Disadvantages of increasing outboard shaft length?

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I may need to buy an outboard for a trailer sailer and am looking at the Tohatsu 6HP Sail Pro. It's available as long and extra long (+5") shaft. Other than needing more draft and slight extra cost, are there any other disadvantages of getting the extra long version? My thinking is that the extra long shaft will keep the prop in the water more if things get lumpy.
TIA
Andy
 
Only other drawback I can see is slightly more drag but I would put up with this to increase grip and help reduce cavitation.

Pete
 
The extra back pressure on the exhaust might make starting more difficult ... perhaps. Its what is always said but I am not convinced.

The extra 5" depth makes a lot of difference in keeping the prop in the water in choppy conditions.

I had a standard ( 15" ) shaft outboard originally. Almost impossible to make any progress under power in choppy conditions.

I now have a longshaft engine... much better.

BUT mine is an Evinrude Yachtwin. It has different exhaust system from the ordinary long shaft engine. It has exhaust outlets higher up and the normal exhaust outlet is blanked off. This eliminates the back pressure issue and by not discharging the gases behind the prop prevents them being drawn into the prop when going astern.
It also has a low pitch high thrust prop more suitable for powering a displacement hull

If your outboard mounting is the "correct" height above the water (15") for a standard/short shaft engine then you will benefit from a longshaft (20") engine. If is normally needs a longshaft then opt for the extra long shaft (25") engine.

Remember if you go for an excessively long shaft you may not be able to raise the engine clear of the water when sailing.
It will be more difficult to stow and transport.

The Sail versions of the Tohatsu engine should have a high thrust prop even if they dont have the modified exhaust system.

 
I used to have a trailer sailor. One problem i remember is when pulling up the anchor, the prop area would lift out of the water. I could tell by the exhaust noise. I often wondered if this would affect the coolant intake?

Choppy conditions and motor sailing, I would go for the long shaft.

Cliff
 
I have a Tohatsu 6hp four stroke, long shaft, bought it 6 years ago. I use it on my 2.9 inflatable and as a back up on the yacht in case the old Volvo decides to stop one day.

I was surprised to see how well the long shaft works on the inflatable, there is no cavitation at all, even when there are waves. The engine is very reliable and it will start mostly on the first pull. No noticeable issues with the back pressure due to the extra length. Solid well made engine; Great choice.
 
I have had a 30Hp Tohatsu 2 stroke and a 20 Hp 4 stroke, both ultra long shafts and I have never notice any back pressure problems in 10 years use also the extra 5 inches immersion really does make a difference in keeping the prop in the water. Worth the extra for ultra long as a main engine and Tohatsu are one of the few that do an ultra long shaft and saildrive which is essentially a bigger prop turning slower as it has a higher gear ratio for displacement hull use.
IMHO Tohatsu are very good engines and cheaper than Yamaha who also do saildrive engines.
Interestingly Tohatsu are the only ones that do Ultra long shaft options in the 15/20/25/30 range.
 
Is it going in a well or on the back?

If it's in a well, once it's in the water any extra depth wouldn't have much more effect, I would have thought.

On the back any extra depth will help in a wind over tide situation or when motorsailing on the wrong tack. Being able to get the prop out of the water is good though, that extra 5" may stop that.
 
I don't think you will get any increase in start up back pressure due to deep immersion as the pee hole in the exhaust being above waterline will cause the pressure in the exhaust/water outlet of the power head to be normal atmospheric pressure.

Thats how my 2 stroke Suzuki works anyway.
 
This back pressure thing gets churned out every time outboards are discussed.

I've had all sorts of small outboards and never had a problem. You are talking about 5 or 6" extra draft. That is about 0.25 psi extra pressure!!
If your motor won't start because of that there is something very much amiss with your motor.
 
Lakesailor said:
If your motor won't start because of that there is something very much amiss with your motor.
Click to expand...

Used to sail a company club boat, a Hunter 701, on a mooring in the Mersey. It had a 5 or 6hp long shaft 2-stroke engine of some vintage. The same engine was used on the dinghy to get to the mooring where it was very low in the water. Never once did it start when cold on the dinghy; we always ended up rowing. Put it on the main boat - started easily every time. On the way back in the dinghy when warm it would start ok. But maybe I'm just imagining it all.
 
Lakesailor said:
This back pressure thing gets churned out every time outboards are discussed.

I've had all sorts of small outboards and never had a problem. You are talking about 5 or 6" extra draft. That is about 0.25 psi extra pressure!!
If your motor won't start because of that there is something very much amiss with your motor.
Click to expand...

I did say I was not convinced !


My Seagull starts even when the water is lapping over the transom so thats a good bit more than 5" deeper than it should be.

But then being a Seagull its starts so easily anyway. Even done it by spinning the flywheel by hand.
 
Last edited:
AngusMcDoon said:
Used to sail a company club boat, a Hunter 701, on a mooring in the Mersey. It had a 5 or 6hp long shaft 2-stroke engine of some vintage. The same engine was used on the dinghy to get to the mooring where it was very low in the water. Never once did it start when cold on the dinghy; we always ended up rowing. Put it on the main boat - started easily every time. On the way back in the dinghy when warm it would start ok. But maybe I'm just imagining it all.
Click to expand...
Not imagining ---- but what you say about the difference between it being cold and warm is a clue to it's condition.
 
Thanks for all replies. I'm persuaded that the ultra long shaft is the best bet, the motor is transom mounted, just need to check the prop clears the water when raised.
 
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