lack of power, or just me.

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lack of power, or just me.

Postby weydagger » Sun Oct 18, 2009 11:21 am

Hi, having moved up from a rib, with one trim / tilt to worry about, I now have a fairline with twin outdrives and twin trim tabs (port & starborad). having only had the baot for about 8 weeks, i am having a few problems.
Boat is twin BMW 6 cylinder petrols with BMW outdrives. The sea trial prior to purchase was good, with the boat achieving a respevtfull 28 knots at around 3200 RPM. After purchasing the boat, the journey back to the soutcoast saw variations between 18 to 24 knots for a given throttle setting, only to be expected, as the sea state was variable.
Experience now is that the baot struggles to achieve 13 knots, when running about 3000 rpm in a mild sea. Whilst in a harbour it achieved 21 knots at 3500rpm, but the boat waw leaning very hard to satrboard. Adjusting trim tans did not seem to make much difference.
What has changed, well the outdrives have been off and refitted, control cables have been reconnected.
Could the props be put on the wrong outdrives, as I have noticed that the outdrives are handed, eg, port drives anticlockwise and starboard drives clockwise.

If anybody has any thoughts, I would live to haer it, as my boating days are not a lot of fun at the moment.

many thanks
PAUL BARKER
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Postby ~Ray~ » Sat Jan 09, 2010 10:44 pm

Personally I have no Idea But I'm surprised none of the members didn't reply. Maybe you have found the "too tricky question"

Best wishes.
Regards
Ray

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Postby plat » Sun Jan 10, 2010 2:39 pm

What is the length and weight of your boat? Also, the hp of your engines?

Paul
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Postby Tico » Wed Jan 13, 2010 5:14 pm

Doubt that the props are on the wrong drive as long as you push the throttles forward to go forward! (if they were wrong then boat would go in reverse)

What power are the engines?.... a sunfury needs around 2x145 hp actual for decent performance. many older petrols will not develop this power if out of tune. The hint is that the revs are down.
For example i had a sunfury with 2x 170 hp that would do 30+ kts if everything was set up right, but any small thing out of tune and it would struggle to get over the hump and exceed 15kts! You really need the power and torque to get over the 'hump' and onto the plane. After that the requirement reduces.... sounds like it wont quite get onto the plane.

Also how are the legs trimmed? I would suggest that you start with them fully down and then lift to approx 0 or +1 only

Could also be dirty bottom (the boat that is) or damaged or barnacled props.
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Postby weydagger » Mon Jan 25, 2010 1:00 pm

Hi,

Thanks for the three responses. The boat is a 28ft Fairline Sunfury, displacing 3.5tonnes and powered by twin BMW B22 engines (190HP each I believe)

The outdrives were rebuilt and refitted August 09 and the boat re-antifouled at the same time.

I have tried various set ups on the boat, eg, Setting Throttles and just adjusting Trim tabs and outdrives, but I still cdo not seem to gain any speed opr power. It struggles to get over the hump, as correctly defined in an earlier thread.

One trim sender is broken, so I don't really know what setting that outdrive si on (can sender be replaced with boat in water?), but no matter where it goes, I do not seem to be able to improve the baot.

I have had the engines checked out and tuned, with both being Ok, al but a slight knock from the port engine.

Is there a correct sequence for trimming a boat. Outdrives then tabs, leave outdrives alone, or just adjust tabs.

Any assistance, always gratefully achieved.
PAUL BARKER
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Postby plat » Mon Jan 25, 2010 5:03 pm

Hi,
When you took the boat out for its sea trial were the fuel tanks close to empty. Was your water reservoir, empty, etc. That weight makes a big difference. That could drop you from 28 to 21 kts. Also, with out-drives, trim tabs become less important than Tilt/trim on you engines. I only use my trim tabs for horizontal stabilization. The Tilt/trim button is designed for more speed. Some boaters with out-drives do use trim tabs but perhaps your is better suited to the Tilt/trim buttons.

Paul
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Postby ms » Mon Jan 25, 2010 7:53 pm

A dirty bottom will have a dramatic effect on speed. a few barnacles will have a dramatic effect,

legs/ propellers are set up so that at speed the flow of water flows out between the legs (contra rotating props) in a single stream.

Is there any fouling - is there a plastic bag hanging on a prop or leg, badly damaged blade, badly fouled props ?.

When outdrives were off were they fitted back (left and right) where they came from. Were the original propellers re-fitted.

When you open up the throttle is the boat trying to lift the nose or drive the bow down. If the latter then trim needs to change to bring the nose up. Frankly you should have enough power to get on the plane without using trim tabs. If you open up throttles quickly you should get a marked nose up .. until you get speed up and the stern starts to lift.

What you've got sounds almost like one prop has fallen off ... !!
martin
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Postby ~Ray~ » Tue Jan 26, 2010 8:03 pm

One prop fallen off.... Don't laugh but that actually happened to me. The guy who refitted them screwed up (excuse the pun)
Easy to check so I suggest you look for the obvious.
Regards
Ray

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Postby enterprise » Tue Jan 26, 2010 8:08 pm

I would check your fuel filters for contamination, any obstruction to the normal flow of fuel will have dramatic effects on power output, easy enough to change the cartridges. :idea:
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Postby JOHN100156 » Tue Feb 02, 2010 9:10 pm

I have a T34 - as has been said, do not discount the possibility of fouling!

Three boats in our MDL marina last year in the Med., were about to have their engines stripped due to similar problems. Until a few of us advised them of the heavy fouling in the area.

First get the boat lifted for a quick inspection and pressure wash, you can check the tabs, props and outdrives then. But wipe its bottom first! 8)
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