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Old 31 May 2020, 11:25   #1
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A little advice please

anyone any ideas on this problem took my boat out today to put a few hours bedding the new outboard in .its a humber destroyer 5.5 with new mariner 115 command thrust mounted second hole from the top .the av plate is roughly 2 inch above the keel line . in the water planing at 16 knots the av plate looked to be just above the water . fully trimmed down . now when i upped it to 4k rpm about 25 knots and tried to trim out it started to lose grip and cavitated revs were up and down to 5.5k rpm with no increase in speed trimed back in and it gripped again . bear with me I am not sure on the size of prop supplied with the outboard ( its a new outboard new prop and i cant check as i left it on a mooring for tonight ) except its a 17p but on the invoice it says supplied with a 19p prop . could this be just too small a pitch prop or is it more likely to need the outboard dropping a hole. thanks
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Old 31 May 2020, 11:41   #2
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For straight line economy/speed trim out until you lose grip and then come back a touch. If you turn tightly you will need to trim down or you will lose grip. Of none of this affects speed etc you may well have too small a prop
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Old 31 May 2020, 12:41   #3
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thanks but its doing it at quite low trim levels quarter on the gauge in a straight line. I will get a number off the prop tomorow
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Old 31 May 2020, 12:55   #4
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If your AV plate is 2” above the keel, it sounds a tad high. Try dropping a hole. It’s very much a trial & error to get things “just so” different engines/props/hulls all react differently.
What was your WOT RPM?
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Old 31 May 2020, 13:04   #5
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hi thanks I haven't got that far yet its a brand new outboard and I'm just running it in so haven't had it flat out yet probably only has 2.5 hours on it ill be back out in it tomorrow another varied hour and ill give it a brief wot run . I was just surprised it started letting go at a relatively low trim out. thanks for the input
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Old 31 May 2020, 16:10   #6
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Quote:
Originally Posted by Pikey Dave View Post
If your AV plate is 2” above the keel, it sounds a tad high. Try dropping a hole. It’s very much a trial & error to get things “just so” different engines/props/hulls all react differently.
What was your WOT RPM?
my thoughts too
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Old 01 June 2020, 14:24   #7
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didn't get chance to do a wot run but the av plate is 2 1/2 inches above the keel line and It looks like if I drop the outboard onto the last hole the top one I will lower the outboard by an inch and a half so I think that's the way to go will give me an av plate an inch above the keel line. im doing nothing with the prop yet its a 15x17p which I think is recommended size for my mariner 115 ct
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Old 01 June 2020, 15:23   #8
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https://features.boats.com/boat-cont...ine-height.jpg. This might help
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Old 01 June 2020, 17:09   #9
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With the boat running at a decent speed the upper plate above the prop should be riding at the top of the water, but just touching the water. So if it is currently above the water the outboard needs to go down. Too deep and it will send spray everywhere. Too shallow and you will have cavitation issues.

Edited to clarify.
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Old 02 June 2020, 00:38   #10
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thanks folks im going to drop the outboard to the lowest setting should sort it
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Old 02 June 2020, 02:44   #11
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thanks folks im going to drop the outboard to the lowest setting should sort it


Don’t go too low. I’d aim for just above the keel with the engine trimmed so that the AV plate is parallel with the keel. Use a long straight edge along the keel to check. The eye can be fooled, especially when the boat is sat on the trailer,
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Old 02 June 2020, 11:21   #12
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Don’t go too low. I’d aim for just above the keel with the engine trimmed so that the AV plate is parallel with the keel. Use a long straight edge along the keel to check. The eye can be fooled, especially when the boat is sat on the trailer,
I did check it with av plate parallel to the keel. the outboard is on its second from the top hole and the av plate measured with a long straight edge is approx. 60mm above the keel line it runs fine fully trimmed in straight and taking turns and fine in a straight line trimmed out until about quarter on the trim gauge where upon it starts slipping this is in a straight line on relatively flat water. I am proposing to lower the outboard onto the top holes in the outboard bracket to lower the outboard by approx. 20-25mm that is unfortunately as low as it will go but will still be 35-40mm above the keel line .I am hoping this will give the prop more bite and allow me to use more trim . thanks for everyone's input I do appreciate all advise

during a search on here to look for similar I found some posts a couple of years ago from Phil m and he was fitting out a new destroyer 5.8 with the same outboard I have and he was asking the same questions although he was worried his transom had been made too deep .anyway he mounted his on its lowest setting and said it was fine no issues even though he was 35mm above the keel line so I am expecting to find the same
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Old 02 June 2020, 12:14   #13
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Have you calibrated the Smartcraft trim gauge.

It's very badly explained in the manual because the only way you can establish where the "Max trim" is, is by trimming out as far as you can with the engine under moderate load, maybe max displacement speed, and save that, throttle back, turn the ignition off (the gauge will continue to register). Run the tilt all the way up and save that as the trailer position. As soon as you do that the gauges will reset and the normal trim position should come in around mid scale.
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Old 02 June 2020, 12:40   #14
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Have you calibrated the Smartcraft trim gauge.

It's very badly explained in the manual because the only way you can establish where the "Max trim" is, is by trimming out as far as you can with the engine under moderate load, maybe max displacement speed, and save that, throttle back, turn the ignition off (the gauge will continue to register). Run the tilt all the way up and save that as the trailer position. As soon as you do that the gauges will reset and the normal trim position should come in around mid scale.
hi I don't have smart craft just the normal quicksilver rev counter and separate trim gauge
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