Mercury 9,9hp 4sk Poor performance on SIB

[Fenlander]

That's far too low so you need to pitch down, remind me did you identify your existing pitch?

[Steve509926]

It is quite low.

Was that rpm reading taken at wide open throttle? What speed was it achieving at that wot? What trim hole did you have the OB in?

Dropping down to a 7.5p prop will bring the rpm up to somewhere closer to where it should be - 5000. to 6000 rpm.

[jimbo341]

F8.5 so 8.9x8.3 prop, not sure what the speed was, probably around 12 knots, trim hole was 1 (closest to the transom).

[Steve509926]

You need to do some proper tests. Trying different packers on the transom to raise the outboard, use different trim holes - I'd suggest No 2 is probably where you should start, record what speed and rpm are you achieving at wide open throttle. Are you getting lots of spray over the transom, is the prop ventilating or losing grip? You need to know where you are know if you want to get the best out of your set up and at the moment you don't have enough information.
And pressure is key in a small sib with a small OB, make sure the pressure is absolutely spot on when you launch and check it half an hour after you've launched. I know my sib and over inflate it, depending upon temperature, before I launch.

Take a look at what I recorded when I did my tests (it is a bit over the top but that's the way I am)

https://www.rib.net/forum/f36/tohats...bie-91068.html

When you have that type of initial data, you then know what is the best height for you OB, what trim hole you should be running in (not always the same if you are 1 or 2 up) and what prop you should have on your OB.

[Fenlander]

Gut feeling is would be well worth trying a 7.5" pitch prop. Probably Polastorm for a decent price.

[Steve509926]

Originally Posted by Fenlander

Gut feeling is would be well worth trying a 7.5" pitch prop. Probably Polastorm for a decent price.

Can't disagree

[jimbo341]

I've had some time to look into this again. I found a good price for the service manual specifically for this motor and considering the boat size and weight the propellar is beneath the recommended pitch, which is ok but I dont think dropping down further is the solution.

Meanwhile, I re-checked fuel lines using clear hosing and found there was a substantial amount of air entering prior to the fuel pump. The fuel tank hose was severely degraded inside limiting flow and the attachment to the engine was slightly loose letting in air. I replaced these and rpm now is around 5000rpm which is better.

(Let me know if I should continue here or start a new thread in engines)

However, the service manual also mentions that if compression test is below 120psi, it indicates a worn engine. Mine is 125psi and when I did a wet compression test it showed 165psi leading me to think that the piston rings are worn or cylinder damaged. I therefore purchased a cheap USB micro camera to look inside the cylinder and this is what I see. Can anyone help with diagnosing these photos? Would just changing the piston rings be sufficient or would the cylinder walls need re-honing or re-boring?

[beamishken]

Originally Posted by jimbo341

I've had some time to look into this again. I found a good price for the service manual specifically for this motor and considering the boat size and weight the propellar is beneath the recommended pitch, which is ok but I dont think dropping down further is the solution.

Meanwhile, I re-checked fuel lines using clear hosing and found there was a substantial amount of air entering prior to the fuel pump. The fuel tank hose was severely degraded inside limiting flow and the attachment to the engine was slightly loose letting in air. I replaced these and rpm now is around 5000rpm which is better.

(Let me know if I should continue here or start a new thread in engines)

However, the service manual also mentions that if compression test is below 120psi, it indicates a worn engine. Mine is 125psi and when I did a wet compression test it showed 165psi leading me to think that the piston rings are worn or cylinder damaged. I therefore purchased a cheap USB micro camera to look inside the cylinder and this is what I see. Can anyone help with diagnosing these photos? Would just changing the piston rings be sufficient or would the cylinder walls need re-honing or re-boring?

Why do you think you have a compression issue when your compression is well above spec? Those bores aren't even worn, you can still see the honeing marks from manufacture. I think you need to look elsewhere.your still down on rpm so looking at propeller type & pitch might be a good idea

[Fenlander]

>>>still down on rpm so looking at propeller type & pitch might be a good idea

My thoughts exactly. Those bores look fine, compression figures against a theoretical spec can be misleading (such as your gauge being out).

You need to resolve the low maximum revs first so I'd still say try a lower pitch prop and don't think of stripping the engine.

Did you check the throttle linkage is fully opening the carb butterfly?

[jimbo341]

Originally Posted by beamishken

Why do you think you have a compression issue when your compression is well above spec? Those bores aren't even worn, you can still see the honeing marks from manufacture. I think you need to look elsewhere.your still down on rpm so looking at propeller type & pitch might be a good idea

Service manual states that "a reading below 120psi might indicate an engine wear problem". Mine is 125psi so close to this although I cant verifty with another gauge.

Also, regardless of the original reading, when i add some oil to the cylinders and reperform the test i get a reading increase of 165psi. The manual suggests "if compression is higher with oil added to the cylinder, suspect a worn or damaged piston, the cylinder and/or rings".

The engine is using alot of oil, and when I open the oil tank cap when the engine is running, there is some smoke/mist and a strong smell of petrol possibly indicating blow-back.

Finally, the service manual propeller chart states that for a gross boat weight of 272-453kg and a length of 3.9-4.9m, 8.9x8.3 is the correct prop. I am running a 3.3m boat with a weight between 200-275kg so it looks like the prop I am running or even higher is the correct match.

I am obviously a novice at this so if you think that the problem is the prop thats an easy solution, but I am a little bit worried that there is a problem with the pistons or rings, if you think the cylinders appear to be in good condition.

Boat is off the water now so I have time for a rebuild if it is worth the effort, but dont want to do it unnecessarily.

[beamishken]

You've obviously been running it overloaded pulling the rpm back which could account for the fuel in the oil & backpressure but your compression is within spec & you'd always expect a higher compression reading with oil in the bore, you reduced the size of the void for a start. Personally I'd get the rpm to the correct reading & give it a few good thrashings with some clean oil in & see how it goes

[jimbo341]

Ok, i will try with a lower prop. However if HP=(rpmxtorque)/2252 if I increase rpm at the expense of torque, does this prove that the engine is running correctly rather than just running at a lower than rated HP.

EDIT: if compression is in spec then I assume torque should be ok

[Fenlander]

>>>if I increase rpm at the expense of torque, does this prove that the engine is running correctly rather than just running at a lower than rated HP.

I believe you are over-thinking this. Whatever its condition an outboard needs to make close to its rated maximum revs when the throttle is wide open on the water. Until you have fitted a lower pitch prop and seen the revs reach much nearer the optimum at hopefully a new higher speed you can't make a judgement on the engine's condition.

[Steve509926]

Originally Posted by jimbo341

Ok, i will try with a lower prop. However if HP=(rpmxtorque)/2252 if I increase rpm at the expense of torque, does this prove that the engine is running correctly rather than just running at a lower than rated HP.

EDIT: if compression is in spec then I assume torque should be ok

I agree with Fenlander, you are overthinking this.
Start with all the easy, and most likely, things that will improve the speed and hole shot of your set up. Prop, outboard/AV height and pressure.
I could be wrong but you do appear to suspect/want there to be a mechanical problem with your outboard?

Edit: the equation is actually HP = (rpm x torque) / 5252

[jimbo341]

Steve,

I don't want there to be a problem, it's just that the motor doesn't perform as well as expected when paired with this boat according to the manufactures specifications in the service manual (prop vs weight vs speed).

I raised the transom height and replaced damaged fuel lines with minimal effect on rpm/speed and the boat still struggles to plain well.

As mentioned previously, maybe I am just expecting too much from a small motor. However, the compression seems to be degraded towards the recommended minimum and I was curious as to what I could do to get the best performance.

I agree with you all its best to try with the new prop, but what concerns me is this is not solving the real problem and this will just lead to further damage later on.

[Steve509926]

Originally Posted by jimbo341

Steve,

I don't want there to be a problem, it's just that the motor doesn't perform as well as expected when paired with this boat according to the manufactures specifications in the service manual (prop vs weight vs speed).

I raised the transom height and replaced damaged fuel lines with minimal effect on rpm/speed and the boat still struggles to plain well..

The manufacturer's specification prop Vs weight Vs speed will be based upon a hard hull boat, not a saggy, soft bottomed sib. If I compare my sib with a hard hulled rib with the same outboard, the rib outperforms the sib by at least 20-30%

Dropping down a pitch will help to achieve much better planing albeit at maybe a lower speed.

[willk]

Originally Posted by jimbo341

The 4 stroke I am using is much heavier than the 5hp 2 stroke I had before, but is no where near twice the performance. As I mentioned, maybe the extra hp isnt sufficient for the extra weight?

OP - IMO you've been given some very good advice in the thread and yet you persist in seeking ever more complex reasons for what you deem poor performance. The quote above shows that you do not fully understand the performance v HP relationship. Doubling HP will not even remotely double performance, however you define it.

Fortunately you HAVE determined RPM at WOT and you're waaaay off where you should be. It's a lowish HP for the size and load, so you need to optimise everything to get the most out of it. As the others have said:
Fully inflate
Light load
Test at WOT in calm conditions - record all speeds and RPMs
Test again at each trim position
Test with different transom heights
Retest with lower pitched prop to achieve an RPM in the upper part of the recommended range.
A small drop in pitch might be enough to get you to a place where the boat suddenly finds a sweet spot and RPM/performance improves more than you would expect from the pitch change.

I spent a bit of time this season doing just this, with three props. I found that there is such a thing as too much weight in the bow which can push the bow down too much and cause excess drag, dropping RPM. You want enough weight in the bow to assist you get on the plane easily, maybe some to dampen undesired slamming, but not too much.