mercury 110 9.8 hp low thrust drive underpowered

[rob.s.uk]

Prop blow out sounds reasonable. The cavitation plate is level with the bottom of the dinghy. The dinghy also has tabs at the rear for stability there just an extension of the inflatable floor. I would imagine when the dinghy bow lowers on the plane the stern raises slightly lifting the cavitation plate out of the water. Do you think hydrofoils on the cavitation plate would make a differance?.

 

[ondarvr]

If it is ventilating (not cavitaing, two very different things) check and see if the motor is mounted as low as it can go, this will help.

And yes, a fin will help with ventilation, it may not be the best solution, but they help eliminate ventilation very well, and that is what they were originally designed to do.

 

[rob.s.uk]

The engine is mounted as low as it will go. Unless I cut a bit out of the transom i don't really want to do that tho. Maybe I should try some fins and see if it makes a difference.

 

[Scott Danforth]

bring a friend or cooler with you loaded with something and see if pushing the stern down a bit helps.

 

[Chris1956]

Sounds like the prop is ventilating. If you look at the prop while running at speed, can you see it sucking air?

 

[rob.s.uk]

It's not that easy to look over the back while running. I'm going to take a friend out to balance the boat so one of us can watch what's happening. I'm pretty sure it ventilating. If it is I think the only solution is to cut the transom down to lower the engine.

 

[Sea Rider]

It's not that easy to look over the back while running. I'm going to take a friend out to balance the boat so one of us can watch what's happening. I'm pretty sure it ventilating. If it is I think the only solution is to cut the transom down to lower the engine.

It's a piece of cake with whipped cream to have a look backwards. Check this :

http://www.rib.net/forum/f50/abc-sib-rib-installation-guidelines-58373.html

Was specially written for Sibs/Ribs starters, when you're lost in limbo boating. How high is your transom measured from middle lower transon to sib's bottom/keel in centimeters ? Follow all suggested guidelines, Need to "determine" if engine sits at the sweet transom height, then work from there raising/chopping down transom accordingly.

For initial water test do it alone, secure fuel tank at bow, if completely filled will have around 23 Kg of weight up front. Sometimes it's good to count with an induction tach when powering sibs with under powered OB's as you can always maximize prop to suit your boating needs.

Correctly inflated means all chambers inflated even with a pressure gauge to manufacturer specs. ? if just by thumb touch, won't be near what tubes need to be inflated to, no matter if feels hard to thumb. You need overall sib rigidity for under powered engine to work as expected. A 3.30 Mtr sib usually uses min 10 max 15 HP OB's transom rated for which are 250 CC , a 9.8 is only 170 CC and that 80 CC difference makes all the nice boating difference.


Happy Boating

 

[GA_Boater]

Rob, try to keep everything to one thread post.

Rob - As Scott said, please work in this one thread. You are getting the same advice in both threads and the prop has been eliminated.

Both threads are now one.

 

[rob.s.uk]

Thanks for the link very interesting. It's not quite that easy to look backwards from the near the centre of the boat where I have found I need to be to plane and see the prop.

 

[rob.s.uk]

Sorry about the two threads I thought I had two separate problems. Thankfully it's been narrowed down to one.

 

[rob.s.uk]

Tried more tests today had a friend with me for added weight also had a 25l can of water for ballast. It didn't matter where we sat it made no difference. I think the cavitation plates to high and the engine needs lowering. When going along the cavitation plates only just underwater and there's a lot of turbulence around the stern. Hydrofoils turned up today so I will fit them and see what happens.

 

[rob.s.uk]

Now I'm really stuck. I've fitted the hydrofoils and because of the stabilizer tabs on the stern the engine won't turn. So my problem is do I cut the transom down and by how much?. Thing is once it's cut there's no going back and what if it makes no difference. I've had a thought I should try the engine on a friend's dinghy and see what happens before I commit to cutting the transom. Any thoughts?. I'm starting to get Fed up with it now it's only a dinghy tender I just wanted an engine to bolt on and use seems a bit of a fuss all this.

 

[rob.s.uk]

Tried it on a different boat today. The lower unit was a lot deeper down in the water so there was no ventilation. Still no difference.

 

[DavidMoore]

Sounds like the prop is ventilating / cavitating. Does it feel like the prop is losing it's grip on the water (the revs suddenly increase and the boat goes no faster or even slower, you have to throttle back for the prop to regain its grip).

Has it always done it? If yes then I think you need to look for a different style of prop. larger diamenter for example.

If not have you hit something with the prop and damaged it?

The only thing I can think of that will stop the problem, and does not invlove changing the prop, is to have more down trim on the engine (the prop is in less aerated water).

I'm sure there are plently of props that will fit the engine, what makes you think you won't find one?.

 

[racerone]

It is normal for the prop to turn and ratchet turning it clockwise in forward.--------But that has nothing to do with it protecting the motor / prop should you hit something.----Not sure where those ideas come from.

 

[flyingscott]

Give us a picture of it on the boat also the cavitation plate right above the prop should be even with the bottom of the boat. And when you marked the prop did you make the mark on the rubber or the metal. The mark needs to be made on the inner metal sleeve with the splines to the outside metal ring. If you made it on the rubber that may not be whats moving Just a thought.

 

[rob.s.uk]

Hi I've only recently got the engine so I wouldn't know if it's hit anything it looks fine no dings or damage. It is doing what you mentioned it accelerates ok then seems to lose grip like you said and I do have to throttle back and try again. Thing is I've only got it on a dinghy so I would imagine it would push it along fine. The chap I bought it from had it on a much bigger rib and he said it flew along so not sure why it won't with mine as I've mentioned before it's only a 3.3 meter tender. Today I tried it on a friend's dinghy 2.5 meter with a lot lower transom to mine. To see what difference it would make and it made none and the av plate was well under the water. When watching it the av plate was never near the surface for the prop to suck in air. The exhaust gas was also exiting the propeller no problem it wasn't being sucked into the propeller blades. The symptoms sound like the hub slipping but it's not as I've marked it and its still in the same place after a run. I thought it must be ventilation but after today's test I'm not sure it is. I think I can get a new propeller on the internet I've been looking but I have no idea what size to try.

 

[flyingscott]

The propeller clicks in forward because of the shifting dog design the ears are ramped on mercurys. The reason is because the tilt mechanism on smaller mercs are attached to the shifter usually you have to put them in forward to tilt them. When you put them in forward not running the ramped dog will slide into gear unlike evinrude/ johnson which need the propeller to be turning to shift into forward.

 

[rob.s.uk]

I will get a picture. The av plate is dead inline with the bottom of the boat. I did mark the inner spline sleeve and the outer housing not the rubber. I thought the stabilizer tabs on the stern which are just an extension of the inflatable floor were causing turbulence around the propeller but after trying it on a different boat I don't think that's the problem.

 

[DavidMoore]

Your right, your tender should fly with a 10 Hp engine, my first boat was a 3.0 tender with a 6hp engine and that would feel like it was shifting with just me in it.

I had exactly the same is​sue as you described on that little tender but it only started after hitting the bottom just once. I never solved it but don't let that discourage you.

BTW My question about hitting the bottom was related to your symptoms, not the ratcheting. The ratcheting is bang on normal.

What are the stampings on the prop?