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Old 03 September 2012, 15:48   #21
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Hi Pluto
Try the trim first, on flat water trim up gradually at speed and see if your problem goes away...if not go and see an experienced outboard guy before ya start moving engines and drilling new holes.
Trim down a bit for turns. At max speed dont come down on the trim too quick, or really before reducing some speed or you may well find the boat will kick off violently in one direction or the other. Probably to port!
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Old 04 September 2012, 12:55   #22
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mine leans to port from 15-30mph,but then levels all the way upto 70mph engine is offset to starboard too. Engine too low can cause it, as can bigger props, engine too high also. Could at worst be a hook in the hull.

Easiest thing to do is adjust weight distribution, if tank is port and fully aft, move it to starboard and forward a bit where it will impact more but 1st move ballast, like sand to get a feel for how much effort you need to go too. Does the boat ALWAYS lean or just at certain speeds? IS console in middle, is helm in middle, is trim pumps, oil tanks all mounted on one side?

Personally I dont like offset engines, and will be setting mine up again, with the engine in centre, height corrected and weight shifted starboard. Racer mate told me too, he knows more than me so I will.
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Old 05 September 2012, 07:54   #23
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I donít want to hijack the thread and while it is a speedboat I refer to, the issues sound similar and perhaps weight plays a part as I am also running a 90hp 4 stroke merc!

It is a 16 foot speed boat (closed bow) and before purchasing it I was concerned that it was stern heavy with the 4 stroke lump and on the first run (test drive) it ran ok and I didnít take a lot of time to play with trim etc as it was a rough day on the Lough and there was just 2 of us in the boat.

With the 2/3 kids (7-8 stone each) in the back, 2 adults in the front it does rise the bow significantly until it eventually planes at around 3000-3200 rpm
(13 ĺ 15 p prop) and will deliver 5900 rpm (WOT).

If I trim out at all even the slightest touch it will begin porpoising which can be quite pronounced. Steering to port is difficult and with just me in the boat it leans to starboard and also when coming of the plane; which is very disconcerting to say the least. I have adjusted the trim tab to starboard with no effect, the steering does lighten a little when on the plane but certainly nowhere near neutral.

In terms of weight distribution the only thing in the stern is a small battery; the fuel tank is in the bow and both are centred.

The engine is offset 2 inches to starboard!

Thoughts-discuss
Many thanks
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Old 05 September 2012, 09:55   #24
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In answer to DJS.I had a Merc 90 4Stroke on my last RIB,and they are a Heavey old lump for that Hp,and if I remember correctly not too long in the Leg. I changed to a 130 e-tec which was Lighter and altered the Whole handleing/performance for the better.
It may be just a case of miss match i.e. too much weight on the Back,but It may be worth trying adjusting the engine hieght.Not being able to trim out AT ALL,and takeing along time to plane may sugest it's too mounted too high?? Either way Good Luck.
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Old 05 September 2012, 12:23   #25
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Thanks Maximus and yes it is a heavy one at 177kg but nice on fuel!

At the min the cavitation plate is just a few mm above the keel of the boat and it is right down on the top of the transom. Certainly cant lower it any further and if I trim out at all its a handful! I Have thought about trim tabs, but may have to accept it is incurable with the current set up.
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Old 05 September 2012, 16:23   #26
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I too had a Merc 90hp offset on a rib..a 5.5m ocean pro. Trimmed correctly no problems and a good sea boat...incorrectly trimmed 'orrible!
I was from memory running a 19inch prop and achieving 6k revs with one up.
Probably you're stern heavy in that speedboat or perhaps the boat's transom angle is wrong and you are not able to trim in enough. Not an uncommon problem with some of the fast fishing boats around here that have had to put a wedge in for the engine to sit against in order to get usable trim range.

Moving weight about is not a viable option for me if the rib were leaning, a central fuel tank, central batteries and seating pods ...what you gonna move? Personally I may use a rib 1 up one day, 5 the next, 3 the day after and unfortunately none of them come in a standard size, or bring a standard amount of cr*p with them and the fuel load is gonna always be different - yet the boat behaves impeccably.
Propellers are not like a rear wheel on a road vehicle-they don't just push forward unfortunately... they also try to pull sideways like a paddlewheel (try reversing a big boat with a big bladed prop to experience that) and the prop shaft is trying to turn the prop one way and, as an opposite reaction, spin the hull over the other way.
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Old 05 September 2012, 16:38   #27
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by the way what make is that "speedboat" ?
There are some good boys over there in both N.I. and Ireland (hope thats all politically correct terminology) for boat courses...may all make more sense if you have a couple of days boat course with somebody over there. Trouble is whenever I see them I seem to wake up with the mother of all hangovers the next day!
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Old 05 September 2012, 18:03   #28
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Originally Posted by Bigmuz7 View Post
Its an offset issue ... you dont have enough of it The worst hull that did it for me was an 8.5,.. and 40 gallons of contained water inboard and on the starboard side would'nt cure it

If its as bad as you say with only a 90 .. it would suggest little or no offset at all.. can you measure the position of your engine on your transom ie how much off centre it is, and report back ?
hi bigmuz7
been out today and checked engine mounting. tis not off set. its mounted dead centre.
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Old 05 September 2012, 18:15   #29
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Originally Posted by gtflash View Post
mine leans to port from 15-30mph,but then levels all the way upto 70mph engine is offset to starboard too. Engine too low can cause it, as can bigger props, engine too high also. Could at worst be a hook in the hull.

Easiest thing to do is adjust weight distribution, if tank is port and fully aft, move it to starboard and forward a bit where it will impact more but 1st move ballast, like sand to get a feel for how much effort you need to go too. Does the boat ALWAYS lean or just at certain speeds? IS console in middle, is helm in middle, is trim pumps, oil tanks all mounted on one side?

Personally I dont like offset engines, and will be setting mine up again, with the engine in centre, height corrected and weight shifted starboard. Racer mate told me too, he knows more than me so I will.
hi gtflash
it mostly leans to port between 20 and 35 knots, and when its accelerating at these speeds. woz out today and took advice of some of the guys on here who said to trim the engine up more and i did. A big difference, i couldnt believe it. i had the engine trimmed in too much. now its not completely gone at all speeds. must shift fuel tank from port side of transom to the starboard.
thanks chaps.
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Old 05 September 2012, 18:44   #30
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