shaft length question, will this work

[justinhip]

Hello,

I have a 23 ft Robalo CC that I want to (need to) install a 150hp yamaha outboard. Eventually, I will buy a new engine that is also better suited power wise to the boat. For now, I have to work with what I have. So, my transom is 23 inches and the outboard is a 20 inch shaft. What kind of problems can I expect. Will it function decent under normal conditions? Any input would b appreciated.

Thanks,
Justin

[marshmat]

Your transom is 23" and you are mounting a 20" motor.
For starters, a 20" longshaft motor usually has a shaft length of closer to 21-21.5 inches. The next size up is a 25" (actually about 26).
Your 23" transom is indeed designed for a nominally-20" shaft. Keep in mind that the prop is at least a foot or two aft of the transom. I'd mount the motor, learn to use its trim function appropriately, and not worry about it unless the prop is clearly ventilating.

[JR-Shine]

You want to measure the distance from the top of the cut out straight down to where the bottom would intersect if their was no transom angle - maybe you did it this way, just checking.

If the 23' boat was built for a 25" shaft (makes sense) then a 20" engine will not work properly.


Joel

[marshmat]

Adding to what Joel said: If your 23" measurement is along the transom, as I suspect it is, then your vertical distance will be a bit less- maybe 21-22", due to the angled transom. If your boat was built for a 25" shaft your transom would be closer to 27" measured along the transom, 26" or so vertical. On a boat this size the engine mount is usually designed so the ventilation plate is an inch or two above the keel, since the prop itself is set back a couple of feet from the keel.
Having said that, a 20" shaft on a 23' c-con is a bit of a rarity; boats this size do tend to be a bit higher in the transom.
Put the motor on its bracket and hold a 2x4 board against the hull bottom adjacent to the keel. The free end of the board should touch the ventilation plate.