Improving inflatable planing

[BGW]

In order to fit on my available drop-down ramp, I bought a tiny aluminum hulled RIB, Achilles HB240, 7-10" long and almost as wide! (Achilles Inflatable Crafts | Boats & Parts https://achillesboats.com/boatmodels/rigidhulls/hb_al/)

I have a mercury 6hp 4 stroke outboard. If the stars align, the boat is unloaded, there is no wind, the sea is flat calm, and I manage to get my butt positioned just right, it will get on plane. Otherwise, fugedabaht it.

The same motor will get a 10 foot catamaran inflatable zipping around at almost no throttle. I have seen projecting plates on bigger boats. Is there any point in trying fold up hull extensions on the aluminum transom?

It is my transport off the boat and I regularly need to go a few miles. The fuel consumption in displacement mode is high and of course its also very slow.

 

[montero]

The same motor will get a 10 foot catamaran inflatable zipping around at almost no throttle. I have seen projecting plates on bigger boats. Is there any point in trying fold up hull extensions on the aluminum transom?

Don't understand "projecting plates" and "fold up hull extensions on alu transom.

 

[BGW]

These are the things I have seen on speedboats: https://www.amazon.com/dp/B00L4HBM6...ref_=asc_df_B00L4HBM6Y&plpRedirect=mhFallback

Technically know as hydraulic trim tabs apparently. I was wondering if something as simple as a piece of aluminum plate at 90⁰ on the transom at surface level would improve the boats ability to plane. One on each side of the outboard leg. 12 or 18" of additional hull surface. Does that make sense?

 

[montero]

It might help a little. But the prop still won't work well.
does your dinghy have a rigid bottom connected to the transom?

 

[BGW]

It is a rigid inflatable boat or rib. It has an aluminum hull and transom. I suspect that the length of the boat dictated the shape: it has almost no 'v' at the front and is wide.

 

[Barry]

Inflate your tubes to the maximum recommended by the manufacturer.
Certainly the aluminum floor is rigid but it appears that some of the tubes are in the water and need to provide planing forces.
Note that if you inflate to max, cold, that in hot sun, you need to bring the pressure back down.

 

[BGW]

Thanks Barry. I have tried that and found it doesn't make much difference. The only thing I've found that helps, sometimes, is to make sure my weight is in the middle. If I sit on one side or the other, it leans quite a bit and the drag of that tube in the water is too much.

After writing the post and then thinking seriously about what would be involved in doing folding hull extensions at the back, I realize its a bit crazy. Better to sell it or just use it until it expires, and get a boat that does what I need.

Someone gave me an old 10ft pvc catamaran inflatable to try out. Not worth saving as it is starting to come apart, but quite revealing just how much faster it is even at very low rpm.

 

[HelmutSheina]

On this thread, member @sailhand is discussing a foam core epoxy glass catamaran tender he has developed and was selling plans for (and may still be). Inflatables only shine in the store before purchase.

Thread 'Catamaran tender to replace rib?'

Jul 4, 2014

Does anyone have any suggestions for a DIY small catamaran dinghy design to replace a rib? The emphasis is on very light weight and stability. My current 10' wood floor Achilles weighs in at around 35kg.

My idea was to have a small electric outboard for going to shore and back 95% of the time, and it would then be able to take a tohatsu 8-9.8hp 2 stroke for fast long range performance when used for exploration etc.

• DennisRB
• Replies: 76
• Forum: Multihulls

 

[montero]

Why catamaran dinghies? Catamarans also have the disadvantage of being sensitive to weight distribution.

Rigid dinghies can also be fast and less sensitive to the amount of supplies.

 

[BGW]

Helmut, thanks for the link. I followed that thread with interest. It looks like a fine dinghy. But I don't have the space to stow a rigid dinghy of any size without compromising access to my sail controls and deck.

 

[CocoonCruisers]

The ends of your tubes remain in the water while you're trying to get on plane i presume ?
These soft rounded tube endings on that model seem quite evil, they will pull the tube right down into the water.
Some vendors are trying to avoid that with sharp cones, some with end caps (https://cpimg.tistatic.com/5256087/b/5/rib-330-inflatable-boat.jpg for example)
If you're not afraid of an ugly hack, anything that will create a sharp edge instead should allow the flow to separate more cleanly.
(perhaps some fairly rigid plastic sheets glued around the bottom of the tube, sticking out a few cm at the rear ?)

Or make a handle extension for the outboard and move so far forward that the tubes are out of the water at 'takeoff'.
2m40 is a little short to play that game though; burying the nose is draggy too ..

Also, lowering the propeller pitch can make a lot of difference on a 6hp.
(Top speed doesn't matter if you can never reach it ... probably your engine never revs high enough to reach its full power.
Lowering the pitch allows the engine to rev higher at lower boat speeds, so you already get most of the power at planing transition speed.)

 

[BGW]

Cocoon, I appreciate that, seems like you understand the problem. I did use an extension for the motor and try moving weight forward but I think the shape of the hull at the front creates more drag than not. It felt like I was plowing. I think the hull shape and the wetted area is just a major compromise to give it a practical load capacity in such a short hull.

I think I will try a different prop. Once the boat is on plane I can maintain it with minimal throttle and it will Rev higher than it ever does in displacement. If that tells you anything. I like the idea of hard plastic extensions to the end of the tubes. Both easy to experiment with.

 

[montero]

6 hp isn't low power. Of course, I'm only familiar with 2-strokes.
The question remains about trim and load on this small boat.

 

[BGW]

Montero, I suspect the hull shape is just a severe compromise. The aluminum hull is almost flat at the front, with a token crease for a 'v'. It doesn't cut through waves but bangs into and over them, kicking spray up everywhere.

A 30-40lb load of groceries under the seat in the middle means it will no longer plane with just me aboard. But as long as I'm willing to just chug along slowly, it can carry two big adult males, 100lbs of water and groceries just fine.

 

[montero]

Have you tried playing around with the outboard trim?