New 4 blade Turning Point Prop...meh

dsiekman

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So after figuring out part of my previous issue was a bad tach (replaced), I was only hitting about 4600 at WOT for about 35 mph. 86 200 blackmax on a 21' Kevlar Hydrasport. Old prop was a stainless merc 23p 3 blade. Went to a 19p aluminum 4 blade. I only picked up about 200 rpm and lost 2-3 mph. I'm only getting about 32 mph out of a boat/motor combo that should do 50ish. Any ideas?
 

QBhoy

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Compression check, I'd think. Doesn't sound right at all. That boat and engine combo should fly and most likely require at least a 21"...23" more likely.
 

89retta

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Your not running on all cylinders. Not a prop issue check for spark on all cylinders and compression.
 

jimmbo

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Is your speed by GPS? With a 1.87:1 gear ratio. engine speed at 4800 rpm a 19 in prop, and 10% slip shows 42mph.

I too think that your engine is not up to par. As mentioned, a compression test is order, also check the timing, also are the carbs opening fully, and does it have spark on all cylinders, the rpms you are getting are consistent to when one plug isn't firing
 

dsiekman

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By gps. Two adults, two kids under 5. .5-.75 tank of fuel.

I agree something seems wrong with the engine. Carbs were rebuilt last fall and I'm pretty particular about stabilizing my fuel. I suppose it wouldn't hurt to swap the filter to be sure I'm not starving it of fuel.

What I don't get is how engine issues have any bearing on speed at a given rpm. If the engine is turning 4800 I should be getting 42ish mph per the calculation. I could see that I would be limited in rpm but don't understand how that has any correlation to the math?
 

dsiekman

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Is your speed by GPS? With a 1.87:1 gear ratio. engine speed at 4800 rpm a 19 in prop, and 10% slip shows 42mph.

I too think that your engine is not up to par. As mentioned, a compression test is order, also check the timing, also are the carbs opening fully, and does it have spark on all cylinders, the rpms you are getting are consistent to when one plug isn't firing

I should be able to visually inspect each plug and see a difference in one, correct? Like covered in fuel/oil vs clean.
 

QBhoy

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I think if it was plugs...you would hear that it wasn't running on all 6. Good place to start, but Defo check compression and go from there.
 

ondarvr

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You can't always tell if a V6 is running on all cylinders by ear, or if one cylinder drops out at higher RPMs under a load, you need to actually check for spark.

Have you confirmed the tach is reading correctly?
 

dsiekman

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You can't always tell if a V6 is running on all cylinders by ear, or if one cylinder drops out at higher RPMs under a load, you need to actually check for spark.

Have you confirmed the tach is reading correctly?

I just replaced the tach. The current readings seem believable.
 

jimmbo

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I only picked up about 200 rpm and lost 2-3 mph. I'm only getting about 32 mph out of a boat/motor combo that should do 50ish


What I don't get is how engine issues have any bearing on speed at a given rpm. If the engine is turning 4800 I should be getting 42ish mph per the calculation. I could see that I would be limited in rpm but don't understand how that has any correlation to the math?

The numbers aren't adding up.
You say you gained 200rpm so that should put you at 4800, then you say the speed dropped, and you are getting 32mph. Well as I said those rpms, gear ratio and some slip should yield approx 42mph.

Is the speed from a GPS or the water pressure speedo?
As asked before, is the tach set correctly? It reads alternator pulses, not ignition pulses.

BTW your engine is supposed to be running between 5000 - 5500 rpm, that would put it around 50mph, but...
Either your engine is not up to snuff, your boat has gained a lot of weight, the hull has issues(hook)
Are you trimming the engine up at higher speeds?
 

89retta

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Did you use the thrust washer that came in the hub kit for the TP prop ? Those thrust washers are specific to the TP prop and must be used.
 

dsiekman

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Speed by gps. Tach set to 6p as I understand it is supposed to be.

I suppose hook is possible as is water logging. She sat for a long time.

One other thought. As old as the motor is and as many owners as it has had, it's possible someone put a lower hp powerhead on it. Is there any way to know for sure?
 

dsiekman

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Did you use the thrust washer that came in the hub kit for the TP prop ? Those thrust washers are specific to the TP prop and must be used.

Yep. Brand new hub, thrust washer, castle nut, etc.
 

dsiekman

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The numbers aren't adding up

Are you trimming the engine up at higher speeds?

Correct, which is part of my frustration.

Yes, trimmed as high/out as she goes. I played with the trim quite a bit. I assumed I'd be able to essentially cause it to cavitate or blow out but it wouldn't raise that high under load. Not sure if that is part of the problem too. Weak trim system? I feel like I'm just grasping at straws at this point
 

dingbat

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So after figuring out part of my previous issue was a bad tach (replaced), I was only hitting about 4600 at WOT for about 35 mph. 86 200 blackmax on a 21' Kevlar Hydrasport. Old prop was a stainless merc 23p 3 blade. Went to a 19p aluminum 4 blade. I only picked up about 200 rpm and lost 2-3 mph. I'm only getting about 32 mph out of a boat/motor combo that should do 50ish. Any ideas?
What model hydrasport?
Given their hull design, I don't think 50 mph is realistic.
 

dsiekman

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What model hydrasport?
Given their hull design, I don't think 50 mph is realistic.

2100CC. 1978/79 (there was some discrepancy between the model number, manufacture dates, and title). It says Kevlar 49 on the side.

I was told by one of the previous owners (not the one who I actually purchased the boat from) that it ran in the upper 50's (via GPS) with a Johnson GT175 on it. Granted, that was a few years back and I am sure the hull was in much better shape. Knowing him, that motor probably wasn't stock, either. That said, he is not really the type to exaggerate, nor would he have any reason to.

Regardless, 50 is waaayyyy faster than I need to go in that boat, I just want to run as efficiently as possible and know that I'm getting what I should be getting out of the motor. On the river where I do most of my boating the speed limit is 35. The bay is a different story but then you encounter bigger waves/chop/wakes in which case 50 is certainly too fast. I'd be happy with WOT at 40 ish and comfortable cruise at 33-35
 

ondarvr

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The speed you want to go is sort of irrelevant to the issue, you need to know it's running correctly, and when it is it should be in the 50mph range. At that point you can travel at any speed you want, but it needs to run correctly first.
 

jimmbo

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A 200 merc should blow a 175 OMC out of the water. A 150 merc would give it a run for its money
Has it ever preformed better for you?
Is the merc a long shaft? I ask because by 86 the 200 was available in an extra long(25") shaft. If somehow an extra long shaft was mounted, your performance would suffer
 

dsiekman

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I believe it is a 25" but the cavitation plate is pretty much parallel with the keel.

I've only had it about a year and just fixed the tach. No, it has not run better previously.
 
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