Odd shaft length, transom riser, square stern canoe

[BoatThings]

Hello everyone. First marine motor for me, first post here.

Got a nice deal on this 1977 6hp Johnson so I figured I'd pick it up for my square stern canoe. Has great compression from what I've seen others posting for similar motors, 87 and 89psi. Wide open ran great, didn't like to idle though. I took the carb apart and cleaned it out with a rebuild kit, purrs like a kitten now.

Shaft length seems to be an odd one to me, unless I'm measuring incorrectly, but this seems to be what the diagrams online show. Looks to be roughly 18", in between standard 15" and 20". Can anybody shed some light on that out of curiosity?

Now, onto my main question, how much should I raise this motor when I build the transom riser? I know it needs to go up, if I try to go WOT there's a lot of drag from the long shaft, and starts bubbling water up over the back of the transom. Should I add 3" so it's like a standard 15"? This would put the cavitation plate about in the middle of the bottom of the transom and the bottom of the keel. Maybe I should bring it up another couple inches while I'm at it, to make the cavitation plate equal with the bottom of the transom? Would be possible to do an adjustable one if I really need to dial it in, but it would be nice not to have to.

What's the consensus?

Thanks for your time.

 

[jbcurt00]

I suspect a 3hp would be better suited to your canoe.

Not sure the cav plate even w the hull bottom rule is applicable on a canoe.

 

[BoatThings]

Maybe a 3hp would be more suitable? I can't imagine it would provide much spunk in a long and wide canoe loaded with camping gear and two people. I'll be reinforcing the transom when I make the riser, and I can always use less than full power on the 6hp. It's a well running motor that I got used for a good deal, so that's what I've got to work with.

Anyone with experience on optimal motor mounting height for a square stern canoe? Or for curiosity sake, know why my motor seems to be an oddball 18" shaft?

 

[hardwater fisherman]

All of my older Omc so called short shaft motors measure around 17 inch. And my long shaft motors measure around 22 inch. There is nothing strange about your shaft length.

 

[Chinewalker]

With that keel, I would leave it deep. If you raise it, then the propeller will be chewing up turbulent water.

 

[steelespike]

Make the transom adjustable. Play with the trim.
I agree a 3 hp is probably a better match.

 

[BoatThings]

Adjustable it is then, I guess. I was hoping some canoe guru would know the prefect height and fill me will confidence to make a fixed riser.

While the 3hp might be a better match as I already conceded, there wasn't a used one available on the island I live on, so I got what I could. I honestly think I'd rather have the 6 anyway. My car is probably a better match with it's stock 210hp engine, but I much prefer it with ~400hp. 3hp would putt me around, but it wouldn't be any fun. Can always use part throttle, it's only slightly more weight than a 3 with onboard fuel, (~10lbs) and the transom will be reinforced for the riser anyway.

 

[jbcurt00]

Boat~car analogies arent accurate

Transoms shouldnt need reinforcement

Outboard powered canoes are intended as putt putts.

I understand why you bought and intend to use the 6hp, doesnt make it wise.

A plan that includes the idea of staying out of the throttle is unrealistic, esp in an overpowered canoe, that may be best described as squirrely, w an outboard and operator at the rear.

 

[BoatThings]

They're not accurate in every sense, but a car with a lot of power would be described as "squirly" as well. It's still completely safe in the hands of a competent operator.

I doubt the canoe is overpowered either. It didn't have a plate when I bought it, it's old, but it's a huge canoe, not one of those tiny ones. It barely fits on my roof rack, it's about 5 feet wide in the middle. I have seen many flat stern canoes, the larger ones, rated from 6-8hp, and some up to 9.9 even. I appreciate your concern, but you're derailing my thread.

When you you raise a transom, it's definitely wise to reinforce it. I don't need to know anything about boats, to know that. It's quite obvious what happens to the force applied to the transom when the motor has a longer lever to act upon.

Just came back from crabbing tonight, got a few keepers. Handled the ocean pretty well on a windy evening. No full throttle of course with the chop, staying off throttle depending on conditions seems pretty realistic to me.

Any more to add about the motor height discussion?

 

[jbcurt00]

Do as you wish, it is your boat to use and power as you see fit, but I stand behind my advice against using a 6hp.

Spend some time going thru other topics here. Lots of topics and posts about using long shaft motors w poor results on short shaft transoms. Both w raised transoms and w/out, on boats intended for small hp motors, 12~14ftr open fishing boats.

Boats arent the same as cars w regards to lots of perimeters, and what you know about 1 doesnt always translate to knowing anything about the other. When you accidently get into the throttle of your car, engine braking and actual brakes help you regain control, neither is present in a boat. If things go sideways you can just stop and get out of a boat.

Good luck w your project and plans. Boat and boat work safely.