Where to start...

[CBC]

I finally got my old boat in the water, got a new tach on it and can't even get close to WOT.

I have a 1975 Larson 1550 Shark tri-hull (dry weight 900 lbs.)
A 1978 Merc. 800 (950 lbs)
18 gal. fuel tank 1/2 full (80 lbs)
2 guys (400 lbs)
prop is al. 14x15
max rpm range is 5000 to 5500.

The problem is I get 6000 rpm at about 3/4 throttle... where do I start?

Thanx,
Crunch

 

[Bondo]

Re: Where to start...

The problem is I get 6000 rpm at about 3/4 throttle... where do I start?

Ayuh,.....

I guess I'd start with a 17" or 19" prop....

 

[CBC]

Re: Where to start...

Took the boat out last night with the intention of seeing just how high the RPM's would go.... seems the last 1/4 of throttle travel doesn?t do a thing, topped out at 6000.

If I understand right, each inch increase of pitch decreases RPM by 200? That means I need to go up to an 18 pitch and they don't make such an animal.

I'm running a 14x15 3 blade AL., don't know much more than that. The number stamped between the blades is 48 77342A-4 15p. This prop is old, pitted, paint worn off.

I guess my real question is, what is my best bet to get my RPM down between 5400 and 5500 in one shot, as we don't have a test program at the local Marinas.

Thanx,
Crunch

 

[steelespike]

Re: Where to start...

If we had your speed the experts might be able to tell if you have excesive slip.gps is best way for accurate speed.The 200 rpm per inch figure is approximate could be more or less.My guess is you might get more drop going from a tired prop.I would do a 17 should get you to 5,600.A little bit of load and you would be right on. If your running light just throttle back.

 

[CBC]

Re: Where to start...

Thanx Bond-0.

I went down and talked to the service manger of a local marina and he said the same thing you did SP, also he said he would exchange a 19 for the 17 I bought if it didn't bring the RPM's down below 5500, so I'm all set for testing this weekend.

I don't have GPS and I'm not interested in wringing all the speed out of this setup, just don't want to run at over rev. and get reasonable economy. There was no cup on old prop., it was 14x15. New prop. has cup and is 13.25x17. I'm thinking it will be close and run much faster. I'll lose a bit on my hole shot but with the 15P it was out of the hole in it's own length... I'll keep the old one in case I ever want to go water skiing.

Thanx again,
Crunch

 

[CBC]

Re: Where to start...

Ran her in the lake yesterday, it was fairly smooth with no wind.... hole shot with the 17P was good but with just me and 1/2 tank of fuel RPM was 5400. Filled up and was down to 5000.

When I was running that old 15 P prop I was at 6000 RPM with just me and a full tank, so I got a 1000 RPM drop by going up 2" in pitch and a cup.

What I'm thinking is to get a new cupped 15P and see where that puts me.... as is, I'm afraid that if I have 2 people or more in the boat, and a full tank, I'll be down around 4000 RPM.

Any thoughts?
Crunch