wrong prop or heavy boat?

[Duckhunter08]

I've got a 1990 astro fish n ski (quickfire 18FS) with a 1990 mercury 150 blackmax engine with a 21 pitch 3 blade stainless pro and a dolphin tale on the motor. Just had boat back from a fresh tune up and runs like a top. The top speed is around 43 mph running full throttle at around 5600 rpms with two people on smooth water and no gear. Ive heard that astros were known to be heavy but im not sure if a prop change would help speed or drag? no jack plate and I do trim the motor up once im on plane to increase speed. Not sure if prop change would help fuel either? Mechanic estimated it should run 55+, Im no speed demon but dont want to own something that doesnt perform to its full potential if im keeping it from it.

 

[MikDee]

Either your tach is wrong, or your prop is slipping? I'd figure you'd be at least 10mph faster

 

[Sea Rider]

What if you trim bit up, give full throttle, once on plane on flat calm no wind waters with well ballanced boat you trim back to neutral ? Would speed remain same as if left trimmed up, have you checked that trim setting at speed ?

Happy Boating

 

[Duckhunter08]

What if you trm bit up, give full throttle, once on plane on flat calm no wind wasters with well ballanced boat you trim back to neutral trim ? Would speed remain same as if left trimmed up, have you checked that trim setting at speed ?

Happy Boating

I reach top end then trim up gaining 4-5 mph if I trim back down It reduces speed

 

[Duckhunter08]

Either your tach is wrong, or your prop is slipping? I'd figure you'd be at least 10mph faster

I would assume so as well. And just had new bushing pressed in prop last summer. Speed is on two different gps units to confirm accuracy

 

[MikDee]

I would assume so as well. And just had new bushing pressed in prop last summer. Speed is on two different gps units to confirm accuracy

Speed maybe right, but then your Tach is wrong! With that prop, and my calc. if it were truly turning 5600rpm, then you should be doing approx. 56mph, -10% slip = 50mph.

 

[Duckhunter08]

Speed maybe right, but then your Tach is wrong! With that prop, and my calc. if it were truly turning 5600rpm, then you should be doing approx. 56mph, -10% slip = 50mph.

I recently got the boat back from a mechanic and he replaced two of the control modules that were causing a few of the cylinders to not get fire. Runs like a top no isssues at all and he said compression was excellent on all cylinders. not sure if i need to adjust throttle or what?

 

[jestor68]

As others have indicated; one of two things is going on here.

Either your prop's hub is slipping, producing about 28% slip, or your tach is way off. If I substitute a reasonable 10-13% slip, it shows only about 4600 rpm at your stated actual speed of 43 mph.

What is the 21 pitch prop you're using?

 

[steelespike]

I don't think the boat is excessively heavy at 1300 lbs dry weight.And more troubling is I think a 21" prop should get a true approximate 5600 rpm.
You might remove the the Doelfin and check the results.

 

[Maclin]

Is this boat new to you? Did it ever run right before this? If so, what were the top speed and RPM's then?

 

[Duckhunter08]

This is the second summer I've ever used the boat. When the speedometer on the boat worked it claimed 53 mph the first yr and 46 the second then it quit working and used gps

 

[Duckhunter08]

Prop is a quicksilver I think. I'd have to look when I get home