Hi all, it's been quite a while since I've posted here.

I'm currently building a 10HP longtail for a friend. Talked him out of buying a Thai kit and in an effort to keep the build cost less than the price of the Thai kit, I decided to go with a straight shaft styled after the Backwater SWOMP Lite. I'm using a Briggs Vanguard 10 HP engine.

My question has to do with the shaft length. Backwater lists the shaft length as 65". I'm wondering if that is the actual driveshaft length or the housing tube length. This motor will live on a 1648 jon boat with 16" transom. I've built a few small longtails but they have all been of the U-joint variety. All of mine were designed to run on the back deck of a double ended layout boat with the mounting height being equivalent to a 12" transom. Even mounted so low and on a 12 degree angle, my tube alone is 60" . When you account for the bearing housing, U-joint, output shaft and the threaded part of the shaft extending beyond the tube, it puts the back face of the prop 70" from the engine case. I can put it on a jon boat with a 16" transom and it works fine but has to run at a steeper angle and I can't imagine it being any shorter. That's why I question the 65" being the actual overall length of the SWOMP shaft. Seems like it would be running at a very steep angle. Anyone know for sure? I have 72" of DOM tube and 84" of 3/4" shaft to play with so I can build it any length, just don't want to build it too short and then regret it.

I'm running a 10hp Backwater on a layout boat with a 16" transom. I will have to measure the drive shaft length tomorrow, but I can tell you it does not run at a steep angle at WOT. Because of the surface tracer cavitation plate, it really doesn't run at WOT at any different angle than the U-joint motors I have run in the past. It does want to pull the cavitation plate and prop down at a steep angle at idle however. I shimmed my cav plate at WOT as I was assuming I was supposed to.

Hope this helps and I will measure the driveshaft length in the morning.

Well I got it built. I appreciate the feedback I got from a couple of people on here through PM. In the end, I decided to build it a little longer than the Backwater for a couple of reasons. First, the boat it is going on is a standard transom, but it has taller gunwales, so it angles up a couple of inches on the back corners. This presented a little bit of a problem when I loaned him a 6.5 longtail earlier this season. It worked fine except it couldn't swing into the boat for towing without removing the motor which is kind of a pain. So I needed to be able to mount it a couple of inches higher on the transom if I want it to swing into the boat, which means a longer shaft is needed. After I started the build, I learned that he was shopping for a bigger boat and I was concerned that if I built it for a standard transom, what if his next boat turns out to have a tall transom? So I built it 72" to the prop and if he does end up with a tall transom later on, all I'll have to do is change the angle of the cavitation plate.

Here are some pics of the build. Sorry it doesn't look too sexy with the metal in the white, but he wants to send it out to be powder coated so I didn't paint it.

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Sir,

Thank you for the follow up photos. Is that a 6.5 HP predator motor or is it the larger 420 cc motor? I couldn't decide from the photos. Did you do something to handle thrust or just push it through to the PtO shaft? Did you use bushings? I thought that the top tubular brace was creative. I liked the transom mount too. Did you build with aluminum or steel? Sorry for all of the questions but I so enjoyed all of your photos that they all were stimulated by.

Mark F. Cheney

Mark, glad you enjoyed the pics, and thank you for the compliments. It's a 10 HP Briggs Vanguard Commercial grade. All steel DOM construction. Handle and upper brace are not DOM though. I don't really think the upper brace was necessary but it gives it a cool look and provides a good two handed handle at the balance point for lifting it on and off the boat.

This had to be built on a budget so there are a few things that I did differently (ie; cheaper) than I would have had I been building it for myself.

First: I have always made my driveshafts from 304 stainless which is about $32 for 6' but he didn't want to spend that so I had to build it from 1018 cold roll at $14 for 6'.

Second: All bushings, no bearings. Personally I don't think this makes it any less durable though and here's why. Sure, bearings run smoother, quieter and cooler, but having upper bearings is more important in a U-joint style because the radial load is so much greater than a straight shaft. Not only is there less radial load on the upper end, but it's just inches from the heavy ball bearing that's already supporting the 1 1/4" crankshaft, so adding bearings at the top doesn't actually add any more support than a bushing does. Some say that bushings will wear out faster but that's not necessarily true. Let a seal fail or become compromised and tell me how long a bearing full of water lasts... (also why I don't like to use lower bearings). I will say that my first U-joint longtail (5 HP) is now 10 years old and I have never had to replace a bushing in it yet but I have replaced the upper bearing once due to water intrusion. My original weedwacker longtail is now 16 years old and also on it's original bushings.

Third: No I did not do anything to take the thrust load off of the engine (the Backwater doesn't either BTW). IMO, addressing thrust load is more important for preventing shaft creep and U-joint binding than it is for taking load off from the crank bearings. Not saying that taking thrust load off from the engine doesn't have it's advantages, because it certainly does. Just don't think it's quite as detrimental to engine life as some believe. Choice of engine will determine if it's necessary, but even the cheap Predator's crank bearings are capable of handling a decent thrust load. On my U-joint longtails I designed my own bearing housing that handles all of the thrust load. Had I built this one for myself I would have likely incorporated that design for the sole purpose gaining a few RPMs.

Forth: PTO coupling. The easiest and most ideal thing to use is a manufactured heavy duty two piece shaft coupling with keyways. This is what I would have used for myself, however, since the output shaft on this engine is 1" and the driveshaft is 3/4" the proper coupling is about $120! My friend obviously didn't want to spend that so I made the coupling from some .250 wall DOM tube turned to size on my lathe. I tapped the end of the driveshaft for a threaded stud that I made from a grade 8 bolt before welding the coupling on. The end of the crankshaft is also threaded internally so the coupling slips over the output shaft and threads up tight. At that point three 1/4" set screws align with the keyway and function as the drive key.


All said and done, I think that it's pretty rugged and should last a long time. I've got $120 in the entire build. Add another $139 for the prop and it's still less than the crappy Thia kit that he was going to buy. Plus it's a real mud motor that he won't have to put a new prop on every outing.