need help selecting the righ prop

[84EdH]

First timer on this forum. I have a 1979 century 19.5ft open bow with a mc1 transom set up I am using a MR outdrive . I have a 5.7 vortec engine that makes around 290 hp. I had done extensive work on this boat with transom repair stringers etc. I have been breaking the engine in and it now has about 25. Hours on it.

Today I put it in the water, hooked up my digital multimeter to use as a tach and used my smart phone gps to measure speed.

Here it is: WOT. 18 pitch aluminum 3 blade prop. 48Mph at 4800rpm. Rpm increased to 4960 but speed never went above 48.

I water ski tube cruise. I think a higher pitch prop would be better. Dont know how much to go. Also dont know how aluminum vs. Ss affects pitch and performance. Help appreciated.image

 

[craze1cars]

Re: need help selecting the righ prop

What is your target RPM?

 

[steelespike]

Re: need help selecting the righ prop

Using a 1.50 gear ratio your numbers are pretty good for the rpm and speed. using 4800 your slip is a very respectable 12%.
I do wonder if the rpm should be higher with a 18" prop. As it stands if you increase pitch you will lose rpm about 200 rpm per inch.
May increase speed but will slow hole shot may lug the motor..I'm not well versed on the larger motors but it would seem to me that
that setup well broken in and setup right should do over 50 at about 4800 with a 21" prop.I could be all wet but it seems 290 hp should do better.
I don't think a multi meter is a good bet for an accurate rpm.

 

[phillnjack2]

Re: need help selecting the righ prop

first of all the alluminium props are total rubbish compared to stainless steel.
the stainless props are made with much more precision and stay in shape better under loads.

you need to look at what the maufacturer states is the max revs for your engine and use this as the target for the
fastest revving and fastest top end speed..
forget about hole shot that will come in with the right prop once you get its size.

i would think rpm should be around 5250 for that engine and would be looking to get atlest a 23 pitch stainless prop.
if the 290hp dont push a 23 prop then its a realy tired engine..

i would think if using the boat for water ski-ing and tubing this same prop will be fine for single skiers etc.
but if you want two or even 3 skiers then a drop down to around a 17 pitch and just keep eyes on the revs so
you dont over do it once on the plane and everyone is up.

go look at a lot of tournament ski boats, these are often direct drive and push 45-50mph easy with enough grunt to
pull 4 or 5 people out the water.

the multimeter is no good for rev counter, it could be wrong, and the phone gps are never that great to be honest.
but 18 pitch alloy prop is going to be bending like mad under power and could be so out of shape anyway.

once you know this engine can twist a decent prop, the world is your oyster as there is so many to choose from for
different applications.
on a campion 195 allante i used to have 3 props...1 for slow river cruising, 1 for average boating including ski etc, and 1
prop for when i wanted to get realy moving on the top speed. so i had a 15 a 19 and a 23 prop. the engine was only a
4 cylinder 165hp volvo and could get this boat to 55mph very quickly.
you have masses of power so i would be wanting to see 60mph plus from that boat with the right prop.

do you know of anyone else with a similar engine and boat to try their props and ask what they are getting ?



phill

 

[84EdH]

Re: need help selecting the righ prop

Craze1 target rpm is 4800. Steelspike, I was using the multimeter as an improvement over the dash tach which is now 35 years old. What is a better option? Phill, yes, I want to get a ss prop, but as you suggest.....which blade which pitch? also, using gps application as an improvement over 35 year old pilot tube hanging off the stern.......until I hit a stick or something and then have nothing! same question, what is a better speedo option? thanks for the help!

 

[steelespike]

Re: need help selecting the righ prop

Best temporary bet on a tach is a borrowed shop tach.
Does your present tach do anything? Try cleaning connections, note the position of the selector switch on the back
operate it a few times to clean the contacts.
a gps is the only accurate every time for sure speedometer.
Some cells have a gps app.
Sure there are speedos that are accurate but how much do they vary,sometimes on one run.

 

[84EdH]

Re: need help selecting the righ prop

yes, my present tach works. I just thought the multimeter would do better....2012 digital technology vs. 1979 technology. the digi. tach read about 200 rpm lower than the dash tach and that got a little worse at high rpm. I did use a smart phone and speed view ap. for speed. I can borrow a shop tach and test it against the others. I thought gps was good, but Phill above thought gps can be shaky.

 

[jestor68]

Re: need help selecting the righ prop

What year is your motor?

I ask because in those several years, the only 5.7L was the 260 hp rated at 4200-4600 rpm( except for the 350 Mag).

A "Vortec" motor has the 4 bolts in the middle of the valve cover(s), for identification purposes.

Vortec motors with roller cams were introduced in 1996.

 

[jestor68]

Re: need help selecting the righ prop

first of all the alluminium props are total rubbish compared to stainless steel.

You have apparently not tried one of newer aluminum props made using "squeeze casting" technology. It's the latest rage. :joyous:

 

[84EdH]

Re: need help selecting the righ prop

my engine is a 1996 or 1997 vortec. for sure. roller cam. I bought a new long block, and built the top end with cast iron high rise intake manifold, edelbrock 1409 carb, new manifolds etc. I had a parts engine from 1995 that I transferred all the pulleys, brackets etc. and used the thunderbold 4 ignition.

 

[84EdH]

Re: need help selecting the righ prop

Update: I tried a ss prop today: 48-74268-21. this is a 13 3/4 inch 21 pitch prop. my 115# first mate was with me this time. using same gps and multi meter tach I got up to 50mph, but engine only turning 4330rpm. my old aluminum prop was 18 pitch and 15 1/2 diameter and gave me (alone) 48mph at 4960 rpm. my target rpm is 4800. I didn't like the way the boat handled at lower rpm with 21 pitch, as I had to gas it more to get to plane at a slow cruise. the hole shot felt ok but without a skier, who knows? I would like to get a ss prop in between these two, that would max wot at 4800, and get the most speed (or whatever I get is ok...).

my problem is that I don't know how much the pitch change with the diameter change played into todays max rpm in the 4300 range.

help appreciated!