Long shaft for dinghy

[avsh]

Hello,
I have a 2.5 meter inflatable boat with a Mercury 4 hp short shaft engine (licensed up to 5 ) In addition I have a Mariner 5 hp long shaft engine and when I sailed with it it seemed that pressure was applied to the transom there were many shifts in contrast to the engine with short shaft. The question is, because the 5 engine has a long shaft, will there be a situation that can damage the transom? The adhesion and connections? Thank you very much.

 

[Scott Danforth]

yes, with a long-shaft, you have a longer pry bar attempting to rip off the transom

 

[QBhoy]

Yeah. It’s not ideal at all really. Very poor running attitude with a long shaft..not to mention usually meaning it throws water at you, over the transom…the cantilever effect under acceleration, has a fair pull on the poor wee transom. Even a short shaft engine usually sits a fair way under the lower line of the hull, on wee boats like this.

 

[FasterDaddy]

Yeah. It’s not ideal at all really. Very poor running attitude with a long shaft..not to mention usually meaning it throws water at you, over the transom…the cantilever effect under acceleration, has a fair pull on the poor wee transom. Even a short shaft engine usually sits a fair way under the lower line of the hull, on wee boats like this.

I have a couple of aluminum row-boats that are rated for up to 15 hp and are 15 inch transoms. Years ago I tried to put a 10 hp with a 20 inch shaft on one of them. It resulted in huge spray in all directions, especially back-splash inside the boat and it lowered top speed noticeably. I tried raising the motor up off the transom by bolting it to the transom so it was above the transom. But I couldn't get it high enough. I still got back-splash. This was probably 20 years ago with a 2003 Mercury 2 stroke 9.9, so unless the motors have changed, I can't see how using a long shaft motor on a short shaft inflatable would have better results. I'd always use a 15 inch shaft motor on a 15 inch transom.

 

[QBhoy]

I have a couple of aluminum row-boats that are rated for up to 15 hp and are 15 inch transoms. Years ago I tried to put a 10 hp with a 20 inch shaft on one of them. It resulted in huge spray in all directions, especially back-splash inside the boat and it lowered top speed noticeably. I tried raising the motor up off the transom by bolting it to the transom so it was above the transom. But I couldn't get it high enough. I still got back-splash. This was probably 20 years ago with a 2003 Mercury 2 stroke 9.9, so unless the motors have changed, I can't see how using a long shaft motor on a short shaft inflatable would have better results. I'd always use a 15 inch shaft motor on a 15 inch transom.

Totally agree. Exactly what I’ve found too. As above

 

[FasterDaddy]

Totally agree. Exactly what I’ve found too. As above

Yeah, that was a day of frustration that I'll never get back again! ;-)

 

[tpenfield]

I've had to run my dinghy with a long shaft motor while the short shaft motor was under the wrench. Not ideal, and it tends to be less efficient because the propeller was too far under the water. Transom load was not a huge concern, more so - I did not win a single race with the long shaft.

 

[QBhoy]

I've had to run my dinghy with a long shaft motor while the short shaft motor was under the wrench. Not ideal, and it tends to be less efficient because the propeller was too far under the water. Transom load was not a huge concern, more so - I did not win a single race with the long shaft.