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Old 10-09-2014, 09:24 PM   #21
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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.
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Old 10-09-2014, 10:54 PM   #22
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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.

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Old 10-09-2014, 11:06 PM   #23
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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.
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Old 10-10-2014, 08:01 AM   #24
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10 ft Livingston

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

Planes easily with 2 adults and two folding bikes
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Old 10-11-2014, 07:53 AM   #25
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Dealer says it's a short shaft so I guess I was mistaken.
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Old 10-11-2014, 10:05 AM   #26
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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.
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Old 10-11-2014, 04:18 PM   #27
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You are referring to the 2 traditional looking trim tabs mounted to your transom?.
What is the length and weight of the RIB?
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Old 10-11-2014, 05:31 PM   #28
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Lehr 9.9 won't plane

The Avon is 9'-2" and weighs 135 lb empty. The Yamaha weighs around 100 lb.
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Old 10-12-2014, 06:36 PM   #29
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Quote:
Originally Posted by manyboats View Post
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?
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Old 10-12-2014, 10:23 PM   #30
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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.
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Old 10-13-2014, 10:18 AM   #31
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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.
I think you have is backwards...
Most of the time if a dinghy has a high bow attitude the motor needs to be trimmed fully down.
The only way a motor needs to be trimmed up is way too much weight in the bow ( move aunt Bertha closer to the stern).
Remember that the hull is trying to climb uphill to get on plane.. trimming up causes the weight to be astern , causing the hull to squat further.

I thoroughly dislike 4 strokes that are under 30hp.. too heavy for most all dinghy's. We had a 10' Zodiac,10hp 2 smoke Yamaha with a wood bottom that coulg plane 4 adults as long as a couple would hang over the bow until on plane.. with 3 it climbed up without gymnastics.
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Old 10-13-2014, 10:56 AM   #32
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I think you have is backwards...
Most of the time if a dinghy has a high bow attitude the motor needs to be trimmed fully down.
The only way a motor needs to be trimmed up is way too much weight in the bow ( move aunt Bertha closer to the stern).
Remember that the hull is trying to climb uphill to get on plane.. trimming up causes the weight to be astern , causing the hull to squat further.

I thoroughly dislike 4 strokes that are under 30hp.. too heavy for most all dinghy's. We had a 10' Zodiac,10hp 2 smoke Yamaha with a wood bottom that coulg plane 4 adults as long as a couple would hang over the bow until on plane.. with 3 it climbed up without gymnastics.
HOLLYWOOD

Our 12' with a 40Hp 2 Stroke does gymnastics if someone doesn't sit in the bow, you have to gun it'll go vertical then plane out. We're thinking of adding one set of these fins on the lower unit or trim tabs.
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Old 10-13-2014, 11:11 AM   #33
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Yep, hard to plane means tilt engine down. As in top of motor further from the bow, prop thrust aiming downward.

If hard to plane with a load, try a prop with lower pitch. Pitch should be stamped on prop like 9x11, which means 9"dia, 11" pitch. Try a 9x10.

Tailfins added on also help getting on plane, especially if it tends to point skyward on the hump. Will lose some top end, but usually worth it on a dink.
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Old 10-13-2014, 11:58 AM   #34
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Has anyone seen or tried these "Lifters" from Maxi Marine? Looks like they increase the effective waterline length and provide lift.

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Old 10-13-2014, 12:17 PM   #35
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Hollywood,
Read my post again and you'll see that's what I said.
The OB engine usually does need to be trimmed down to get the bow down utilizing the thrust angle to overcome the way too bow high attitude. But the prop is most efficient w it tilted up a tad .. moving parallel to water flowing through it.
I will agree w you totally about the heavy 4strokes. They should have made 15hp and under outboards exempt from the emission requirements. The heavy 4 stroke OB is a very bad mismatch in small light boats.


Larry M,
Now that's a trim tab that will probably work and probably w less drag than the small ones at high angles. Not seen that.
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Old 10-13-2014, 12:27 PM   #36
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Has anyone seen or tried these "Lifters" from Maxi Marine? Looks like they increase the effective waterline length and provide lift.


Looks like a good idea, I'd just worry about accidentally knocking one off loading the dink on the boat deck.
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Old 10-13-2014, 12:40 PM   #37
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Another huge advantage of trim tabs for some/most dingies to me would be side to side trim...rarely is a dingy running level without a lot of shifting around.
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Old 10-13-2014, 02:32 PM   #38
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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.

With a small outboard that planes, the AV plate below the keel distance is more 1/4 to 1 inch. 1-2 inches below is fine for boats that do not plane. Larger planing boats with higher hp and stainless props should be 0 to 2+" ABOVE the hull line.
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Old 10-13-2014, 02:34 PM   #39
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Another huge advantage of trim tabs for some/most dingies to me would be side to side trim...rarely is a dingy running level without a lot of shifting around.
You are absolutely right, but in the case of Smart Tabs and Lifters, the side to side trim (or pre-load) is only adjustable dockside or out of the water. It works well once you find the right adjustment if the loading is about the same. The main advantage of the tabs though, is for getting and staying on plane, and keeping the ride level.

I'm a big guy and it does the job for me on a very small Avon RIB.

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Looks like a good idea, I'd just worry about accidentally knocking one off loading the dink on the boat deck.
As for the tabs getting in the way, I would worry about the Lifters too. The Smart Tabs can be retracted (lifted a little above horizontal) for stowage or launching.

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Old 11-01-2014, 06:44 AM   #40
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Our 9.9 Yamaha 4 stroke and our old 9.9 Merc 4 stroke planed the 10' Avon with 400#, no problem.
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