Lehr 9.9 won't plane

[rclarke246]

Got a new Lehr 9.9, long shaft, with a new AB 9' aluminum hull RIB.
Planes fine with just me but won't with 2nd passenger.

Dealer has been notified. Thinking about a different prop and a few other adjustments to the engine, like a slight tilt (bringing prop up slightly) or perhaps putting a spacer (1") on the top edge of the transom.

 

[bligh]

I am not sure I would re prop the motor. It's probably optimally propped for torque. Check the motor stops where the motor tilts all the way down. Make sure the stops are all teh way down and the motor can tilt farther down (past vertical). You could also get a plate (planing plate, thrust plate, speed plate, whatever). Those usually help get you up and out but may hurt your top speed slightly.

 

[healhustler]

Man, that's hard to believe. My Merc 9.9 2 Cycle and 9 ft. Caribe has no planing issues with two aboard, 400 lbs. The Lehr reportedly had a little more umph than my engine, although has some more weight too. Stay in touch about this. A Lehr was on my want list before full time cruising.

 

[Xsbank]

Long shaft is a lot of drag...

 

[Nomad Willy]

In the old days a 10hp OB would plane a 14' boat w several people in it. And at that time they measured hp at the crankshaft ... not the prop.

 

[FlyWright]

I second the advice from bligh. A 'whale tail' can drastically improve planing by providing some lift at the transom. Don't know if there's much difference between manufacturers. Maybe others have more experience there.

Davis Instruments Whale Tail XL DAV-448, Hydrofoils

 

[Conrad]

We have similar issues with our 9.9 Yamaha on our 10' livingston. Planes fine with one on board but if you have two adult males putting the weight at 400 pounds or so, it cavitates when you try to get up onto the plane. Our suspicion is leaning toward the hull design, not the motor.

 

[Spike]

I have the same motor on an 11.5 Zodiac Zoom with a wood floor. My boat may be lighter than yours but we plane fine with 2 adults on board. Any more than that...don't even bother. Love both the boat and motor so far!

 

[Larry M]

Maybe the water line. We had a 5 h

 

[rclarke246]

It has a "Hydro-Shield" fin,mounted horizontally below the prop.
The dinghy weighs only 100 lbs., before battery and 11 lb. propane tank. The engine weighs about 100, too. I'll guess 250 lbs. before passengers.
Thanks for the input.

 

[HopCar]

My experience is that if you want a boat to plane, you won't be happy with less than 75% of the maximum rated hp of the boat. No body is unhappy if they put the maximum hp.

Putting a long shaft engine on a short shaft transom degrades performance a lot. You've got five extra inches of motor creating drag. My guess is that the long shaft is the problem.

The fin also creates extra drag.

 

[Moonstruck]

Putting a long shaft engine on a short shaft transom degrades performance a lot. You've got five extra inches of motor creating drag. My guess is that the long shaft is the problem.

The fin also creates extra drag.

That was my first thought. Long shafts are usually for sail boats. Did the dealer recommend the long shaft?

 

[Xsbank]

There seems to be an echo in here...maybe I'll leave for a while and work on my heater re-plumb. Bye

 

[rclarke246]

Yes, the dealer recommended the long shaft and the fin.

 

[Donsan]

The RIB spec normally identifies if a long or short shaft should be used but my recollection is most called for a long shaft.

 

[Nomad Willy]

IMO the low prop and thrust line acts like a lever and pitches the bow way high. In that attitude the hull has so much drag planing is not possible.

I suspect your dink may plane fine judging from some of the preceding comments of skippers that have similar boats.

The dealer that recommended the long shaft should belly up to the bar and provide you w a short shaft engine at no charge. And w much groveling and apologizing. The engine manufacturer (I'm sure) recommends the short shaft for a boat w a 15" transom. Call the engine manufacturer first if you like.

 

[Larry M]

Eric's right. Go back to the dealer. The AB 9' Al RIB specifies a short shaft (15").

 

[rclarke246]

Thank you all.
I've emailed the dealer with the idea that the shaft is too long.
Will let you know the outcome.

 

[DavidM]

Well, ignoring the shaft length for now, which I agree may be contributing to the problem, a fin and a prop change can work wonders.

I had a 9' Caribe RIB with a 9.9 Tohatsu, 2 cycle. As with yours it could plane with one but not two or more. I first put a flatter prop on and it could plane two. Then I put a Doel Fin on and it could plane two, our 40 lb dog and groceries for two weeks aboard.

Check to see if Lehr makes a flatter pitched prop. If they do, then it is for a reason and it will help. Four strokes have a higher torque band and need to rev up to accelerate.

Also to convince yourself, check to see if the engine has a rev limiter. These are set just beyond the maximum hp point. Rev it in neutral to the limit and note the sound pitch. Compare it with your boat at wide open throttle. If it is significantly lower, you need a flatter prop.

If you really want to check it out analytically, do what big boats do. Get a phototach, remove the cowl and put a piece of reflective tape on the flywheel. It will take some gymnastics, but you can measure rpm underway. Compare it to spec at WOT and if it is more than 1,000 rpm off then you definitely will benefit from a flatter prop.

David

 

[HopCar]

"Check to see if Lehr makes a flatter pitched prop."
Some Yamaha props fit the Lehr 9.9. You have to match the spline count and shaft diameter. Take your current prop to the dealer to be sure you get the right one.

 

[SCOTTEDAVIS]

I have the same boat (Aluminum 9.5 AB RIB) with a 9,9 Yamaha 4 stroke it planes with two (I'm 250 wife is 135) but I added the Dolfin fin and it made a noticeable difference.

My RIB is rated for a 15hp but I wanted less weight.

 

[ben2go]

I have seen this before.The cavitation plate on the motor should be 1 inch below the bottom of the transom and the engine should be able to tilt 15 degrees down to help push the bow down when on plane.

 

[Nomad Willy]

To get the maximum thrust the propeller shaft should be parallel to the water flow through the prop. With any angle of attack of the hull the engine will need to be tilted up (a bit) not down to achieve best thrust. However a loss of thrust by tilting down may be necessary to overcome too much weight aft.

 

[Great Laker]

10 ft Livingston

9.8 hp Nissan, short shaft, standard prop, hydrofoil plate

Planes easily with 2 adults and two folding bikes

 

[rclarke246]

Dealer says it's a short shaft so I guess I was mistaken.

 

[LarryM]

I have a set of Smart Tabs installed on my Avon 280 RIB with a Yamaha 9.9 4-stroke. They have a swiveling upper bracket that allows them to be retracted when not in use. They are individually adjustable to compensate for trim, and are fully automatic. The boat planes easily with two passengers. When running at speed they are almost completely retracted (deflected upward) and supposedly create less drag than a fin permanently mounted to the outboard. The down side is that they are more expensive and complicated than a fin. Everything is a trade off I suppose.

 

[rclarke246]

You are referring to the 2 traditional looking trim tabs mounted to your transom?.
What is the length and weight of the RIB?

 

[LarryM]

The Avon is 9'-2" and weighs 135 lb empty. The Yamaha weighs around 100 lb.

 

[rclarke246]

To get the maximum thrust the propeller shaft should be parallel to the water flow through the prop. With any angle of attack of the hull the engine will need to be tilted up (a bit) not down to achieve best thrust. However a loss of thrust by tilting down may be necessary to overcome too much weight aft.

Just to be clear, when you suggest "tilting up, a bit, not down..."

Do you mean tilting slightly in the direction where the top of the engine moves toward the bow?

 

[Nomad Willy]

rClark246,
Yes.
The outflow of water aft of the transom is not parallel to the boats keel, bottom or horizon. The water has been pushed down by the boat and is in the process of rising up (or leaping up may be more accurate) immediately after it leaves the bottom of the boat at the transom. It's close to parallel to the horizon but not.

So for the prop it's most efficient w the engine tilted very slightly up ... w the top of the engine slightly closer to the bow. One should tilt up also to compensate for the angle of attack of the hull and slightly more for the rising water.

If one's boat is trimmed badly (as many or for some types most) w too much angle of attack the hull has so much drag that extreme OB tilt angles are needed to reduce the hull drag enough to achieve a planing attitude and speed. RIBs are so short and wide that a reasonable planing attitude is hard to achieve. Trimming the boat w weight and balance is preferable but if that fails draggy cavitation fins and power reducing trim angles may override all else and plane the boat.

If one has reasonable length to beam ratios. hard chines and reasonable trim weight wise planing is easily attained.