TOPIC: swaping lower unit on fat fifty

Hello everyone, first time here. I am trying to upgrade my 58 fat fifty with some parts from a 61 Johnson 75hp. I'm swaping the reeds and am wondering the ability to swap the lower units. They are both long shaft motors. The boat is a 65 sea ray 500, 15 foot runabout with a hull weight of 650 lbs. If this is doable, what prop is recommended since there is a difference in final gear ratios? Thanks in advance, DJ roszina

The later lower unit will bolt right up. I'm thinking a 10X11 will do well. The small gearcase turns the prop very fast - so you have to keep the pitch low enough to get the boat to plane out. If the prop tends to cavitate, getting a cup put on the prop will help

Thanks for the info, but as I got into the Jonson I decided to transplant the entire powehead, midsection and lower unit into the "fat fifty" as I love the looks of the 58 cowling. It will have a magneto ignition, and no charging system for now as I am not sure of the control box. I'm wondering if a modern voltage reg/rectifier from a motorcycle could be spliced into the alternator circuit? Thanks again, and happy boating.

Update; the transplant has been made. It was more involved than expected when it came to going from battery/coil distributor to magneto. The pulleys are different, the dist has 33 teeth, and the mag has 32 teeth. There for you need to also swap flywheels and flywheel shrouds. Then the bendixs are different heights and also need swapping. The 75hp carb, which has fixed jets and a different throttle cam seemed to make the motor run really rich, so on went the fat fifty Carb and linkage. In the barrel the motor came to life beltching smoke and running rough. I fould two sets of plugs before the black gunk expelled itself, and the motor would stay running. Today I took it to Lake Michigan and she ran very good except that the throttle felt funny. The fat fifty would get me to 28 MPH on the GPS, now the 75 got me to 35mph...nice . At the dock I found that the plastic thingy at the base of the verticle linkage post had broken yielding only 3/4 throttle. At home I fixed it with the spare from the other motors linkage. I can't wait to get her out again. I will post the results, so far I am very happy. Thanks for the motor frog.

DJ , your welcome and iam glad that it is working out for you. let us know more on how it is coming together. frog

Update to the update;
Here is the latest news on the 61 Johnson 75hp disguised as a 58 Evinrude "fat fifty". The 75hp carb was reinstallled with the 75hp linkage and 50hp magneto. The cam profile was not condusive with the fulcrum arc of that mag. Therefore, I found a 75hp mag on ebay, and soon it was delivered to my door. A quick spin on the bench vise revealed no spark. Luckily the spare points and condenser I purchased from frog along with the motor were the ticket and the reassuring zap, zap, zap, of the spark plugs spurred me onto installing it asap. Now the carb, linkage, and magneto were in full harmony with each other. I trailered the Sea Ray up to my nephews place on Castle Rock Lake and off we went. When up to temp I adjusted the low speed screws, did a few cycles of the throttle and gearbox with everything checking out so-far. Giving her the ponys resulted in a little less than desired responce to how quickly she would plane out, but once up she cruised beautifully up to a max 35mph. Throttling back to 30mph resulted in a very relaxing ride still on full plane. We cruised about 35 min. and headed back to the launch. She idled in very nicely, and we cycled the gearbox a few times before shutting her down. Upon loading onto the trailer by lining her on we popped her out. Much to my surprise the prop was missing. There was no damage to the shaft and we hit nothing in our travels. I stripped down to my skivies and with fishing net in hand tried to no avail to retrieve the lost prop...it had to be right there at the launch. Temps in the 60s with water temps in low 50s allowed me to only go in to my "you know whats". We then tried from the pontooon boat, but realized there was a 7-8 foot hole where the other boats had powered up the launch. I know its down there and will return with longer retrieval gear soon. I did locate a prop and install hardware locally. Hopefully a more suitable prop for better "holeshots". That is it for now, Happy Boating.

Way to go DJ, glad it worked out too bad about the prop though, hope you find it.
Bob

And at least it happened at the boat launch!!! You couldnt have been any luckier loosing a prop.

Dean

Thanks guys, being new to this classic boat / motor endeavor I also have a 40lb thrust trolling motor and BIG battery just in case. Happy Boating.

I used to have a habit of painting my props fluorescent orange or yellow, although I never lost one. Maybe I'll start painting them again...just in case...

Well good luck, I once bought a brand new shiny pole holder, bright red in fact. I installed the base and one day I was fishing and water visibility was a crystal clear 20 ft or so. Right to the bottom where I could see my pole holder lying next to an old stump.

I was able to retrieve the prop late November last year. The water clarity improved and i brought with me 2 6 foot screw poles and taped them to my fishing net. Took a little but was able to locate and scoop her out. I will have the Sea Ray out this weekend on Castle Rock Lake here in Wisconsin.

How about posting a photo of the motor? How nice is the cowling? It's a rare bird.

Verne

Here is a partial view, all original, except for bungee strap. I had her out on the lake today for fist time since replacing low speed screw seals and being layed up for winter. She started great but was a little cold blooded when throttling up at first. Once fully warmed up she ran great. I will have to adjust the auto-choke. After some idle quality adjustment she purred like a kitten during idle and low speed. She is as smooth as most modern engines.

djroszina wrote:

........ She is as smooth as most modern engines.

And much more "classy", IMHO. Nice work DJ, glad to hear the relaunch went well!

Nice job on the swap - it's the best way to use a fat 50 just use the bottom pan and the cowling.

In a nutshell, the fat 50 parts used in the conversion / swap were the cowling, front and rear lower pans, transom bracket, flywheel with magneto pulley, recoil housing, bendix,external front throttle linkage, and rewiring for the use of the fat 50's wiring harness, control box, and dash ignition. The only "loss" is the 75's charging system. But, I have a 31 series AGM battery, magneto which is self powering and doesn't draw electricity, and a recoil starter. Sweet and simple.