Installed 2nd New Engine

[383 BRAVO]

Lenny/registre I Installed The New Engine Which Mercruiser Sent Through Warranty And I Had A Chance To Run The Boat. I Pulled A Prop Off Another Boat Which Was A 4 Blade 23p Mercury Revelution Prop. Wot W/trim I Was Only Able To Reach 4550-4600rpm. Rpm Is True However I Was Using The Speedo Gauge And Was Seeing 65mph Did Not Use Gps Yet.so Speed May Not Be Accurate. Should It Have Turned Up Passed 4600 Rpm? Should I Try A 21p Or 23p 3 Blade? This Motor Is Rated For 48-5200 Rpm. Could The Hp Be Over Rated?

[onesubdrvr]

Are these engines broken in already? If not, you will see a few hundred additional RPM's when it's broken in. Also, you may want to try a smaller pitch prop.

LastTango has a newer 18, with the 6.2mpi engine (I believe rated for 320hp), and the Bravo drive. He runs mid to high 60's (65-67), so you're probably pretty close. I'd break it in properly, then start playing with props.

Wayne

[383 BRAVO]

I Pretty Sure Mercruiser Ecm 555 Does Not Have A Break In Mode. I Dont Think Breaking It In Will Give Additioinal Rpm. Some Ecm Burn A Chip After So Many Hours.

[onesubdrvr]

I Pretty Sure Mercruiser Ecm 555 Does Not Have A Break In Mode. I Dont Think Breaking It In Will Give Additioinal Rpm. Some Ecm Burn A Chip After So Many Hours.
Well, what I'm trying to say, is after I rebuilt my 350, I could only turn my 22p Turbo I to 5000rpm,.... after about 10 hours, I noticed I was turning closer to 5700rpm, speed had gone up as well (rings seating, etc.)

Just a thought, I'd give it 20 hours or so of run time then play with the props a little, but I think you're pretty close, like I said, 66-68.

Wayne

[MOP]

Something still does not seem right, I have a 22 with a mild 383 maybe 360hp +or- I turn 5150+ or- with a Turbo 23" 4 blade best 65.7 full tank and skinney me. I feel you should be able to swing a 25" on and 18 and hit on or about 70. Lets back track, get about 5-6 hours on it set timing at 32 total and lets see what happens, I am at 31 but will kick it to 32 come spring. I know that you know your stuff knowing the yard you run but have to figure this one out something is a miss.

Phil

[Lenny]

I agree with MOP's "a-miss" statement. You first need to dress warm and DEFINATELY borrow a hand held or whatever GPS. It will get rid of that variable in the equation as you have gotten rid of all the others.

The TOP speed with that combo and RPM's ( and the 1:50 gear set you say you have and have verified) is about 66.79 mph (GPS) NOT factoring in slip. I would think you are seeing a true 60-62 (maybe) with that set-up. You are missing out on about 600 rpms/ and about 8mph (before slip calculations)
I truly feel you should see 70-72 mph.

When you put the BRAVO on, did you glass in the transom and RE-CUT the hole or did you simply bolt it into place where the old hole (ALPHA) was?

I have an old 18, with probably about 280 ponies out of a 350, and spinning a 24" prop (very, very high slip mind you) at about 5200 now. I see about 58-59 GPS mph with myself/gear and full fuel. It has NO trouble pulling this prop, in fact, I can spin it (cavitate it) on demand, and with no real ponies.

What is the MERC horsepower rating for this combo? 350hp?, 340?, etc etc.

You should be able to pull a 25" prop with that motor and easily see 5000 rpms.

We definately still have some missing pieces. :yes:

[smokediver]

do you happen to have bottom paint 383 ?

[383 BRAVO]

On the ecm 555 there is no timing all you do is set phase timing. Yes the boat has paint which will be removed soon. today I will try a 21p mirage and a 23p mirage. The engine is rated at 330hp and is a 6.2mpi. Gear ratio is diffently 1.50. If I go from a 4 blade 23p to a 3 blade 23p how much rpm should I gain. Does anyone with the 18 donzi have problems with water/exhaust revursion.

[boxy]

Is bottom paint not a 3-5 mph suck on your top end ??

[MOP]

I wonder if we are starting at the wrong end of this, groggy hang over thought! Any chance of a fuel restriction?

Phil

[MOP]

Is bottom paint not a 3-5 mph suck on your top end ??

Steve though bottom paint is damn ugly if properly applied I doubt much speed loss, true if its old and flaking it would be like dragging a bucket through the water. But a nice fresh job should not hurt much at all, I raced blow boats and for many years I would spend days wet sanding the bottom paint. Then I saw what a stippled bottom did for Conners in the Americas Cup in Austrailia, being and I could haul out any time for free I decided to try a rolled bottom I also gained a slight bit of speed from thr stipple of the rolled paint provided. We have some very active one design fleets here and now most have gone from waxing, soaping and what ever SMOOTHING tricks to sanding the bottoms with 220. Slick smooth is not fast, the barrier layer must be broken. A perfectly smooth golf ball is nearly 10 MPH slower in flight then the modern stipple ones.

Phil

[boxy]

Slick smooth is not fast, the barrier layer must be broken. A perfectly smooth golf ball is nearly 10 MPH slower in flight then the modern stipple ones.
Phil

The dimpled golf ball is still "slick smooth" the surface is just dimpled to reduce air friction.
If the bottom paint is fresh, and in good shape, then maybe it might not make a difference, but if it is old and mossy you might as well throw a couple of sea anchors over the side.
As far the sailboat theory the co-efficient of drag is an equation above my head, but I'm sure dimpling works better on a non-planing hull, than it does on a high performance v hull. I do agree that wet sanding the hull will help break surface tension, but wet sanding with 220 can't be compared to bottom paint.

[MOP]

Steve could not agree more, I would love to see a test of stippling the last 1/4 of the bottom of a high perf boat. I know the stippling works at high speeds, Boeing Aircraft has been applying the rivulet surfacing to jets for a long time. Years back I inquired about it, it is a thin Mylar that is just stuck on. Would like to find a source and try a little to see if and how much it would help.

Phil

[smokediver]

I think it's the bottom paint ...Think about how heavy it is , with primer and how many coats ? I don't care how new it is if it the 5th coat there is about 200 lbs of paint and primer on there ! I agree with breaking the surface tension and all but it is too a point . Kinda like friction loss in a hose . As the velocity increases so does the turbulence of the water .