Outboard - long or short shaft?

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GrahamM376

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Our 6HP 4 stroke Suzuki short shaft is underpowered for the inflatable and I had to fit wings to stop cavitation. I bought a 9.8 Tohatsu off ebay which arrived today and turns out it's a long shaft, not short shaft as advertised!

Obviously I could reject it and send it back but wonder how it would perform on the inflatable. May have difficulty rejecting if I've had it ion the water. It's going to ground earlier being 5" deeper but the extra depth may get rid of the cavitation problem.

Any thoughts? Anyone using a long shaft?
 
I dont know if you are talking 2t or 4t. I had a 2 t long stroke and tried using it on our dinghy. It did not like being immersed deeper than designed and ran like a dog.

I converted it to short shaft. In my case the leg had an extension that could be removed. The water tube and gear linkage were easy to cut down and the drive shaft was replaced for a short shaft. (alternatively the long drive shaft could have been cut short and welded.
 
GrahamM376 said:
Our 6HP 4 stroke Suzuki short shaft is underpowered for the inflatable and I had to fit wings to stop cavitation. I bought a 9.8 Tohatsu off ebay which arrived today and turns out it's a long shaft, not short shaft as advertised!

Obviously I could reject it and send it back but wonder how it would perform on the inflatable. May have difficulty rejecting if I've had it ion the water. It's going to ground earlier being 5" deeper but the extra depth may get rid of the cavitation problem.

Any thoughts? Anyone using a long shaft?
Click to expand...

Advantage is that it will likely help your cavitation problem and will therefore have more bite.
Disadvantage is that you will need to be aware it will ground a little earlier than your old motor. Sell boat shoes and buy boots, sorted :)
 
TSB240 said:
I dont know if you are talking 2t or 4t. I had a 2 t long stroke and tried using it on our dinghy. It did not like being immersed deeper than designed and ran like a dog.

I converted it to short shaft. In my case the leg had an extension that could be removed. The water tube and gear linkage were easy to cut down and the drive shaft was replaced for a short shaft. (alternatively the long drive shaft could have been cut short and welded.
Click to expand...

The current Suzuki DF6 is 6hp 4 stroke, the Tohatsu is 9.8hp 2 stroke. Both engines weigh the same at around 27kgs. The DF6 is a lemon!

I've now emailed 3 parts suppliers for Nisan/Tohatsu to find out the cost of new and second hand parts.

Doing some more Googling, there are a lot of negative comments about using long shaft due to poor starting because of increased exhaust back pressure and, worries about extra leverage on transom. However, I'm looking for practical experience rather than theory.

Thanks to those who have replied.
 
Watch the extra forces on the transom do not start ripping it out...

Seen it happen to one or 2 dinghys they can come unglued over time...
 
TSB240 said:
I dont know if you are talking 2t or 4t. I had a 2 t long stroke and tried using it on our dinghy. It did not like being immersed deeper than designed and ran like a dog.

I converted it to short shaft. In my case the leg had an extension that could be removed. The water tube and gear linkage were easy to cut down and the drive shaft was replaced for a short shaft. (alternatively the long drive shaft could have been cut short and welded.
Click to expand...

I agree regarding deep immersion affecting running. I have a 2 stroke Mariner 3.3 which refused to start when we had 4 adults in our dayboat but started perfectly well with 2. Of course, with hindsight I would now raise the bracket for starting.
 
The leverage of having the prop lower down may twist the transom backwards!
How much longer is the shaft?
Can you raise the motor a little by putting a block on top the transom? (be very sure the motor is totally secure).
 
GrahamM376 said:
Our 6HP 4 stroke Suzuki short shaft is underpowered for the inflatable and I had to fit wings to stop cavitation. I bought a 9.8 Tohatsu off ebay which arrived today and turns out it's a long shaft, not short shaft as advertised!

Obviously I could reject it and send it back but wonder how it would perform on the inflatable. May have difficulty rejecting if I've had it ion the water. It's going to ground earlier being 5" deeper but the extra depth may get rid of the cavitation problem.

Any thoughts? Anyone using a long shaft?
Click to expand...

I have a 9.9 Suzuki short shaft with less then 30 hours but bought a new sailboat boat that requires a long shaft so I am looking to either convert this one or trade someone for a longshaft. What did the 9.8 run new?
 
boomdiada said:
I have a 9.9 Suzuki short shaft with less then 30 hours but bought a new sailboat boat that requires a long shaft so I am looking to either convert this one or trade someone for a longshaft. What did the 9.8 run new?
Click to expand...

Wouldn't want another Suzuki (bad experience) so not interested in trading for one, sorry.

Looked at cost of converting the Tohatsu to short shaft and parts are around £250-£300. Not done anything with it yet, have been away sailing. 2 locals wanted to pay me 800 euro for it but it turned out they're not allowed to fit it to a Portuguese boat because I don't have complete paper trail from new! Crazy situation here, if your engine breaks down on a Portuguese registered boat, you can't fit a spare as that would entail reregistration.
 
Short / Long Shaft motors

Hi everyone;
New here but I have something to ask. I've read a few of the comments about the use of long shaft motors on short transoms. I have a long shaft Suzuki DT2 motor that I got with the Nordica 16 I purchased last spring. I haven't had the sail boat in the water yet (trailer issues) but I have used the motor.

In a test with a water filled garbage can everything ran great.
On my 12' row boat, I found the motor over heated quickly. It would start and run great then I could hear the motor labour and I knew something was wrong.

Unfortunately I've been on a learning curve with this motor which = $$ spent.
I overheated to the point of melting the fuel tank (lucky I didn't go up in a blaze 'cause it wasnt' a great situation.)
I replaced the fuel tank and the impeller suspecting it was the obvious problem - no water = over heating.

I've had the motor a part after a second run out and had the same over heating problems. I inspected all the ports, cleaned them out, installed a new water pipe (the old one broke trying to get the lower shaft off the base of the motor). I took the motor out again with these new parts and still had the same problem! @#$!!

I've thought about this now enough to wonder. If the water column in the shaft is too high (long shaft too deep in the water) would this cause enough back pressure from the engine exhaust which ports directly down the shaft to keep water from moving up the tube into the engine. It only has a small port which it empties out of into the shaft. And I admit as others have experienced, with the water exiting below the water line with only very tiny spits of water out tiny holes for any inspection, by the time it starts to over heat; damage has occurred. This is frustrating because I know this motor was deemed very reliable and solid, as older motors go.

Any thoughts or opinions??
Thank you,
Eric, Nova Scotia
 
TSB240 said:
I dont know if you are talking 2t or 4t. I had a 2 t long stroke and tried using it on our dinghy. It did not like being immersed deeper than designed and ran like a dog.

I converted it to short shaft. In my case the leg had an extension that could be removed. The water tube and gear linkage were easy to cut down and the drive shaft was replaced for a short shaft. (alternatively the long drive shaft could have been cut short and welded.
Click to expand...
Hi,
New here. I posted another comment about this but I'll tag on to here as well.

I have a Suzuki DT 2 outboard, long shaft. I've had a heck of a time with it being able to stay cool. I've replaced the impeller, replaced the water tube and cleaned out all the ports. My feeling is it doesn't like being long-shafted on a short transom, as you suggest.
I suspect the back pressure from the engine exhaust enters the water port that runs out of the engine and down the shaft. Less area above the water inside the shaft would create greater pressure, I would assume. I think this is part of my problem.
It is frustrating this motor ports its exhaust water under the water line, just above the prop. It is impossible to tell if the motor is receiving cooling water until too late.
The pee-holes are so tiny, nothing can get out, really. I'd consider drilling them out bigger to help port some of the exhaust if I didn't think it would affect the motor when I put it on my 16' sailboat. Any thoughts???
Eric, Nova Scotia
 
If it's overheating to that extent then the cooling water passages are partially blocked. Usually the best fix is to strip the motor and physically clean them.
You can fanny about as much as you like but until they are clear it will overheat.

From a web forum
it is designed to "spit"!
when the cooling water leaves the water jacket, it enters the exhaust tube and joins up with the exhaust gases, and the bulk of the cooling water exits under water out the exhaust pipe! the two "spitting" holes are just there to relieve back-pressure in the exhaust tube!
Click to expand...
Which tells us two things.
1 The immersion problem isn't a problem

2 There is probably no water exiting the water jacket. If you take the powerhead off the leg you find the channels available to clean out. The inlet side and outlet side both have hole through to the head and these clog up. Best to not only clean them but ream them a bit bigger.
 
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I have never before seen any suggestions before that a longshaft engine mounted on a "short shaft" transom will suffer overheating as a result.

The usual objection is that the back pressure may make starting difficult... but I'm not so sure that's even fact.

It is not unusual to find longshaft engines being used on small sailing cruisers that in theory would accept a short shaft engine in order to prevent the prop being lifted from the water when the boat pitches in choppy conditions.

IMO the pump, at least if its the usual flexible vane type, should produce adequate pressure for the water exiting the power head to overcome the increased exahaust gas pressure inside the leg.
 
I agree with you Vic. The deep immersion theory is just that, a popular theory. If your motor will start in a couple of pulls there isn't opportunity for back pressure to build up. The difference between short and long shaft is only a nominal 5". If the increase in pressure in 5" of water is enough to stop your engine starting, you should be looking for other issues.
 
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