Prop issues

[vegas_storms]

Took the new to me 89 172 Cobalt out with GPS today(Mercruiser175) and with the 17 degree prop WOT is only 36.3(I would have thought even with that prop it should have been faster(only 2 of us on the boat today)

3000rpm - 17mph - 35% slip

3500rpm - 23mph - 26% slip

4500rpm - 36.3pmh - 7% slip - used a calculator to figure this out, theoretically would the slip stay the same at 21 degrees?

I have a lot more motor left at 4500rpm but did not want to push it above what I believe is close to the max rpm's for the motor.

I have a 19 degree SS prop that i should be getting back from the shop this week.

Questions

1. I understand the 17degree is the wrong prop but do these figures seem correct? Cruising speeds seem pretty low.

2. Mechanic said this thing should WOT around 45 is this feasible with a 21 or 23 degree prop?

3. Boat is primarily on Lake Mead, so higher cruising speed for the size of the lake would be a big plus.

4. If I'm getting roughly 3.5 mpg at a cruising RPM of 3500 and 23 mph, would that equate to the same mpg if say with a 21 degree prop I got 30mph (with the same rpm's)

5. 3 or 4 blades - don't really know what the difference is.

6.My manual says a 13.75 inch prop, can I use a larger one?

7. when i turn the prop by hand I hear what I believe is a metal grinding noise

Good news was I got up on plane in under 2 seconds with the trim tabs down and could keep her on plane down to 14 mph.

Basically what prop should I get a 21 or 23,(since I already have the 19SS) and what kind of top end, cruise speeds do you guys think I will be able to achieve.

Thanks again

 

[vegas_storms]

Re: Prop issues

15 views and no opinions

 

[Dhadley]

Re: Prop issues

When you say a 17 degree prop we assume you're refering to a 17" pitch and not degree of rake, right? Pitch is measured in inches, rake is measured in degrees. For the moment we'll assume you're using a 17" pitch prop.

You say you have throttle left at 4500 and we'll further assume you're looking for no more than 4600 rpm, right? Depending on how much throttle you have left will determine if going to a 21 or a 23 would be best. Since you didn't test it at wide open we don't know what the slip factor is at wide open. However it doesn't matter at this point. We know (assume) you need the top rpm to come down.

The best thing is to stick with your plan of trying the 19" prop next since you already have it. Let's see how that prop works and go from there.

 

[tmh]

Re: Prop issues

Of course, there is no way to guess how much you had left to get to true WOT so one can't know what prop pitch you can turn at proper WOT. As was recommended, try the 19" and see if it still has lots left at 4600 or whatever or even run it up to WOT if you can for a few secs to check how far it goes (within reason).

Four blade usually improves hole shot and low-mid speed accelleration, but typically lower WOT speed. Because you are way underpropped, it is hard to know what performance areas you'll want to improve once you are "close" on the prop pitch. As you move up in pitch, your hole shot will change a bit also so you'll have to guage how much.

It's difficult to guess what your WOT speed will be with the proper prop because you don't really have any WOT data to go on. IF you can turn a 23" prop near the 4500 rpm then you should be well into the 40's for speed. Even a 21" at 4500 with 7% slip you'd be into the 40's easily I think.

 

[vegas_storms]

Re: Prop issues

Thanks for the feed back guys.

I'll try the 19" SS and run it up to WOT for a couple seconds and get back to you guys, so you'll have the correct information. Was afraid to run it any higher, but if a couple seconds won't hurt it, then I'll give it a try.

Dhadley - The prop has 17 on it, so yes I guess it is 17" pitch and not degree(still learning).

 

[Dhadley]

Re: Prop issues

Correct, that would be 17". They don't put the rake angle on them. Keep us posted on the testing.

 

[Texasmark]

Re: Prop issues

Just a sideline note; not butting into the dialogue.

I bought this boat and it was severely underpropped. I can remember the tach going to 7500 rpm my first time out coming out of the hole, before I could shut her down. Couldn't believe my eyes and this boat was 2 years old.

I tried to calculate another prop, but like stated herein, I didn't know what my numbers were at WOT......didn't want to go back to those rpm's or maybe even higher. The prop was 17".

I guesstimated a 21. Well, changing that much pitch caused the performance of the rig to change dramatically (much more efficient running) and first time out I immediately found that the 21 wasn't enough. The goal posts were moving.

So I went to a 24 and things are where they should be.

So, as you get closer to where you need to be you may have to take more than one shot at it to get what you want.

HTH

Mark

 

[vegas_storms]

Re: Prop issues

Thanks for the info guys,

Going out in the morning with a 21" pitch aluminum prop. We'll see if I still have some rpms left above the 4800 range, then I will return it and get the 23" pitch. Prop guy told me my setup couldn't handle a 23" so we will see tomorrow what the new numbers are.


Cheers

P.s. Guess I got lucky, my old prop just slipped right off, no pulling or yanking required

 

[vegas_storms]

Re: Prop issues

Well, what do you know the prop guy knew what he was talking about:redface: . So 23" is definitely out of the question.

Went out this morning

43mph - 4450rpm
30mph - 3500rpm
25mph - 3000rpm

89 17ft Cobalt 4.3 mercruiser 175 with Alpha 1.
Dry weight - 2475
Fuel - 190
3 people - 520
misc - 300

Total - 3485 pounds

I would figure this would be close to the final weight, correct me If I'm wrong.

Thinking with the rpm's I was getting at WOT I should maybe go down to a 20" pitch. What do you guys think about that. I like the way the 21 handles but another 200rpm at WOT seems optimum. I figure this will bring my top speed down a little(rpms up 200) but from what I've read here seems like I want to be in the middle of the 4400-4800 range.

Does anyone have any ideas to get more speed? Really just looking for a higher cruise 3000-3500rpm. I guess this old boat can only do so much .

On a different note - The new prop 14.25x21 Quicksilver Aluminum seems to shutter or create more vibration at certain settings(motor trim and/or trim tabs). It does seem to have a sweet spot for every speed but it seems very small. Does this seems right to you guys?

On a good note my fuel mileage did increase significantly
21" pitch 3.6 mpg(51 miles), with maybe 50% WOT versus 3.2mpg(31 miles) with the 17" pitch just mostly cruising(5-10%WOT).

Thanks for all the great help!!!

 

[tmh]

Re: Prop issues

hmmmm...problem is, most 3-blade props come in odd pitch #'s only, so next step down may be a 19". A 4-blade 20" may not get you anything in added rpm, but maybe different "feel" you may like better (or may not). I guess you can have a prop shop re-pitch a 21" to 20" (I think) or maybe you can find a 20" 3-blade.

Oh, I see tha was an alum prop you tested. You could get a bit more out of the boat with SS, but there are always other considerations.