25" shaft on 20" transom

[reelfishin]

I've been looking for a good 20" shaft 100 to 150 HP outboard for some time now, today I came across a near mint 1974 Evinrude 175 HP V6 but it's a 25" shaft. There are two problems, the boat is rated only at a 150 HP max, and this is a 25" shaft. If I use the lowest set of holes, it the anti cavitation plate will set about 1/4" or so below the lowest point of the hull. Second, I have a non running 150HP parts motor from 1974, both of these motors look nearly identical other than the HP designation, can I detune the 175 enough to be safe? What were the differences in the 150 and 175? They both weigh within 10 pounds of each other so the 175 isn't adding any more weight than the 150 would, and if I can mount this high enough I don't see any reason why I can't make it work?
I got the 175 for free and it came from a clean, running boat. I ran the boat myself to the dock where we trailered it away. The hull on that boat is just not what I want so I plan to pull this motor for use on another boat.

I know that overpowering a boat can be dangerous, as well as illegal, but I don't see the real problem here since weight or stability won't be an issue.
I also don't often run my boats hard, I never run at top speed, there's just no room to do so here since I fish mostly the river and back bays.
I just hate to not put such a clean motor to good use. I'd like to here some other opinions here on both the 25" shaft on a 20" transom and the 175 vs 150 hp issue.

 

[tashasdaddy]

Re: 25" shaft on 20" transom

Jack Plate and use it. put the 150 cowl on it.

 

[Dhadley]

Re: 25" shaft on 20" transom

2 problems -- There is no 175 in 1974 and there were no 25" motors in 1974. Get the model number so we can "see" what we really have.

 

[reelfishin]

Re: 25" shaft on 20" transom

the model number is 175749S, the motor measures roughly 26" from the top of the anti-cavitation plate to the top inside of the motor bracket. It's currently on a 1973 Hydrasports cuddy cabin, it's one of a twin pair. Both motors are identical. The transom of that boat measures 24" and the motor is mounted in the lowest hole and sits about level with the lowest point of the hull on that boat if mounted dead center, they both sit below the hull the way they are mounted now on that boat since they are set off to each side.

The motor is blue in color with a silver band around the cover with large chrome plastic "EVINRUDE" letters on each side. There's no hp designation on the motor cover other than the 175 logo on the rear of the cover. I could easily change that and leave the original cover.

I hate to use a jack plate on an older transom, its not soft or rotted, but it is a 25 year old boat. With the maximum weight motor it alone will be testing the transom's strength. I'd hate to push my luck. The boat had a 90HP V4 on it when I first saw it, that seemed large to me for such a basic boat, the 175 will make this boat feel like a rocket ship. I hung a 70HP on it just for test purposes, and it ran ok, it was able to plane out but wouldn't break 30 mph or so. The 175HP will take lots of restraint. I ran the Hydrosports in on only the one motor since the left side throttle cable was broken, it was plenty strong with just the right motor, having twins on that boat isn't needed, and it sits pretty low at the stern at rest. It's close to the scuppers with two guys standing at the rear when docked. I am curious how the Renken will sit with that motor in place. (adding the jack plate will amplify that additional 100lbs that the V6 will add over a V4). These motors both look to be very low hours and the original owner had said that he had bought the boat as a left over with those on it, the boat's was sitting in a barn in storage from 1979 to 2007, it was put in the water after he decided to step back down to a smaller boat to save fuel, but I guess there were to many other problems with the boat and he gave up on it. Running wise it's fine, but things like cables, trim and fuel tanks and lines have all been problems. Both fuel fill hoses are rotted, preventing use of either in hull tank, and the fuel lines to the motors were both petrified, they replaced those and added two portable 6 gallon tanks as a temporary solution. The carbs were cleaned buy a local dealer and both motors were gone over. All cylinders have like new and equal compression.

I did find it odd for two older motors to be so long, usually the only 25" shaft motors are later models. The mid sections of these two motors are longer than that of the 150 HP part motor, it's model number is a 150840S. The 150 measures 21.5" from plate to bracket. That came off another boat and has a broken rod and a huge hole in the block. That motor was a freebie someone gave me and I saved it for parts.

 

[Bondo]

Re: 25" shaft on 20" transom

If the Powerheads will interchange,........

Put the Good Powerhead on the Shorter Midsection........

Then you'll Have what you Need...........

 

[reelfishin]

Re: 25" shaft on 20" transom

I considered swapping the power heads, but the 150 lower and mid are rough and they are the newer style. I am not sure if the power heads will swap or not. These are both from different eras of motors. The motor brackets are different, the electronics are different. I also hate to mess with a motor that runs so well as it is, these two 175's are real clean and both run like new. The 150 is pretty rough and has been run in saltwater all it's life, (has lots of pitting). I think I may just try it even if it sits a bit low, my guess judging by the way it sits on the Hydrosports and by measuring the transom on my Renken, it looks like its going to be within a 1/4" of being even with the bottom of the hull, it will probably sit that far below the lowest point using the lowest set of holes.