Transome height and shaft length.

[DocTide]

Quick question, my transom height is 18in. Does that mean I have to go with a 20in shaft? My transome is a bit odd shaped, and heard that drag and the wrong shaft length will decrease performance of the engine.



Heres how my transome looks. I can measure the inside of the small length horizontal portion if that matters. Since the Tohatsus' have clamps as well, this might be an issue?

Thanks.

 

[JB]

Re: Transome height and shaft length.

Is the 18" the height measured on the surface of the transom or vertically from the top? Some transoms are tilted enough to make the vertical distance shorter than the surface height.

Both long and short shaft outboards can be adjusted to place the AV plate higher or lower. Which to start with is not obvious to me in your case, but I think I would start with a short shaft mounted as low as possible or with the AV plate slightly above the level of the bottom (or keel.)

What Tohatsu outboard on what boat? That could make a difference.

 

[Sea Rider]

Re: Transome height and shaft length.

According to your drawing, you are in the middle of nowhere transom land, if placing a S engine, prop will be too high, if placing a L engine, prop will be too low with splash issues at back transom. Performance will vary depending on the hull type and transom height, Do you have the engine already, or plan to buy one accordingly. In your particular case, what would be easier/cheaper for you to do, cut transom down and use a S engine, rise transom upwards and use a L engine ? With actual transom height both S & L engines will move that boat, but won't perform as expected at speed & close turns.

Happy Boating

 

[pvanv]

Re: Transome height and shaft length.

Japanese OB's are actually almost 2" longer than the "nominal" 15" shaft length. So a Short Tohatsu is usually OK (perfect) on a 17" transom. As mentioned, 18" is kind of a "no-man's land" for transom height. You will need to install and test for the correct jack height, but a Short is likely what you will want. If the anti-ventilation plate runs too high at speed, you will need to lower the top of the transom accordingly. That said, you can jack a Long motor with blocking, possibly up to the height of the main part of the transom, but that may be 3" or more, so... to run a Long motor, you may need a jack plate.

 

[Sea Rider]

Re: Transome height and shaft length.

Japanese OB's are actually almost 2" longer than the "nominal" 15" shaft length. So a Short Tohatsu is usually OK (perfect) on a 17" transom. As mentioned, 18" is kind of a "no-man's land" for transom height. You will need to install and test for the correct jack height, but a Short is likely what you will want. If the anti-ventilation plate runs too high at speed, you will need to lower the top of the transom accordingly. That said, you can jack a Long motor with blocking, possibly up to the height of the main part of the transom, but that may be 3" or more, so... to run a Long motor, you may need a jack plate.

Yeap, you are right on the overall ideal, I would go for a S shaft, test at wot and make some small transom height modification if needed.

Happy Boating

 

[DocTide]

Re: Transome height and shaft length.

Well, as mentioned, I just wanted to go to dealer and have a new one installed. I assumed it would be a short shaft, but now that seems troublesome. I dont plan on ripping the engine by going super fast and making tight turns, but incase I need it... This bums me out a bit, because it sounds like it is going to make the instal more difficult and longer, increasing the overall price of the engine! Well see.

 

[TOHATSU GURU]

Re: Transome height and shaft length.

You are over thinking this...A little. It's an imperfect world. Go with the short and be prepared to be told that the transom needs a little minor trim. Maybe an additional $25.00 worth of a dealers time and aggravation.

 

[DocTide]

Re: Transome height and shaft length.

Good to hear, and I will still be giving you a call for this when the time comes. Stocks been taking a hit but I will be calling you before December to get this thing put on. Thanks again everyone.

 

[Sea Rider]

Re: Transome height and shaft length.

To end the post, you have a aluminum, wooden or fiberglass transom ? Which HP were you thinking installing ? Is it possible to test a Tohatsu short shaft engine and see it's performance, definitely will work better than a L one.

Update, cut another 18" fiberglass transom down to 16" and placed a Tohatsu short shaft at back, now performs fantastic, excellent top speed, close turns with no prop aireation. It's the way to go.

Happy Boating