F-Ribs And Sibs - Inflatable Boat Community

Hey there folks,

I bought a second hand 9.9hp outboard from Andy B (Thanks, bud, it's awesome! this is not a complaint about the engine in the slightest) and today I took it out for a spin with my wife and our godson (he's 2 and half)
With all of us in it, and neither my wife or I are small, we struggled to get more than about 6 knots. It appears that it doesn't matter how much power you have, my elderly c310 won't displace at much more than 4.5 knots without burning through fuel very quickly.
Anyway, I can live with that - the boat is small and we are heavy so it is to be understood.

I dropped SWMBO on the beach at Calshot and popped out on my own. Holy smokes it shifts! I only hit 18 knots but it felt VERY squirrely. I adjusted the trim which made it a bit better but it still feels like I am skating about on the water.

The 310 has a wooden floor with a small inflatable keel but is essentially flat bottomed.

Any ideas how I can increase stability? The outboard has a small hydrofoil installed - Andy B may have more details.

18 knots may just be the limit of the boat.
All boats have a physical limit that you'll struggle to exceed, no matter how much power you stick on the back.
The instability again is down to the boat opperating at it's limit I'm afraid.
You could try sitting on the floor rather than the tubes to centralise the weight.

18knots (20 mph) is not bad out of an old 3m Zodiac mate. And like you say, you is a big chap.

That all sounds perfrectly reasonable! I’ll just slow down then!

Cheers, Andy!

Once again - loving the engine!

In displacement mode the max hull speed for a 3.1m waterline length hull is 4.27 kts (formula is speed in kts = square root of waterline length in feet X 1.34). However big the engine is it will not go faster than that speed unless you go into planing mode.

Hi Chris,

That’s excellent information. Thank you!

This is exactly what I have been trying to find out. I have a 3.1m airdeck Yam by Zodiac which is rated to 8hp on the plate but the manufacturer re-rated the boat to 10hp. I've checked with an insurer and they said that they would accept up to 10hp with no problems but want to know what speeds I'll get. Maybe 8hp would be enough to get decent speed and if your boat is performing at maximum hull speed maybe 8hp could get mine there.

I am running pretty much the same 9.9 yamaha on a 2.9 lodestar sib. I'm scaring myself at 17 knots...which is the absolute limit of the set up really. Really these set ups are best for cruising at 10-12knts. Anymore and its just noise, splash and fuel being wasted.

They are superb engines and that one Andy had was mint....

I run my Honwave T38 with a Suzuki 9.9 convertered to 15hp (4 stroke) and best I’d managed was 21 knots, I borrowed Martins 20hp and best with that was 23knots, I think I’d found the hull speed as it’s was getting very livley and unstable , so with even 25 % more horsepower I only gained 2 knots,

So as others have stated, your hull is only designed for a certain top speed