Merry Fisher 805 top speed?

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Steve 1

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Hi i have recently boughtn a 2006 merry fisher 805 with the Land crusier engine, it starts an runs really well, however it will only rev to 3,200 rpm and i can only get 15 knots going with the tide. it does'nt make any difference if i alter the trim tabs. The speed reading is from my Raymarine C70 so one would assume this is correct. The boat was serviced at the begining of the year by the previous owner, and the bottom is quite clean. Can anyone shed any light on why i am getting poor speed.

Thanks
Steve
 
Might be worth asking on the motor boat forum.
Do you know what it should rev to? Did it previously go faster, if yes is the hull clean? If no is the prop pitch too much?
 
gordmac said:
Might be worth asking on the motor boat forum.
Do you know what it should rev to? Did it previously go faster, if yes is the hull clean? If no is the prop pitch too much?
Click to expand...

I have see somewhere else that it should rev to 3,600,when i have the survey done on it before i bought it in March it acheived 21 mph at 3,200! i know the previous owner did the service him self so maybe i should get it serviced. I will ask on the motor boat forum.

Thanks

steve
 
I'd check that the fitted prop is the same size / pitch recommended by Merry Fisher. Is there much smoke from the engine at these speeds? Usually I'd say that if there are mechanical issues (boost leak, sticking turbo wastegate etc), they're normally associated with secondary problems (smoke, rough running etc)

I'd check the basics and if you have a trusted marine engineer, seek their advice on the boat.
 
When i first start the engine from cold it is a little smoky but this soon subsides, no no smoke at speed and generally runs very well, as you said maybe get a marine engineer to look at it.
 
Personally, my stab would be prop pitch and /or trimming issues plus despite the boats 'only' been in since April, you may be surprised how much slime may be attached to the bottom of the boat. This may be to the affect of 2 or 3 kts.
 
I agree, give her a scrub and polish the prop - probably worth up to 5kts! Incidentally, your chart plotter is indeed giving an accurate speed (why should GPS lie?) but it is speed over ground which tells you very little about boat performance. To get a better idea of what's going on, do a full throttle one way and note best speed, then turn 180 deg and do it again - average out the results and you have removed the effects of tidal stream.

Rob.
 
I will try full throttle both ways at the weekend, it will be interesting to see the results, mybe then i will get it lifted for a good srub. I did polish the prop before i put her in the water, it is amazing how quick things can grow. I didn't realise it could make up to 5kts difference.

Steve
 
As others have said, clean the bottom and prop. Also in your two posts you have compared 15 knots and 21 mph. 15 knots is over 17 mph, so you may only be 3-4 mph/knots out, and a dirty prop alone can easily do this. Just slime on the hull without any obvious weed will also slow a boat down.
 
I polished the prop when i bought the boat in mid April, so it been in the water for over 3 Months. i am surprised at how fast things start to grow on the bottom. I am also not sure how to get the most from the trim tabs, i suppose i just need to play with them at different speeds.
 
Hi Steve,

I have a 2008 MF805 with the Nanni (Landcruiser) engine. On a flat calm day I can get around 19.5Kts. I don't have trim table on mine. Don't remember the RPM at that speed but probably around 3400

Cheers

rich
 
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