Mercury 9,9hp 4sk Poor performance on SIB

[jimbo341]

I am using an Mercury 9,9hp 4stk engine on a 320cm aquaquick SIB and am struggling to get up to speed or on a reasonable plain. Yesterday there were just 2 people in the boat with light gear on a fairly flat sea and we reached around 20km/h in optimal conditions with the engine at full throttle.

The motor height to transom base is as shown in the photo and I am using an F8.5 prop. I checked both the spark plugs with a spark plug tester and they seem to be firing ok and are not fouled. Oil was changed recently.

Am I expecting too much from this engine boat combo or is there a problem with my set up or engine? Any tips would be greatly appreciated!

[Fenlander]

I tend to think in kts so your 20Km/h is about 11kts, that's well below the 15kts or so I'd expect having achieved that 2-up in a 3.8m sib with the identical Mariner version of your engine and using the standard 8.5" prop. I didn't note the revs at 15kts but they were 5350 at 14kts. That engine has a max rev range of 5-6000 which is very broad but I did comment at the time a slightly smaller pitch prop might help get the best performance but the standard was near enough for us so we didn't change.

To get the full picture you need to fit a small £15 tach and find out where the revs are compared to the rated maximum range.

A usual question is are you sure the SIB is fully up to pressure using a gauge you trust. Also it's hard to see from the angle of your image but the outboard looks a little low, a starting point is the AV plate above the prop to be in line with the bottom of the boat.

Also check the twist throttle fully opens the carb linkage/butterfly.

How is it getting onto the plane, OK or a slow bow up process?

[Steve509926]

Hi Jimbo, as Fenlander has said, I think you need to check the rpm at wot to determine if you have the correct prop fitted or not. I dropped from a 8.5p to a 7.5p and found it now runs within the rpm range.

I am going through a similar scenario with a 9.8hp Tohatsu 2 stroke at the moment.

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

One up it is performing well, but 2 up with kit it's about the same as you are experiencing.
One thing I have noticed is trimming out is critical, if the passenger is sat a little to far forward getting and staying on the plane is difficult. I have a very broad beam sib which isn't helping, I can't find any details about your Sib (weight/measurements) so it's difficult to make comparisons.

[jimbo341]

Thanks Fenlander for your reply, I´ve ordered a cheap tach off Aliexpress which although not the best will hopefully give me an indication of rpm. Also I ordered a compression tester to see if the cylinders are ok. I will report back once I get the results.

Regarding the boat pressure, it´s at 0.25 bar which is recommended by the manufacture and 0.4 bar in the inflateable keel to add a bit more stiffness. I checked these on the water using a bosch easypump to adjust which should be reliable.

Getting the boat to plane is slow, and it always feels like the bow is more comfortable staying up, even when I shift all the weight (fuel, anchor, children) forward. It seems like the engine power isn't enough to offset the weight.

In the meantime I will add a piece of wood at the top transom to lift the engine slightly as per your advice. If the weathers good tomorrow I can test and will let you know, thanks again!

[jimbo341]

Originally Posted by Steve509926

I have a very broad beam sib which isn't helping, I can't find any details about your Sib (weight/measurements) so it's difficult to make comparisons.

Thanks Steve, I have an AQ320PL. I cant find the exact specifications but the length is 320cm and the width is around 151cm. weight is around 55kgs. Is this broad beam or do you mean the width of the chambers?

I had a quick look through the thread you linked, very informative! 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?

Do you mean an 8.5*7.5 prop? if the compression is ok then this would be the next step in my troubleshooting. The boat has a long holeshot this could be the solution.

[Steve509926]

151cms is quite narrow as compared to my sib which is 180 cms wide, so I don't think it's anything to do with it being wide. It is heavy for a 320 at 55kg.
I think rpm is the place to start to ensure you have the correct prop.
To put trim/balance into perspective, we moved the passenger front bench/seat back 15 cms and it made a huge difference getting on the plane (I know this is counter intuitive), I think we had to much weight forward which made the sib bow heavy pushing water in front instead of riding up/over and onto the plane.

[Croolis]

Just for info, I have the same engine with standard prop and the fastest I've ever got out of it on my 3.2m, two grown men in it, was 20mph on a totally flat, slow river (being naughty). In very minor chop on the sea I get less, but I've not tried very hard or monitored it accurately either.

EDIT: here's an amusing anecdote though - I got the boat out the harbour onto the water and powered up, then couldn't get any more than like 12mph out of it. Spent ages wondering what was going on, spark plugs fouled or something. Then leant over the back of the boat to check for weed around the prop... and realised I hadn't pulled up my transom wheels.

[Steve509926]

Originally Posted by Croolis

Just for info, I have the same engine with standard prop and the fastest I've ever got out of it on my 3.2m, two grown men in it, was 20mph on a totally flat, slow river (being naughty). In very minor chop on the sea I get less, but I've not tried very hard or monitored it accurately either.

EDIT: here's an amusing anecdote though - I got the boat out the harbour onto the water and powered up, then couldn't get any more than like 12mph out of it. Spent ages wondering what was going on, spark plugs fouled or something. Then leant over the back of the boat to check for weed around the prop... and realised I hadn't pulled up my transom wheels.

That's fast for a 9.9 Croolis especially 2 up, 20mph is 17 knots. I don't get much more than that with my 20hp efi solo.

[Croolis]

Originally Posted by Steve509926

That's fast for a 9.9 Croolis especially 2 up, 20mph is 17 knots. I don't get much more than that with my 20hp efi solo.

Yeah, as I say, on a straight bit of the Trent, totally flat water, downstream. Upstream 18mph. This is all using a phone app to measure, of course.

[Fenlander]

I guess a 2-way average of what equates to 16.5kts is about right as compared to our 15kts with the same motor Croolis will have a less draggy hull than us and it's likely he was carrying less weight than ourselves plus dog and seagoing kit, extra fuel etc. We've never ever had the Aerotec in totally flat water either. Last time we saw flat water was about 16yrs ago with our Zodiac on Loch Duich.

[Steve509926]

That's twice I've heard Aerotec and draggy hull, I was always under the assumption that the Aerotec was a relatively efficient hull. So the 3.2 airdeck Boatworld won't be as "draggy"?
Slight thread drift but would a T38 be less draggy than the Aerotec?

[Fenlander]

It is the impression of those that have used Aerotecs that they are a bit down on ultimate speed from the flatter bottom SIBs. I remember our Zodiac lightweight flat air floor models being a few kts faster on calm water. And as Chipko mentions in the Aerotec thread loads down on the cat hull types.

[Croolis]

Originally Posted by Steve509926

That's twice I've heard Aerotec and draggy hull, I was always under the assumption that the Aerotec was a relatively efficient hull. So the 3.2 airdeck Boatworld won't be as "draggy"?
Slight thread drift but would a T38 be less draggy than the Aerotec?

It's this one. 36kg boat. Single piece air deck type that looks like a SUP, inflatable sausage keel.
https://boatworld.co.uk/boatworld-ai...RoCJDgQAvD_BwE

About 165kg of grown men, few kilos of stuff, transom wheels, no anchor on board.

[jimbo341]

Thanks for everyones help so far,

As suggested by Fenlander, I lifted the motor a 2 inches so the AV plate was inline with the base of the boat. I tested this on Monday in fair conditions,
a slightly choppy sea with some breeze, with just myself (85kgs) and a 12l fuel tank about half full.

Max speed was up from 11 to 13 knots which is some improvement but still 5mph less than what some of you are managing. The weight was probably 60kg less than before which could account for some of this. Handling was more squirrelly but this could have been due to the higher speed and wind/waves.

Holeshot was reduced but the engine RPM did sound bogged and struggling at times.

So I'm getting closer to expected. I'm just waiting for the tach to arrive to measure revs and can then make the decision about replacing the prop.

As a related issue, how can I make the transom height increase permanent (see attachment)? Using a block of wood as a spacer means the outboard clamps are half off the top of the transom plate and so not well secured. Can I just epoxy glue a 2 inch piece of wood, same thickness, to the top of the transom then re-screw the mounting pads 2 inches higher? The inner one is aluminium so this would provide added support.

[Steve509926]

Hi Jimbo, good you've got some improvement but I think a lot of that will be down to only being one up and less weight in the bow.

Don't make anything permanent until you are 100% satisfied that you've got the best out of your setup.

2" - 50mm packer on top of the transom is a lot. Did you try anything thinner? If you did what was the performance like?

I've just taken another look at your pic in #1 and I can't see a trim pin, is there one there and if there is what number trim hole have you had it in whilst doing your tests?

[Fenlander]

Yes basically what you've said in the last paragraph but I would change the outer pad for plywood though to give a bit of extra support.

As Steve says be sure this is the correct lift with a couple more runs out before going to all the trouble. As long as you're not suffering loss of prop grip (ventilation) in turns or certain wave conditions then the transom raise is fine.

See posts in the range 69-78 this thread for my solution which is now some 6yrs old and has never shown any signs of coming loose. Having said that as I'm a belt and braces guy I do bolt the outboard on each trip as we go out and push quite hard in lively sea states.

https://www.rib.net/forum/f50/aerote...e-71528-2.html

[jimbo341]

Ok, so the compression tester arrived and it’s reading 125psi on both cylinders with the throttle wide open on a cold engine. I can’t find a range from mercury but else where 120-140 has been quoted as acceptable. Does this sound correct? If so does that indicate some wear which is normal on a 2008 engine but not enough to account for the poor performance?

[Steve509926]

Hi Jimbo, have you checked the rpm yet to make sure you are running the correct prop?

[jonp]

Its a pretty small engine, which sits very low causing heaps of drag. Obviously prop pitch will help but don't expect to much from a small motor.

[jimbo341]

Originally Posted by Steve509926

Hi Jimbo, have you checked the rpm yet to make sure you are running the correct prop?

Tachometer arrived. It’s the timorn 1 cable type. Max rpm was 4500 whether that was one up on the plane or two up. Max rpm for this engine is 6000 (written on the case). Should I go for a new prop, if so which size?