Outboard shaft length.

[munster1967]

I recently purchased a 2.8 meter. Inflatable dingy. Round tail. The transom sits on top of the tube and measures 13.5 inches deep. But from top transom to bottom of dingy measures 20 inches when inflated. Do I need a long shaft for such a small inflatable. ??

 

[munster1967]

This is the inflatable. With transom attached.

 

[VicS]

I recently purchased a 2.8 meter. Inflatable dingy. Round tail. The transom sits on top of the tube and measures 13.5 inches deep. But from top transom to bottom of dingy measures 20 inches when inflated. Do I need a long shaft for such a small inflatable. ??

Yes it seems you will need a long shaft outboard, but borrow a short shaft engine and try it.

 

[munster1967]

Thank. You I tried 2 , a 15 inch shaft which didn't work and a 17.5 which seems to work.

 

[VicS]

17.5" is an unusual size. Long shaft engines are usually 20"

 

[munster1967]

17.5" is an unusual size. Long shaft engines are usually 20"

Yes. It's a cheap 1.3 cc 4 stroke . Thingy.
Not a lot of power in it. A friend uses it on a kayak.

 

[munster1967]

17.5" is an unusual size. Long shaft engines are usually 20"

I can buy a tothatsu 3.3 2 stroke. Shaft length is 22 inches when measured. Is this then Too long.

 

[Tranona]

That type of dinghy is normally made for short shaft. It may not be exactly the normal 15 or 16" as measured but a long shaft will not work well. Once you get in the dinghy the waterline will shrink and the motor will be submerged enough to work. Some of these odd makers clearly have no clue about international standards! All the Chinese PVC inflatables follow the standard short shaft dimensions.

 

[munster1967]

That type of dinghy is normally made for short shaft. It may not be exactly the normal 15 or 16" as measured but a long shaft will not work well. Once you get in the dinghy the waterline will shrink and the motor will be submerged enough to work. Some of these odd makers clearly have no clue about international standards! All the Chinese PVC inflatables follow the standard short shaft dimensions.

Ok thanks for your reply. . It's a Ukrainian inflatable. 2.8 meter . A 15 inch doesn't work but a 17.5 seems ok. . A 22 inch tothatsu long shaft seems very long for it.

 

[VicS]

Ok thanks for your reply. . It's a Ukrainian inflatable. 2.8 meter . A 15 inch doesn't work but a 17.5 seems ok. . A 22 inch tothatsu long shaft seems very long for it.

The small Tohatsu engines are listed as either 15" or 20 " transom height

 

[Tranona]

Ok thanks for your reply. . It's a Ukrainian inflatable. 2.8 meter . A 15 inch doesn't work but a 17.5 seems ok. . A 22 inch tothatsu long shaft seems very long for it.

As I said it is not a standard size for normal petrol outboards but there are plenty of electric that have either longer shafts or variable length. If you want to use a 15" you may be able to cut the mounting board down, although from the photo it looks like it might be a plastic moulding.

 

[munster1967]

As I said it is not a standard size for normal petrol outboards but there are plenty of electric that have either longer shafts or variable length. If you want to use a 15" you may be able to cut the mounting board down, although from the photo it looks like it might be a plastic moulding.

Thanks for your reply.
It’s a wooden transom. 4.25 inches deep. I think I need 3 inches to take the clamp. So I could take 1.25 inches off it. I still think a 15 would be too short . But it might work.

 

[snowbird30ds]

Whatever the waterline when laden the prop still needs to be below the bottom of the dinghy in clear water, I would try a longshaft.

 

[Tranona]

Thanks for your reply.
It’s a wooden transom. 4.25 inches deep. I think I need 3 inches to take the clamp. So I could take 1.25 inches off it. I still think a 15 would be too short . But it might work.

If the anticavitation plate is below the bottom of the boat it will work. With inflatables the biggest problem is too deep immersion when the boat is loaded particularly if the helm sits at the rear which is why a long shaft is not a good idea. Not only does it raise the potential for back pressure in the engine but it increases the draft of the boat and lowers the point of thrust which can cause handling problems.

A lot of thought went into the design of inflatables and matching outboards in the early days (1960/70s) and just about all the mainstream makers follow the standard established then.

 

[Mark-1]

FWIW I regularly use a Tohatsu 3.5 2T long shaft and a Mariner 5hp 2T long shaft on inflatable dinghy and a rigid dinghy. They both work fine.

By definition they must put more levarage on the Transom but I've yet to have a problem.

 

[munster1967]

If the anticavitation plate is below the bottom of the boat it will work. With inflatables the biggest problem is too deep immersion when the boat is loaded particularly if the helm sits at the rear which is why a long shaft is not a good idea. Not only does it raise the potential for back pressure in the engine but it increases the draft of the boat and lowers the point of thrust which can cause handling problems.

A lot of thought went into the design of inflatables and matching outboards in the early days (1960/70s) and just about all the mainstream makers follow the standard established then.

I've since looked up some videos on you tube re the same dingy and a lot of guys saying the had to not only lower the transom but reinforce it too, with only a 2.6 HP attached. I think i will sell it on again and buy a inflatable with a fixed transom.

 

[billskip]

If the anticavitation plate is below the bottom of the boat it will work.

^^^^ this and old rule of thumb 1 inch below.....

 

[munster1967]

^^^^ this and old rule of thumb 1 inch below.....

Thank you, for your reply.