Long or short shaft

[Craigponder]

Currently have an older Duraboat 14 and cant read the plate on the boat as to whether its a L for long or S for short transom. The transom measures 16". My 1960 Evinrude Fastwin 18 hp, measures 18" to the cavitation plate. It works well on this boat.
I'm thinking of getting a new Tohatsu 20hp and they come in 15" and 20" lengths. Seems I'm at a loss as to what to think. Here is from their website and still I cant hone in on what would be correct. Any comments would be great. So basically I have an existing boat with a 16" transom and wonder which new Tohatsu would be correct, the 15" or 20"?

" If you are trying to match a motor you already own to a new boat your choice is determined by the distance from the mounting surface of the motor to the anti ventilation plate on your motor. The distance will be approximately 15 inches (short) or 20 inches (long). select a motor to match. If your motor is between those dimensions but definitely less than 20 talk to your dealer. Most likely you will get a short. It takes at least a 20 inch motor shaft to perform well on a long transom boat."

 

[racerone]

A 15 " transom is for short shaft motors.---Your 1960 Fastwin would have a 5" spacer if it was a long shaft.-----A 16" transom is also considered short by many builders.-----Order a shortshaft ( 15" ) motor from Tohatsu

 

[440roadrunner]

You want to measure it "plumb" rather than down the incline of the transom. If you "Google" (search) this there are some charts, short is around 15 from ventilation plate to top of clamp area, long is around 20. "It varies."

 

[Craigponder]

Thanks you guys. Great info. I found this at online outboards as well. Clears up what you spoke to racerone.

 

[Sea Rider]

If plan going for a Tohatsu 20 HP short shaft motor, need to check if powers the boat efficiently. Trim it to 90 deg, with deck load well distributed go for a wot run, once on plane, pull your head side of transom and check at which lower leg's plate is the hardened water flow paasing by.​

These motors have a long gap in between plates, would be ideal for the water flow to skim right under the small upper plate, that setting works top for in all water conditions, whereas the lower AV plate setting is only good for straight flat water courses, prop will aerate on choppy, windy and close tight turns at speed much worse if the prop has been maximized to rev more with a less pitch from the factory delivered one...

Happy Boating
​

 

[Texasmark]

Those engines sit low. I had a 1956 18 Fastwin I bought new so I know. Get any vintage sales brochure and check pictures of those old OMCs sitting on a trailer with the engine in the normal running position. You'll see the AV plate below the bottom of the transom. I never dug into the cooling system on mine but it may have to do with the water associated plate on the side, just above the AV plate or the fact that the water PU is short and is situated just under the rear of the AV plate. or the fact that the AV plate was small with rounded corners. Since 30 MPH was probably tops for those old girls, it doesn't make a whole lot of difference.

 

[Sea Rider]

Only a wot run as stated will indicate where is the water flow paasing at plane that's under the upper small plate or over the AV plate. Both plate settings will dial different water performances, being the first one the best overall permormer in all water conditions with modern Tohatsu motors.

On a dry installation with motor sitting flat on transom won't know where will the water flow phsically pass by as the boat once on plane could float more or less with prop thrust varying accordingly. There's the idiotic idea that could achieve a higher rpm and speed increase difference with the AV plate setting. When tested a Tohatsu 30 HP motor with maxed prop to run slight over 6K, the rpm difference achieved playing with both plate's settings was only 70 rpm more and with that low rpm increase don't expect to achieve a wow speed increase to count with a speed machine to fly you direct to Mars.

Happy Boating
​

 

[QBhoy]

If plan going for a Tohatsu 20 HP short shaft motor, need to check if powers the boat efficiently. Trim it to 90 deg, with deck load well distributed go for a wot run, once on plane, pull your head side of transom and check at which lower leg's plate is the hardened water flow paasing by.​

View attachment 353662

These motors have a long gap in between plates, would be ideal for the water flow to skim right under the small upper plate, that setting works top for in all water conditions, whereas the lower AV plate setting is only good for straight flat water courses, prop will aerate on choppy, windy and close tight turns at speed much worse if the prop has been maximized to rev more with a less pitch from the factory delivered one...

Happy Boating
​

Hi searider. Always love your posts and we share a common love of inflatables too, from memory…but honestly, that picture you’ve posted above has the height set up a little low and likely wrong. Far too low in the water for any average sports boat use. Even Running with the AV plate level with the lower hull would be considered a sensible and conservative set up on most average boats. And would never lose grip of the water at that. But that picture above is seriously low set, beyond even that. Just my opinion.

 

[Sea Rider]

Hi QBhoy, to clarify :

The posted pic which corresponds to a S size tiller/clamp, manual trim, tall 4.0 inch gap plates distance motor same version for which the OP is requesting tech advice. That setting works spot on in All Open water conditions, that's from flat glassy to medium chop windy sea conditions and at tight close turns at speed, whereas the AV plate setting only works OK on flat calm no wind water conditions such as a closed bays, ponds, river at straight water courses. Outside that ideal boating scenario the prop will aerate badly at speed on straight water courses and tight close tuns.

What you guys don't understand is that on those kind of motors there's a tall sharp edge that goes from the upper small plate to the lower AV plate where the hardened water flow is perfectly cut at any portion of that edge, will add that these motors have been max propped to run at their max wot rpm range as loaded, at that low setting the prop achieve its best forward thrust. Doesn't matter if there's a slight lower leg drag increase, the motor is already running a maxed prop which has cancelled out the tiny lower leg drag increase. It's impossible to quantify how much drag is increased with just 70 wot rpm difference achieved on water tests between both plate's height settings.

I'm so sure about my proven posting that made a public $ 500 dollar bet to anyone that proved me the flat hardened water flow exiting under the aft hul while skimming around the AV plate setting worked much better compared to the neat performance the small upper plate setting achieved which have been promoting ever since. What nobody took into account is the country's geographycal location and the local open water condition which varies greatly between oceans. Personall don't give a crap where the AV plate sits with respect of the boat's aft hull on a dry installation, for me it's all about at which lower leg is the water flow passing by at plane when at full throttle and go from there fine tunning the motor/transom height setting if in need to modify it. Lets see if the OP finally reports what was asked as a starter...

Happy Boating
​

 

[QBhoy]

The posted pic which corresponds to a S size tiller/clamp, manual trim, tall 4.0 inch gap plates distance motor same version for which the OP is requesting tech advice. That setting works spot on in All Open water conditions, that's from flat glassy to medium chop windy sea conditions and at tight close turns at speed, whereas the AV plate setting only works OK on flat calm no wind water conditions such as a closed bays, ponds, river at straight water courses. Outside that ideal boating scenario the prop will aerate badly at speed on straight water courses and tight close tuns.

What you guys don't understand is that on those kind of motors there's a tall sharp edge that goes from the upper small plate to the lower AV plate where the hardened water flow is perfectly cut at any portion of that edge, will add that these motors have been max propped to run at their max wot rpm range as loaded, at that low setting the prop achieve its best forward thrust. Doesn't matter if there's a slight lower leg drag increase, the motor is already running a maxed prop which has cancelled out the tiny lower leg drag increase. It's impossible to quantify how much drag is increased with just 70 wot rpm difference achieved on water tests between both plate's height settings.

I'm so sure about my proven posting that made a public $ 500 dollar bet to anyone that proved me the flat hardened water flow exiting under the aft hul while skimming around the AV plate setting worked much better compared to the neat performance the small upper plate setting achieved which have been promoting ever since. What nobody took into account is the country's geographycal location and the local open water condition which varies greatly between oceans. Personall don't give a crap where the AV plate sits with respect of the boat's aft hull on a dry installation, for me it's all about at which lower leg is the water flow passing by at plane when at full throttle and go from there fine tunning the motor/transom height setting if in need to modify it. Lets see if the OP finally reports what was asked as a starter...

Happy Boating
​

Each to their own for sure. I’ve spent a bit of time messing with engine heights. Just thought I’d add what I’d found to be best set up. My 19ft cuddy with merc 115 ct is currently running at its best with the AV plate around an inch or so above the lower line of the hull. On the other end of things…my wee cvx16 with its merc 150 has it run about just over 3” above. Set up for straight line speed. Would I be turning that boat fast ? Not at all. The 19ft will turn and still grip at any speed. Even at its full speed of 45 mph it won’t lose grip. Only the guy in the seat will.

 

[Sea Rider]

What works for you running a larger heavier boat with a large 115 HP motor at your particular leg's height setting doesn't work for me while runing large sibs/ribs with usual medium light (L) 18-30-40, 2 stroke maxed propped motors sitting on the back. Let's leave it there. Merry Christmas!!

Happy Boating​

 

[QBhoy]

What works for you running a larger heavier boat with a large 115 HP motor at your particular leg's height setting doesn't work for me while runing large sibs/ribs with usual medium light (L) 18-30-40, 2 stroke maxed propped motors sitting on the back. Let's leave it there. Merry Christmas!!

Happy Boating​

Merry Xmas to you too searider.