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Old 06 August 2013, 10:53   #1
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Country: USA
Town: Southeast PA
Boat name: Heritage
Make: Sea Ray / Achillies
Length: 10m +
Engine: 6HP Merc 4stroke
Join Date: Aug 2013
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Ventilation/Cavitation Help Needed

I used to have a Merc 5HP 2 stroke that ran like the devil and got my dingy up on plan with no issues. It was stollen.

I bought a nearly new Merc 6HP 4 stroke from a dock friend. It works fine at lower speed but

1. digs way in, bow rises making it near impossible to get on plan

2) if it does get on plan it cavatates/ventaletes on and off. By this I mean it seems to loose the water to keep going and regains it. Motor revs of course when this happens and it looses almost all speed momentarily.

3) generally seems to have far less power than the old one (I miss it).

What are my options? Will a different prop help (Merc Black Max). If so, what pitch?
Will a hydro foil help?
Does it sound like there is something wrong with the motor?

The boat is a 9 foot Achillies inflatable with an inflatable bottom. Its in good shape, relatively new.

Please help.
Jack
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Old 08 August 2013, 12:28   #2
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Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
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Posts: 1,706
Firstly, inflate sib's tubes and keel to it's max recommended working pressure, that's with a gauge. Float sib alone, trim engine to be perpendicular (90) to water surface, sit on middle deck as forward as possible, go full wot, check if with over or outter transom water splashes. If so engines needs to be height corrected on transom, sits too low. If still cavitates, aireates your engine is way too high on transom, bad news will need to chop transom down..

Bear in mind that a 4 strokes 6 HP engine is much heavier than same 2 strokes brother, that's why you need to sit more up front than with previous engine to compensate heavier engine weight.

To play with pitch sizes will need to install an induction tach to check max wot rpm with you alone and lightly loaded sib. Try to achieve this "sweet height" engine/transom condition by placing wooden shims untill correct position is achieved.



Happy Boating
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Old 08 August 2013, 14:59   #3
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Country: UK - England
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With the boat out of the water the anti-cavitation plate on the engine should be 1-2 inches below the bottom of the keel.

If it is lower you need to raise the engine on the transom, with a block of wood. If it is way too low you might have a long shaft engine which is not suitable for your boat.

If the engine is too high then you need to lower it of course, although I would be reluctant to cut down the transom.
So then you may need to change the engine for a long shaft one.
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Old 08 August 2013, 15:07   #4
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What trim setting are you using? Sounds like you might need to go up a hole or even 2. A picture of the motor on the transom (out of the water) would help.
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Old 08 August 2013, 15:24   #5
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This is pretty much what it should look like.
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Old 09 August 2013, 18:43   #6
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Boat name: Nautile
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Don't think he's got a long shaft engine for a nine footer sib. What's stated on any engine owners manual is not cast on stone, it's just a refference from where to start, most boaters takes for granted that all engines brands matches perfectly all boats transoms, big mistake. Will depend on the hull shape, tube diam and transom height. The only wat to achieve best engine-transom height is going for a wot test and under trial and error optimize best engine-transom height match.

Check video, the guys on that boat had same issue, now at opt engine transom height and that's running at just medium throttle.

Happy Boating
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Old 11 August 2013, 04:28   #7
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He bought the engine from a "dock friend" so it could be anything.

Especially if it was an aux engine from a larger boat.

If it looks like the picture l posted when it is fitted to a small inflatable boat then it is almost certainly the right shaft length for his craft.
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Old 13 August 2013, 23:23   #8
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lightening: how many inches below the bottom of the transom is the anti-cavitation plate in that pic?
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Old 14 August 2013, 07:52   #9
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About two. I am told that the smaller plate should be about level with the bottom of the transom.
It pretty much is, the angle of the photo makes it look higher. On my boat I needed a 2cm block of wood under the engine mounting to achieve this.

Before I did that there was a lot of splashback.
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