Trying to determine proper engine/ trans/ prop pairing

[Will Steinbeck]

I sea trialed a 1972 Chris Craft Commander Sport Express. It has twin 2015 Captains Choice 330 hp and 1:1 trans. Not sure about prop info. The boat cruises at 18 mph and tops out at 23. That seems dismal! Does anyone have knowledge about the proper pairing of transmissions and prop size for that hull? Or can you send me to someone who does have that knowledge? We really like the layout of the boat. Perfect for the Pamlico Sound and OBX

 

[Mad Dog 2]

How big is the boat, weight and length? What does the tach read at cruise and wide open throttling?
Do you have pictures?

 

[Will Steinbeck]

It's 28 feet and an 11 ft beam, 30" draft , dry weight at 8000 lbs, 17 mph at 3000 rpm. We never went WOT.

 

[Mad Dog 2]

What is the factory approved full RPM for that engines?
You should do another sea trial and confirm for your self it does “boat cruises at 18 mph and tops out at 23.” and the engines reach full rated RPM. This would give you a clearer idea of the engine and prop pairing.
Couple hundred bucks to have a boat mechanic go on sea trail for his comments and advice would well worth the money.
I assume fiberglass and not wood.

 

[Will Steinbeck]

It's fiberglass.
Captain's Choice 5.7 MPI Engine

Displacement:	5.7L/350 c.i.d.
Rated Power:*	330 hp@5000 rpm
Cruising RPM:	Idle-4000 rpm
Max RPM:	4800-5200 rpm
Fuel System:	Multi-Port

 

[jimmbo]

I sea trialed a 1972 Chris Craft Commander Sport Express. It has twin 2015 Captains Choice 330 hp and 1:1 trans. Not sure about prop info. The boat cruises at 18 mph and tops out at 23. That seems dismal! Does anyone have knowledge about the proper pairing of transmissions and prop size for that hull? Or can you send me to someone who does have that knowledge? We really like the layout of the boat. Perfect for the Pamlico Sound and OBX

It's 28 feet and an 11 ft beam, 30" draft , dry weight at 8000 lbs, 17 mph at 3000 rpm. We never went WOT.

It "Tops Out at 23", yet you claim you Never went WOT...?

 

[Will Steinbeck]

Correct. It was stated by the owner it will top out at 23. I'm not going to explain the days events. All I asked was with the information I gave, does that combination seem optimal.

 

[jimmbo]

We will know something once we are told what the WOT Rpm is.

 

[Scott Danforth]

the only way to size props....... is mash the throttle at WOT, then report the RPMs and speed. you need to prop to get the motors to the upper end of the WOT RPM window

for a carbureted SBC or BBC, that is in the 4600-4800 RPM range.

 

[jimmbo]

According to the Chart provided, the WOT Range for that Engine, was shown as 4800-5200

 

[Will Steinbeck]

Thanks for that. Now I'm waiting for the owner to give me the WOT speed and what the RPM are. Chris Craft is offering zero assistance. They don't respond to any requests. Before I go back for another sea trial in better weather, I need to know if this current setup is worth pursuing.
I'll post the WOT info when I receive it.
Thanks again.

 

[Scott Danforth]

The Chris Craft company of today has nothing in common with the company of 1972. The name has been bought and sold many times since then, and the records didn't follow the name

 

[Will Steinbeck]

And anyone involved with the company back then is probably long retired or dead.

 

[Mad Dog 2]

You have:
Tested speed of 17 mph at 3000 rpm
Reported boat cruises at 18 mph and tops out at 23
5.7 MPI Engine mfg spec 4800-5200 rpm
Recommend “upper end of the WOT RPM window for a carbureted SBC or BBC, that is in the 4600-4800RPM range.”
So you have unknown potential for mph from 3,000rpm (17 mph) to 4,800 rpm (X mph)
1800 rpm to gain more mph (WOT)
Cruise Rpm would be up to 4,000 rpm at X mph
At what speed and rpm does the boat get on plane?
(I assume the hull will plane)

The mechanic could bring test equipment to verify the boat’s tach. Tach’s can be off (some boat tach can be reset from the back of the tach with a small screw driver).
If you haul the boat you can get the prop size and pitch (owner may have receipt that show the prop sizes) and see if the bottom is clean.
I was on a test run on a customer boat and it would not get up to speed. We hauled it and found a tire on the prop. Other times sail ties, pot line and etc.

 

[Will Steinbeck]

Hey MD 2
The owner got back to me.
3000 rpm @ 17 knots (not mph as I thought-- GPS was on knots)
WOT 4000 rpm @ 25 knots he recalls??
(Is that near optimal with twin 330 hp??)

Prop size could be?? 13 12 and the gearbox is definitely 1:1
Where the boat is docked in Edenton NC is largely fresh water. It does have that velvety fuzz growing and it is referred to as a deep V

What do you think?

 

[Scott Danforth]

only turned the motor 4000 RPM and stopped giving it throttle or did it only turn 4000 RPM with teh gear ratio and prop? Because the first scenario is someone too chicken to open the throttle, the second is over-propped scenario which kills motor.

Until you @Will Steinbeck take the boat out, and give it full throttle, as in mash it against the stops to get the WOT RPM you are currently turning. there is no way to even guess at a prop.

are you turning the WOT RPM's per the spec for the motor. if your spec in post #5 is correct for your motor, you need to be spinning 5000 RPMs

 

[jimmbo]

That Velvety Fuzz is a Speed Killer, and needs to be removed.

 

[Will Steinbeck]

only turned the motor 4000 RPM and stopped giving it throttle or did it only turn 4000 RPM with teh gear ratio and prop? Because the first scenario is someone too chicken to open the throttle, the second is over-propped scenario which kills motor.

Until you @Will Steinbeck take the boat out, and give it full throttle, as in mash it against the stops to get the WOT RPM you are currently turning. there is no way to even guess at a prop.

are you turning the WOT RPM's per the spec for the motor. if your spec in post #5 is correct for your motor, you need to be spinning 5000 RPMs

According to the owner, WOT and staying on it turns 4K. That's not enough. The specs from the other post are correct. So it's not propped right? But the 1:1 gearbox is good?
I guess I'm asking if the gearbox can stay and I just experiment with the props?
If my wife didn't like the large open layout I probably would skip this boat.

 

[Scott Danforth]

what pitch props are on the boat now? jumping 1000 RPM in props is about 4-6" in pitch you may not be able to prop that much

since the motor is a replacement, is the throttle opening fully?

right now there are a few million boats for sale as the marine market is stalled both new and used. as long as you dont have to sell, you can almost name your price

 

[jimmbo]

I would verify that the Tach is accurate-set correctly.