Mercury 150 slow at full throttle

[sfestiva]

Hi all,
I fitted a mercury 150 four stroke on 18ft searay, but at full WOT (5000rpm mercury smart gage) it is only going 22knots, to me this doesnt seems right, shouldnt be faster? hole shot is perfect, planes very quickly (smart tabs added)

- replaced flo torq sleeve (was little bit cracked on each 4 edges)
- i think the motor is setup too low?
- the prop is 4 blades SS

Any ideas?

Much appreciated!!

 

[Bondo]

- i think the motor is setup too low?
- the prop is 4 blades SS

Ayuh,.... Welcome Aboard,.... How low is the motor mounted,..??

What is the pitch of the prop,..?? Are ya trimmin' it up to gain speed,..??

Is the bottom of the hull straight, 'n true,..??

 

[fhhuber]

A few things that could be going on... some more likely than others.

Bad angle on the engine wasting power.
Trim up until the nose of the boat is bouncing then down just to smooth it out. That should be right. This angle varies a little with load (weight of people and stuff in the boat, including fuel) and distribution of the load.

Trimmed too far down is GREAT for hole shot. (up to a point, but usually you can't go too far because the transom stops the engine from trimming down that much) It keeps the nose from rising as much... but you need to re-trim as soon as its on plane.
Trimmed down too far forces the nose of the boat into the water and adds tremendous drag. You should be planing on the back couple of feet or so of the hull.

Bad angle on the trim tabs... can help with hole shot (keeping nose down) and act like brakes at speed.

Assuming you are trimming the angles correctly...
Most likely: You could find a different prop that works better... a little less pitch and more total blade area would be my first guess.

Most unlikely: Prop slipping on the hub. I'd expect your RPMs to be screaming up if that was it.

Overloading the boat will destroy your ability to go fast... but then I'd expect bad hole shot.

So... Expect it to be engine trim angle and/or prop selection.
Or the speedo reading wrong... Check with a cell phone GPS app.

You should have the power to break 50mph...

 

[sfestiva]

No Title

Thank you Bondo and fhhuber!

I am attaching picture of the engine installation, the trim tabs can be seen as well.

I eliminated the spunprop condition replacing the flotorq2 sleeve (after inspection it did require replacement).

The hull is in excellent condition, but needs antifouling (being in water since December)

As for the prop, i am not sure what pitch, it is standard came with engine. Note, the 150 mercury on the previous 19ft hull was at least twice the speed when compared to when installed on the searay.

I used the phone/GPS and it measures max at 22knots or 25mph.

If I raise the engine by at least 2 engine mount holes would this improve the speed? Overall the hole shot is perfect and very quick, and the boat planes within few seconds. I trim up the engine once plane but doesnt increase the speed much.

Really appreciate your comments!!

 

[fhhuber]

RPM is a little low I think. (not sure for that engine) that kind of says to me that you probably aren't pulling excess air into the prop.

Since you were pushing a bigger hull faster with the same engine and prop then its probably tilt and/or height. But can't be 100% sure over the net.

I would kind of expect that the engine being too high is less likely than too low.

Set the engine and tabs at the angles where you have them when trying to plane...
Get a pic that shows just the back about 1 ft of the hull to a little behind the prop, direct side view so the anti-vent plate is just a line.
Can't tell much about the angles or height from those pics.

I don't know a lot about the tabs... but intuitively those are drooping a LOT.... like flaps on a plane trying to slow it down for landing.

The hull looks clean enough to not be an issue.

 

[Faztbullet]

Running it at speed with that bimini top up will slow it down also.We found out years ago that the F/S models hulls(Stratos/Procraft/Hydra-Sports) that just removing the windshields added 6-10 mph to speed..

 

[schematic]

overproped.....try a lower pitch 3 blade.

 

[jimmbo]

Ditch the parachute above the boat, ditch the 4 blade. With one less blade to cause drag your rpms should rise, get friendly with a dealer who will let you try several props, on the condition you will buy one and not bang them up