Transom height for 25hp 2006 tohatsu

[kevinwd1]

I'm looking at replacing my old Johnson with a short shaft 25hp 2006 Tohatsu 2 stroke but can't find what the specs are for the shaft length.My current motor measures 17.5 inches with is just right ,any shorter and I could have problems.I know that short shafts vary from about 17.5 to 15 inches(15 inches would be to short).If someone could let me know the length of shaft it would be appreciated

Regards Kevin

 

[pvanv]

You will be fine. Modern Japanese motors are about 2" longer than the old spec.

 

[kevinwd1]

I looked at the Tohatsu manual and it states that the cav plate should sit 30 to 50 mm below the bottom of the boat. The boats transom height of 17" does not seem to suit either short or long shaft motors. The boat is a 4m aluminium hull and goes well with the current motor (Johnson short shaft) set with the cav plate level with the bottom of the boat. I would like to know what level people have their Tohatsu motors set at. I don't want to get a second hand motor have to make alot of adjustments because Tohatsu motors need to sit lower than my current Johnson. Any advice appreciated

Regards Kevin

 

[pvanv]

The factory recommendation is to be on the safe side, but is often too low. Rule of thumb is 1 inch above to 1 inch below. You must do on-water testing to find the sweet spot. I often find that I need to raise the 25/30 an inch on some transoms. I do that by trial and error with various spacers (1/4" at a time, ranging from 1/4" to 2") on top of the transom until I get the best performance (highest) without sucking air into the wp intake. Then drill the mounting bolt holes and lock it in there. Because of drag and splashing, low is slow; high is fast. But too high, and you won't get enough cooling water at speed. You want the surface of the water to be flowing between the large antiventilation plate and the smaller splash plate. Sea Rider has a [long] post that describes his process in detail.

 

[Jon Boat Jim]

I have heard that all Nissan/Tohatsu's tend to splash a lot (lots of spray). I had to raise my short shaft, 18HP, 2 stroke almost 1.5 inches to minimize the spray. I still have more spray than I like, but I'm living with it. I would have to rebuild the transom to raise it any further.

 

[Sea Rider]

Tohatsu/Nissan are real water splashers compared to Yam /Suzuki. Have tamed the beast moons back as currently work with these engines on different boating applications. It's very difficult to make a perfect engine/transom match, usually a transom height optimization is mandatory, check this example : that's the sweet height for any Tohatsu engine.

http://www.youtube.com/watch?v=oBeZjaA76VM&feature=youtu.be

Be aware that If water flow passes over small upper water deflector plate will produce over transom water splashes, in that case shim transom and test engine at speed untill over transom splashes dissapears. When that happens will have matched the perfect height. Once there will achieve the best prop thrust, impecable performance on flat, slight windy choppy seas including tight cornering on a dime.

Happy Boating

 

[kevinwd1]

The specs for short shaft give a dimension of 435mm from the bottom of transom mount to cav plate.

My transom measures 430mm,so it looks like cav plate will sit level with the bottom of the boat which gives no room for adjustment.Just have to hope it works first time




























dimension

 

[Sea Rider]

What's stated on Owner's Manual is plain theory as hull shape/design has not been taken into the slight consideration whatsoever. BTW, which type of boat do you intend to install than engine on ? Dry engine installations will not always match correct transom heights to provide opt engine thrust and overall top boat/engine water performance.

Someone said it's a good start, well it's the one and only position to sit engine on and test before optimizing engine height with whatever you need to do. But that will depend if being inside of the 99.5% of boaters that thinks the initial set up is ok for them, boat is moving along or part of the 0.5% who thinks a engine height optimization is mandatory to get the most out of portable engines as opposed to higher HP engines that compensates better the less performance of portable engines.

Happy Boating

 

[kevinwd1]

I brought a second-hand 25hp Tohatsu motor and sat it on the boat. It sits at exactly the same height as my old motor so all be equal should work fine. My problem now is try to get cables, control box etc. to get the throttle and gear shift to work. Unfortunately my old Johnson controls aren’t compatible. I have an old Mercury blue band control box that I’m hoping to use if modern cables will fit it. Does anyone know where you can buy (online), only the parts that connect the threaded part of the control cable to the throttle and gear shift on the motor

Regards Kevin

 

[Sea Rider]

Kevin,

Check : http://www.boats.net/parts/search/Tohatsu/Outboard/2003/M30A4/REMOTE CONTROL/parts.html

Happy Boating

 

[pvanv]

If your control uses standard Morse cables (with 10-32 thread), just get the proper clevises, nylon washers, and R-pins from your Tohatsu dealer.

 

[kevinwd1]

Well I took the boat for its first run with the motor and it ran perfectly with spray hitting upper water deflector plate similar to the YouTube link above. There was heaps of water coming out the tell tail and was luke warm. The water coming out the hole at the back of the engine was quite hot. I could keep my hand under it but only just. Is that OK? How much water should come out that hole .The amount of water coming out the hole was the same at both low and high speeds but didn’t seem as much as I would expect.
I have managed to fit the Johnson forward controls the outboard the only issue is that throttle control is opposite to normal. That is you pull the throttle lever back to accelerate. This will take a bit of getting used to but saved me at least $300. Any feedback on the amount of water that should flow out of the rear outlet would be appreciated.

I'm thinking replacing restrictor plate behind carb to make the motor a 30hp and was wondering if you get much more top speed or does it mainly just give you more load carrying capacity
Also what sort of oil do these engines run best on.I've just been using the cheapest I could find on my old motor with no problems

Regards Kevin

 

[Sea Rider]

Kevin,

Some issues :

-If you have matched the vid example, congrats, the engine is sitting at the sweet transom height.

-Buy a new carb gasket which is unrestricted, check if engine was factory delivered with timing set to 25 deg, if at just 20 move timing to 25. Some engines comes factory delivered with 20 and some with 25. Is your current 25 HP the latest upgrade D2 version ?

-Once at 30 HP will have slight more top end speed and better torque to transport more weight than with previous 25.

-Water should come through water port warm enough as not to burn your hand, there is a relationship between water temp, throttle and quantity. For your peace of mind install a new thermo and gasket speciallly if that engine has been used in salt water, probably previous owner was not a big fan of flushing it after use.

-Use any good TCW-3 oil brand of your prefference, mix it at 50:1, go for a higher octane fuel and run carb dry after each outing if engine will not be constantly used on a daily or weekly basis.

Happy Boating