Prop size for 73 Slickcraft w/Twin 140 nMercs

[black sheep]

Currentl prop size is 13 3/4" 21P. WOT 3900rpms @ around 38mph.
The elevation is at 3600 feet. Boat loaded with gear and 3-4 people. What prop size and pitch would be the best to bring it into the recommended rpm range? Is the current prop sizes causing to much of a load on the twin merc 140'S. I can back the rpms down to around 3400 3600 and keep plain to cruise up the lake. I was thinking a 14" 19P prop would operate good. What do you Suggest would work the best?

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

Boat model is a ss-235

 

[hwsiii]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

Black Sheep, i need to know the year of the motors as well as are they outboards, what manufacturer of prop and model of prop and gear ratio. And if you could also give us the basic weight of the boat and the allup weight as normally you run it.
Welcome to the forum.

H

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

1973 slickcraft SS-235 twin mercruiser 140hp 3.0L sterndrives. Current props RH 48 78122 A40 21P I believe may be tha wrong props because the castle nut does not slip on to the drive splines. I would like to find a lower pitch prop to attain the recomended rpm range around 4600-4800. I don't know what the weight of the boat is. Thanks for your reply and help.

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

I do know it holds 60 gallons of fuel and has a 5 gallon water tank.

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

Found this Info


1973 SLICKCRAFT/AMF
SS 235
Stern Drive Power Boat


Length: 22'
Model Name/Description: SS 235
Boat Type: Stern Drive Power Boat
Hull Material: Fiberglass
Beam: 8'
Engine: 2
140HP each
Gasoline
Net Weight: 3160

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

I found this info on a plate in the boat.

people gear 2185 lbs

fuel 1675 lbs 57-60 Gallons

out drive numbers
right 3470217
left 3470351

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

WOT range is 4200-4600rpm not 4600-4800rpm. I'm guessing a 14X19 prop would improve performance and fuel economy.

 

[hwsiii]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

Black sheep, I show you need to change to an 18" pitch in the same model of prop. But you stated you thought you had the wrong prop because the castle nut didn't fit right. I don't understand what you meant by that statement, can you post a picture showing the problem.

H

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

At this time I do not have a picture. I posted earlier on this subject @ http://forums.iboats.com/showthread.php?t=301223.

I have a question reguarding the WOT RPM's. If my recommended WOT RPM range is 4200-4600 and I have a 18 and 19 pitch prop wich prop would provide better performanc and economy? I assume for my application the 19 pitch would run at 4300rpms and the 18 at 4500rpms. Is it better to lean toward the upper end of the WOT RPM range?

Thanks for your input I may be able to post a picture of the castle nut later.

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

hwsii, I have a friend who owns a boat identical to mine and he runs 18 Pitch props on his slickraft. He stated his boat runs about 35 MPH. I thought you might want to know. It sounds like your software program works. I wish I could tell you what his rpms are. I will be replacing my props with 19 or 18 depending on what end of the WOT range you reccommend 4300 or 4500.

 

[hwsiii]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

Black sheep, it is always better to run at the upper end of WOT as it develops less strain on all parts of the motor. Hope this helps and thank you. Could you please get bak with us on the actual WOT speed when you change as I keep a database for these statistics.

H

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

I ran the slickcraft with two different prop sizes at the same time a 21P and a 19P. the 21p ran at 4000rpm WOT and the 19P ran at 4400rpm WOT. I am going to purchase two new 19P props. I was just wondering which prop to purchase. i am considering a michigan wheel 14x19 , 14.5x19 and a quicksilver 14x19, 14.5x19 also a michigan vortex 14.5x19. I was wondering what the difference is betweeen the regular michigan wheel and the vortex. Which diameter would be better a 14" or 14.5" I noticed a note saying larger diameter is used mainly after 1997. I would like to purchase a new thrust washer castle nut and washer and nut. I understand the quicksilver requires a hub assembly which would include these items.

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

I purchased two michigan wheel vortex 14.5X19 props. WOT RPM was 4550-4600 and the speed was 39-40mph. The Vortex Props require the hub assembly which includes the thrust washer, hub, castle nut, lock washer, and lock nut. I am pleased with the performance and hub assembly of the vortex prop. Thanks for your help.

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

What size of 4 blade Prop would best maintain the recommended RPM range for the Twin 140's?

 

[hwsiii]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

What is wrong with the Vortex's.

H

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

Nothing is wrong with the 3 blade vortexes that I installed. I am very happy with the way they perform. I am just curious what size of 4 blade would work best and what the differnce if any would be.

 

[hwsiii]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

I think I have the wrong gear ratio for your boat, do you know what it is.

H

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

No.

 

[black sheep]

Re: Prop size for 73 Slickcraft w/Twin 140 nMercs

I want to try a set of 4 blade alluminum props what size should I buy? My current set up is two michigan wheel vortex 14.5X19 props. WOT RPM was 4550-4600 and the speed was 39-40mph. Thanks for your help. What WOT RPM would a 17 and 18 4 blade be?