke mud motor. Newer four strokes are much more energy efficient than 2 strokes, I would expect the 2 stroke, at 10 HP output, would use 25% - 35% more fuel than a modern OHV four stroke producing the same power.

 

I think there is no direct comparison here. Outboard motors are rated at the shaft, not at the crankshaft. So a 10Hp outboard is actually a little more after the gearing while a 10 Hp long shaft-long tail will have less Hp at the prop side after losses in the bearing.

The outboard thrust line is almost parrallel to the waterline increasing efficiency while a long tail is forced to have an angled thrustline which could affect planing or trim angle.

 

It seems to me that the long tail is a third world solution to getting boat propulsion without having to rely on costly first world technology. Most any old engine can be converted to a long tail with relatively simple additional parts by unskilled workers without access to the outboard engines we can get so easily.

Along with these advantages, the biggest disadvantage I see is the necessity of needing a hand on the thing at all times its running. It looks very primitive to me but if you like the idea of being so involved in propelling the boat, go for it. For general use, I suspect that the novelty will soon wear off.

 

Once I was thinking. Why not mount the engine low, connect the driveshaft thru the hull via a constant velocity joint secured to the transom and covered by a rubber boot. Then you have a near horizontal prop shaft, maybe even an surface drive? A simple lever would move it side to side, up or down.

I know that Arneson balked at this idea claiming that CV joints won't take the load but then he used something similar. Use a bigger CV joint then.