Modifications to my Mercruiser?

[spoolinawdtsi]

Anyone have some good tips on upping the hp on my 5.7 mercruiser. I have a 24 ft cabin, aft cruiser seems to struggle on plane with 4 adults on it. Runs bout 36 mph, any upgrades like cam, roller rockers, prop(stock still)etc help the boat move to bout mid to upper 40mph range?

 

[lime4x4]

Re: Modifications to my Mercruiser?

what is current prop?? Also what speed and rpm are u at when the boat comes onto plane??

 

[180shabah]

Re: Modifications to my Mercruiser?

An additional 10-12 mph in a cruiser is unrealistic. you will need AT LEAST another 100hp or so, which means a big block and Bravo drive. Your alpha won't handle it.

Better hole shot is very achievable though.
step 1. toss your existing prop and get a four blade.
step 2. Trim tabs get them and learn to use them.

 

[spoolinawdtsi]

Re: Modifications to my Mercruiser?

4 blade prop make that much differ? Previous owner said it would run 44-47 but i've yet to see that. Did he over exagerate or is that a common speed for this size cruiser?
just sucks I can't have no more than 4 adults on board an two of them have to get in the cabin before it will plane out! Boat will not plane out if all 4 of us are on deck, an the boat just seems to hold the same rpms. This boat is a 89 cruiser with omc outdrive, boat has trim tabs an the wing device that goes on the lower unit.

 

[Cuyose]

Re: Modifications to my Mercruiser?

I have never heard of a single engine 24 ft cabin cruiser getting remotely close to 47 mph. He was dreaming when he quoted that.

Trim tabs or different prop are gonna help you get on plane, but no prop change is going to get you to 47 mph unless that boat is made of balsa wood!

 

[QC]

Re: Modifications to my Mercruiser?

We need the RPM and prop pitch info. Lose the Hydrofoil. Do you know how to use your trim tabs? Also you said 5.7 Mercruiser and OMC drive. Care to ellaborate? What HP? You carrying a bunch of water, fuel, stuff? If so get rid of some. The more info you give, the more complete answers you will get

 

[spoolinawdtsi]

Re: Modifications to my Mercruiser?

lol k I'm new to the cruiser scene. I didn't know what an average speed is for this size cruiser, It usually does 32-33 but it has hit 36-37 on a glass water. I assume this is average?? If I do the prop change to a 4 blade will it plane quiker an still run bout the same speed? It's a 260 hp cobra outdrive not sure on size an pitch of prop seems to run around 4500 rpm but doesn't seem to accurate cause it floats a grand or so. Any Ideas on what to get? Boat weighs 4500
I always leave the trim tabs down I try an adjust them but seems like the boat will lean dramatically when i do this

 

[180shabah]

Re: Modifications to my Mercruiser?

Which is it?

Merc or Cobra???

 

[spoolinawdtsi]

Re: Modifications to my Mercruiser?

It's a 89 5.7 260hp mercruiser with omc cobra outdrive
So the hydrofoil is slowing the boat down, an the trim tabs will do the job of the hydrofoil for planing the boat out???

 

[180shabah]

Re: Modifications to my Mercruiser?

Get rid of the hydrofoil, it is a waste. You already have trim tabs to help lift the stern of your boat.

Yes the hydrofoil along with deployed tabs WILL slow your boat down. When the stern is lifted too far, the bow drops and plows through the water. When I purchased my boat it had a hydofoil on it, I won't go into all of the details but I picked up 7mph when it was removed. Don't expect that kind of gain on a boat the size of yours, mine is only an 18' BR.

 

[QC]

Re: Modifications to my Mercruiser?

If you adjust the tabs (plural) and it always leans, then maybe you have one that isn't working? Can you make it lean either way on command? If so then they are both working. Test them on the trailer. Move one and go look, move other then go look. Starboard switch should work port tab and visa versa. Go to the Bennett site and read the op manual . . .

Once you are up on plane start letting them both come up equally. If you have independent switches hold them both down (should make tabs go up). Hold them for a count of 1 second at at time and see what that does, then another second, then another etc. This way you let the boat react before you mess it up . . .

Also, you absolutely must verify that your tach is accurate before we talk propellers. It is the ONLY way to do it. Annnnnnd we have to know the existing pitch, so we can tell you where to go. It should be stamped on the end of the hub. I am guessing there is a 15 somewhere there, but it could be 13 or 17 or anything for that matter.

 

[spoolinawdtsi]

Re: Modifications to my Mercruiser?

I know they both work cause it will lean either way working the switchs indapendant, I've lowered both during take off an ounce the boat plans out I hold both up switchs at same time but always seems like one must work faster than other an boat goes to an extreme lean. Main ? does the tabs need to be trimmed all the way up at cruisin speed, then adjust one tab to counter the lean?
I'll pull the hydrofoil off this weekend, along with finding out the pitch of prop.

 

[QC]

Re: Modifications to my Mercruiser?

The hydrofoil may be causing ther lean . . . The tabs do not have to be anywhere for anything. They are best down to help you get on plane and as far up as you can for the best top speed, but slightly down may work best for your boat at cruise. This is a trial and error deal, no truly firm rules.

 

[MikDee]

Re: Modifications to my Mercruiser?

Just as a reference here, I had a 78'- 24' SeaRay Cuddy with a 260hp mercruiser, with thru hull exhaust, and it would only do 40mph fully wound up with it trimmed out, and this boat was considered fast for what it was, I was lucky to do that,,, the only thing that would have made a big difference, was if I had dropped a big block in it, but even then maybe not even a 10mph gain probably. As it was I was taching 4500rpm, with a 14X17prop.

 

[spoolinawdtsi]

Re: Modifications to my Mercruiser?

Thanks guys for all the great info!

QC Now that u say that about the hydrofoil, I realize the one on my boat is made of hard plastic an noticed before how one side was little weaker than other side. It will be takin off this weekend, an I will be working with the tabs!!

What rpm is considered redline on these motor an what is a good cruising rpm?

 

[paulrfrancisco]

Re: Modifications to my Mercruiser?

lol k I'm new to the cruiser scene. I didn't know what an average speed is for this size cruiser, It usually does 32-33 but it has hit 36-37 on a glass water. I assume this is average?? If I do the prop change to a 4 blade will it plane quiker an still run bout the same speed? It's a 260 hp cobra outdrive not sure on size an pitch of prop seems to run around 4500 rpm but doesn't seem to accurate cause it floats a grand or so. Any Ideas on what to get? Boat weighs 4500
I always leave the trim tabs down I try an adjust them but seems like the boat will lean dramatically when i do this

Mmmm.. I have twin 300hp 454's boat is a 30' cruiser and weighs 9500lbs... about double yours. plus an equivelent of 40 hp more (based on half my boat) and I cruise at 30, and WOT is only 43 on the GPS.. I'm propped well and RPM are on target... I wouldn't expect much more out of yours... check for excess weight, clean bottom, correct trim, prop in good shape... (it could look good, but really be distorted).. Alluminum props flatten under heavy load.. try stainless...