Prop Change

[dadstinny]

Gday all i am about to go from 8 to 10 inch pitch prop on my suzuki 15hp after the boat manufacturer recommended it.

About to head away but the part has finally arrived after being on back order and is being installed a day before i leave so a bit rushed.

Is it advisable to do a water test after the prop change?

[chris69]

Yes it might be a good thing to do as adding 2inches of pitch will drop about 400rpm,or take the old prop with you just incase changeing a prop is not hard theres only a split pin and a nut that holds it on.

[frosti]

could make a huge difference , i just re powered my 4.0mt makocraft with a 30hp 4 stroke Suzuki , took it out and it wouldn't even get up on the plane with 1 person . took it back and the changed the prop 2 pitches down . now with 2 people it jumps straight up on the plane and it fly's

[dadstinny]

could make a huge difference , i just re powered my 4.0mt makocraft with a 30hp 4 stroke Suzuki , took it out and it wouldn't even get up on the plane with 1 person . took it back and the changed the prop 2 pitches down . now with 2 people it jumps straight up on the plane and it fly's

Interesting so you went down 2 inches?

My situation is that with a heavy load we need more bite to get us on the plane as the 8" prop is just to small.

One up or two up the 8" has plenty of bite but with the kids + gear just not enough.

Sacrifice is less torque from the word go but overall more bite should get us there quicker.

This is what the boat manufacturer has recommended based on how much we are loading the boat with.

Boat is a 3.7m fibre glass roof topper.

[Moonlighter]

I am not sure what you mean by needing more "bite" to get you on the plane? Perhaps you could explain?

If the boat is slow to get on the plane now, it will almost certainly be worse with 2" more pitch, especially when you are loaded up with gear etc

in fact, if that is what situation is, then the logic of what you said is exactly the opposite, in very general terms, it works like this, assuming that the prop diameter stays the same:

less pitch = better acceleration, but less top end speed
More pitch = less acceleration, but more top end speed

[frosti]

i were just doing what the experts recommended , it didn't
sound right but it sure worked .

[dadstinny]

I am not sure what you mean by needing more "bite" to get you on the plane? Perhaps you could explain?

If the boat is slow to get on the plane now, it will almost certainly be worse with 2" more pitch, especially when you are loaded up with gear etc

in fact, if that is what situation is, then the logic of what you said is exactly the opposite, in very general terms, it works like this, assuming that the prop diameter stays the same:

less pitch = better acceleration, but less top end speed
More pitch = less acceleration, but more top end speed

Suffering from cavcitation at start to plane and some times mid to wot.

Boat manufacturer tested this theory on the same motor.

[Noelm]

Motor height right?? Changing props can be trial and error, might be some sort of height problem making the motor rev but go nowhere? (or cavitation as you refer to it)

[dadstinny]

Motor height right?? Changing props can be trial and error, might be some sort of height problem making the motor rev but go nowhere? (or cavitation as you refer to it)

Hey Noelm we had a big discussion about this a year ago when I got the boat and at the time the manufacturer dropped the transom height.

This at the time resolved the issue but we found in choppier seas / heavier load the problem persisted.

The boat manufacturer has been testing and suggested the higher pitch to overcome this.

Dropping transom height did somewhat resolve the issue and now it is just fine tuning it.

Not a mechanic myself just going off there recomendations.

[dadstinny]

The last saga for those interested....

http://www.ausfish.com.au/vforum/showthread.php/196625-Motor-Height-Setup

Back on topic sounds like I should be ok to not water test until I get the boat to a situation with more expected conditions.

So Will let memechanic change the prop and test it over the trip in a variety of conditions.

[Spaniard_King]

less pitch = better acceleration, but less top end speed
More pitch = less acceleration, but more top end speed

This is not always the case. If the engine is overloaded a smaller pitch may well allow the engine to rev so it in fact makes a lot more HP and hence an increase in top end speed is acheived

[Moonlighter]

That is why I prefaced my comment with "in very general terms".

As you say, when the propping is really, grossly, wrong, then other things may result from pitch changes.

Anyway, it now appears the OP has different issues, cavitation or ventilation. Possibly, engine height issues still in play, as is the prop. And the hull design being very flat may be playing a part too.

[Fed]

Get some string & check that hull for hook, check it at the chines, the keel and a couple of places in between.
I was concerned about hook in the other thread..

So far the whole saga just isn't adding up for me, I hope I'm wrong.
But then you said it was fixed.
When I saw the pictures it sure looked hooked to me both at the chine & the keel but pictures can often deceive you.

I agree by increasing the pitch you will increase the slip.
Put a tiny tach on the motor & reduce the pitch as far as possible having regard to the WOT RPM (I'd prop it to the very upper RPM limit). This will reduce the slip.
Check the hull for hook first though.

(What's the tiller steering feedback like?
There doesn't appear to be any trim tab on the motor and you may be compensating with extra positive trim, correct me if it's not the case.)