Optimizing a longtail setup

I started a longtail thread in the outboard subforum but since realized that most longtail talk takes place here. Apologies for starting another thread, but the thread over in outboards doesn't seem to be generating much interest. I'm interested in longtails as simple and cheap alternatives to outboards rather than "all terrain boat motors," and won't be operating in mud or water all that shallow in general. My goal is to power as large a boat as possible with a 10 HP longtail as that is the power restriction on my local lakes. That said, I probably could get away with as much as 13.5 HP since I believe, at least with regard to Briggs and Stratton, the outer appearance of the motors are the same and a decal change would be all that would be needed.

From what I've seen there is a disconnect between Thai style longtails and American style longtails. Everything I've read on Thail style longtails stresses the need to have as shallow an angle as possible in order to push the boat forward (rather than up) in the most efficient manner. Hence, I've seen recommendations of shaft lengths in excess of 7', or more if you have a high transom. American longtails seem to not only use shorter shafts, but also many times incorporate universal joints to make the angle even steeper, which, at least as far as I can see creates a double problem of a shaft that is pushing the transom up, rather than forward, and a direction change which creates power loss. It would seem to follow that the combination of the two results in a lot of inefficiency. I've read plenty of complaints from folks running relatively large (over 10 HP) engines on relatively small (12-14') boats about how the boat is not fast enough. I'm wondering if they fitted a long direct drive shaft on the same motor (maybe lowering the transom mount a bit as well) what kind of improvement they would see?

A lot of the designs I've been looking at are simple plywood dorys, built for efficiency in the days when power was unavailable or unreliable and the boat might need to be rowed with a heavy load on board. My plan is to test a 10-13.5 HP longtail with a shaft length of at least 7-8' (not sure what I should be thinking in terms of prop size yet) on an 18' x 6' beam dory to see how it performs. I should also add that I'm not looking for massive amounts of speed either, just 10-12 MPH around the lake. One question I'd also like to delve into a bit more is whether or not a setup like this would benefit from some reduction? A centrifugal clutch and a larger drive gear could easily be fitted to a setup like this to provide some reduction and presumably a torquier, slower turning prop.

 

I would consider three things before deciding to build a long tail motor. The first would be will I need reverse. The second would be noise, air cooled motors are very noisy. Another problem is exhaust fumes in the boat if you have a tail wind and your not going fast enough. I built an outboard using an 11 horse Briggs on a Chrysler 12.9 horse outboard leg. I had reverse but the prop was not big enough for the Briggs RPM and it was very noisy and had the exhaust fume problem. I could have probably fixed these problems with a bigger prop and running the exhaust under water but a friend gave me a 30 horse outboard and now I have replaced it with a 70 horse Merc. that I bought for $500.00 off from Craigslist. You could probably find a used water cooled 10 hp outboard for less than you could build a long tail.

Dave T

 

10-12 mph there is the problem ...you need to be on the plane at 15 mph + or not on the plane at 5 mph that bit in the middle is the hump you need the power to get over.
Dont froget to gear down your motor 3 or 5 :1 and ensure you have left or right hand props to play with