Prop Size Pitch and Slip

"I probably can find out soon

"I probably can find out soon enough by taking my props to a shop and get some advise; but many of you may help me with this quest.

If you change from say a 17" pitch prop to a 19"pitch prop with a slight cup; then, at low RPM's such as 650-1000RPM, would there be much increase in speed. At 650 RPM, I already idle at 4.1MPH. (makomark?..where are you) "

"Going up 2" in prop pitch

"Going up 2" in prop pitch and switching to a cupped prop will likely increase your speed at 650 RPM and you will likely get a better hole shot. BUT, it will likely drag down your WOT RPM's several hundred RPM's, possibly as many as 400 - 500. You want to keep your WOT RPM's at 6000 +/- 300."

"I will assume you are running

"I will assume you are running a 3 blade prop (??)

If the 17 pitch is running well for you currently you could switch to a 17 pitch 4 blade (or even the "hi-5" blades) which will give you better "bottom end".

Alternately, moving up in diameter will have a similar affect, it is just less understood and there is far less data on the actual effect.

The extra blade only shaves 50-100 rpms off your max and if you go with a cupped model, maybe another 50 or so.

It should give you a little more speed at comparable rpms (although the motor will work a little harder to turn the extra blade(s), thus burn more gas in the process as well).

While I interchange my props depending on the type of boating I`m doing on a given outing I have never compared the speed differences between 3 or 4 blade, cupped or un-cupped for a given rpm other than at the top end so can`t say if the change would be minimal or even noticable..."

"You will go faster with more

"You will go faster with more pitch at idle, especially with those injected engines that will maintain an idle rpm. The cup will not make any difference at idle. How much will be driven mostly by the wetted area of the hull and the load you place on it.

I doubt you will make the 'hole shot' any better with the pitch increase & that WOT figure may be NA."

"OK here's more of the POO

"OK here's more of the POOP! Manuf suggested WOT is 4600 on 2003 Mercruiser 8.1L S MIE. I am now at 4985 RPM; 33.1knots; 17 x 17 Federal 3-blade props. Tranny is Zurth direct with 1.51 ratio. As you see I can afford coming down on the WOT. The boat originally ran with 310HP carburated engines; now I'm running 370hp engines with MPI. I got HP to spare. Dry wgt is 11,500 but my DATA above is based on loaded gas, 4 people and gear at approx. 14,700lbs. YES! I don't want any more speed at idle, but is there a reasonable way to UP the crusing speed at get to the FISH quicker? Need to add that the props sit in a half tunnel and I do not believe I can increase to a 18" diam. prop."

"is 4600 the middle of the WOT

"is 4600 the middle of the WOT band (it is usually given in a range)?

So you repowered and didn't change the wheels? Check with the hull mfg on the diameter being max'd out?

The gear is either a HURTH or a ZF; never heard of a Zurth.

What rpm do you normally cruise? How fast is that? How fast would you like to go?

If cost isn't an issue, a helicopter will get you there as fast as anything else."

"ENGINE RANGE IS 4400-4600; SR

"ENGINE RANGE IS 4400-4600; SR used 17x17C with the 7.4L, 310 HP; Sea Ray used a 18" diam prop for the diesel option running at 2800, I believe 18" on gas engine higher RPM may cause cavitation in the tunnels.
Manuf of tranny is ZF Hurth Marine Model ZF630A; direct drive.
I cruise (LOADED) at 18kts/21MPH @ 3400RPM; (at half gas and 2 people, I can still maintain 18kts but @ 3000-3100RPM) Approx. 24kts @ 4000RPM (LOADED)and 29kts @ 4750RPM (LOADED); top WOT now is 33.1kts @ 4985RPM; (WOT = 385RPM over mnf recommend)

YEAH! I'd like to UP my cruising speed a few knots but don't want to idle speed any faster; IS that possible?

Also FIY:
Engine produces 370HP @ 4000RPM; with 502 torque ft/lbs."

It ain't cheap but you can check into trolling valves for the ZF's....last time i looked they were ~1500/ea for the bigger gears. you can dial down the troll speed to whatever you desire. Also, ask for the 'electronic smart box', the one that closes the valves if you move the throttle. They will pay for themselves the first time you exercise them.

If you wanna spend more money, I've heard of, but never seen, gears with two forward speeds.

Maybe the prop shop has another idea?

Interesting! Didn't know about the trolling valve alternatives. I have a good repor' with the Hurth Service Tech and he can advise me. Meanwhile, your input is always appreciated.

http://www.perfprotech.com/store/product/Trolling-Valve-ZF63A-3312199009,65151.aspx

At least it is a viable lead. I've dealt with them in the past but not recently. Tech support was good (knowledgeable) but response was a little slow.

If you go this way, you'll still want to add some pitch. These things will let you crawl if necessary - perfect for striper trolling in the winter.

The valves bleed off (lower) the pressure of the oil being applied to the clutch plates. This allows a controlled amount of slipping (slowing) to occur. Engine speed is capped at 1000 rpm during valve operation so the clutches don't burn up/warp. The "smart box" eliminates the operator error by ensuring the valve returns to "normal" before allowing the engine's speed to change whenever the throttle is moved. (suspect this is easiest to add to the "full electronically controlled" engine packages; may only be available for this installation, too.)

hi folks- i damaged my 17p prop on a sand flat and im wondering if i have to stay with a 17p. i dont know anything about props or much about boats, so i need alot of advise. the 17p worked fine but i found that i had to have her wound out to get much speed, can i go to a different pitch and get more speed? thanks alot guys....cal ps- my boat is a 23ft sunray cierra with a 470 merc

How about telling us what kind and size of engine and the type set up you have - inboard, inboard/outboard, or outboard? What is the prop diameter and what was the performance of the old 17P prop in terms of rpm's at full throttle (WOT)?

In general, a lower pitch will give you higher rpms. Some engines have an RPM limiter that will not allow you to exceed a certain WOT threshhold no matter what size prop you have. If you are talking about an outboard, at WOT you will want to be running in the middle to upper limit of the manufacturer's recommended maximum rpm's.

I think he's at the point of diminishing returns already - a 470 pushing a 23 ft anything isn't gonna be 'fast' by any standard. I'd be more inclined to have the prop that provides the recommended WOT and stop there. If that doesn't provide enough speed, get another boat.