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Old 06 May 2016, 05:38   #1
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Boat name: Arimar 360
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Arimar 360+Mercury 25 EFI

Hello,

I have a small dingy, a Arimar 360 Topline with a Mercury 25 EFI from 2007. It makes 22 knots with 2 people (150 kg). From 75 % trottle and upwards it only makes 1 knots more.

What am I doing wrong?

I will check my propeller, what size it is. Cannot find Mercurys spec since it is the standard one.

Will try to optimize the trim since it do not have electric trim, but have trim foils in the back end of the hull.

Can you specify your specs for speed, engine, boat size? That would be helpful so I know if this is standard speed or if something is seriously wrong. Thought 25-28 knots would be reachable?

Regards Joakim
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Old 06 May 2016, 07:27   #2
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You do need to find out the prop pitch. Also you need to put a small rev counter on the outboard to see what revs you are making at full throttle.

I have this one, there are many similar available...

Digital Hour Meter Tachometer Adjustable Resetable Job recall max rpm + service | eBay
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Old 06 May 2016, 12:21   #3
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Two solutions solved the problem:

1. The trim was way off.
2. I had some water in the hull, changed the valve in the back and no water inside anymore.

Went from 22 knots to 27 knots. I forgot to check the prop unfortunately but still it do not increase speed when going 75+ % of throttle. Think it is caviation/increase of pitch that will solve this? I THINK I should have 9 inch size and 11 in pitch as standard?

Regards Joakim
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Old 06 May 2016, 12:32   #4
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I found a picture and I have a 9.75 inch, 11 inch pitch, 3 blade propeller with model name 48 896896a40 11p.

Max speed: 27 knots, 6000 rpm 1 person (2 not tested)
75 % rpm = 25 knots
Boat levels out at 9-10 knots

Very quick acceleration.

Boat weight approx 110 kg. Engine 71 kg. My weight 70 kg.

Any suggestions?


Regards Joakim
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Old 06 May 2016, 16:23   #5
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>>>Any suggestions?

Not really now you have sorted your problems... sounds a near perfect setup. Will be interesting to see if a second person brings the revs down. If not I guess you could go up to a 12" or 13" if speed was your priority.
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Old 07 May 2016, 01:58   #6
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I checked the Mercury prop page and for my setup they recommend a Four bladed Spitfire 9.3 inch/11 pitch prop. Almost the same size (9.75/11) but four bladed should get more bang for the fuel consumed at the 75+ % throttle...

Regards Joakim
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Old 12 May 2016, 03:05   #7
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Tested the Spitfire yesterday, 24,5 knots maximum (27 before). Hole shot and curv handling/speed better. Do not know which prop should be the primary though. Thought it would be closer top speed though, so a bit disappointed.

Regards Joakim
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Old 12 May 2016, 07:36   #8
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I thought a 4 blade's advantage was all in mid range performance not top end. Also you normally go down a pitch when moving to a 4 blade so by keeping to a 11" in the 4 blade you have gone up in effective pitch and it's perhaps not ideal for making maximum speed any more??
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Old 15 May 2016, 08:43   #9
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Quote:
Originally Posted by JoaAmb View Post
I found a picture and I have a 9.75 inch, 11 inch pitch, 3 blade propeller with model name 48 896896a40 11p.

Max speed: 27 knots, 6000 rpm 1 person (2 not tested)
75 % rpm = 25 knots
Boat levels out at 9-10 knots

Very quick acceleration.

Boat weight approx 110 kg. Engine 71 kg. My weight 70 kg.

Any suggestions?


Regards Joakim
How are you measuring revs?
I'd be surprised if you can get much more out of it, 27 knots in a 3.6m boat sounds about as fast most people would want anyway.

An 11" prop at 6000 rpm (with a 1.92:1 gearbox) would give a theoretical maximum speed of 28.3 knots. You'd expect 10% slip, so you are doing well. At 75% revs it should be impossible to go more than 21.2 knots, so either your RPM or speed doesn't add up.
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