Dinghy Outboard Behavior

[thedigitalpro]

Hi.

I would appreciate some advise as I am generally new to motor boating. I have purchased a 3 meter inflatable small tender (Air Deck) with a mercury 6hp outboard.

We (2 persons) took it out on the river for the first time last week and had a gentle trip up and down hamble river. Everything was good but one thing we noticed.

We are having an issue whereby when I increased the throttle to anything over 20% the boat struggled to keep up with itself. I can only describe it to feeling like the prop was out of the water then entered the water creating a quick burst of power. We also experienced the same feeling when we had a little wash from passing vessels.

I have checked the outboard all over. It was positioned correctly on the back of the rib. My only theory is that the tilt pin was not positioned correctly and was kept in the default position. I realised after that the engine should have been positioned perpendicular to the water which it was not. Could this be a good theory?

Would appreciate any advice as we cannot think of anything else it could be.

Regards

Steven.

 

[ST840]

I was going to ask whether or not you had checked the manual trim position. But you beat me to it. Have you adjusted and tried a re run?

 

[thedigitalpro]

Hi thanks for your reply. I plan to try this tomorrow but wanted to double check before I set out.

 

[Seastoke]

How far is the prop under the boat

 

[thedigitalpro]

It's a short shaft outboard and positioned without obstruction so technically as low as it can go.

 

[Seastoke]

It's a short shaft outboard and positioned without obstruction so technically as low as it can go.

Can u turn the prop. When in gear

 

[Graham376]

I can only describe it to feeling like the prop was out of the water then entered the water creating a quick burst of power.

If it's a similar feeling to slipping clutch on a car, then the prop to hub bonding may be allowing slip. If you can, mark the prop nut and prop with a file, take it for a run and then see if the marks still line up. If they don't, a new prop will most likely be needed, most small ones can't have new hub bonded in.

 

[thedigitalpro]

If it's a similar feeling to slipping clutch on a car, then the prop to hub bonding may be allowing slip. If you can, mark the prop nut and prop with a file, take it for a run and then see if the marks still line up. If they don't, a new prop will most likely be needed, most small ones can't have new hub bonded in.

Hi I can check this. I forgot to add this this is a brand new outboard. Not sure if that's makes any difference?

 

[thedigitalpro]

Can u turn the prop. When in gear

I'll check this thanks. The outboard is actually brand new. Less than 2 hours running time.

 

[Seastoke]

Must be trim then. Does it try to lift front end up

 

[paulajayne]

Sounds like a short shaft outboard on a long shaft transom.

 

[thedigitalpro]

Sounds like a short shaft outboard on a long shaft transom.

Hi I did check this and my understanding was for a small inflatable Quicksilver Air Deck 300 that a short shaft was the recommended outboard.

 

[thedigitalpro]

Must be trim then. Does it try to lift front end up

I believe so. We did try to shift weight towards the centre of the boat (me at the back, passenger in the centre) but it did not make much difference.

 

[Portofino]

6 Hp ain’t enough basically.

 

[Rappey]

I can't see the tilt pin allowing enough adjustment to get anywhere near the prop coming out of the water or close to it .
I did fit a long shaft to a 14ft racing catamaran and experienced your symptoms but could also feel acceleration and de acceleration of the boat at a set throttle speed.
Turned out the outboard needed raising by 4" ,which is the difference between a long and short shaft. I also gained 50% more boat speed ..
Other than the dingy partially folding under power I've no idea.
Take someone that knows their stuff. They should get able to get to the bottom of it.
Ideally your outboard anticavitation plate should be in line with the bottom of the boats keel and no deeper.

 

[ashtead]

We have a quicksilver air deck and acquired a new Yam 6hp last year. It works fine with a short shaft -might you be having issues due to too much weight in stern? Maybe a tiller extension might help move weight forward?

 

[thedigitalpro]

Great advice everyone. Some real pointers here for me to try. I'll seek a solution and report back here on the outcomes. Thank you all for your help.

 

[Hot Property]

You should easily be able to plane with one person with your set up.

If it's a brand new motor then don't forget to run it in according to the manufacturers advice....

 

[thedigitalpro]

We have a quicksilver air deck and acquired a new Yam 6hp last year. It works fine with a short shaft -might you be having issues due to too much weight in stern? Maybe a tiller extension might help move weight forward?

Hi. How many notches did you have to offset the tilt of the outboard if you don't mind me asking?

 

[thedigitalpro]

You should easily be able to plane with one person with your set up.

If it's a brand new motor then don't forget to run it in according to the manufacturers advice....

Absolutely. I've run it only for a short period and hardly worked the engine due to this behaviour. Thank you.