Odd prop sizing / gear ratio question

[Will.drift]

This may be a little bit of a odd question but hope I can find the answers I am looking for. I have a 1994 Glastron ssv195 which was originally equipped with a 4.3 4bbl mercruiser and what I assume was 1.84 gearing? The boat came without a leg and motor so now I have swapped in a 220hp 2bbl 5.0 and a 1.5 gear ratio alpha one gen 2 outdrive . With a 21 pitch aluminum prop I can hit 37 mph wot with a normal load of people and fuel tank. However I can over rev it if I want and it will still pull harder. This is where it got a bit strange I put a 23 pitch prop on and the boat would barely rev above 3000 rpm and not accelerate... and on a prop slip calculator it’s showing something crazy like 30% is jumping to 1.47 gears from the proper 1.65 going to make all the difference I feel the boat should be Seeing closer to 50mph with a decent cruise speed . Any help/ suggestions would be much appreciated thank you.

 

[Texasmark]

Using the prop slip calculator is the right way to tune in a prop-setup-engine-boat relationship as I see it and the way I do it. The whole shebang is nothing but a screw. You have torque to twist the screw, threads at a certain pitch, diameter and material. The gear train is a train and the engine rpms, gear ratio, and prop pitch are all linked together. If you start changing prop types you start throwing a curve ball. Gotta keep it apples and apples to get any kind of intelligent response from your equipment.

Over revving and continuing to increase speed tells me that your gear train could stand a little more resistance to the available torque, aka changing 2" in pitch...equal to 300-400 rpm drop and keeping you out of the redline while maintaining your prior speed achieved over revving. Something else has changed.

Changing from a normal 15% (for a number) slip at WOT to 30% automatically tells me you have an erroneous input. If you cut back on the throttle and the speed slowed down and the boat sank deeper in the water and drag shot up then fine, but not when everything is the same and you just added 2" to the pitch and things went to hell!

 

[Will.drift]

I’m going to retry it as the props have only changed in pitch , same oem prop other wise. My only other change I have was I just rebuilt my fuel pump due to diaphragm failure and flooding of the carb. I will re try and let you know how it goes.

 

[dingbat]

and on a prop slip calculator it’s showing something crazy like 30%. Is jumping to 1.47 gears from the proper 1.65 going to make all the difference. I feel the boat should be Seeing closer to 50mph with a decent cruise speed . Any help/ suggestions would be much appreciated thank you.

In theory, you've put an additional 20% load on the motor between the gear reduction (~11%) and prop change (~9%).

 

[jimmbo]

The boat came without an engine or outdrive. You installed a V8 and outdrive. The outdrive you installed has 1.5:1 ratio. A 21 " prop with those gears will be doing mid to high 50s when the engine is running in its WOT range of 4400 - 4800 rpm. What rpms are you getting? Did you recalibrate the tach for 8 cylinders from the 6 for the older engine? You did use an actual Mercruiser engine, not something from an auto wrecker?
What condition is the engine in? Compression? Timing set correctly and advancing? Is the Throttle Cable adjusted so the carb is opening fully?

 

[Will.drift]

Yes .low hours fully dressed mercruiser engine , Has maybe 100 hours on it. It can probably achieve 5200 rpms + and runs great.I didn’t know a tachometer calibration is necessary? Or possible. Throttle linkage and timing set correctly. Could it be as simple as that ? Actual rpms reading too low on tach so I’m not pushing it hard enough?

 

[jimmbo]

If the tach is set for 6 cylinder(3 pulses per rev) and you have an 8(4 pulses per rev), it will be reading higher than the engine is running

I have an 5.7 with a 1.51:1 ratio, and turning a 20" Raker. I think your SSV 195 is the same hull as my SX 195
https://www.youtube.com/watch?v=VepbXm5RFDI

 

[Will.drift]

Wow that helps a ton and I see the selections on the back of the tach now, really glad I joined this forum. You guys are very helpful

 

[Texasmark]

Wow that helps a ton and I see the selections on the back of the tach now, really glad I joined this forum. You guys are very helpful

Very tender adjustment, probably need to push in, feeling the detent and then turning to desired number. Releasing and it should pop back out if not nudge your tweak a bit either way till it does....little locks (ridges) in the plastic to keep your setting once set.

 

[QBhoy]

Hi there. If it helps you get somewhere...I can almost without doubt tell you that you will not be able to over rev that engine with a 1.47-1, a 5.0 2 barrel and a 21” prop. Not a chance. The prop or hub must be under question if it is.
If you could pull even 4800 rpm with a 21” and that ratio, you would be at or about 60 mph, at a guess and without checking the prop slip.
Best of luck.

 

[Will.drift]

Good to know , the tach setting issue is going to be #1 I’ll be able to get the proper numbers now and be able to calculate my prop slip correctly. Just wanted to make sure my “improper “ gearing change wouldn’t be a large factor in this issue. Might take me a week or two to get back on the lake here. 55-60mph would be awesome

 

[QBhoy]

Good to know , the tach setting issue is going to be #1 I’ll be able to get the proper numbers now and be able to calculate my prop slip correctly. Just wanted to make sure my “improper “ gearing change wouldn’t be a large factor in this issue. Might take me a week or two to get back on the lake here. 55-60mph would be awesome

Best of luck with it. Although I’d say that 60 mph genuine GPS speed is a bit of a mile stone to achieve with anything other than a stripped race hull and big outboard....or plain and simple huge horse power on any other boat.
Your glastron is quite a light built boat to be fair, but even with the 5.0 and perfectly propped I think you will do really well to see over 55 mph gps in it. 50 will be possible running light and trimmed up with a really good 3 blade like an enertia, tempest or laser 2. Be great to see it at between 50-55. You’ve done well if it does.
Keep us posted.

 

[jimmbo]

You won't see 60 with your set up. I get there but with a bigger engine. 50 - 55 will be very good for 5 liter

 

[QBhoy]

You won't see 60 with your set up. I get there but with a bigger engine. 50 - 55 will be very good for 5 liter

Agree. And nearly 20ft of hull. Not naturally a fast one either, really. Light though.

 

[jimmbo]

The 195 is about 18 1/2 - 18 2/3 ft.

 

[Will.drift]

Ok so I finally had a chance to get the boat out this time, it’s running great and changing the back of the tachometer to 8 cyl mode made a drastic change , it now only sees 4000rpm wot with the 21 pitch at 40-42 mph , I’m going to drop to a 19p if I can get my hands on one soon to Dial in my rpms

 

[QBhoy]

Excellent. That’s more like it. I think you may just struggle with a 19” too. Maybe get away with a 19” ally into the lower rpm range but probably not a stainless. 17/18” would be my guess at being right. Perhaps.