prop depth

[gregtobin]

I have a Merc 1150 with a 17p prop on a 17.5 foot fiberglass boat. WOT gives me about 4400-4800 RPM and I'm planning to test a 15p 4-blade SS prop.
Before I do this, what should I know about adjusting the height of the motor so that the prop is at the correct depth?

I want maximum hole shot, not necessarily top speed.

Thanks
Greg

 

[WillyBWright]

Re: prop depth

I really can't explain why, but I've found 4-blade props on the old Merc inlines to be poor performers in every aspect.

 

[steelespike]

Re: prop depth

Not an expert:: Your a little short on your rpm range rec 5,000 to 5,500. but a 15" pitch 4 blade prop will improve low end performance and probably take away a little top end and may keep you out of your rpm band.If you go with a 15" 3 blade it will improve hole shot and may give you a little top end and get you into your rpm band at about
4,600 to 5,000.Motor needs to be in the area of 5,000 .Lugging below recommended rpm is not good for the motor.
Typical motor height is with the antivent plate even with the bottom of the boat.Then you can raise it a little at a time until just before ventilation becomes a problem.If it has regular clamps just add 1/4 inch sticks on the top of the transom.Just be sure of a good grip and that motor pumps water.
If you can gain an inch you could get 2 or 3 mph and a little more hole shot.
Before doing all this you may want to do a compression check and be sure the tach is accurate.

 

[Texasmark]

Re: prop depth

Hole shot without affecting anything else on a bass boat type hull, high power outboard thing is porting of the prop. Works!

4 or 5 blades can cost you top end. Porting costs you nothing on the top end with a 3 blade HP SS prop.

Case you missed it, porting allows air (exhaust gasses) to pass over the prop in the hole where torque requirements are most stringent and hp hasn't developed as yet because of limited rpm's trying to move the water out of the way and get on top.

Porting allows the prop to ventilate/cavitate, hence air reduces the density of the water over the blades and the engine can come up to rpm's much faster due to the lighter load on the blades. Since rpm's are a key ingredient in HP, HP develops rapidly and as a result the prop spins up fast. The boat responds favorably and hole shots are stellar.

Once on plane the water flowing across the outside of the hub seals off the holes and the exhaust gasses exit the rear of the hub in the normal fashion.

Paying attention to the transition is liken to a manual transmission going into overdrive. Speed increases and engine rpms drop roughly 20%ish sort of thing. Really cool to hear and feel it and as a result, the boat, now in the reasonbly high mph by now will accelerate even more as rpms recover.

HTH

Mark

 

[gregtobin]

Re: prop depth

I guess I'll need a little more explanation here:

1) Steelespike, would a 3-blade 15p prop give a better hole-shot than a 4-blade 15p prop (all other features that same)? And, what is the anti-vent plate? In general, do you get a better hole shot when the motor is raised higher (but not tilted)? How do I make sure that the tach is accurate?

2) Texasmark, Can you explain what is meant by porting the prop? and how do you do it?

Thanks!
Greg