2.1 meter (6.8ft) Rib getting on the plane

[Fuss]

Does anyone have any experience with a small rib getting it to plane with more than one adult.
Mine only planes with one adult, would hydrofoils on the outboard help?
Any other ideas?
The next size up is 2.3 meters (7.5ft)

[Sailmonkey]

We’ve got a 2.4 meter folding transom RIB.

With just me and the 5hp OB I can fly. Add a second individual and I’ve not been able to get it out of the water. I’ve played with tilt, prop pitch, as well as fins. It’s just not long enough to plane with 5hp and two people.

I’ve been itching to try it with 8hp (listed max hp). But the opportunity hasn’t presented itself.

[Fuss]

It might work with 8hp but mine doesn’t, that’s why I though of trying the outboard fins. I know 2.1m is very short but it is so impressive planing with one person that it would be great if I could just manage to get it to plane with 2.

[a64pilot]

Couple of things, first on that little boat another person is a huge increase in weight proportionally.
Second the find work by generating lift on the stern, also drag of course.
However moving your weight forward does pretty much the same thing, so if you moving forward won’t make it plane, it’s not likely the fins will either.
I’d suggest a cheap tachometer so you can prop it so that it will reach near max RPM with two aboard, it will over rev with just one of course.
But it it’s not revving up to near max, it’s not making the power, so you have killed it twice, first by a huge weight increase, and then by bogging it down and not letting it make full power

[akprb]

Ain't gonna happen (easily or consistently).

Bought a new AB 8' with aluminium hull. Tried a 9hp but was just too big and scary fast, tried a 2hp and too slow. Ended up with a 6hp which was just right but as far planing goes a one person deal :-)

[a64pilot]

Why not stay with the 9, just not run it scary fast?

[stewie]

You could try to increase the waterline length by adding a piece of wood, maybe about a foot long, to either side of the outboard screwed to the back of the transom on a couple of brackets each side and keeping the weight forward.

[Montanan]

Perhaps install lifters to provide greater surface area and to raise the transom and level the RIB.

Reference to one such product and video.

MAXI MARINE - Detachable Planing System for Inflatable Sportboats/RIB (Lifters - special trim tabs)

Link to video of a 200+ pound person in an 8.5 foot RIB with a 9.9 hp Johnson engine equipped with lifters achieving full plane ride:

https://www.youtube.com/watch?v=xrIehHGrEW0

[Boston Blackie]

A bit of drift but we love our Walker Bay Genesis 310 folding transom RIB with a 9.8HP Tohatsu. Planes fine with two adults and some gear. Very stable and well mannered.

[Oceanride007]

Not much mention here of 2 stroke versus 4 stroke, can't buy new 2 stroke in this country anymore, would suggest that 4 stroke is heavier and would probably cost you more roughly 2 hp. Would suggest even that with 4 stroke need 3.0 metre



Mine is 2.5m with 6hp 2 stroke, Won't plane with groceries, have a new carby sized for 8hp not tried yet. Another option is to go for a smaller o'board for those times when you have extra person or lots of gear or speed restricted.




Did try a 9.8Tohatsu (2 stroke) works but a bit twitchy.

[Chuck Hawley]

The Surface area of the hull is frequently as important if not more so than the horsepower of the engine. I tried a 3.4m air-floor sportboat with a 5HP four stroke and it flew. Got on a plane easily, probably ran at 12-13 mph, and was a delight. The same engine on a 2.6 m would have been very difficult to plane.

The other issue is the rigidity/smoothness of the hull surface. If you get sort of a pressure bubble under the floor with a boat that either doesn't have enough pressure in the keel or which just has a sloppy floor, it makes it very difficult to plane. That's why smaller RIBS are generally better at planing: the hull stays in the correct shape.

Small increments in length make a disproportionate difference in how a RIB or sportboat handles. A foot makes a world of difference, and two feet is amazing. This is true whether the boat has one, or two, or its maximum number of riders.

Newer generation 4-stroke OBs in the 8 and 9.9 range are much nicer than the original generation. But you really have to have a 2.8m or greater boat.

Chuck

[akprb]

Originally Posted by a64pilot

Why not stay with the 9, just not run it scary fast?

Good question, weight. The little RIB looked like it was always doing a wheelly :-)

Was just too heavy. I thought about buying a Yamaha 2 stroke while in the Bahamas but finally just settled for a used 6hp.

[tarian]

Originally Posted by Boston Blackie

A bit of drift but we love our Walker Bay Genesis 310 folding transom RIB with a 9.8HP Tohatsu. Planes fine with two adults and some gear. Very stable and well mannered.

I have this exact set up as well very nice

[Reefmagnet]

Hydrofoils could help, but then they can become dangerous with one up because they can lift the rear of the boat out of the water, which causes it to slide out of control.

Planing ability drops off very quickly on small ribs. It's the "hole shot" that becomes the problem. Short of throwing more horsepower at it, which can cause the rib to be overpowered with light loads, you can try with a finer pitched prop. IMO, the standard size supplied with the outboard tend to be too course a pitch for typical dinghy use. You could also experiment with the trim settings on the outboard.

[WSMFP]

I had an 8ft 2 RIB and it would just barely plane with a 8hp Yamaha two stoke. That 8HP was my favorite dink engine so far but the smaller RIBs will never be fast no matter how you power them when loaded.

As mentioned its all about the planning surface.