4.3L MPI Prop Question

Joined
Aug 17, 2008
Messages
4
I could sure use some help. I have a 2006 Crownline with a 4.3L MPI with a 1.62 Alpha 1. With the stock Merc 3-blade(14.25 x 21P) I push 4700-4800 rpm and 53-55mph trimmed out. My problem lies in that this boat has a horrible hole shot(struggles to get on plane). It struggles to pull a deep start slalom skier weighing 175lbs(points the nose to the ski and spins). It is trimmed all the way down. I had a 2002 Glastron SX175 with a 3.0L that could pull a skier better than this thing.
The sympton is that it seems like the boat ca't get enough water around the prop to push it. By easng into the throttle it seems to help. I have tried a Alpha 4 (14x20) although it helped we are still not their...I also push close to 5,000 rpm with that. My brother has a Sea Ray with the 4.3L 1.81 Alpha 1and when using the Alpha 4 (14x20P) he can literally pull the rope out of your hand. Please help as I'm extremely disappointed. Should I be going in the opposite direction(higher pitch)? Please help, at this rate this boat will be sold next Spring.
 

Tail_Gunner

Admiral
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Jan 13, 2006
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6,237
Re: 4.3L MPI Prop Question

Nice boat's crownline's but the 1.62 gears are up there a bit....from my limited experience there v8 gear's...

But here's a start...21P blade alum @4750...so first getting the boat to the top of the rpm range of 4950 by dropping 1 pitch thats a 20p alum 3 blade. Now going to SS you drop a bit more about 200 rpm and if you go to 4 blade's another 100

1. 20p 3 blade alum @ 4950



2. 19p 3 blade ss @4900..... Best top end

3.18p 4 blade ss @ 4950 Better midrange lower planing speed's

4.17p 4blade ss Pull your arms out....:cool:

3.
 
Joined
Aug 17, 2008
Messages
4
Re: 4.3L MPI Prop Question

Tail_gunner,

Thanks for the response. The 4.3L MPI & the 5.0L make the same ponies. Mercruiser also gears them the same, 1.62. Although, I'm guessing there is more torque with the 5.0L. The WOT for the 4.3L MPI is 4400-4800 and I'm on the high side already. Any other thoughts? I appreciate the help.
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
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6,237
Re: 4.3L MPI Prop Question

Tail_gunner,

Thanks for the response. The 4.3L MPI & the 5.0L make the same ponies. Mercruiser also gears them the same, 1.62. Although, I'm guessing there is more torque with the 5.0L. The WOT for the 4.3L MPI is 4400-4800 and I'm on the high side already. Any other thoughts? I appreciate the help.


Well this is only a personal preference based on a few suggestion's from a few people on this forum who really are in the know on this subject. The top end of a 4.3 with a roller cam is 4950 that is where your rev limiter step's in and stops the party. One can go into a long discussion on this topic but it starts and end's for me knowing that i spend little time @ wot and lots of time everywhere inbetween.

Unlike a car a boat has one final gearing....no shifting just one. The heaviest demand's come from a dead stop to plane, propping a boat so it can reach it's peak also mean's it more efficent out of the bottom end....does 200 rpm make a diff..yes if you take into account the cost of a prop and just what it can do for the overall performance for the boat.

You should spend litte time @ wot compared to the time you spend at or just on plane. In my particual case a 4.3 gi... i have found using a 4 blade give me 34 mph at 3200 the recomended crusing speed for the motor. That is 200 rpm better than a 3 blade's that i have tried and where i spend 70% of my time....I should also note that the boat did lose 1.5 mph @ wot...personally who cares...im never there more than 5 minute's and 54 vs 55.5.......:p

All this talk is based on a 3000 19' boat with 1.66 gears running a 20p 4 blade. I normally would say a Stilletto Bay Pro 2 in a 18 pitch(they have big cupping) would hit the number's and performance your looking for but your 1.62 gearing suggest different and that's strickly a opinion.

WH has given some very good info above on the ss Michigan wheel which might work quite well or a Solas Titan in a 17...you might lose a litte top end say 3-4 mph but for the rest of the time id bet youd be happy

1.http://boatpropellers.iboats.com/Me...r=2004&**********=359663567&*******=607244007

2.http://boatpropellers.iboats.com/Me...r=2004&**********=359663567&*******=607244007
 
Joined
Aug 17, 2008
Messages
4
Re: 4.3L MPI Prop Question

I agree with your thought process on the WOT. Can you please clarify your recommendations 1-5(with five being the greatest success of what I'm looking for). Please reference brand/model as well as I think the diameter plays a role in what I'm experiencing. Thanks in advance as this is very helpful and buying props at retail for testing purposes is very expensive.
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Re: 4.3L MPI Prop Question

How large is your boat, length width weight and what degree of hull do you have. Sorry but i must ask do you trim the drive accordingly when launching and when runnning at wot.....All in at launch and out at wot that is...:redface:...gotta ask

That 20 pitch 4 blade alpha took the boat to it's top i think, did you notice any very slight misses while running at 5000...if not you may want to check you tach first to se if it is accurate within 100 rpm...that mpi motor has a rev limiter built in at 4950...hence the question any misses at @ wot.

It also brings up another good solution if everything else is accurate... that is trim tab's, they lift a boat out of the water very effectivly and are quite a bit cheaper

Just question's that will help anyone responding here.... aka a bowrider..deck boat..lots of holes here..;)
 
Joined
Aug 17, 2008
Messages
4
Re: 4.3L MPI Prop Question

Bowrider
18' length
92" beam
3000lbs
37 degree hull

The boat is trimmed accodingly pending upon the use.

No way to track the tach...this boat is practically new(30hrs). I never hit the limiter with the 14X20P 4 blade.

Hopefully this plugs all the holes.
 

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Re: 4.3L MPI Prop Question

If it were me and i wanted the best of all world's. i would pick a 18p 4 blade stilletto bay pro 2 and compliment the package with Nauticaus trim tab's. The Stilletto has a huge amount of stern lift, making for lower plane time's and lower plane speed's while at the same time at high speed it create's enough lift that your hull will catch some air reducing drag and increasing top end speed. The tabs simply create leverage futher lifting the boat on launch..quicker times and even stablize @ high speed....I have a boat that is almost a twin to your's and the baypro 2 gave it enough lift that @ 55 it would chine walk and the tab's were just enough stablity to control that..

http://boatpropellers.iboats.com/Me...r=2004&**********=359663567&*******=607244007


Those Titan's are supposed to great prop's, you could use anything from a 17 stongest hole shot at the cost of top end speed..... 18 for overall performance and adding tab's along the way surely would help out more than you might think....

Rember on the Titan's they have a exchange program here....and i have heard they are Merc's knock off's ..revolution 4 and 3 blade are Mirage's...but that is pure speculation and conjecture..;)

Sorry for the long winded statement.....I/O's dont have as much flexability as a O/B...so its the little things that count.or you can play with...;)


Umm that is a Merc drive Alpha with gen11???
 
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