Merry Fisher 705 Performance

crossy

crossy

Well-Known Member
Joined
10 Jun 2010
Messages
178
Location
Bradford on Avon, Boat in Poole
www.horizoncharterboats.co.uk
Hi Folks,

I recently upgraded to a Merry Fisher 705 with a Yanmar 4BY150 engine. I haven't been out on it a great deal lately due to the weather and other commitments but I went out yesterday and the performance was not the greatest.

At 3600rpm (rated cruising revs) I was gettting around 14.5knots across the tide and with the throttle fully forward I was only just getting 16 knots @3700rpm. Just myself, my wife and the mother-in-law so I would say a light to medium load.

I'm sure that during the sea trial in January I was cruising at 17 knots across the tide and flat out was somewhere nearer 20 knots with three blokes (myself, broker and friendly surveyor) on board.

The boat was already coppercoated by the previous owner so I am assuming it doesn't have a dirty bum since it went in in early Feb.

The thing that concerns me is that a) I can't cruise at 16-17 knots as I will cover some miles and b) that I can't get to 4000rpm as this suggests that all is not well somewhere.

I was just wondering whether there is anyone else on this forum with this boat or knowledge of this boat that could advise/help me?

Cheers,
Andrew
 
crossy said:
Hi Folks,

I recently upgraded to a Merry Fisher 705 with a Yanmar 4BY150 engine. I haven't been out on it a great deal lately due to the weather and other commitments but I went out yesterday and the performance was not the greatest.

At 3600rpm (rated cruising revs) I was gettting around 14.5knots across the tide and with the throttle fully forward I was only just getting 16 knots @3700rpm. Just myself, my wife and the mother-in-law so I would say a light to medium load.

I'm sure that during the sea trial in January I was cruising at 17 knots across the tide and flat out was somewhere nearer 20 knots with three blokes (myself, broker and friendly surveyor) on board.

The boat was already coppercoated by the previous owner so I am assuming it doesn't have a dirty bum since it went in in early Feb.

The thing that concerns me is that a) I can't cruise at 16-17 knots as I will cover some miles and b) that I can't get to 4000rpm as this suggests that all is not well somewhere.

I was just wondering whether there is anyone else on this forum with this boat or knowledge of this boat that could advise/help me?

Cheers,
Andrew
Click to expand...

Andrew

If you have previously achieved 17knots then its still there for the taking. I would say smaller lighter boats like yours are very prone to issues with weight distribution.... so IMO I would have a think where everyone was and any gear that was on board / placed when you first did your sea trial and cruised at 17knots, then compare this to when you could only achieve 14.5. Including water tank, full or empty? Fuel tank full or empty... cockpit locker.... full of cruising gear... dinghy...outboard, liferaft etc etc.

From Feb to now there won't have been much marine growth as you say, water still cold so I doubt there has been a sudden build up which wiould scrub that speed off..

...if no luck.... is there any vibration through shaft... could you have hit something with prop etc etc.. but start simple.. get weight forward and see what happens.

BTW, 4000rpm sounds a tad hight, is that what it achieved previously at WOT?
 
Firefly625 said:
Andrew

If you have previously achieved 17knots then its still there for the taking. I would say smaller lighter boats like yours are very prone to issues with weight distribution.... so IMO I would have a think where everyone was and any gear that was on board / placed when you first did your sea trial and cruised at 17knots, then compare this to when you could only achieve 14.5. Including water tank, full or empty? Fuel tank full or empty... cockpit locker.... full of cruising gear... dinghy...outboard, liferaft etc etc.

From Feb to now there won't have been much marine growth as you say, water still cold so I doubt there has been a sudden build up which wiould scrub that speed off..

...if no luck.... is there any vibration through shaft... could you have hit something with prop etc etc.. but start simple.. get weight forward and see what happens.

BTW, 4000rpm sounds a tad hight, is that what it achieved previously at WOT?
Click to expand...

Hi there and thanks for the reply,

If anything the boat is now lighter than it was during the test as I have offloaded 110' of chain.
I do get a bit of vibration when in a following sea I get stuck in that slack bit behind the swell, only slight though.
The water tank is definitely fuller now than it was during the trial. Would 50l of water make that much difference though?

Cheers,
Andrew
 
maverickofpoole said:
Andrew

The hull maybe coppercoated but was it abraded before it went in the water? secondly your prop wouldn't of been coppercoated is this fouled? Other than that all I can suggest is a word with nautibusiness (Tom) he is the Jeaneau guru

Martin
Click to expand...

Hi Martin,

Yes, I was very tempted to have a word with Tom but I have pestered him quite a bit lately so thought I'd try this route first...

Not quite sure about abraded, I thought once a boat was copper-coated that was sort of it? Have I missed something?

Looking at the Alderney trip on PBSBAC with great temptation, but not at 14 knots!

Andrew
 
When I was selling my last boat it could not reach maximum speed, Later in the day I pulled the boat out of the water to find that there was part of a fishing net wrapped around the propeller.
In your case I would put the boat against a wall, wait for the tide to go out and have a look. You cant form an opinion without knowing all the facts.
PS. Nice Boat.
 
50l of water may not make that much difference, 110ft of chain would though!!....The weight of that chain would have been pulling that bow down a treat.. perhaps that is the issue, or indeed as others have said some netting round the prop etc would also pull boat speed down but I think I would get some weight up forward.... simple jobs first :)

I assume you have trim tabs?
 
Last edited:
Hi There,

Yes, there was about 150' of chain up in the pointy end when I got her, a bit of an overkill.
I swapped about 110' of chain for 500' (ish) of rope so I can have a smaller about up front on the windlass for shallow anchoring (up to about 50' depth) and then a large amount on a second anchor for anchoring in up to 150' for fishing.

If anything the weight should have come down and thus improved performance.

I do have bennets tabs, I had them fitted before I put her in the water. The only other thing I haven't checked is whether they are working. They have blown a fuse before and if they have done so again but lodged in the down position (I didn't buy the indicators) that could slow her down a bit I guess.

Thanks for all the suggestions.
Andrew
 
crossy said:
Hi There,

Yes, there was about 150' of chain up in the pointy end when I got her, a bit of an overkill.
I swapped about 110' of chain for 500' (ish) of rope so I can have a smaller about up front on the windlass for shallow anchoring (up to about 50' depth) and then a large amount on a second anchor for anchoring in up to 150' for fishing.

If anything the weight should have come down and thus improved performance.

I do have bennets tabs, I had them fitted before I put her in the water. The only other thing I haven't checked is whether they are working. They have blown a fuse before and if they have done so again but lodged in the down position (I didn't buy the indicators) that could slow her down a bit I guess.

Thanks for all the suggestions.
Andrew
Click to expand...

If the tabs are set in the correct position they won't slow you down, but I just think a shaft drive boat like yours (and mine) needs help keeping the bow down. Weight & trim tabs will help..
 
I have a MF705, and she usually cruises around 15Kn at 3300rpm, and 20Kn at 4000rpm, which is throttle flat out. Wind and tide have significant impact on the speeds, as does weight distribution. I do tend to keep fuel and water fairly full.

I do not have trim tabs, so this could be an issue for you.
 
I used to have a 635 and performance was definitely affected by hull condition (clean/dirty) and even more if the prop was dirty. Worth having a quick look just make sure you are not fouled up with growth or even some netting/line etc.
 
Last edited:
What conditions were you out in.
Best speed will always come in flat calm any waves or swell will affect your speed.Bashing into waves will slow you down and trim a bit off your top speed.
 
Hi All,

Well I went out mid channel yesterday and the best I would get was 15.5 knots flat out with the tide up my bum, 13 knots cruise.

I had the boat taken out of the water when I got back to the marina and it is clean as a whistle, the coppercoat is doing it's job.

The only thing I can think of is that there is some sort of fault with the engine which is preventing it from reaching WOT and has caused it to lose a bit of power. I'll get the local engineers to plug a computer into it to see of there are any fault codes logged.

The only other thing is the weight of chain I removed from the nose but I the TABS sort out the trim for me so this shouldn't be an issue?

Andrew
 
Tabs will trim the boat, but they create drag as they do it.

An interesting experiment would be to put the chain back (if you still have it) and see if it was helping. Weight in the right place can make you go faster, sometimes.
 
FlowerPower said:
Tabs will trim the boat, but they create drag as they do it.

An interesting experiment would be to put the chain back (if you still have it) and see if it was helping. Weight in the right place can make you go faster, sometimes.
Click to expand...

Having eliminated most other things I am starting to agree with that viewpoint. I think i'll get some weight from somewhere and have a bit of an experiment.

Andrew
 
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