Outboard Shaft lengths as S , M Long urg

[Capt Popeye]

Well owing to changing my boats again I am up against getting the right Shaft Length OB : On researching Dealers I get Short , Medium Short , Long , Extra Long which is really limited use when ones Stern measures up as an Inbetween size , the dealers say best measure my OB Shaft lengths then ;

Might ask why them Dealers cannot measure up a stock Outboards Shaft Length and put that in their adverts ?

Maybe a lot of Dealers used to sell cars or nappies before ? urg

 

[Tranona]

The 2 basic lengths for petrol outboards are short 15/16" and long 20/21" measured from the top of the mounting board to the static waterline. Pretty obvious which is needed for a specific boat if it has a built in mounting pad on the transom or in a well. The extra short is rare, I think only the Seagull 40 minus which was 13" for use on Avon dinghies, but suspect some of those throw away adapted chain saw motors may be shorter. Extra long is for sailing boats with high transoms or swinging brackets and usually achieved by adding a spacer between the upper leg and the gearbox. Heights vary from model to model but usually 3 or 6" spacers.

So ignoring the extremes it is very simple. Only 2 sizes clearly different.

Hope this helps.

 

[Capt Popeye]

The 2 basic lengths for petrol outboards are short 15/16" and long 20/21" measured from the top of the mounting board to the static waterline. Pretty obvious which is needed for a specific boat if it has a built in mounting pad on the transom or in a well. The extra short is rare, I think only the Seagull 40 minus which was 13" for use on Avon dinghies, but suspect some of those throw away adapted chain saw motors may be shorter. Extra long is for sailing boats with high transoms or swinging brackets and usually achieved by adding a spacer between the upper leg and the gearbox. Heights vary from model to model but usually 3 or 6" spacers.

So ignoring the extremes it is very simple. Only 2 sizes clearly different.

Hope this helps.

Hi there Tranona , very sorry for the llooonnngg delay in responding , but been a tad ill then getting better : Might I state that you are the first to state measuring the Outboard Shaft Length needed from Transom top to the static Waterline . every one else clearly states , measure from Top of transom to bottom of craft /boat , which , as in my case is a bit different : THanks for your comments , me going to measure both distances on my Boat , then think about it ?

I find that s the Outboard is mounted on the Boat Stern the weight makes the actual Stern sink lower into the water , so effectively placing the OB baffle plate lower into the water than the Boat Waterline

 

[Tranona]

Of course it does, depending on the amount of buoyancy in the stern. The measurements are for the static waterline

Properly designed boats intended for use with outboards have their transoms made to one or other of the "standard" lengths - short or long. Their is enough difference (5-6 inches) to make the difference obvious. Detailed measurement really only becomes an issue when using a transom bracket on say a small yacht or as a standby on a small motor boat where you need to measure so that you get the bracket mounted correctly. For example a high transom might need a long shaft engine so that the power head is accessible from the cockpit so the bracket has to be attached to the transom so the outboard is at the correct height when in use.

Suspect you are seeing a problem that does not really exist.

 

[jamie N]

My Honda 2.3 came as a long shaft unit, which was OK in flat water on my Folkboat, but would cavitate in any swell, so I had it lengthened and put in an extra Honda spacer piece, 6" long, which the manufacturer uses to lengthen the shaft from standard to long.
The only difficult bit was to have the shaft lengthened, but getting a local engineering firm and showing them the item was simple enough, and it works perfectly.

 

[ChromeDome]

Standard measuring:



For special uses and other hull designs it can be different, of course

 

[VicS]

Well owing to changing my boats again I am up against getting the right Shaft Length OB : On researching Dealers I get Short , Medium Short , Long , Extra Long which is really limited use when ones Stern measures up as an Inbetween size , the dealers say best measure my OB Shaft lengths then ;

Might ask why them Dealers cannot measure up a stock Outboards Shaft Length and put that in their adverts ?

Maybe a lot of Dealers used to sell cars or nappies before ? urg

Tranona has explained the difference between short (aka standard ) and long shaft engines..

For a small sailing boat, however, you will find that one size longer than indicated by the height of the transom top, or mounting bracket, above the water is advantageous otherwise you will find that motoring in choppy conditions becomes difficult , if not impossible, due to the prop lifting out of the water as the boat pitches.

Engine sold as "sail" versions tend to be 20" or 25 " shaft lengths for this reason. They usually also have a smaller pitch high thrust propeller which is suitable for a slow displacement hull.

 

[ChromeDome]

[.......]

 

[CapPugwash]

Well some manufactures have different definitions I found recently. Also, trying to find the measurements on dinghy manufacturers websites is bewildering difficult!

 

[Tranona]

Standard measuring:

View attachment 159074

For special uses and other hull designs it can be different, of course

That works for V bottom but not for displacement boats where waterline is the base used.

 

[Tranona]

Well some manufactures have different definitions I found recently. Also, trying to find the measurements on dinghy manufacturers websites is bewildering difficult!

Can you give examples of different definitions (apart from electric which are different. Almost all dinghies are short shaft except a few small ones which might be ultra short such as the Redcrest and Redstart although ordinary short shaft works OK on them. These standard dimensions have been around for decades and it is really difficult to make a mistake when there is 5 or 6 inches difference between a short and long shaft.

 

[Snowgoose-1]

Can you give examples of different definitions (apart from electric which are different. Almost all dinghies are short shaft except a few small ones which might be ultra short such as the Redcrest and Redstart although ordinary short shaft works OK on them. These standard dimensions have been around for decades and it is really difficult to make a mistake when there is 5 or 6 inches difference between a short and long shaft.

Depending what website is used, the sizes are around 15" for a short shaft, 20 " for a regular shaft and 25" for a long shaft . I just looked In my shed. The Evinrude 4hp is 17" and the 4hp Mariner is 18 .5 ". I agree that when sizing an outboard for a well or transom , it's important to find out the exact dimensions .

 

[Tranona]

Depending what website is used, the sizes are around 15" for a short shaft, 20 " for a regular shaft and 25" for a long shaft . I just looked In my shed. The Evinrude 4hp is 17" and the 4hp Mariner is 18 .5 ". I agree that when sizing an outboard for a well or transom , it's important to find out the exact dimensions .

What are you measuring. Both those manufacturers use the standard measurements from top of mounting to waterline.

 

[Snowgoose-1]

What are you measuring. Both those manufacturers use the standard measurements from top of mounting to waterline.

Hi
I'm measuring the outboards from the inside top of the mounting bracket to the top of the anti cavitation plate of the outboards.

 

[Dellquay13]

Hi
I'm measuring the outboards from the inside top of the mounting bracket to the top of the anti cavitation plate of the outboards.

The top of the cavitation plate sits below the waterline. The leg narrows as it goes up from the cavitation plate with the water pump impeller sitting inside the leg above the cavitation plate. The impeller needs to be below the waterline to work. Where the leg stops tapering and goes straight up is generally where the waterline should be on outboards small enough to be lifted on and off the boat, or up to about 15hp.
The cavitation plate should generally be either level with or 1-2” below the bottom of the transom, for the propellor to be in clear water undisturbed by aeration or bubbles caused by flow passing under the hull as the boat moves forward and to allow reverse thrust to go under the hull and not ineffectually push on the transom. This should apply to fast planing hulls, displacement speed hulls and tenders.