Help – Do I need a standard or short shaft?

[Dtwo1946]

I’ve posted a couple of times recently about the transom on my boat being raised to accommodate the long shaft Yamaha outboard that’s on it. I got some really helpful replies but some seemed to think that the boat was designed for a short shaft motor whereas I was under the impression that it needed a standard shaft.

The boat is a 1970s Aqua Jeep built by Chartermark Marine.

Can anybody out there give me a definitive answer so that I get the right shaft length whether I replace the whole engine or just the leg and drive shaft? Standard shaft or short shaft?

I don't know how to estimate the leg length required by measuring the gap from the transom top to the waterline because it's twin hulled. When it comes up on the plane the bottom of the hull centre may be marginally out of the water at high speed. Any advice on how to estimate it if nobody knows the answer?

 

[Fantasie 19]

I’ve posted a couple of times recently about the transom on my boat being raised to accommodate the long shaft Yamaha outboard that’s on it. I got some really helpful replies but some seemed to think that the boat was designed for a short shaft motor whereas I was under the impression that it needed a standard shaft.

The boat is a 1970s Aqua Jeep built by Chartermark Marine.

Can anybody out there give me a definitive answer so that I get the right shaft length whether I replace the whole engine or just the leg and drive shaft? Standard shaft or short shaft?

I don't know how to estimate the leg length required by measuring the gap from the transom top to the waterline because it's twin hulled. When it comes up on the plane the bottom of the hull centre may be marginally out of the water at high speed. Any advice on how to estimate it if nobody knows the answer?

Start here:

http://www.onlineoutboards.com/Help.html?Question=Shaft-Length

I'm no expert but really there are only two lengths of o/board - short and long - there are a few 'extra long' shafts about, but basicaly you have either long or short (and short being the most common it could probably be called "standard")

Measure the length as per the web link and you'll know what length engine you need...

It's generally cheaper to buy a new engine than try and source the parts to do a conversion - but I'm used to old 2 strokes, it may be different for newer 4 strokes...

Someone with infinitely more info than me will be along in a mo' to set me right....

 

[jfm]

Oooh, I remember the aqua jeep 430 very well. My father nearly bought one. We're talking around 1975-6. Is yours yellow?

I remember pretty strongly it needs a long shaft (20 inch). The builder supplied it rigged with engines up to the 80/85hp merc and in those days (as opposed to earlier) they only came in long shaft. For this boat, you do not want to measure the distance from the top edge of the transom to the bottom of the boat/keel, as you would with a normal boat. That is for the reason you say - it is a bit like a cat and rides on its semi twin hulls. In fact, there isn't a particular place to measure to.

All that said, I seem to recall seeing plenty that had the engine jacked up with a wooden shim block, about 2 inches, form rough memory. Only the bigger engines, not the 40s that many were supplied with. The builder might even have done this oem - it was a fast boat with a small bit of air entrapment and it makes sense to lift the engine (technical term is X dimension)

Anyway, if you're getting something around 80hp then I'd get a long shaft, but drill the mount holes so you can jack it up, or even start with it jacked 2 inches higher than "on the transom". If you're getting a smaller motor eg 40-50hp, then get a longshaft and just plonk it on in the usual way, not jacked up. Anything <40 is too small

Just on terminology:
Short shaft (aka standard shaft, but hardly anyone calls it that these days) is 15inch
Long shaft is 20 inch
Extra long is 25inch (big and recent motors only)
XXL is 30 inch (ditto - a recent thing for massive engines)
All measured from the top edge of the transom to the anti cavitation plate (the plate/soffit that sits right on top of the propeller

 

[Dtwo1946]

Thanks but because it's twin hulled and sails almost like a cat when it's on the plane I can't measure transom top to keel in order to establish the shaft length.

Start here:

http://www.onlineoutboards.com/Help.html?Question=Shaft-Length

I'm no expert but really there are only two lengths of o/board - short and long - there are a few 'extra long' shafts about, but basicaly you have either long or short (and short being the most common it could probably be called "standard")

Measure the length as per the web link and you'll know what length engine you need...

It's generally cheaper to buy a new engine than try and source the parts to do a conversion - but I'm used to old 2 strokes, it may be different for newer 4 strokes...

Someone with infinitely more info than me will be along in a mo' to set me right....

 

[Dtwo1946]

Thanks for all of this. It really is a great help and I think it's cleared the matter up for me.

Yes, it's mostly yellow but some arse painted the sides silver before I owned it! You're dead right about the engine being jacked up. Mine's up 3" on a double marine ply plate. See the pics.

The engine's a rock solid 1986 Yam 50HP 2 stroke, definitely a long shaft, and it flies when you open it up. I think I'm going to stick with it. I was going to change it for an engine with PTT but I'm now leaning towards a used PTT unit. Trouble is they're rare and expensive.

It seems I've got the option to lower it to the transom if I wish. I've kept pic's of other Aqua Jeeps seen on ebay and the engines, though obviously long shaft, aren't jacked up. I think I'll leave the motor where it is and check out the condition of the marine ply plates when I take the engine off to fit the PTT unit.

Thanks again

Oooh, I remember the aqua jeep 430 very well. My father nearly bought one. We're talking around 1975-6. Is yours yellow?

I remember pretty strongly it needs a long shaft (20 inch). The builder supplied it rigged with engines up to the 80/85hp merc and in those days (as opposed to earlier) they only came in long shaft. For this boat, you do not want to measure the distance from the top edge of the transom to the bottom of the boat/keel, as you would with a normal boat. That is for the reason you say - it is a bit like a cat and rides on its semi twin hulls. In fact, there isn't a particular place to measure to.

All that said, I seem to recall seeing plenty that had the engine jacked up with a wooden shim block, about 2 inches, form rough memory. Only the bigger engines, not the 40s that many were supplied with. The builder might even have done this oem - it was a fast boat with a small bit of air entrapment and it makes sense to lift the engine (technical term is X dimension)

Anyway, if you're getting something around 80hp then I'd get a long shaft, but drill the mount holes so you can jack it up, or even start with it jacked 2 inches higher than "on the transom". If you're getting a smaller motor eg 40-50hp, then get a longshaft and just plonk it on in the usual way, not jacked up. Anything <40 is too small

Just on terminology:
Short shaft (aka standard shaft, but hardly anyone calls it that these days) is 15inch
Long shaft is 20 inch
Extra long is 25inch (big and recent motors only)
XXL is 30 inch (ditto - a recent thing for massive engines)
All measured from the top edge of the transom to the anti cavitation plate (the plate/soffit that sits right on top of the propeller