Merry Fisher 855

Mark Allum

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I am a new owner of a 2013 855 with twin Yamaha F150 engines, the boat runs very well but it cavitates badly on acceleration until it gets to 20 knots +, it doesn't seem to make any difference how fast you accelerate or whether trim tabs are applied etc, the props have 15 written on them which one company said seems about right, are there any other owners out there who could confirm whether this is normal or not, the boat revs out to 5800 & achieves 31 knots flat out, many thanks..
 

Canopy Locked

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No expert, but here's my tuppence worth

Height - are the motors set at the right height - see here Suzuki 115 Correct Height?

Prop sizes F150 WOT is given as 5000 - 6000 rpm so you're in the ball park if you can get 5800 RPM, however I think propping is a bit of a black art so it's definitely worth researching a little more and if at all possible get some different props to try
 

KINGFISHER 8

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The engines have to be trimmed right in when you set off … that's to say, as near the transom as possible. When you get up to 20 knots trim them out a bit. Engine trimming is important. Play with the trim tabs afterwards to get the correct running level i.e. bow up or down but I should leave them alone until you're up to speed, just worry about the engine trim. The height will be right and probably the props.
 

Kawasaki

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Thats good advice Kingfisher5
Been on lots of these type of boats with twins and singles
Always advocate that the motors are trimmed fully 'in' until on the plane at about 20 kts with an 855 or similar
Then adjust the motors to suit
Then the trim tabs
Motor height is very important too
As robemchug said props are deffo a black art but appears you are not far out there
2 Questions
1 does the boat have a 'list'?
2 what type of trim tabs have you got
'Conventional' or the electronic vertical 'blade' type?
 

Mark Allum

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Thats good advice Kingfisher5
Been on lots of these type of boats with twins and singles
Always advocate that the motors are trimmed fully 'in' until on the plane at about 20 kts with an 855 or similar
Then adjust the motors to suit
Then the trim tabs
Motor height is very important too
As robemchug said props are deffo a black art but appears you are not far out there
2 Questions
1 does the boat have a 'list'?
2 what type of trim tabs have you got
'Conventional' or the electronic vertical 'blade' type?
[/QUOTE
Thank you for your replies, I have got the engines fully trimmed in on start up, if you try & trim the engines out at any speed the engines cavitate & the speed drops off & the engines rev higher indicating cavitation. The boat is fitted with standard tab type trim tabs, yes it does sometimes 'list' but only when the wind is on the port or starboard bow, applying too much trim tabs causes the speed to drop off a knot or two.
 

Mark Allum

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The boat is completely level in the marina & at 20+ knots I can see the cavitation plates are just under the water line, is this correct ? Someone told me to check the 'Toe' of the engines, apparently there should be 1" difference between the steering eye & the centre of the props, with the props being 1" narrower so that they are pulling in 'clean' water. Not sure if its important but the props are painted with antifoul by the previous owner, don't know why they did this because when the boat is not in use they are tilted up & therefore not in the water, I was thinking of removing them & polishing them back to original ?
 

Kawasaki

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Ok
So that’s eliminated any ‘list’ snags
You’ve lost me on the ‘toe and steering eye’ bit!
antifoul on the props won’t make a heck of a difference on its own
Presume the hull is clean?
 
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