Why won't my outboard go any faster & do I need a new one?

[MissFitz]

When I bought my Sonata this year, it came with an old 4hp long-shaft Mariner two-stroke outboard. It had been in a shed for a while so I got it serviced before bringing the boat back to Brighton. It did good work on the delivery, including getting me most of the way across Lyme Bay, but all at a maximum of 3.5kt. Any attempts to increase the speed cause the revs to go up & down madly for no apparent reason.

This would be fine if I was berthed somewhere with a modicum of shelter - but in Brighton I often need to be able to motor out into wind & a fairly heavy swell, which I'd be happier doing with more oomph in the engine. (I can't sail in & out as a) it's banned & b) more importantly, there's a massive wind hole right where you start to hit the swell. Which is not so bad coming in but a problem going out.)

I've read somewhere that hoping to get more than 3.5kt out of an old two-stroke is optimistic. Is that right? If not, should I spend more money on getting the one I've got seen to or invest in one in better condition? Or would I be better off getting a newer four-stroke for local sailing/racing & is that going to cost me a fortune?

A slight complication is that I do at times (eg RTI race) need to be able to take the engine off the back & stow it under the cockpit floor, which is obviously much easier with a light two-stroke (I'm small & not particularly strong). However, I'm fairly sure I can leave it on for local racing if necessary.

Any help much appreciated as always.

 

[RichardS]

I think Mariner are a good make and 2T engines are powerful for their light weight when they're running well.

The problem is that any 4T engine of similar or greater power is going to be much heavier. You could perhaps pop into a chandlers and ask to lift a few outboards (they will all be 4 strokes) around 5HP and above to see whether you could cope with it.

I suspect that you will come to the conclusion that it's worth getting the Mariner sorted. 2T's are very simple engines so there probably not that much wrong with it but whether it's worth repairing probably depends upon how far up the rev range you were when it starts hunting. If that happens at only 50% throttle then the engine would probably have plenty enough poke for you if you could get it to 100% for short bursts. On the other hand, if it starts hunting at 90% throttle then there's not much more to give anyway and looking for a second hand 2T might be more worthwhile.

Richard

 

[macd]

Any attempts to increase the speed cause the revs to go up & down madly for no apparent reason. .

Overall, I echo Richard's advice. However, the particular phenomenon you mention above, assuming the prop isn't simply responding to sea-state, could be due to fuelling issues or, possibly, crankcase seals on their way out. This can allow air into the crankcase and thence the combustion chamber, leaning-off the mixture and producing erratic bursts of higher revs. If this is the issue, it can only get worse and needs sorting.

 

[TSB240]

It did good work on the delivery, including getting me most of the way across Lyme Bay, but all at a maximum of 3.5kt. Any attempts to increase the speed cause the revs to go up & down madly for no apparent reason.

This would be fine if I was berthed somewhere with a modicum of shelter - but in Brighton I often need to be able to motor out into wind & a fairly heavy swell, which I'd be happier doing with more oomph in the engine. (I can't sail in & out as a) it's banned & b) more importantly, there's a massive wind hole right where you start to hit the swell. Which is not so bad coming in but a problem going out.)

I've read somewhere that hoping to get more than 3.5kt out of an old two-stroke is optimistic. Is that right?

Any help much appreciated as always.

I would suggest you look at what prop you have on this engine. It sounds to me like you need a sail drive prop if available. We used to use our 4 hp 2T on our similar Trapper ts 240 with similar results. The engine was able to push a lighter dinghy at much more than 3.5 knots . It just acts like an egg whisk when you apply too much power and the prop cant get a grip on the water.

 

[Pete7]

I would suggest you look at what prop you have on this engine. It sounds to me like you need a sail drive prop if available. We used to use our 4 hp 2T on our similar Trapper ts 240 with similar results. The engine was able to push a lighter dinghy at much more than 3.5 knots . It just acts like an egg whisk when you apply too much power and the prop cant get a grip on the water.

This ^^^ plus is the propellor down below the transom in clear water or does the transom block some or all of the propellor.

 

[Teko]

Ideally you want a sailmate engine or similar with a low pitch prop like this https://m.ebay.co.uk/itm/mariner-4h...658871?hash=item1ed4599277:g:qcsAAOSwsYpaGY9c

 

[lw395]

A fine pitched prop might help the engine rev without 'losing grip' or cavitating.
A good outboard retailer might be able to help.
The engine will need to be revving quite hard to develop its max power.

 

[VicS]

When I bought my Sonata this year, it came with an old 4hp long-shaft Mariner two-stroke outboard. It had been in a shed for a while so I got it serviced before bringing the boat back to Brighton. It did good work on the delivery, including getting me most of the way across Lyme Bay, but all at a maximum of 3.5kt. Any attempts to increase the speed cause the revs to go up & down madly for no apparent reason.

This would be fine if I was berthed somewhere with a modicum of shelter - but in Brighton I often need to be able to motor out into wind & a fairly heavy swell, which I'd be happier doing with more oomph in the engine. (I can't sail in & out as a) it's banned & b) more importantly, there's a massive wind hole right where you start to hit the swell. Which is not so bad coming in but a problem going out.)

I've read somewhere that hoping to get more than 3.5kt out of an old two-stroke is optimistic. Is that right? If not, should I spend more money on getting the one I've got seen to or invest in one in better condition? Or would I be better off getting a newer four-stroke for local sailing/racing & is that going to cost me a fortune?

A slight complication is that I do at times (eg RTI race) need to be able to take the engine off the back & stow it under the cockpit floor, which is obviously much easier with a light two-stroke (I'm small & not particularly strong). However, I'm fairly sure I can leave it on for local racing if necessary.

Any help much appreciated as always.

The wild fluctuations in revs may be due to the prop lifting from the water as the boat pitches, due to not being mounted deeply enough. I had this problem with a standard shaft engine which was solved with a long shaft, putting the prop deeper in the water. I would investigate this first before splashing cash on a lower pitched prop although that should give improved performance under power.

Personally I would want a 6 hp on a 23 ft boat but that is likely to be somewhat heavier.

I have a 6 hp longshaft sail power engine on a 19ft boat. It never fails to push the boat through the water ........ but over the years it has become very heavy. Certainly I can no longer lift it on an off its bracket singled handed.

 

[MissFitz]

The wild fluctuations in revs may be due to the prop lifting from the water as the boat pitches, due to not being mounted deeply enough. I had this problem with a standard shaft engine which was solved with a long shaft, putting the prop deeper in the water. I would investigate this first before splashing cash on a lower pitched prop although that should give improved performance under power.

This is a problem as even with a long-shaft the prop is only just in the water & comes out any time someone goes forward of the mast. However the fluctuations in revs happen even in completely flat water with all the larger crew members standing at the back of the cockpit, so I don't think it's that!

 

[thinwater]

^^ Yes, that's the problem. Even in flat water, it is running so near the surface there is no water pressure.

Longer shaft, go slow, or mount the engine lower. A larger engine will work a little better because it will be turning slower, but not much.

Transom-mounted outboards suck.

 

[Engage]

I used an old 2 stroke mariner on my 19 footer and at full throttle could squeeze out 5kn and a bit more if sailing, 6kn was breaking the records. I had never experienced the revs going up and down other than the occasional cough. I changed to a Mariner 4 stroke and it is significantly heavier. As you may appreciate it is not so much the dead weight but how awkward it is on a slippery and moving deck. If I could have kept the 2 stroke I would have. I would suggest you make all economic efforts to getting it fixed. Try running it in a tank to see if you can duplicate the problem.

 

[VicS]

This is a problem as even with a long-shaft the prop is only just in the water & comes out any time someone goes forward of the mast. However the fluctuations in revs happen even in completely flat water with all the larger crew members standing at the back of the cockpit, so I don't think it's that!

Fair enough

Some info ... viz serial number to exactly identify the engine would help but is the prop driven by a shear pin or is it a rubber bonded hub with a splined drive. If the former check the pin but ifi broken they generally let go completely.

If rubber bonded hub maybe " spun " They will grip at low power but then slip at higher power. Marking prop and shaft is the way to check for a spun hub.

Not slipping out of gear due to a worn clutch dog?

Check fuel system for leaks.

 

[stuey_two]

This is a problem as even with a long-shaft the prop is only just in the water & comes out any time someone goes forward of the mast. However the fluctuations in revs happen even in completely flat water with all the larger crew members standing at the back of the cockpit, so I don't think it's that!

An adjustable outboard mount as supplied by eg Marinestore, Marine Super Store or Marine Supplies Direct might get your prop further below the surface but probably won't fix fluctuating revs problem.

 

[MissFitz]

Some info ... viz serial number to exactly identify the engine would help but is the prop driven by a shear pin or is it a rubber bonded hub with a splined drive. If the former check the pin but ifi broken they generally let go completely.

If rubber bonded hub maybe " spun " They will grip at low power but then slip at higher power. Marking prop and shaft is the way to check for a spun hub.

Not slipping out of gear due to a worn clutch dog?

Check fuel system for leaks.

It know it doesn't have a shear pin but not much beyond that, will look into all those suggestions, thanks. Btw should I be peeved that I paid £130+ for a service & this doesn't get picked up/fixed?

 

[ProDave]

I would echo the comments that you want a bigger motor, if you can find one another 2 stroke.

I have the mariner 4 2 stroke on my 18ft 6 boat and that tops out at 5 knot in flat water, less into a swell. It's a fact that I can sail my boat faster than it can motor, suggesting even I would benefit from a bigger motor on a smaller boat than yours.

 

[crewman]

I have a 4hp 4 stroke on my Sonata. It gives about 5 knots in flat water GPS speed not log. Very long bracket so prop is reasonably well submerged. Still comes out in rough seas.

 

[ghostlymoron]

I thought that most boats can sail faster than auxiliary engine. I had a Mariner 4 2 stroke on my Leisure17 which could do about 4.5kn. I could sail at 5kn.

I would echo the comments that you want a bigger motor, if you can find one another 2 stroke.

I have the mariner 4 2 stroke on my 18ft 6 boat and that tops out at 5 knot in flat water, less into a swell. It's a fact that I can sail my boat faster than it can motor, suggesting even I would benefit from a bigger motor on a smaller boat than yours.

 

[GrahamM376]

It know it doesn't have a shear pin but not much beyond that, will look into all those suggestions, thanks. Btw should I be peeved that I paid £130+ for a service & this doesn't get picked up/fixed?

I agree with VicS that the rubber bush is the most likely culprit, I've had it happen. Not surprised a service doesn't pick it up unless you tell them there's a problem as it's usually still possible to turn the engine over by turning the prop but, with the torque generated in use, it just spins above certain revs.

 

[Capt Popeye]

Humm there are Extra Long Shaft outboards available, they are certainly longer than the standard Long Shaft, fairly rare, but often at a good price because there are fewer boats that need them, indeed they are a real bgr on a standard Long Shaft hull.

 

[Heckler]

It know it doesn't have a shear pin but not much beyond that, will look into all those suggestions, thanks. Btw should I be peeved that I paid £130+ for a service & this doesn't get picked up/fixed?

If it hasnt got a shear pin then its the rubber bonding gone and the shaft is spinning but the prop isnt keeping up.
VicS is very good at ferreting the info out about the parts to make sure it is a bonded prop. The servicing would t have picked up this issue.
Stu