Need Help picking bravo3 props for 7.4mpi

[Iaintslow]

I have 2001 maxum 29ft with single 7.4mpi bravo3 3×3 20 pitch. Its absolutely terrible top speed 31mph and rpm max out with only 65-70% on the throttle shifter. It feels like it struggles to plane out sometimes but once it does it pretty much maxing out rpm fast. Marine shop said we underpropped but could not recommended anything. I was looking at 3×4 24pitch what you guys think or its too much for single 7.4?
thanks

 

[tpenfield]

Can’t provide any guidance without knowing the RPM that you are reaching at WOT.

A 29 foot boat with a single 7.4 is a dog and there is probably not much that can be done. It should have twin engines.

 

[alldodge]

If its a 2900 SCR it shows weight at 7300 dry and agree need your WOT RPM

 

[Sea Rider]

Need to find out what's the min-max wot rpm range factory stated for that engine, go for a full wot water test with min 2 up and check max achieved wot rpm. Will know how much over propped the engine could be.

Personally would go towards same 3 blade prop model with less pitch if need to pull wot revs up towards middle to max wot rpm range than going blindly for a different 4 blade one that isn't known which max wot rpm will achieve as you already count with a 3 blade starter prop with well known max rpm achieved.

Find much easier to go for a top prop maximization from current starter prop on a second try and eventually on a third try if the ( +200,-200 rpm per pitch rule of thumb doesn't work as expected.

Happy Boating

 

[Maclin]

It may be difficult to find out what RPM's the 7.4MPI can attain if it is up against the rev limiter now at 2/3 throttle. Going from 6 blades to 7 blades is interesting, but you may end up having to roll the dice without the true top RPM attainable with that rev limiter in play.

 

[Sea Rider]

When at top speed of 31mph and rpm "max out" with only 65-70% on the throttle shifter, did the rev limiter kicked in ? if so, at which wot rpm ? in that case need a higher pitch prop to lower wot rpm to around middle to max rpm for the rev limiter not to kick back again if that's what's been doing lately.

Happy Boating

 

[Faztbullet]

You need some big trim tabs and add some cup the inside prop only

 

[Iaintslow]

I hit 4500- 4600rpm which is max and still have about 30% of the throttle left on the shifter. Rpm don't bounce off like it would in neutal it just stays there. Thats in open ocean with pretty choppy water

 

[alldodge]

I hit 4500- 4600rpm which is max and still have about 30% of the throttle left on the shifter. Rpm don't bounce off like it would in neutal it just stays there. Thats in open ocean with pretty choppy water

If your sure you have more throttle available (this means your at 4600 rpm and you have given it more throttle and hit the rev limiter or the rpm goes way above 4600) then you have 29% prop slip and need to go up in pitch.

The issue is you have 29% slip. so the prop is not holding very well. If it was holding around 20% (and 20 is not even good) at that rpm you would be doing 34 MPH.

So starting to thin your motor is either tired or not hitting on all 8

 

[tpenfield]

Not sure what you mean by 30% left on the throttle. Does that mean you can move the throttle 30% more and nothing happens? Or does it mean the throttle won't move past that spot?

 

[Iaintslow]

Not sure what you mean by 30% left on the throttle. Does that mean you can move the throttle 30% more and nothing happens? Or does it mean the throttle won't move past that spot?

yes i can move throttle more and nothing happens

 

[Iaintslow]

If your sure you have more throttle available (this means your at 4600 rpm and you have given it more throttle and hit the rev limiter or the rpm goes way above 4600) then you have 29% prop slip and need to go up in pitch.

The issue is you have 29% slip. so the prop is not holding very well. If it was holding around 20% (and 20 is not even good) at that rpm you would be doing 34 MPH.

So starting to thin your motor is either tired or not hitting on all 8

Rpms wont go above 4600 it has safety this is the limit for this engine, problem is i hit 4600rpm with only 70% throttle on the shifter. The other 30% doesn't change anything since engine is already at max rpm

 

[alldodge]

Rpms wont go above 4600 it has safety this is the limit for this engine, problem is i hit 4600rpm with only 70% throttle on the shifter. The other 30% doesn't change anything since engine is already at max rpm

If your tachometer is accurate then the motor is just running out of steam, the rev limiter is in the 4750 area. Need to look into the health of the motor with compression and leak down test.

Measure fuel pressure = 43 psi and should stay about there throughout the rpm range
Timing at 8 BTDC
Plugs MR43LTS gap .045
Look down inside the Y pipe to see if the exhaust shutters are there
You should also have an exhaust tube and not an exhaust bellows, the bellows is restrictive with a B3
Look at the plugs to see if they are running rich

 

[tpenfield]

It is a bit of a guessing game, since it would be hard to tell how much more RPM you would get out of the remaining throttle range. You could try a 22 pitch set or a 24 pitch. I don't see a 29 foot single engine boat being able to handle much more pitch than that. You might get into the mid-30's for speed.