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Discussion Starter · #1 ·
Hi my boat project was previously a 16ft sailing boat and has quite a high transom and low keel.
I have a long shaft 8hp outboard, but the cavitation plate is still well shy of the bottom of the keel.
Rather than removing a chunk of transom to lower it, can I use a mounting plate like they use for ancillary engines, with an extra support added to transom where bolts go in.
Or is this bad practice ?
Extra long shaft outboards are hard to find and probably still would require transom modifications, but less removal.
 

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Hi my boat project was previously a 16ft sailing boat and has quite a high transom and low keel.
I have a long shaft 8hp outboard, but the cavitation plate is still well shy of the bottom of the keel.
Rather than removing a chunk of transom to lower it, can I use a mounting plate like they use for ancillary engines, with an extra support added to transom where bolts go in.
Or is this bad practice ?
Extra long shaft outboards are hard to find and probably still would require transom modifications, but less removal.
;);Personally I can't see 8hp ripping itself off the bracket but to be sure I'd ask the seller or the manufacturer first. ;)
 

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Whats wrong with where it is now?
The advice for outboard mounting heights is that the anti ventilation plate is inline with the bottom of the hull where further adjustment may have to be made. At top speed the anti vent plate must be 1-2" under the waterline.

Heres where the hull ends and the keel starts, which looks to be spot on for the anti vent plate height.
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You will only ever get displacement speeds out of that hull, i.e. pushing a boat through water. The water is simply moved out of the way to accomodate the hull, and falls back as the hull moves through the water.
Like this.
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The water at the stern will not have a massive deep hole like on a planing vessel but will in all lilelihood stay at exactly the same level whilst underway as it is at rest.

Try it. See how the boat lies in the water first. If it needs adjusting then do so. The plate should always be under the water.
You could maybe make an argument about moving it port or starboard to be out of alignment with the keel and in cleaner water, but there shouldnt be any need and you would have to then adjust for any pitching that could occur possibly taking the anti vent plate out of the water.
 

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Discussion Starter · #4 · (Edited)
Hi Many thanks for the replies - really appreciate the hands on good advice and time taken. Seems I hopefully might be alright then 😊
From internet searching, I was under the impression that the cavitation plate had to be in line with the bottom of the keel, not the bottom of the transom. The boat has a high freeboard as it was designed to carry 6 sea cadets and a rudder person.
I just have a drain plug hole and the drop keel cutaway to glass, then I will take your good advice and give it a go before getting the angle grinder out 😊
 

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Hi Many thanks for the replies - really appreciate the hands on good advice and time taken. Seems I hopefully might be alright then 😊
From internet searching, I was under the impression that the cavitation plate had to be in line with the bottom of the keel, not the bottom of the transom. The boat has a high freeboard as it was designed to carry 6 sea cadets and a rudder person.
I just have a drain plug hole and the drop keel cutaway to glass, then I will take your good advice and give it a go before getting the angle grinder out 😊
I think thats where people get confused. The keel on a planing hull is literally the midline and in "keel" terms non existant.
A planing hull rides on the water and has to rise up from being in it. It digs a hole in the water to do so and its important the prop stays under water and clear from obstruction for max speed and efficiency, hence you drop it if need be.
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Theres a limit to how low you can go and too much leg in the water can be a bad thing- drag etc.

On displacement boats they dont have this rise as they dont ride ontop of the water but move through it, so it just is not practical and isnt needed. They also dont make outboards big enough for some keels!
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Try it. If it needs to be moved then buy a bracket. But odds are high it wont need it and would make very little difference.
 
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Yes definately try the boat with it first and try the bow as A exall says.
Sorry OP, I spelt pitching as "litching" earlier, edited now if it wasnt clear.
You will only know once you try and id definately try before you buy in this case.

An adjustable bracket can be one of the spring loaded ones with a suitable rating, with 4/5 height steps, or it can be a type that can be raised and lowered with a spanner, aka Jackplate.
 

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Don’t worry about the stuff you read about getting the engine height correct - with a 60hp on a 16ft planing hull it can make a huge difference - with an 8hp on a displacement hull, as long as the prop is fully submerged you’ll be fine. I lent my 9.8 short shaft to someone with a 16ft displacement boat that was meant to take a long shaft - the prop was totally wrong (I use it on a small inflatable that does almost 20 knots) and the AV plate didn’t even reach the bottom of the transom let alone the bottom of the keel - it got the boat to its maximum hull speed no problem.
 

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As others have said it looks fine id try it. Also being tiller controlled you'll be sitting aft and with the down force when motoring it will be plenty deep enough in the water
 

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If it should turn out that it's catching aerated water from the keel (as mentioned earlier, I don't think it will) just move it to to the port side of the keel a bit (port so that it gives you more room for the tiller) - this size engine on this size and type of boat, it doesn't need to be central. Once you have established the best position, add a wooden pad below the gunwale so that the whole of the bracket sits on a flat surface. Until it is mounted in this way, add a safety line just in case it works loose, so that you don't lose it overboard.
 

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agree with above. ive a 14ft open boat with a 5hp outboard and the prop is about the same level as yours and no problem . you could have got away with a smaller outboard ( 5 or 6 hp } as have been said its a displacment boat and as with mine any thing over 1/2 throttle will only lift the bow with no extra speed.
 
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