New boat, help appreciated

SimonR · 18 July 2021, 14:57

Hi one and all, joined the site today in the hope of any information please.
First of all I’ve recently bought a 3metre Aquafax with a 15hp Honda engine.
Took it out yesterday for only the second time and have found that at anything over than about quarter revs the prop is coming out of the water before being able to get on the plane. I’m assuming I’d need a longer shaft outboard?
The 15hp is quite heavy for lifting in and out of my car, (I transport the dinghy uninflated in my car) and see there’s not a great saving on going down to 10hp.
I’m wondering if further downsizing to say a 7 or 8hp wouldn’t make the dinghy too slow, would I get on the plane?
The other query if I may pick the collective knowledge here is I’m moving to Portugal in September and am hearing from a local there that I may need to register the boat there and have some form of training/exam?
Any help greatly appreciated.
Hope your all enjoying this fabulous weather.
Cheers,
Simon

Poly · 18 July 2021, 17:21

I'd be surprised if a 3m SIB needs a long shaft. Measure the transom. Where is the anti ventilation plate relative to the bottom of the "keel" / transom?

It doesn't really make sense to me that the prop can be in the water properly, but then be out the water when the boat is not yet planing. People have previously reported issues with things like the boat seeming to get a bubble of air under it / bending in the middle and 95% of the time they are solved by the correct inflation process.

I suspect an experienced helm, lightly loaded in good conditions with carefully weight distribution etc can get a 3m boat on the plane with 8hp. Very few people would chose that when afloat - and only go that way if the "out of the water" limitations were the issue.

I'd pop your Portugal questions in a different thread with a sensible title because there are a few people with Portuguese experience but who might miss this thread.

SimonR · 18 July 2021, 18:13

Hi Poly
Many thanks for your reply. I will set the dinghy up tomorrow and take some measurements.
I suspect you may well be right regards inflation. I bought the dinghy second hand and it came with an electric car pump that maxes out at under 3psi. The side and front tubes seem fine, possibly the keel under inflated?

Fenlander · 18 July 2021, 18:44

Hi Simon welcome to the forum. Is this an air or alloy floor SIB? As Poly says there is no condition we could imagine where the prop can come out of the water at displacement speed. Does it just feel like that or do you see it above water?

Sadly a 15hp Honda at 47kg is a very heavy outboard and not the very best choice for a 3m SIB. If you are lightly loaded the outfit would probably feel a lot more responsive with something like a 26kg 2-stroke Yamaha 8hp.

Having said that with advice from here we should be able to get that Honda pushing you onto the plane.

SimonR · 18 July 2021, 18:50

Hi Fenlander,
Thanks for your reply, the dinghy is air floor, I think maybe it was under inflated at around 3psi? The problem I’ve had is the drag of the boat at the prop is causing cavitation I think it’s called?

Poly · 18 July 2021, 18:59

Quote:

Originally Posted by SimonR

The problem I’ve had is the drag of the boat at the prop is causing cavitation I think it’s called?

Cavitation is an often misused term for a very specific circumstance. Whilst it might be possible you have cavitation, I'd take the time to spell out exactly what you were seeing, hearing, feeling. How you know the RPM etc. I don't know what the floor of your boat is meant to be inflated to - but if its drop stitch 3psi does sound too little.

Fenlander · 18 July 2021, 19:00

Ahh makes more sense. Most air floors run around 10-12psi. You need a new pump and gauge. Limp air floors cause dreadful problems of prop grip.

A prop drawing in air and losing grip is called ventilation but so many folks do refer to it as cavitation which is something different.

SimonR · 18 July 2021, 19:38

Will buy a new pump tomorrow, weathers looking good, can’t pass this up!
Fingers crossed.

Fenlander · 18 July 2021, 20:03

Bear in mind after you put the boat in the water it will need a recheck as the water cools the floor/tube air and lowers pressure. Also to bear in mind at normal start pressure pulled up on a beach in this weather it could over-pressurise unless fitted with PRVs so you might need to let some out then re-inflate before setting off again.