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Mercruiser Magnum 454 wonbt go over 3000 RPM

N

netprouk

New member
"Hi
I hope somebody can help

"Hi
I hope somebody can help me! I have a 37ft Sunseeker Tomahawk fitted with twin Mercruiser 454 Magnum engines, The boat was built in 1989 and the engine powers Bravo 1 legs with Mirage 25p propellors. Both engines start up on the button within a couple of turns of the starter motor and run well except that when they are under load they won't go past 3,000 rpm. They both appear to be running absolutely fine apart from this issue, does anyone have any idea what might be wrong? The boat has just been out of the water and had new props fitted and has been re-antifouled but this hasn't improved the issue at all"
 
does it run smooth while at th

does it run smooth while at the 3000rpm break point or is it stumbling.

Sounds like you have a fuel restriction. Check the vent line to the tank and make sure it is clear. Make sure you have new filters as well.
 
"Hi
Many thanks for your repl

"Hi
Many thanks for your reply, the boat seems to run fine with no signs of fuel starvation even at 3000 rpm but i will replace the fuel filters and check the tank vent"
 
"25 inch pitch props sound lik

"25 inch pitch props sound like too much pitch on a big heavy boat like that.
Those props would be right for a top speed of 75 mph with 1.50 ratio drives.

Did it always have these props?
What is your WOT speed now and before?
Depending on weight, I would think that boat should be able to hit 50-60 mph with twin 454's, and that would mean your props should be more like 19" or 21" pitch.

In other words, you may be way over-propped.

Rod"
 
"Hi Rod, She has had these pro

"Hi Rod, She has had these props since i bought her and is currently only managing 20 - 25knots when both engines are flat out at 3000 RPM, The props have been totally cleaned also and are Mirage stainless steel props which look brand new after there refurb last month."
 
Jason;
What ratio are your dr

Jason;
What ratio are your drives; 'cause something does not add up.
Have you ever had them over 3000 rpm?
I don't even want to guess what you burn and hour.

Rod
 
Jason:

These gents know the

Jason:

These gents know their marine engines. I would go over the service that was performed from your receipt and talk to the mechanics that performed the work. They may have changed something or forgot to readjust something. It sounds like you are dragging an oversize anchor preventing the boat from reaching the RPMs it did before. Trim tabs?
 
""just out of water"

""just out of water"
Ever had water in hull? Water logged foam etc?
Has a hook or soft spot developed on bottom of hull?

Food for thought,
kurt"
 
"Hi Rod / Kurt
The ratio of t

"Hi Rod / Kurt
The ratio of the drives is i believe 1.5 although i can't confirm as unfortunately i don't have the owners manual and the drive legs were replaced with reconditioned units a couple of years ago by the looks of them. The reconditioned units don't have any markings on them but are in tip top condition as is the hull and trim tabs etc; Everything appears to be completely in order there is no OSMOSIS and the hull has just been given 2 good coats of International Interspeed Ultra Antifoul Hence i am totally at a loss as to what is wrong, Sunseeker state that the boat should do 50 Knots yet she won't even go up to half that according to the speedlog and an independent GPS. Really appreciate your help here as i am totally lost as to what the issue is!"
 
25" props are still way to

25" props are still way too much for 50 knots with 1.5 ratio drives.
More research is in order.

With your engines pinned at 3000 rpm; this means they are likely only making about 200 hp each since hp is almost a straight line function of rpm.

What props do Sunseeker recommend?
Whats the dry weight of the boat?

Rod
 
"Hi Rod,
I made the enquiry a

"Hi Rod,
I made the enquiry about the props today to Sunseeker - so i will let you know as soon as they come back to me, the boat weight 5.5 tonnes unlaidened - Really appreciate your help on this

Regards

Jason"
 
Try a 19p or 21p props and see

Try a 19p or 21p props and see if you hit 4600 to 4800 rpm wot (check merc spec for wot rpm) as this is where you will see upper hp numbers. your torque should be hitting top around that 3000-3200 rpm mark. As Rod has stated
it sounds like they are way to steep to enable
correct rpm at wot. Get out the phone and see if you can find someone that will do a try before you buy props or loaners.

Kurt
 
"Jason;
I just ran a check an

"Jason;
I just ran a check and based on 11000 lbs weight and 2 x 350 hp; you should expect about 40-41 knots flat out.
The best props look like they would be 18". If your local dealer will let you try before you buy, I would start with 19".
This all depends on the drive ratios being 1.50; so that is the first thing to confirm.

Rod"
 
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