Wellcraft 170 American underpropped?

[SunburntJon]

I recently bought this 1983 Wellcraft 170 American with the Mercruiser 140 horse 3-liter engine. It runs well, gets up on the water quick and tops out at 42 mph at full trim and 5500 rpm. That's a little high for WOT I think, as peak hp is listed at 4800.

I have a SE Sport 300 cavitation plate and a Quicksilver 48 16312 A5 stainless prop (14.5x15p).

My complaint is that the boat struggles to get a skier up. I had expected a strong hole shot out of this boat and am feeling disappointed. Considering the higher-than-normal WOT rpms, my thought was this boat may be a bit under propped. Was considering a 4-blade, 16p.

Am I thinking in the right direction? Is 16p still too shallow? Maybe 17p would be better? Should I get the 4-blade or stick with a 3? You can probably tell from the age of the boat that my budget is limited. Would like to have the best shot at getting the performance I want without having to experiment with multiple props...

Thanks for your help!

 

[matt167]

WOT at 5,500 would be perfect I think, but a 4 blade will get a better pull on a skiier

 

[Scott06]

I recently bought this 1983 Wellcraft 170 American with the Mercruiser 140 horse 3-liter engine. It runs well, gets up on the water quick and tops out at 42 mph at full trim and 5500 rpm. That's a little high for WOT I think, as peak hp is listed at 4800.

I have a SE Sport 300 cavitation plate and a Quicksilver 48 16312 A5 stainless prop (14.5x15p).

My complaint is that the boat struggles to get a skier up. I had expected a strong hole shot out of this boat and am feeling disappointed. Considering the higher-than-normal WOT rpms, my thought was this boat may be a bit under propped. Was considering a 4-blade, 16p.

Am I thinking in the right direction? Is 16p still too shallow? Maybe 17p would be better? Should I get the 4-blade or stick with a 3? You can probably tell from the age of the boat that my budget is limited. Would like to have the best shot at getting the performance I want without having to experiment with multiple props...

Thanks for your help!

verify your tach it may be off as 5500 rpm this engine wont live very long , 4800 is the upper limit.
3.0 s are not noted for their power its only 135/140 hp. First is the engine in good operational condition ? Compression ok, ignition components serviced , fuel system carb in good working order? I assume it is if you are getting 42 mph top end, thats about all you will get.
strong holeshot is not normally associated with a 3.0. I had very good luck on a similar boat (‘91 sea ray 170/3.0) running michigan vortex 4 blade 16” for skiing &18” for general use and tubing. But i wouldn’t describe the holeshot as strong, rather adequate to pull me up on skis (280 lbs…)

May not do any better than a15” 3 blade except getting wot rpm down.
you may also want to try without the foil , mixed reviews on them

 

[Scott Danforth]

3.0s will warp the head at rpms over 5000

When I had tha Avanti, i had a 15p for water sports and trolling, a 17p for general use, and a 19p for long cruising

There just isnt enough power out of the 3.0

When pulling skiers, I normally had to move the spotter to the bow to get the boat on plane

Did I mention the generally isnt enough power from the 3.0

 

[SunburntJon]

Thanks for the good advice! Next question: Mercruiser prop guide wants to know if my lower unit is 2.0 or 2.4 ratio. Does it matter? They're both 15 spline, right?

 

[Scott Danforth]

Thanks for the good advice! Next question: Mercruiser prop guide wants to know if my lower unit is 2.0 or 2.4 ratio. Does it matter? They're both 15 spline, right?

your drive is a 1.98:1. the 2.0 and 2.4 is for outboards

and yes, it matters

 

[QBhoy]

There is little chance that you’re actually doing 5500 rpm. If you were…you’d be on here asking where the best place to get a new engine is. Reliable as they are…it wouldn’t last long at those rpm. Especially given its age.
mid think if you are getting a genuine gps 42mph…you’re about or close to as good as any 3.0 gets. Very impressive even. But at that speed…she won’t be doing those rpm. Especially if you say she is slow to plane. Any prop that would in theory allow such an rpm…would result in the acceleration being equivalent to a surface to air missile out the hole. Not the opposite. I’d even suggest that you may even be on the verge of over propped. Meaning that 4500 might be more like the real rpm..or below that. Try flicking the tachometer when it suggests that you’re at 5500. See what happens

 

[SunburntJon]

There is little chance that you’re actually doing 5500 rpm. If you were…you’d be on here asking where the best place to get a new engine is. Reliable as they are…it wouldn’t last long at those rpm.

Never really ran it more than a few seconds at that rpm. Not slow to plane. Just lacks the pull I expected under load.
The tach is a little inconsistent, but pretty easy to tell when it's reading high and as you stated, a couple taps brings it right back down.
Speed I recorded is as shown on the fish finder. Checked by gps today. It is accurate. The boat speedo does not work.
Today I recorded 37 mph at 4800 rpm. Didn't try to ski, just pulled all the kids around on the tube.
I got to thinking...the other day when we tried to ski we had 4 on board plus the skier. Maybe it just felt weak because we had it overloaded. Feels notably quicker with just 2 on board. Maybe we will give it a shot next time the wife and I go out alone.
Still interested in the WOT thing though. I would like to get it so max rpm comes down a few hundred and corresponds more closely to WOT. Still trying to decide what pitch/config to use. According to my research, 5500 with 15p/3 blade should correspond to approx 4700-4900 with 16p/4 blade... or 17p/3 blade (?).
Assuming the 4 blade (should?) give me a better hole shot...

 

[QBhoy]

Never really ran it more than a few seconds at that rpm. Not slow to plane. Just lacks the pull I expected under load.
The tach is a little inconsistent, but pretty easy to tell when it's reading high and as you stated, a couple taps brings it right back down.
Speed I recorded is as shown on the fish finder. Checked by gps today. It is accurate. The boat speedo does not work.
Today I recorded 37 mph at 4800 rpm. Didn't try to ski, just pulled all the kids around on the tube.
I got to thinking...the other day when we tried to ski we had 4 on board plus the skier. Maybe it just felt weak because we had it overloaded. Feels notably quicker with just 2 on board. Maybe we will give it a shot next time the wife and I go out alone.
Still interested in the WOT thing though. I would like to get it so max rpm comes down a few hundred and corresponds more closely to WOT. Still trying to decide what pitch/config to use. According to my research, 5500 with 15p/3 blade should correspond to approx 4700-4900 with 16p/4 blade... or 17p/3 blade (?).
Assuming the 4 blade (should?) give me a better hole shot...

More like it. I’d think