PROP BLOW OUT?

[imported_FLYGUY]

I have a 1977 17' terry bass boat w/ a merc 175 hp.
no markings on 3 blade sst prop, but careful calc says 24-25 pitch x 14.25 dia. ventilation plate pretty near bottom of boat. problem is getting to plane. step on gas, bow rises, get to around 3000 rpms and motor takes off screaming to 5800 rpms while i just sit there for 4-5 seconds. then it 's on plane and everythings fine except I can only get to 4900 rpms and 47 mph. so i am over propped, right? so my local master mechanic gave me a merc alum 23 pitch X 12.75 dia. and said i would be fine. i questioned dia. and he said dia doesn't mean that much. i think he was wrong because i played sit and spin with that prop and couldn't even get up on plane. so now what? any help would be greatly appreciated!

a little more info! heavy boat, wet foam, possibly over powered, can't read max plate, so probabally stern heavy!

 

[72SideWinderSS]

Re: PROP BLOW OUT?

FLYGUY
Welcome to iboats.
Alot of your issue could be trim angle being to positive (out).
Do you have your trim set all the in (down) before taking off ?

If you take the prop nut off you might find a number under the washer, that's where alot of props are marked.

jimmy

 

[Ron G]

Re: PROP BLOW OUT?

Is the prop on there vented?sounds like your blowing out quit a few of the bass boats with bigger motors will do this a little.you are over proped if you max rpms 4900 you could raise your motor but i dont think itll get know were near were you need to be a 23 may get ya there with raising the motor.by therory a 21p would be closer to getting you to the max rpms.

 

[Texasmark]

Re: PROP BLOW OUT?

Just shooting from the hip, I think the overpower comment is correct in causing the hole shot problem. My experience with props is that when you hammer down on them they either move forward, or look for the path of least reisistance. Course if your prop is ported, the ports will be blasting away as the water is still behind the prop's exhaust and can't get away fast enough.....which will do what yours is doing as Ron mentioned. BTDT

I think that you have so much power that you hammer down and the boat doesn't have time to start moving before the prop has created a vacuum and the softest spot is up (where air is) and ventilation occurs. So a softer throttle at hole shot might help.

I remember the smaller Terry boats of the 70's but not familiar with the hull on the larger craft. 175 just seems to be a ton of power for a 17 even if it is a BB.

Agree that a prop that blows out with the trim too far out on hole shot may not blow out when you trim in some. As a matter of course, I can deliberately make mine blow out on hole shot by trimming out past vertical a few degrees. But my prob doesn't completely blow out. It drives up to about 6 grand and holds it. After a couple of seconds of screaming, and a little porposing, I settle out at the normal WOT rpms of 5600 and I am gone.

On the alum, it may not have been cupped and the blades are thicker. Both are not good if you have been running SS. BTDT....my "test" alum is back in the box sitting on the shelf.

Agree you are overpropped. My mfgr's data says the 175 top end is 5600. You have a long way to go buddy. Buttttttttt

But something must be wrong. The 175 gearbox is 1.87:1. You said WOT is 47 @ 4900 and prop is 24-25. Well the BAM prop slip calculator says:

24P 4900 rpm's 1.87 gearbox theoret 60; for 47 slip is 22%

25P 4900 rpm's 1.87 gearbox theoret 62; for 47 slip is 24%.

Man your slip is humongous or your gauges suck! I'm going to bet your tack is the culprit. You can't be 900 rpm's low on a 17' padded boat with a 175 and a 24P prop unless you are dragging a sea anchor or something.........or the engine isn't pumping out 175 hp.

My 2c

Mark

 

[Crownie2]

Re: PROP BLOW OUT?

Mark - are you sure they didn't use a 2:1 ratio back then? Just asking - I know the newer ones used 1.87:1.

Sure works better in the calculator.

Bob

 

[imported_FLYGUY]

Re: PROP BLOW OUT?

FLYGUY
Welcome to iboats.
Alot of your issue could be trim angle being to positive (out).
Do you have your trim set all the in (down) before taking off ?

If you take the prop nut off you might find a number under the washer, that's where alot of props are marked.

jimmy

 

[imported_FLYGUY]

Re: PROP BLOW OUT?

before i take off i trim all the in. also it looks like the numbers on prop are worn off.

 

[72SideWinderSS]

Re: PROP BLOW OUT?

FLYGUY,
When you said the "ventilation plate pretty near bottom of boat. "
How far above / below is it, I'm pretty near the river, but still it is 1/2 mile away.
Is your SST prop vented i.e. holes in the body in front of the blades (transom side). If so, how large are the holes 7/32", 3/8", 17/32".
The more info you can provide the better the diagnosis. I am by no means an expert on setups or props. I do face a similar senrario with my 16' Sidewinder.
My setup:
16' Sidewinder SS (v hull with strakes)
115 hp Johnson minor mods Manual trim
21" Turning Point Prop. (measured avg pitch over the cup=23")
A/V Plate 2 3/8" above the bottom
WOT 5400 - 5500
GPS 54.3 mph 55 mph

If I slam the throttle WFO the prop breaks loose, rpms jump to 5200 - 5300, and will eventually hook up at about 30 - 35 mph. Then it is off to the races.

If i open the throttle to about 3/4 and hold it there, the rpms climb almost imediately to 4500 and hold, once I reach 35-40 mph I open her up the rest of the way.

I don't have Power trim so my setup is a compromise between hole shot and top end. When out sking i drop the motor 2" and run my 19" MWC.
Hope this helps.

jimmy

 

[Texasmark]

Re: PROP BLOW OUT?

Crownie, I got it out of the Merc sales brochures for '94 thru '97. You could be right, but I have no records for '77. My tower was 115 hp '89 model and it was 2:1.

Just assumed they kept the same gearbox, but that could be a misconception. :%

Mark

 

[760panther]

This sounds like exactly the problem I am having with my boat. To start out. I am very green at boating I have a 1960 Whitehouse with a 1960 75hp Evinrude v4 on it. From what has been described as blow out is what I am observing. Can someone please tell me what prop I should start with? Boat runs fine up to 25mph then 'free spins' the prop

 

[jimmbo]

This a 10 1/2 year old thread, you should have started a new one.
You are very limited in choice of props. At most OMC offered 5 or 6 props. What prop is Currently on it? Open a new Thread to further this discussion