I've acquired a 3hp Johnson (JW-21E, which I believe to be 1966?) that I hope to use on a 12 foot dinghy I'm building. The thrust of the prop shaft on this motor angles down at what looks like an excessive degree (about 10 degrees in fact, with the motor sitting level.)I've no experience with outboards, so I'm having trouble deciding what kind of mounting bracket to design for optimum performance. I hope to build in some adjustment ability, but don't know what range of thrust angle (with reference to waterline) I should aim for. The boat is supposed to be capable of planing with as little as 2hp. (I know, I know )My (naive?) assumption is that when the boat is riding level - on plane or not - the thrust should be pretty close to horizontal. But this would mean having this motor tilted significantly forward. I'm puzzled about the apparent downthrust built in to the lower unit: Most motors appear to have the lower shaft at about 90 degrees to the column?Any advice/info will be much appreciated.
You got one of the shallow water drive engines, Edzell. It was an option on some JWs. The angle is deliberate to allow the engine to be tilted far out and still draw water for cooling and thrust straight back. If I recall correctly, the tilt pin can be set for optimum prop angle.I never owned one of them, but I have seen guys maneuvering canoes and jons in as little as 6" of water while duck hunting using shallow water drive JohnnyRudes.That is a specialized leg on a great little engine. You might be able to swap it for a conventional 90* leg engine with someone who wants/needs the shallow water drive, but I would keep it.
The shallow water/weedless drive works very well at keeping your propeller free of weeds and lines. I've got a '72 and regularly use it in some very weedy waters that choke up conventional outboards (we run them in the same waters). I have yet to clean anything off the prop.The thing is built to hit the bottom gracefully too. That'll be pefect for a dingy where performance isn't a big deal. After all, the 3hp engine isn't really a 'performance' outboard. I think they originally made the 90Â° drives for the sailboat 3hp engines.
Thanks, JB & Paul, for explaining the reason behind the downward shaft angle on my Johnson 3hp. If I understand aright, performance at "normal" draft, with the motor level, is sacrificed(?) for shallow-water ability with it tilted forward.But my boat is a sail/row/motor dinghy (http://www.bandbyachtdesigns.com/spin.htm if you're interested.) With board up it should draw scant inches and I shouldn't need to tilt the motor till I'm virtually on the beach. So it sounds like I have the wrong lower unit if I want to get the most knots or hours per gallon motoring. I guess I could try to devise a bracket that has the motor permanently tilted and at appropriate depth, but that seems like it would present various difficulties besides being a bit ugly.SO: anyone interested in swapping a "sailboat" bottom end for the one I have?
We're always interested in someone who's building their own boat here. I can tell you now, it really won't be making much of a difference. Propshafts on inboards (sail or otherwise) are pitched downards at a similair angle afterall. You *don't* want to angle it so the propshaft is parallel with the bottom of the hull because the "cavitation" plate above the prop will become a brake in the water and slow you down. At least that's been my experience with them.I think what you've got is really ideal for your boat. You would be hard pressed to find a lighter, smoother running self contained small outboard like that. If a 3hp 90Â° drive comes along and you feel like swapping it, go for it. But if I were you I wouldn't even bother checking ebay. A word of caution - since they were used for larger sailboats, most of those 90Â° lower units are configured for 20" shaft 3hp engines or longer. I *think* you can swap the driveshafts to make it shorter but I'm not sure.PS - a couple links: Tom Travis' 3hp pageAnother tune up page:maxrules.com/fixtuneitup.htmlPPS - 3-5hp seems crazy big to me. 'Course, I'm not the designer though. My brother's 24' Shark is only rated 3-6hp.
Hi Edzell, Having run both versions of that motor, I can tell you that you really won't notice a difference. The right angle drive motor is geared differently (12:25) and typically propped differently for more low-end grunt, kinda like driving in 1st gear. The weedless models are geared a little taller (17:28) and swings a different prop. Performance might be noted as different if you were to load down the boat with several hundred pounds worth of people and gear, but on a small dinghy it's not likely to happen...- Scott
Great advice & info - thanks Paul & Chinewalker. I'ts good to hear from someone with relevant first hand experience; and the web links will be useful.Paul said "3-5hp seems crazy big to me."Well, I'm sucked in by the designer's claim that the boat will plane plus the fact I already inherited the motor. I'm about 30 miles up-inlet from a gorgeous sailing area and it would be nice if I can access it without taking a week's food along, or else trucking the boat across some rotten logging roads to get there. We'll see. But I also have (blush) an old Seagull that will be fine when there's no rush.Again, many thanks."It's only a boat - go ahead & build it."- But I forget who wrote that.
The Shark planes too. Made 7 knots on a beam reach last week vs 6. knots hull speed. I see your point though. I don't think the 3hp will plane it off, but perhaps with the Seagull too...(If the 3hp doesn't choke on the Seagull's smoke that is. )Could be wrong though, hard to say since it's supposed to be so light.