Transom height with reference to outboard

[davidpbo]

When transom height is specified for an outboard, is it with reference to the waterline or bottom of the boat?

My enquiry relates to an outboard in a well on a sailing cruiser.

 

[Tranona]

Waterline. Outboards are made in mainly two standard lengths (plus some extra long) of 15" and 21". Hopefully the designer and builder of your boat has followed convention and installed a mounting board with one or other of the heights. Then, hope you have bought the correct shaft length motor for your boat! This is a the most fundamental dimension to check before splashing the money on a motor.

Quick check with a tape measure from the top of the board to the waterline will confirm.

 

[davidpbo]

Waterline. Outboards are made in mainly two standard lengths (plus some extra long) of 15" and 21". Hopefully the designer and builder of your boat has followed convention and installed a mounting board with one or other of the heights. Then, hope you have bought the correct shaft length motor for your boat! This is a the most fundamental dimension to check before splashing the money on a motor.

Quick check with a tape measure from the top of the board to the waterline will confirm.

The reason I ask is that bracket height to waterline is 450mm which puts it in standard/long shaft territory however the engine that came off is 800mm from prop centre to bracket rest point which would I guess make it an extra long shaft maybe used because the well is offset.

 

[Tranona]

That is better with a long shaft - and yes 800mm is an extra long shaft.

Expect the designer set the board a bit lower to avoid the prop coming out of the water if motorsailing on the wrong tack. The potential problems with the motor being too low, particularly if you have a lot of weight in the stern is that the motor ends up too deep leading to backpressure problems and exhaust fumes rising up the well. Running an outboard in an offcentre well is rarely satisfactory except in flat calms.

 

[VicS]

Waterline. Outboards are made in mainly two standard lengths (plus some extra long) of 15" and 21". Hopefully the designer and builder of your boat has followed convention and installed a mounting board with one or other of the heights. Then, hope you have bought the correct shaft length motor for your boat! This is a the most fundamental dimension to check before splashing the money on a motor.

Quick check with a tape measure from the top of the board to the waterline will confirm.

That is better with a long shaft - and yes 800mm is an extra long shaft.

Expect the designer set the board a bit lower to avoid the prop coming out of the water if motorsailing on the wrong tack. The potential problems with the motor being too low, particularly if you have a lot of weight in the stern is that the motor ends up too deep leading to backpressure problems and exhaust fumes rising up the well. Running an outboard in an offcentre well is rarely satisfactory except in flat calms.

I think you will find that "shaft lengths" are: standard (aka short) shaft 15" ( 381mm) , long shaft 20" ( 508mm) and extra long 25" (635 mm) . There are also a small number of models available with a 30" shaft which I think are used on the curious American pontoon boats and perhaps a few other applications.

Some of the small Hondas seem to differ and generally I think Hondas measure them differently although the end result may be the same as other makes.

The reason I ask is that bracket height to waterline is 450mm which puts it in standard/long shaft territory however the engine that came off is 800mm from prop centre to bracket rest point which would I guess make it an extra long shaft maybe used because the well is offset.

I agree that your 450mm appears to fall between the 15" and 20" lengths but presumably the bottom of the boat is somewhat lower. If that coincides with the depth of the anti-ventilation plate of a 20" model you are about right. Its best not to go too deep as Tranona says, other wise back pressure on the exhaust may cause starting difficulties.

well worth looking at the possibility of routing the exhaust issuing from the relief ports to some place outside the boat or you may suffer from the fumes. The outboard wont like breathing in its own exhaust gases either.

As Tranona says your 800mm indicates an extra long shaft model.

 

[davidpbo]

Juries out on venting through transom. I have decided to try it without first.
Under way it is not a problem. Idling for a while I tend to lift the lid, one side of the locker is open at cowl level.