THSCC Forums

First you must figure what the average load on the boat will be. How do you plan to operate the boat, how many people and how much equipment will you normally have on the boat. Thats your base line for how much weight the prop will have to push, and how fast do you need to get to speed. Pulling skiers you want good accel, hence increase diameter. But theres a limit on how much diameter you can increase, the tip of the prop must be more than 11% of the diameter of the prop from the cavitation plate or the prop will cavitate. Almost any prop shop can increase the pitch for you and I feel the best method is to cup the prop which does both, change the effective pitch and decrease slip. Cupping the prop should drop the RPM, but your local prop shop can steer you better on that than I can.

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2004 C5(415whp,390ft/lbs),
1997C5,1997Trans Am, 1986 C4,
1990 Miata, 1976 MGB,1997 Protege, 1989 MR2

Welcome to the wonderful world of props! Bernie is spot on with the statement of what you want to do with your boat. Yes, stick on a performance prop and get that extra 5 mph. But do not plan on taking the family water skiing or tubing with it

You kind of have to clear your mind of the auto / marine comparisons. There is a reason GM uses Merc as a test bed. A marine engine is under tremendous loads for a long period of time. Try running your car at redline for 5 minutes. Stick to the manufacture's RPM limit. If you don't break the motor, you will drastically reduce the outdrive's life. That is the real weak point.

If the boat is running well and gets on plane (with family on board) with the current prop, I would stick with it for a while. Just keep the motor at 4800 RPM Are you sure it is the prop that should be on the boat?

Stainless steel props are the bomb. But they get really expensive and it is a trial by error type of thing. I used to have 3 different ss props for my 20' Regal. Depending what I was doing that day dictated which prop. If you are just starting out boating or on an unfamiliar lake, stick with aluminum! SS will not give when you hit something, the outdrive absorbs the shock and breaks. The aluminum prop will absorb the shock and save the outdrive.

Oh, one last thing, carry a spare aluminum prop or one of the plastic ones. It will save you a tow someday.

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Jim Pastorius
2008 Silverado VortecMax
1992 Camaro CMC#92
2002 BMW R1150R

2009 3rd Place CMC Mid-Atlantic Championship
2009 CMC Hyperfest Winner

reminds me a CL ad i saw last night: http://raleigh.craigslist.org/cto/1916892342.html

The car has a LT-5 double overhead cam 350 C.I. that was partnered with GM, Lotus, Mercury Marine. The aproval test was that it had to surpass 100 hours at wide open throttle, before GM was going to release it.

Bernie thanks for the info. The "screw in sand" mental image puts it all in perspective for me! Based on your an Jim's thoughts, I'm planning on sticking with the factory sized prop. I want a good holeshot and I have no need to go over 45mph. Just means that I won't ever be doing WOT cruising since it would be over-revs.. Which is a good thing for both the motor and the outdrive!

Just for my intellectual curiosity, can you make a guess about the following:

14.5x19p (current)
holeshot = good
top speed ~50
WOT RPM ~5000

change from current to 15x19p (larger diameter only)
holeshot = worse? same? better?
top speed = same?
WOT RPM = same?

change from current to 14.5x21p (more pitch only)
holeshot = worse
top speed = about the same
WOT rpm = lower, probably 4800

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

change from current to 15x19p (larger diameter only)
Better holeshot
top speed about the same or slightly higher due to less slip
WOT less than the 5000

change from current to 14.5x21p (more pitch only)
same or less on the holeshot
higher if the engine can achieve rated RPM
WOT less and probably less than 4800 (2" is a lot of pitch)

Here's some more food for thought---small changes can be made on aluminum props by a competent prop shop. But as Jim stated It's a good Idea to have an extra one on board. As mechanically inclined as you are you will be able to fix most things that break on the boat. But a prop is a prop and if it breaks due to what ever cause (grounding, stump or other submerged object or just plain fatigue) you're screwed
Increasing the diameter is what I'd be looking at or cupping the prop.
increasing the diameter will require another prop, making your old one the spare. Cupping the prop is done at a prop shop and requires that the leading and trailing edges of each blade is rolled toward
the direction of the incoming flow of water. This reduces slip and gives the prop more efficient movement through the water
your current prop slips about 44% you could reduce the slip by increasing the diameter, I believe that you'd increase speed also.
This slip rate can be calculated by RPMxPitchx60/63360 divide the result into your actual speed and subtract from 100% that figure is your % slip, the less % slip the faster you go.
All props slip, but an increase in diameter on your prop would help I'm sure.
a peak efficiency prop is said to have square proportions that is diameter =s pitch, increasing your pitch would make the slip worse so I wouldn't recommend it.
One last thought be sure you have at least 11% of the diameter of the prop as clearance between the prop and cavitation plate. or the prop may cavitate. Also its not a bad idea to have the prop checked to insure the pitch is still correct. A grounding in soft silt or mud can straighten out the blades and still keep the prop in balance.

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2004 C5(415whp,390ft/lbs),
1997C5,1997Trans Am, 1986 C4,
1990 Miata, 1976 MGB,1997 Protege, 1989 MR2

I was about 8 years old and at the helm of my aunt and uncle's boat cruising down the middle of the lake at maybe half throttle. My uncle was hovering, but letting me drive. Then the boat just goes "wuuuumph" and next thing you know we're going about 2MPH at the same RPM. What the?

Limp back to the dock and pull her out and the prop is totally trashed. Just hit something submerged. Day over. My uncle bought a new prop, but had the prop shop "fix" that one for about half the cost of a new prop. And he kept it as a spare. Dunno if he ever needed one after he had one, though.

Oh, and he had two trailers stolen at the boat ramp in as many weeks. Both locked up pretty well.


--Donnie

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My Blog

Good info thanks Bernie!

FYI I was alarmed at the 44% slip number, one piece of information that you didn't have is that I have an overall ratio of 1.68:1.

So a 19" pitch prop at 5k rpm on 1.68 drive yields theoretical 54MPH, actual 50MPH, so I'm getting 7% slip. From what I have been reading that's pretty good.

I'll check the distance available for up-sizing...

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

Mike, I'm going to go out Sunday. You going out this weekend?

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In need of car.

Yep, Sunday as well. We've sent out feelers to 4 families also, 2 with boats, maybe we can all meet up. We usually get out around lunchtime, so don't go too early! I should have my new trim tabs installed, can't wait to try them out. Boat ran perfect (except one unexplained stall) the last 2 times out.

FYI most trips we've been beaching here:

http://local.google.com/maps?q=35.68980 ... 48323&z=15

It's a great little cove, usually not crowded, and there's a little bit of sandy beach right there.

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

Do you have a depth finder? My navionics program shows that area as being shallow with flooded timber, so I've stayed away from there. You pass right buy a danger buoy, right? Now that I know you can get back there, I'll give it a shot.

Send me a text message on your way out. We should stay out until 2:30-3:00.

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In need of car.

It's shallow near the shores but I consistently see 10+ feet away from shore. Been all over the place back there and I haven't flipped the boat yet There are a few danger buoys right at the entrance but I have passed by many times and seen lots of other pass by too with no issues.

Maybe during droughts I should stay away. I'd like to see your navionics. Does it show individual trees and their height/depth? Or just general warnings?

My marine radio is on order but I plan to try to coordinate on FRS/GMRS. Want to pick a channel? We usually stay from about noon-5pm

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

No, it doesn't show individual trees, just gives a warning of the area.

I'm not sure I can find my old FRS radios. I'll look for those tonight.

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In need of car.

For those following along, I caught up with Mike today on the lake. His boat is quick, especially for the price!! After a little " comparison testing", I think I will have to buy some new impellers for my jet drives. Anyone have some knowledge on those? I noticed today that from a dead stop I get a bit of cavitation if I go full throttle too quickly. I've been reading and it looks like new impellers will help with that issue.

Mike, do you think they will chase us off if we set up a course with buoys and start running laps around them?

P.s. Next time, you're sitting in the front!

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In need of car.

Update.

Smart tabs are FRICKING AWESOME. Solved most of my remaining problems/gripes about the boat. Since it's an ass-heavy (iron block v6) small (18') boat, it's always been a handful in some cases with porpoising, high bow raise, etc.

Who would have thought that two 12" plastic plates and two springs and shocks would have done all this. I was amazed yesterday, it's a totally different boat...

- Probably 50% less bow rise when getting on plane
- On plane even faster than before
- Planes at 12 MPH versus 18 before
- Easier to maintain slow planing speeds (tubing little kids)
- Feels more stable when turning, responds better to input.
- Low speed bow wander at no-wake / idle is GONE, tracks perfectly straight
- Eliminated the stern wave coming over the stern and into the boat when dropping throttle suddenly
- No loss of top speed (well maybe 1 MPH)

And the greatest thing: Porpoising and crashing over wakes and rough water is 95% reduced.

My daughter (3 year old) would cry and have a fit about "I don't like the bumps!" every time we were out before. Yesterday we went out on the open part of the lake in 10 mph winds and a lot of rough chop. Ride quality was superb. She fell asleep. Son was sitting in the open bow eating an apple ... before the tabs he would have been holding on for dear life.

Best $120 I have spent on the boat so far.

Not my boat, but you get the idea:

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

I've seen conflicting reports on whether or not those would work on a jetboat. It seems like they would, but I don't know. The ones in that picture are mounted where my jets are, so I would have to mount them further out, which might reduce how well they work?

FWIW, I found that I'm missing some sealant along the intake grates for my pumps. Supposedly this can cause a bit of cavitation, so I'm going to pull those apart and reseal them over the winter. I'll probably blast the paint off of them and have them powdercoated, since they have a little bit of corrosion starting to form.

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In need of car.