Prop pitch

[88 206]

O.K.,  here goes guys.  I have an 88 206 bowrider with 320 hp 5.7 efi motor.  Very slow out of the hole with more than 2 aboard. WOT rpms 4200 fully trimmed,  it seems over propped to me. it is running a 21p ss mirage prop.  How much help can I expect from going to a 19 pitch 3 blade?  should I consider a 4 blade? I can find a 4 blade in 20 pitch , or 18 pitch.     any  recomendations?    thanks.

[Vigilance]

Not a prop issue-That should have no problem spinning a 21 pitch.

[alswagg]

Unless you changed your engine, no 206 came with a 320 hpEFI engine, especially in 1988.  Maybe a typo?  Any how a standard engine 5.7lt 4bbl would have had 260 hp @ 4,400 rpm's.  Droping from a 21 pitch to a 19 pitch can yeild a 400 rpm increase at WOT.  If you did swap out the engine, a 320 hp current production Vortec would spin up to 5,300 rpm @ WOT if proped properly.   Al

[88 206]

Definately not a typo, 320 hp 5.7 efi, I am the 3rd. owner, and the po and the original owner are friends and I have met him, he ordered the boat new in 88 with the high performance pkg. which consisted of this motor and alpha super sport out drive. he told me he put on the 21p prop, does not remember what came on it standard.  if it is not a prop issue, any ideas? it starts and runs excellent, revs right up in neutral,  just seems doggy when tyrying to plane up. after on plane and trimmed, it runs and sounds good.  rpm's just seem low to me at wot.  I have tuned up the engine, and it seems better.(cap rotor wires fuel/water seperator.)  even at idle, docking speed is too high. I don't know the gear ratio on this outdrive.       Thanks,  Matt

[DaleM]

I would think that boat with that engine drive combo would spin a 23" or better prop for ultimate performance.  Something not right in the set up.  That's a ton of HP in a 21' boat!!

[88 206]

well, I put on a 19p 3 blade prop this weekend,  whole new boat. Easily planed out with 4 adults and 4 small kids. I cannot believe the difference. it was a little too choppy to really open her up, but I will try this weekend. I want to see wot rpms at what speed.  I'll post after.   Thanks,  Matt

[alswagg]

Ok, but in 1988 EFI was not even available in ANY marine application.  Was not Coast Gaurd approved yet.  I am glad your 19 pitch prop helped.  Al

[DaleM]

Something still not jiving here.  320 HP in a 21' boat needing a 19" pitch prop, NO WAY!!  When was it tuned up last?  I think you're running on 7 and not 8 cylinders.  Low rpms is just a symptom and the prop with less pitch helps the sadly running engine get the boat planing and running in a way that seems right.  The 300 hp 7.4 L in my 232 sounded great, but ran slow to plane, slow top end and low rpms.  Found a small cut in a wire that was arcing to ground.  A little black tape was the fix for the day but I probably could have put a prop with less pitch on her to get it running "right".  In either case things just don't add up on your 206 with that engine.  I'd look someplace else yet. 

[88 206]

alswag, I am holding, and reading the orig. 1988 mercruiser catalog which has a chart listing all mercruiser applications for 1988 and under performance products lists 320hp efi with alpha supersport outdrive(which I have) or bravo 1  outdrive. dependant on application.

[88 206]

Dale, I tuned it up when I got it, new cap rotor,wires, plugs, fuel filter. it definately is running on all 8. I have never seen the listed max rpm  with the 21p prop(spec. 5000 to 5400)  any Ideas?  low fuel pressure? efi problem? the listed drive ratio is 1.50./1    thanks,  Matt

[DaleM]

Matt I'm just comparing it to all I've seen in my boating life.  When you put HP like that into a smaller boat she just goes.  Now maybe the age of the boat will make it heavier due to construction methods of the time.  Maybe the things you brought aboard make it heavier than what I'm used to seeing.  Maybe the engine is starting to show it's age or maybe the newer (late 90's and up) engines are just more efficient at getting the HP to the prop.  Either way I'm not saying what you have going on isn't right, just not the norm for my experience.  Specs don't usually list a max with a specific prop but for the engine overall.  Timing?  Also try www.midwestboatparty.com for some info.  There's a gearhead section there with some pretty informative guys.  They have way more experience than I ever will with high performance equipment.  Post your info there and see what comes back.     

[frodo13056]

I had an 88 206 Cuddy with the Merc 350 / 260 HP with a 19 pitch stainless. The boat would run around 50 MPH at close to 5000 RPM with the 19 pitch and  closer to 55 MPH with the 21 pitch (if memory serves me correctly!). I alternated between the 21 pitch stainless and the 19 pitch depending on what I was doing for the day. If I was just cruising, I'd use the 21 pitch. If doing watersports, I'd use the 19 pitch.
If your boat now runs better with the 19 pitch then my guess is that your 21 pitch may have been re-pitched to 23. Most prop shops can re-pitch a prop up or down a size and you'll never know because the stamp on the prop does not get changed.  But, even with the 23 pitch and 320 HP - it would be a bit slow out of the hole but you should have been running mid-60 MPH.
Another factor is that the SS Alpha drive is about 2 1/2 inches shorter than a standard Alpha One drive - so that means that your drive is not down as far in the water (good for performance due to less drag) but harder for the boat to plane out due to less leverage on the hull. Your power package is for top end performance - not hauling lots of people, gear, etc.

[alswagg]

I have done some research on this.  I stand corrected, Mercruiser did build some 320 hp EFI small blocks in 1988.  These were not to be sold to the general public but to race teams and to proto type engineering.  The injection system is unique only to Mercruiser.  Most of the parts are no longer available.  If it were mine I would remove the engine assy completely and store for display.   Extremly rare motor.  Especially in a 1988 which meant the engine was built in 1987.  Again it was meant for racing application only.  Should push the ol 206 past the 62mph mark upwards of 65 with correct propping.  Al

[DaleM]

Right there is why I love this forum and the people that make it what it is!!  Alswagg, who I'm sure is quite busy this time of the year, took a little of his time to research someones situation.  Great info, thank you sir!!!

[88 206]

Thanks, guys. all I am looking for is this to run properly and to do what I want it to do. I have all the origional paperwork from when the boat was ordered, right through to the service for storage last winter,  right down to how many times the oil was changed ( always had synthetic oil in it. )   It still seems like I should be getting more from it, I have still not been able to get it out and see where I'm at wot.  we'll see hopefully tomorrow and I'll post. I have a place on Sebago lake in Maine and I have to travel for internet service when I'm up here.  I'll let you know. thanks,     Matt