prop questions

M

MBAKER

Petty Officer 1st Class
Joined
Jun 9, 2008
Messages
289
Replaced old 305 with vortec 350. Went out with the same 17P prop we had, and it would run to 4800 without problem and still have throttle to go. I know I need a different prop but trying to find some guidance on this. This was with light fuel load, and only 3 adults, so with normal load we will prob be 750# heavier. We mostly just cruise around, and do some tubing/skiing etc, top speed is least of my concerns, I would rather sacrifice top end for a little more low end power. It also seems like in the past the boats have felt better handling wise etc, when slightly under propped just keeping in mind you have to hold them back at the top end.



Here are my questions...

The prop we had was a Michigan Vortex 17P 3 blade, and would consider another because I can get them locally very reasonable cost. At the very least it needs a 19 and I think it would pull a 21 but again Id rather have to pull it back a little on top and not work the engine as hard in the midrange. Also feels like it overpowers the prop a little on take off, will a little more pitch also help grip better?



Is it worth considering a 4 blade prop? Maybe a 20P? should take a little more rpm than the 19 out of it. How would a 4 blade affect lifting characteristics, more stern lift or less etc?



How does Michigan compare to Solas or Mercury etc.... I planned on staying with the Michigan because it would be an easy comparison on pitch changes, but Im not stuck on it.



Do manufacturers guidelines usually tend to error on the side of too little or too much prop? The boat manufacturers recommend prop for a 350 in this boat is a 21P, but Im not sure if they play it safe and under prop a little to help prevent detonation etc, and let the rev limiter stop it. Or if they error on the side of over prop to keep from going to the rev limiter?
 
S

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: prop questions

If the company suggests a 21 it should be a good place to start.You would be smart to try to find a dealer that will let you try some props with the understanding your buying.Probably require a sizable deposit.
The Solas al props use squeeze casting technology And have been predictable performers.
The Amita is a regular hub,the Rubex has a removable hub.
There should be others along with more detailed info.
Would help for us to know your boat details also if its a sound broken in motor you can run it briefly beyond 4800 to get the
wot rpm and gps speed or maybe the rev limiter won't let you.
4 blade props are said to improve hole shot,stay on plane better at reduced throttle, handle better at speed and around the dock.
They usually lose a little top end.
Yes it is normal to drop an inch of pitch going to a 4 blade.
I think the manufacturer recommends props that will operate within the motors rpm range,
If you were ordering a prop with insufficient information its better to err on the light side.
 
M

MBAKER

Petty Officer 1st Class
Joined
Jun 9, 2008
Messages
289
Re: prop questions

I have been reading some thoughts about hubs and some thoughts are the pressed rubber hub is a better because it cushions more. I have used both, switched to the insert type just to make swapping props a little cheaper, but have no strong feelings either way. Any thoughts on pressed vs removable hub?

Its broken in, and I wouldnt have a problem over revving it a little to test, but the rev limiter stops is. when you back off a little to keep it off the limiter, its at about 4800, and still has 1/8 or so throttle travel left to go, all without doing any trimming up.

I had done some reading about the 4 blades saying that on a sterndrive they were so deep in the water they didnt benefit as much as a application closer to the surface. I dont mind a little slip on take off, letting the engine rev up into power quicker before it bites. Kinda like a loose torque converter in a car. But I think what i have now is excessive.
 
M

MBAKER

Petty Officer 1st Class
Joined
Jun 9, 2008
Messages
289
Re: prop questions

MAxum 2100SR bowrider, roughly 21' length, 101" beam, Weight w/ gear and passengers 4500+/-. 1.47 drive.

My questions were intended to be more general in nature, I know a more specific prop cant be recomended without a better test.
 
M

MBAKER

Petty Officer 1st Class
Joined
Jun 9, 2008
Messages
289
Re: prop questions

aluminum please...

top speed... dont remember for sure but I think somewhere around 42-43 mph at 4800 rpm with the 17P light load, and max trim. Problem is I think the 305 engine was weak all along. With a 19P (mfr rec for 305) it would only turn 4000 rpm with 2 people. A good tuneup and step down in prop got it up some though.
 
Tail_Gunner

Tail_Gunner

Admiral
Joined
Jan 13, 2006
Messages
6,237
Re: prop questions

Guessing the 19 would be very close a 20 maybe to its hard to get much closer.
 
jestor68

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: prop questions

According to the Mercury Marine prop selector:

It selects for your application a choice of: in 3 blade aluminum; 14.5 X 19 or 15.5 X 18(my choice). In 4 blade aluminum; 14.5 X 18.

Details include: calculated weight of 4690; with 18 pitch you should get 4600 rpm @ 46.3 mph. 19 pitch is basically the same outcome.
 
M

MBAKER

Petty Officer 1st Class
Joined
Jun 9, 2008
Messages
289
Re: prop questions

Everytime I tried the prop selector it alwas gave me something like a 15 or 16 pitch, which wouldnt even be close. I treid inputting by weight and also by length and almost always ended up with 16" +/-.

With the old 305 engine with same load and fuel, fresh tuneup and 17P prop it would get to 4000 and you could feel it falling off It would still climb but slower rate, then you could trim up and it would slowly get to about 47-4800. With this 350 and the 17P it goes to 4000 easily, and if you put the hammer down it goes to 4800 like nothing, doesnt even feel like its starting to struggle yet and all without any trim. Based on that and the way the boat 'feels' and is stilll ready to go with more throttle I think it would go 5500 (maybe more) easily without any trim yet. When I input these numbers and use 5600 for rpm it reccomends something like a 20P, which seems more in line. I have found a 19P that I could try, its old and a little beat up but not bad. I will try it and see what happens and go from there.

I should have mentioned... This was a built engine not a crate or reman. We blue printed the block, used stock roller cam but heads were ported a little bit and we put an edlebrock intake and carb on it. We dynoed it to break it in, and did a couple pulls. It made 350HP and 400# TQ (with headers) so it might be a little stronger than a stock 350 from mercruiser.
 
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