Merry Fisher 805 cruising and max speed

molyneux

molyneux

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My MF805 only reaches 20 knots max with a Nanni 200hp.
The bottom is clean having just been antifouled, the engine just serviced by a Nanni dealer and the engine will pull the 3600rpm max.
I have a keel protector on the boat which may slow it slightly.
I hear other owners getting as much as 24 knots. As my Nanni pulls its maximum revs, I cannot see how I can improve the speed.
I would be most grateful to hear from anyone with knowledge of 805's that could shed some light!
 
If you boat is acheiveng max revs, then you could perhaps increase the pitch of your propellor. this is the only thing you can change.

Does your boat hit max revs easily? And does it run into the limiter or does it struggle to get there. If it is easy then a bigger pitch prop would give you greater top end but slower acceleration
 
The boat has the standard propeller that came with the boat.

I have noticed that it struggles to get to maximum revs sometimes. It revs to 3000 but all of a sudden picks up to between 3400 & 3600. More often 3400

The book says it should make 24 knots and 18 knots cruising.

May be a bit ambitious on mine though unless there is something I am not doing.

When I say 20 knots max. That is with the tide!
 
Do you have trim tabs fitted and if you dont, were they on the options list?

If you have them fitted, try trimming the bow down when at full speed. if you can get the baot a bit more on the planne rather than nose up you should find a bit more speed if not already doing this.
 
They are not on the options list.

This is something that I have looked into though.

I'm also petty sure that they are not recommended on the 805 because of the hull design although I have heard of owners that have fitted QL tabs to them.

I would be very interested to talk to owners who have already fitted them.

They are recommended on the 925, my boats bigger brother.

I am puzzled!
 
A large piece of galvanised metal from the hull about 6' connected in 2 places and ending under the rudder but not connected. I can just about lift it!
 
I have a Raymarine C70 with GPS fitted and a Humminbird Side imaging fishfinder with GPS. The speed is measured through the Humminbird. The Raymarine reads slightly less.
 
20kts with tide is less than 20kts which is a long way off the stated maximum. Do you have any heavy gear on board eg diving tanks. Can you explain more how the engine revs to it's max. Does it get to 3600rpm all the time or only occasionally? Does it only reach 3600rpm when the hull is clean or after a few weeks fouling as well?
Have you had the prop checked for any damage?
 
Can get 18 knots on the GPS against the tide though!

I can't help thinking that the boat should be peaking between 22 & 24 knots though.

Hoping to receive some replies from 805 owners to find out what heir speeds are.
 
No heavy gear on board!

I came back into the water at the end of December. The bottom was full of barnacles and weed. I was only pulling 3000 rpm and peaking between 10 and 13 knots. This was without doubt the state of the bottom.

The boat was taken out 3 coats of gel protect 1 primer then 2 coats of hard racing. The engine was fully serviced by a Nanni engineer. new cutlass bearing, pump timing altered and prop refurbished. (cleaned as it was pinking in places but not damaged) Came back like new though! The Boat has less than 110 hours use!

The boat hit 20 knots with the tide when she went back in. The revs were 3600 which is what she should be pulling max.

I have sinced noted that at full throttle the boat moves to 3000 then slowly increases it's speed and revs to between 3400 & 3600. Certainly not immediately responsive.

Any ideas?
 
[ QUOTE ]
20kts with tide is less than 20kts which is a long way off the stated maximum. Do you have any heavy gear on board eg diving tanks. Can you explain more how the engine revs to it's max. Does it get to 3600rpm all the time or only occasionally? Does it only reach 3600rpm when the hull is clean or after a few weeks fouling as well?
Have you had the prop checked for any damage?

[/ QUOTE ] No heavy gear on board!

I came back into the water at the end of December. The bottom was full of barnacles and weed. I was only pulling 3000 rpm and peaking between 10 and 13 knots. This was without doubt the state of the bottom.

The boat was taken out 3 coats of gel protect 1 primer then 2 coats of hard racing. The engine was fully serviced by a Nanni engineer. new cutlass bearing, pump timing altered and prop refurbished. (cleaned as it was pinking in places but not damaged) Came back like new though! The Boat has less than 110 hours use!

The boat hit 20 knots with the tide when she went back in. The revs were 3600 which is what she should be pulling max.

I have sinced noted that at full throttle the boat moves to 3000 then slowly increases it's speed and revs to between 3400 & 3600. Certainly not immediately responsive.

Any ideas?
 
From another MF805 owner (i.e. real life rather than theories):
Nanni 200, WOT = 3,650rpm, 21 knots

All other factors as quoted by OP (but includes rope-cutter).

Edit: no hesitation in getting to WOT, throttle lever moved steadily to max as speed increases, not just pushed wide open.
 
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