Building a '93 Caravelle 1750 Classic Bowrider

[Reserector_]

The 17P prop was an improvement over the 19P. Slightly better holeshot, slightly higher speed, and slightly higher rpms. Rpms came up around 300-ish from what I can see in the videos.
Based on that, I just ordered another used OMC prop. This time it will be a 15.5 x 15P (3 blade alum.). Everything the same except for the pitch.

 

[chevymaher]

Getting a little collection going like I got huh?

 

[Reserector_]

I will likely sell the 19. In the end, all I need is one prop that works well, and a runner-up as a spare.

 

[chevymaher]

I only got 6 of them. I gave one to a buddy that was a double.

 

[Reserector_]

New prop to try. This one a 15P x 15 1/2
It is about an inch lathe in diameter than my other two props, but if it fits, it ships.


I started with a 19P, then a 17P and now a 15.

The boat should leap out of the water on launch, now. Lol. I figure the extra diameter will reduce slippage.

I'm hoping that this will get my rpms up. Just trying to get above 4K at WOT has been a challenge so far.

I found some good used 13P props in my price range on eBay, so I ordered one. That may be next if the 15 doesn't hit the sweet spot.

When I'm happy with the prop, I want to try it without the fin on the lower unit.

 

[Reserector_]

So it seems that the boat runs at around 33-35mph with a few different props; 19P, 17P and 15P a slightly different diameters. The only thing that seems to change is the rpm.
This last time out, I stepped down to the 15P and it gave me 34MPH at around 3,500 RPM.
My next prop to try is a 13P. At this point, I don't expect any more speed. Just higher RPM. More efficient??

 

[Scott06]

So it seems that the boat runs at around 33-35mph with a few different props; 19P, 17P and 15P a slightly different diameters. The only thing that seems to change is the rpm.
This last time out, I stepped down to the 15P and it gave me 34MPH at around 3,500 RPM.
My next prop to try is a 13P. At this point, I don't expect any more speed. Just higher RPM. More efficient??

Would question if your tach is accurate if only getting 3500 rpm with a 15p, unless you have the wrong gear ratio drive for your engine. what is max rpm for this motor ? You should be up around 4400 or more rpm.

I also found the same thing with a couple different 21, 22, 23" props- you hit a wall where you need more power to go faster, just the rpms change.

 

[Scott Danforth]

You should be about 4400 RPM with the 19p with just you on board. About 4600 RPM with the 17p and about 4 on board and the 15p should redline except pulling a skier

 

[Reserector_]

It should be around 4,400 or so, as you say. Without knowing the HP and torque graph for the GM 3.0, I can only assume that I am running on torque at this point. I expect the HP to increase beyond torque at some point and get me past this plateau if I can get the RPMs up.
It is a '93 OMC Cobra drive, which was the last of that era. It is identical to the one that they used behind the 4.3L v6, so it is likely geared tall for the 4 cyl.
The previous owner couldn't recall what prop it had, and there was not a prop in the pile of junk that I got from him, so I had to start from scratch. Someone here suggested that I try a 19P, so that's where I started.

 

[Scott Danforth]

If the gear ratio is 1.84:1 you will be about 15p. The RPMs I stated are with a 1.94:1

 

[Reserector_]

If the gear ratio is 1.84:1 you will be about 15p. The RPMs I stated are with a 1.94:1

That's what I have, the 1.84:1

There may also be a power (engine) issue, but I don't want to get ahead of myself. You see, it is a 1982 block and head (large ports on the head) with a '93 manifold. (Smaller ports) There is a bad mismatch that surely interferes with flow, but to what degree, I have no idea.
I looked into getting an old-style manifold, but they did not use the two-barrel carb, and there were other major differences.
I'm not all that interested in putting a later model head on it, but if I could get one cheap enough, I might.

The truth is, I am content with 35mph if that is all this arrangement will give me. I just want to make sure that it is at least propped right.

 

[Scott06]

That's what I have, the 1.84:1

There may also be a power (engine) issue, but I don't want to get ahead of myself. You see, it is a 1982 block and head (large ports on the head) with a '93 manifold. (Smaller ports) There is a bad mismatch that surely interferes with flow, but to what degree, I have no idea.
I looked into getting an old-style manifold, but they did not use the two-barrel carb, and there were other major differences.
I'm not all that interested in putting a later model head on it, but if I could get one cheap enough, I might.

The truth is, I am content with 35mph if that is all this arrangement will give me. I just want to make sure that it is at least propped right.

Yeah there is an issue with power given the combo you state.

If the tach is accurate prop down until you get rpms up to 4400 or so.

I believe Barr makes an adapter for the head and manifold mismatch you have. https://www.michiganmotorz.com/mercruiser-exhaust-manifold-adapter-plate-kit-high-output-3-0l-engine
Same part is on ebay for $129 or so

not sure if it can flip to go the other way new manifold with old style head? Might have to drill some holes for manifold bolts.

What carb was on the old style manifold? Had only seen 2 bbls on any 2.5/3.0 from OMC,VP, and Merc

If the engine was 100% in perfect shape- you probably would only hit 38-40 mph. The amount of times you can actually use WOT ...who cares. I had a 91 Sea Ray 17 ft with 3.0 LX/135 hp (with later HO head) and that is about what I got. I ended up propping down to ski and tube.

It is a worthwhile endeavor to get the rpms up so you are not lugging the engine.

Did you ever get your lock N stick crack repair completed?

 

[Reserector_]

I agree. It's fast enough for the small lakes in my state. I just don't want it lugging.
Yes, I finished the crack repair with the Lock-n-stitch. It worked out well.

That adapter plate is interesting. Too bad it goes the wrong way for my situation. But it has me wondering about the "High Output" verbage. does the newer engine put out more power than the older engines? I have never been able to find any HP or torque information on any of the Merc or OMC versions.
If that was the better way to go, I don't think it would be hard to find a complete cylinder head from someone's freeze-cracked 3.0L and buy it cheap. Easy swap. Something to think about.

 

[Scott Danforth]

Could always get the 3.0 derived LS headed motor from Blueprint Engines. It's 340hp.

 

[Scott06]

I agree. It's fast enough for the small lakes in my state. I just don't want it lugging.
Yes, I finished the crack repair with the Lock-n-stitch. It worked out well.

That adapter plate is interesting. Too bad it goes the wrong way for my situation. But it has me wondering about the "High Output" verbage. does the newer engine put out more power than the older engines? I have never been able to find any HP or torque information on any of the Merc or OMC versions.
If that was the better way to go, I don't think it would be hard to find a complete cylinder head from someone's freeze-cracked 3.0L and buy it cheap. Easy swap. Something to think about.

the 2.5 and 3.0 had ratings ranging from 110-140 hp over the years ( like 60's thru now). Throw in that the ratings went from flywheel to prop hp at some point it is overly complicated.

When my folks bought the 91 sea ray it had options of 125 hp (3.0L) and the 3.0LX at 135 hp. If I recall there may have been 115hp option but I assume that was the last of the 2.5l engines.

really it is kind of stupid to have so many hp combos of essentially the same engine... Really if you want more power get a 4.3 you already have the drive ratio...

Are you at high altitude? if so you may run out of prop . Be interested to see of the 13" gets you over the hump right now its like passing with out down shifting...

 

[Chris51280]

with the 15p prop at 3500 you should have only made 27mph with the 1.84 ratio. So thats physically impossible to be at 35mph Something is off. Your tach or speedo. If it is GPS measured I would say the tach is off. At 4400 you should be able to get 33mph with the 15p prop. Pending all is healthy

 

[Reserector_]

with the 15p prop at 3500 you should have only made 27mph with the 1.84 ratio. So thats physically impossible to be at 35mph Something is off. Your tach or speedo. If it is GPS measured I would say the tach is off. At 4400 you should be able to get 33mph with the 15p prop. Pending all is healthy

The highest I recall was 34 on the gauge. 33 on GPS.
Here is a screen shot from the video. Looks like just over 33MPH on the gauge at 3,250 RPM
The video is posted up above in a previous post. It SOUNDS about right for that reading.

 

[Reserector_]

I also have one of these on the lower unit. I don't think it's necessary. I wonder how much drag that creates?

 

[todhunter]

I'm not sure if mine was a bad install from the PO, if I hit something small with mine, or if it's just the risk you take running a whale tail, but one side of mine broke off this summer and took a chunk of the cavitation plate with it. I'm going to weld in new aluminum plate where it chunked out.

 

[Chris51280]

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