Stuck between pitch

[DeepSiks]

Can’t decide between 3 blade 15.6 x 19 or 15.6 x 21; both Turning Point Mach 3 OS (SS)

Current issue: 6100 rpm at WOT (24 kts) 4500 rpm at cruise (19 kts) trying to get to cruise of 3200 rpm and WOT around 5000.
Current setup: 26’ Cruiser, 260 Holiday, 8.5’ beam, 5200 + 400 in load= 5600 gross weight
5.7L Vortec 350, 312HP, OMC stringer drive, 4 blade dual pitch 15 x 15 (15/17 pitch) Turning point aluminum propeller.
Not sure of the rpm change from 4 to 3 blade. I understand that each degree of pitch increase will decrease 200rpm some say 300.
Should i switch to 19 pitch? 21 pitch? Or am I way off the mark? Please help if you can and thank you in advance.

 

[QBhoy]

Are you actually getting a verified 6000+ rpm ? If you are..or have…it’s incredible that the engine has survived the ordeal. You’ll be at least 1000 rpm over any known rating for it.
Perhaps it’s not a standard engine and you’ve uprated it to survive the higher rpms ? Sounds like it, when you say it’s producing more power than any normal or standard rating usually given from that engine.

 

[DeepSiks]

Are you actually getting a verified 6000+ rpm ? If you are..or have…it’s incredible that the engine has survived the ordeal. You’ll be at least 1000 rpm over any known rating for it.
Perhaps it’s not a standard engine and you’ve uprated it to survive the higher rpms ? Sounds like it, when you say it’s producing more power than any normal or standard rating usually given from that engine.

Definitely not stock. Rpm and HP are verified.

 

[QBhoy]

Definitely not stock. Rpm and HP are verified.

Ok. Good stuff. Is she built to take 6000 rpm ? Presumably you’re tying to lower the rpms a bit? What’s the rpm range now that the engine has been upgraded internally

 

[racerone]

Perfect pitch.------That is when you throw a fiddle into a dumpster and smash a set of bagpipes.----Sorry, could not resist.

 

[DeepSiks]

Definitely not stock. Rpm and HP are verified.

Oh

Ok. Good stuff. Is she built to take 6000 rpm ? Presumably you’re tying to lower the rpms a bit? What’s the rpm range now that the engine has been upgraded internally

she is built to withstand an RPM of 5600, I want WOT to be at 4800-5000. 6,000 was a split second and the main cause for concern. I have since gone back through the engine. I should be cruising around 3200 but I am running 4500 at cruise (60-75%).

 

[jimmbo]

Without knowing the Engine's Power and Torque Curves, it becomes a problem to determine a WOT Rpm Range. A stock 5.7 of that era of OMC had a WOT range of 4200-4600 rpm. Yours is a Vortec, and even stock units have a range of 4400-4800.
OMC Stringer Drive. That Drive was Overstressed when handling a stock 260hp 5.7... Good luck

 

[QBhoy]

Oh

she is built to withstand an RPM of 5600, I want WOT to be at 4800-5000. 6,000 was a split second and the main cause for concern. I have since gone back through the engine. I should be cruising around 3200 but I am running 4500 at cruise (60-75%).

Excellent. Glad the internals can take the kind of rpms. Vortec 350’s have a few different rpm ranges. 4400-4800, 4600-5000 and just occasionally, 4800-5200 or close. 260-300hp at their best form.
I might be cheeky enough (but with the best intentions), to suggest that the weak point with your set up, would be the outdrive you’re running the power through, perhaps. That said…some of the omc/Volvo Penta collaboration outdrives were well engineered

 

[Scott Danforth]

Check your tach. Valves float and cam curve runs out long before 6100 RPM

 

[DeepSiks]

Definitely not stock. Rpm and HP are verified.

Oh

Ok. Good stuff. Is she built to take 6000 rpm ? Presumably you’re tying to lower the rpms a bit? What’s the rpm range now that the engine has been upgraded internally

she is built to withstand an RPM of 5600, I want WOT to be at 4800-5000. 6,000 was a split second and the main cause for concern. I have since gone back through the engine. I should be cruising around 3200 but I am running 4500 at cruise (60-75%)

Excellent. Glad the internals can take the kind of rpms. Vortec 350’s have a few different rpm ranges. 4400-4800, 4600-5000 and just occasionally, 4800-5200 or close. 260-300hp at their best form.
I might be cheeky enough (but with the best intentions), to suggest that the weak point with your set up, would be the outdrive you’re running the power through, perhaps. That said…some of the omc/Volvo Penta collaboration outdrives were well engineered

Thank you for the advice. I had a machinist who specializes in OMC outdrives build it for me. I actually really like the drive now that I solved the trim issue. Instead of trusting OMC I added electric stops to the trim and a head tank to the reservoir . Trim system works like a dream now.

 

[DeepSiks]

Check your tach. Valves float and cam curve runs out long before 6100 RPM

Will do. Thank you

 

[DeepSiks]

Excellent. Glad the internals can take the kind of rpms. Vortec 350’s have a few different rpm ranges. 4400-4800, 4600-5000 and just occasionally, 4800-5200 or close. 260-300hp at their best form.
I might be cheeky enough (but with the best intentions), to suggest that the weak point with your set up, would be the outdrive you’re running the power through, perhaps. That said…some of the omc/Volvo Penta collaboration outdrives were well engineered

Without knowing the Engine's Power and Torque Curves, it becomes a problem to determine a WOT Rpm Range. A stock 5.7 of that era of OMC had a WOT range of 4200-4600 rpm. Yours is a Vortec, and even stock units have a range of 4400-4800.
OMC Stringer Drive. That Drive was Overstressed when handling a stock 260hp 5.7... Good luck

not stock, not from that era. I appreciate the heads up though.

 

[DeepSiks]

Can’t decide between 3 blade 15.6 x 19 or 15.6 x 21; both Turning Point Mach 3 OS (SS)

Current issue: 6100 rpm at WOT (24 kts) 4500 rpm at cruise (19 kts) trying to get to cruise of 3200 rpm and WOT around 5000.
Current setup: 26’ Cruiser, 260 Holiday, 8.5’ beam, 5200 + 400 in load= 5600 gross weight
5.7L Vortec 350, 312HP, OMC stringer drive, 4 blade dual pitch 15 x 15 (15/17 pitch) Turning point aluminum propeller.
Not sure of the rpm change from 4 to 3 blade. I understand that each degree of pitch increase will decrease 200rpm some say 300.
Should i switch to 19 pitch? 21 pitch? Or am I way off the mark? Please help if you can and thank you in advance.

I absolutely love the convo and can’t thank everyone enough for the words of wisdom. In either case, 19 or 21?

 

[jimmbo]

You do realize that what makes Power in a Car/Truck application, is rarely transferable to a Marine Engine. I am referring to the Bumpstick. I have seen far too many Boats with warmed over engines, where the Owner, is boasting that his motor can make X number of HP at whatever Rpm. Maybe it did do that on a Dyno at the Builders. Yet when in the water, the boat is a Real Dog, and the Lowest HP version of the Stock Marine Engine performs way better, and is not a difficult Job to Drive

 

[QBhoy]

Oh

she is built to withstand an RPM of 5600, I want WOT to be at 4800-5000. 6,000 was a split second and the main cause for concern. I have since gone back through the engine. I should be cruising around 3200 but I am running 4500 at cruise (60-75%)

Thank you for the advice. I had a machinist who specializes in OMC outdrives build it for me. I actually really like the drive now that I solved the trim issue. Instead of trusting OMC I added electric stops to the trim and a head tank to the reservoir . Trim system works like a dream now.

Great sounding idea/improvement there. Never owned an omc, but enough of the VP 80’s era drives, to know that they suffered inherent issues around easily allowing water ingress to the gear oil, by way of their seals at the drain plug at least. Perhaps more specifically a human influence flaw, with the exact reason for it usually being the o ring getting nipped or pinched slightly, if care isn’t taken when tightening up the plug screw..making certain the o ring is able to compress and seal uniformly. Easily done otherwise. I always used to make sure it was water tight, with a blob of sealant over the whole plug screw head area, once she was tightened up each time. Anyway…don’t know if the omc suffers the same design character or not…but thought I’d mention.
Anyway..you’ve got my full attention at the moment…tell me more about this propeller you have currently. Specifically perhaps referring to the detail around being a dual pitch thing. Is it a fairly unique oddball progressive pitch design, perhaps? As in its stepping up in pitch progression from leading to trailing edges ?
Regardless, I think it’s perhaps not to be trusted or assumed as a good or ideal indication, in terms of a given or known reference/starter for ten…aiding the hunt for a suitable prop.
On another note…big fan of the cruiser holiday models. Lovely boats and great hulls.

 

[QBhoy]

Oh

she is built to withstand an RPM of 5600, I want WOT to be at 4800-5000. 6,000 was a split second and the main cause for concern. I have since gone back through the engine. I should be cruising around 3200 but I am running 4500 at cruise (60-75%)

Thank you for the advice. I had a machinist who specializes in OMC outdrives build it for me. I actually really like the drive now that I solved the trim issue. Instead of trusting OMC I added electric stops to the trim and a head tank to the reservoir . Trim system works like a dream now.

Is she by any chance a 2670 model ? Trying to picture your exact model. Perhaps wondering if I’ve came across a cruisers model similar, that would maybe have an omc drive fitted originally. Not an expert by any means, but known a fair few cruisers models from the 80’s and 90’s era…all of them featured VP or mercruiser outdrive(s)…alternatively driven by shafts/velvet drives/v drive arrangements, on the models around 30ft and above…like the rogue models that size.

 

[jimmbo]

I will admit that the OMC Stringer, had a Vastly Superior Trim System, not Partswise, but in how the Change of Trim occurred., By raising/lowering front of the Engine, the resulting Arc of the Outdrive was a much better way, than having the Pivot Point at the Double U-Joint on the shaft going into the Outdrive, used by Mercruiser, Volvo, the later OMC drives. That said, the rest of the Stringer Drive leaves a lot and I mean a lot to be desired

 

[DeepSiks]

You do realize that what makes Power in a Car/Truck application, is rarely transferable to a Marine Engine. I am referring to the Bumpstick. I have seen far too many Boats with warmed over engines, where the Owner, is boasting that his motor can make X number of HP at whatever Rpm. Maybe it did do that on a Dyno at the Builders. Yet when in the water, the boat is a Real Dog, and the Lowest HP version of the Stock Marine Engine performs way better, and is not a difficult Job to Drive

Just looking for help with the propeller pitch. If the info I provided doesn’t match your expectations, I am sorry. However, the info is correct and I just need help deciding how far to pitch up. The measured HP was not at 6K that is what it spun to, because of the lack of propeller pitch. As stated, I have since rebuilt the engine again. Now I am trying to avoid the same problem. And yes, I do realize. No this is not a car engine. Engines are built to application (when done right). This is a marines engine.
Further; for those that keep doubting the HP, a stock 5.7L Vortec puts out 315 easy all day. The inly ones that didn’t were in respinse to fuel shortages during the reign as the go-to for GM. (Some were even dumbed down to 140 HP). I promise, I understand engines. However I do lack experience with propeller pitch. Hence the problem.

 

[brodmann]

going from 19 to 21 pitch should reduce RPM's by 200-300. With that motor's potential, you may need to go to a 4 blade or a larger diameter if the drive allows for it. Sounds like you've got a monster of a motor on your hands! I've recently gone back to aluminum props. They are lighter and can spin faster due to lighter weight and less mass. Also, aluminum props actually have a little "flex" at hard acceleration and then straighten back up once speed increases. This works great for getting up on plane a little quicker. None of that should be a problem for you though. I'm thinking a 21 pitch is not enough for you and that set up. I have no idea what going from 3 to 4 blade does to RPM's. There are a lot of better sites than this for getting this type of information though.
This site might be a good one for you to visit and use their free prop calculator to determine what works best for you. Not to make you feel unwelcome, but judging from the responses to your question, this might work better. Please share your results and let us know what you end up with.

Free to use Propeller Sizing Calculators

www.vicprop.com

 

[jimmbo]

At what rpms does the Cam chosen, provide best Horsepower, and best Torque?

What is the Overall Gear Ratio of the Outdrive?