Correct engine shaft length for dingy

[Binman]

Measurement from transom to the bottom of the Keel is 17 inches, I'm using a standard shaft engine on a Suziki 3.5 outboard, dingy 9ft. Would a short shaft engine be more suitable?

 

[VicS]

Measurement from transom to the bottom of the Keel is 17 inches, I'm using a standard shaft engine on a Suziki 3.5 outboard, dingy 9ft. Would a short shaft engine be more suitable?

Bottom of the keel is not relevant

Transom top to waterline is more relevant. Match that to "shaft" length

Modern outboards pretty much come out as 15" short (aka standard) shaft , 20" long shaft.

A few manufacturers use the term "standard" shaft. Most call them "short" shaft.

Short/standard shaft ( ie 15") will almost certainly be OK for your dinghy.


IIRC Honda measure them slightly differently .
The only truly short shaft engine that I know of is the Seagull Featherweight ( it' s about 13" IIRC)

 

[Scoresby]

As a rule of thumb, the cavitation plate wants to be about 1" below the bottom of the keel. Shortening the shaft will noticeably reduce drag at planning speeds and reduce your draft. The downside is that your prop might cavitate in a chop.

 

[VicS]

As a rule of thumb, the cavitation plate wants to be about 1" below the bottom of the keel. Shortening the shaft will noticeably reduce drag at planning speeds and reduce your draft. The downside is that your prop might cavitate in a chop.

Anti-ventillation plate I guess you mean. Cavitation is something different

But why as deep as 1" below the bottom of the keel. Would be problem for an outboard used s an auxillary on a deep keeled boat, or even your average bilge keeler for that matter

 

[Binman]

Thanks VicS, short shaft it is for water line.

 

[VicS]

Thanks VicS, short shaft it is for water line.

Just be sure that its deep enough to ensure that the waterpump is not starved of water. Probably the most important consideration for an a outboard on a small dinghy/tender.

 

[Scoresby]

But why as deep as 1" below the bottom of the keel. Would be problem for an outboard used s an auxillary on a deep keeled boat, or even your average bilge keeler for that matter

I assumed that when the OP referered to keel, he meant the lowest part of the transom on his dinghy. 1" below the keel on a fin-keeler would be some whacky sort of set-up for sure.

 

[Binman]

I measured the distance from Transom to keel as 17 inch, once in the water it would be a lot lower,engine and me.so I'm sure you are right saying short shaft will do fine, but will report back what I have got to the water from transom later.

 

[VicS]

I measured the distance from Transom to keel as 17 inch, once in the water it would be a lot lower,engine and me.so I'm sure you are right saying short shaft will do fine, but will report back what I have got to the water from transom later.

I reckon if the water level is between the black lines on the lower part of the leg it'll be about right.

A little deeper will be Ok but if its much shallower you might starve the water pump

 

[ghostlymoron]

If you have a Honda 2.3, you don't need to worry about the water pump - its air cooled!

 

[VicS]

If you have a Honda 2.3, you don't need to worry about the water pump - its air cooled!

Yep , looks pretty well air cooled