Interesting prop situation

[Slwevo]

So I'm experiencing something quite different then what I'm reading on forums about switching props and here is my situation.

I have an 89 cobalt condurre 192 bowrider with a 4.3 mercruiser alpha one. The prop that was on it when I got it was a ss 3 blade 19p. After getting the engine up to snuff and everything working perfect for a season I decided I wanted to change the prop for a better holeshot. I decided on the 14x19 hustler which is aluminum because my buddy swore by it and has the same engine as I do. Anyhow , my experience has been the 3 blade was a little slow to take off but got a top speed on my boat with just me in it of 42mph at about 4300 rpm, which I feel is slow but that's another topic..well I put the 4 blade on there with the same pitch and was expecting a little lower rpm at wot with a little less top end but a lot better holeshot and handling. Well I dropped from 42mph to 31mph which I feel is huge loss and the rpm actually went up to about 4600 on my tach and felt like it was touching the limiter..so my question to my confusion is why did my rpm go up and speed get dramatically reduced.. do I need a 14x21 pitch ??

here's another story. The 3 blade ss i measured to be around 13.25 or 13.5 x19 and the 4 blade is bigger at 14x19. Before i added the new prop i put a stingray classic fin on the back and that made it useable for pulling tubes and holeshot was great compared to without it..all tests mentioned were with this installed. So should I get the 21 pitch 4 blade to better match rpm and top end or just go back to the 3 blade and be happy with what worked? Any input appreciated

 

[Sea Rider]

What's the min-max wot rpm range for that engine factory stated ? Usually higher revs dials less top end speed and less revs dials more top end speed.

You can achieve ot bettert hole shot with a 3 blade alum or SS prop provided that engine is propped right to achieve max wot rpm or slight over as usually loaded while losing slight top end speed.

Happy Boating

 

[Slwevo]

Well it's the 175 hp 2bbl carb motor. I'd think somewhere around 4200 and 4600 but I can't find any info on the older engine on a quick search. I'll have a look on the engine tomo and see if the sticker Is still there

 

[Scott06]

I'd remove the stingray fin and try it. Agreed it doesn't fit that rpms would increase and speed would drop. I don't think you have a rev limiter on the T bolt 4 . Couple questions have you verified the tach is speed via gps ? Are you sure engine is in good working condition tuned up? You make a comment on top speed with original prop "that's a different story"- is there an issue here? Would think a 19 ft with 175 hp wouldn't do much more but could be wrong.

 

[Sea Rider]

Some escenarios :

Which engine is it, a 4.3 Mercruiser or a 2 strokes 175 HP-OB. If we asume both revs with well calibrated tach at max 4300 wot rpm range, you're there at the max rpm, would stay there. OTOH if max wot rpm would be 4600, you're 300 wot rpm off, going for say one less pitch or even re pitching current one while maintaining same 3 blade prop sill surely dial close to max wot revs.

To fine tune dilaing best prop need to know its max wot rpm range. How loaded do you usually like going boting with ? If with a near fixed load much better dialing best prop performance whether wanting top hole shot ot top max speed.

Personally don't like doel fins, if they don't ride parallel to water level when combo is on plane will achieve water drag and slow combo and rpm down, very notorious on portable engines, besides are huge collectors of water garbage, you name it LOL!!

Testing new second starter props as per theoretical recomendations could end being costly as all props must be water teested as loaded to fine tune what you're looking for. Personally like maximizing any current prop factory delivered with engine and go from there fine tuning pitch for engine to rev to its max wot rpm range or slight over it as fixed loaded for any boating application.

Happy Boating

 

[Slwevo]

The engine Is the 4.3 mercruiser inboard. The reason i put the fin on there is because without it the boat cavitated so badly it was hardly enjoyable to ride in, I attribute that to hull design I guess. Put the fin on there and its smooth as ever so I wont be removing it. But anyways my question is should I go to 14x21 or the other way with 14x17? Reading all these numbers gives me a headache after a while Haha

 

[QBhoy]

Ditch the doel fin first of all.
Are you sure that motor isn’t supposed to be 4400-4800 rpm ?
if you are only getting 4300 at wot with a 19” (presumed a 1.8 or 1.6 drive) it would be unusual..even with the 2 barrel 175hp. Was that trimmed up ?
there is also the slight variant in that the original prop you had was tampered with ? Just a thought.
Anyway, basically all else being healthy, I would of imagined that boat with that engine and a 19” prop would rev right out at wot and trimmed up...bouncing off the limiter.

 

[QBhoy]

Also worried that you felt you had to put the fin on. Shouldn’t be cavitating if all is original and set up as designed. Cobalt have been making boats for a while now and even in circa 1989...they did.
Given what I know about the usual set up of similar boat and engine set ups...I’d say either a 19” or 21” would have been spot on...with the 19” requiring caution not to over Rev. Prettt sure it shouldn’t be under revving like yours.
What was 3 blade stainless prop you took off it ? What make etc ? I’m suspecting this is throwing you....or your tacho is not so good...or both ?
all the best.

 

[Sea Rider]

If an OB sits at the sweet transom height, should not cavitate when combo runs at wot on flat calm water cond provided that's trimmed to ride parallel to water level and deck weight evenly distribited. Better check what's the max wot rpm range for that engine before wild guessing which prop is ideal, props needs to be water tested.

Running a maximized prop that achieves max wot rpm range is a day/night difference...

Happy Boating

 

[Maclin]

If an OB sits at the sweet transom height, should not cavitate when combo runs at wot on flat calm water cond provided that's trimmed to ride parallel to water level and deck weight evenly distribited. Better check what's the max wot rpm range for that engine before wild guessing which prop is ideal, props needs to be water tested.

Running a maximized prop that achieves max wot rpm range is a day/night difference...

Happy Boating

Searider, the engine and drive is a Mercruiser I/O, 4.3l 175HP with 1.81 ratio. The first post established that (well, the ratio is a guess but accurate). Engine rpm range is 4400-4800. There will be no height adjustment in play.

 

[Maclin]

Slwevo Are your MPH numbers from a GPS?

Reason I am asking, the numbers from the 19p SS #blade show almost exactly 0% slip in a basic prop calculator. And since your RPM is not quite into the 4400-4800 range that would indicate that a lower pitch prop is needed when comparing same to same (SS 3blade to SS 3blade). That assumes the engine is capable of producing close to the 175hp rating.

The the 4blade numbers with the 31mph are more like it is a 15p, because when a 19p is used the calcer shows 32% slip, very high for a 4blade, Aluminum or SS. When using a more normal 10% and letting the calcer come up with the pitch it shows a 15p.

That is why I have been hesitating to participate, numbers are way off between the two props if the pitch and speed and RPM's are accurate. Also I was guessing on the drive ratio at 1.81. Also there is a chance that the RPM range is more like 4200-4600 as it is the 2bbl and the lower 175 rating, I had trouble verifying the range.

But definitely do NOT go to a 21p in any configuration or material.

The 1980 Cobalt Coburre 192 is heavy at 2850lb (from NADA). I suspect with the limited 175hp version of a 4.3 that it may have actually needed a 17p if you ran it when loaded down, especially for towing sports.

 

[Slwevo]

Thank you for the responses. Let me throw a few more things out there I noticed, the 3 blade is a quicksilver 19p that looks like it was damaged before and reshaped so when I measured it it was right about 13in, the new aluminum 4 blade is brand new 14x19. With the 3 blade i could get rpms to about 4400 loaded down, so i have no complaints with my old prop. My concern is how much different the new one is and what it's doing compared to what I thought it would do based on what I've learned I should go with. The engine is rated to 4200-4600 wot. The 4 blade hits the top end of that range and feels like limiter with just me in it. I normally run with about 6 people in the boat so I'm sure that would drop it down. My concern is that using a 4 blade the same pitch I though would not make me lose as much top end as say dropping down to a 17p like most folks when going to 4 blade. And yes my speed is all GPS not the speedo. I'm sure my tach isnt 100% accurate but it's pretty dang close after rewiring and cleaning all connections it feels like.

14x19 4 blade 32mph just me 3/4 tank 4600+rpm

13x19 ss 3 blade 41mph 2 people 1/2 tank 4400rpm

basically the whole idea was looking for 4 blade performance when pulling tubes but not cutting off thaaaaat much top end.. so with the rpm I'm turning should I go up to 21 on the 4 or just say screw it it was running fine before?

 

[Slwevo]

But also why is everyone so anti fin? I think it made a huge difference... without it it took me like 10 seconds to plane out, now it's like half a second

 

[porscheguy]

Alpha gen 1 is a 1.84 ratio but that’s no big deal. My Wellcraft came with the same engine/ratio. I never had a chance to see how that engine ran since the block was cracked when I bought it. My new engine is a much newer version from ‘02. Mine came with a 21P aluminum prop.

Anyway. The operating range on the engine is 4400-4800rpm. I just st checked on my old flame arrestor cover.

The fin you just installed tends to hinder performance And produce a lot of drag unless the outdrive is trimmed to the optimal point. They place a lot of stress on the anti ventilation plate. And if they were a good idea, Mercury would incorporate it into the casting process. And finally, when you’ve got speeds and engine rpm jumping all over the place, you don’t add band aid accessories to correct the issue. If anything, you remove BS so you can establish a baseline and go from there.

1. Remove the fin. If the boat cavitates excessively without it, you need to figure out why. Cobalt is a decent brand and shouldn’t have this issue unless there’s something wrong or operator error.
2. What are you relying on for data? Speedo or gps? Factory tach? Your numbers are so inconsistent, I’d first speculate that you’re receiving inaccurate data.
3. How many hours on the engine? Are you certain it’s running properly? Timing set right? Plug leads going where they’re supposed to? Exhaust flaps intact?

Props are part f the fine or final tuning. They’re the last issue you deal with. By the data you’ve provided, you’ve got something going on higher up chain than just a wrong prop.

 

[Sea Rider]

Personally would buy a brand 3 blade 13 x 19 SS prop, If old one with 2 up achieved 4400 wot rpm expect to extremely lug the engine with 6 up. One thing is propping right a boat for a fixed load, other where load varies too much and kills wot rpm while maintaining same prop dialed for a light or fixed load.

If load will vary that much constantly, go for a 13 x 18 or 13 x 17- 3 blade SS prop whichever one is available to pull wot revs up when with 6 souls and watch tach when lightly loaded for engine not to over rev or rev limiter to kick in.

Happy Boating

 

[QBhoy]

That set up should Defo never have less than a 19”. Something isn’t making sense to me. Strange one.

 

[89retta]

But also why is everyone so anti fin? I think it made a huge difference... without it it took me like 10 seconds to plane out, now it's like half a second

They create drag while underway. Get yourself a set of smart tabs and get rid of the fin. The tabs will do the same without the drag and your boat will perform better all around

 

[Scott06]

Why anti fin?

I too got similar improved holeshot results as you with a fin (searay170/3.0 engine), thought it was great until it snapped my cavitation plate. It does create drag , since something doesn't add up with the numbers so to asses what's going on here.

As pointed out by others I think there is another issue as the four blade shouldn't drop speed so much and raise rpm. I typically run 4 blades (either same pitch or drop 1" vs three baled) and lose 3-4 mph top end, get much better holeshot and lower planing speed, and rpms are same of 100-200 rpm lower.

 

[Maclin]

Clamp a set of visegrips in the middle of a 6x12 piece of plywood at the narrow side, like where the leg is on a boat. Hold the pliers with one hand at the end of the grips like where a fin would be, and try to manipulate the board with no help from the other hand. Now grab the edges of the board thumb and forefinger, both hands, or use 2 sets of visegrips, simulating tabs --- it should be easier to get it to get it in the position you want, much more balanced.

 

[Slwevo]

I had the fin on there for over year so it hasn't been recently added, and yes it was cavitating very badly which is why I went for it. I am aware of how to trim it out so I tried different settings to see how it responded. Without the fin I couldn't come off just about all the way down without it bouncing all over the place. Now the sweet spot is about right in the middle of my gauge, maybe a hair less of trim up.
but in any case I do understand my numbers are all over the place, which was the reason for my questions. But thank you all for input I'm going to try a few more things and just see where it gets me.