Which way for a little more top speed?

[barman84]

54 mph at 4800 rpm wot

Prop Size?
21p 14 ?
OEM Volvo prop

My redline per manual is 5200.

This was GPS tested lastnight on smooth water. It is an H203 Four winns with a 5.0 260hp motor.

I would love to step up to a four blade but not sure pitch and diamter.

 

[Philster]

Re: Which way for a little more top speed?

Step up to 4-blade = little bit less top speed

If you stick to three blades and step down one level in pitch, you are gonna be at max operating RPM at WOT and a couple of MPH faster if it works out just right (no guarantees on top speed). You could have the current prop tweaked by a prop shop and they can add a couple hundred RPM to your WOT runs, so that you are in the middle of the max operating range (so that when you are loaded up, you aren't lugging the engine).

Umm... important clarification: You're taking SS, right?

 

[steelespike]

Re: Which way for a little more top speed?

I can't be too specific about your setup.But a 4 blade is not the selection for more top end.Generally a 4 blade improves low to mid range performance.Stays on plane with less power,handles better at the dock.But they generally produce slightly less top end.You have a 20ft boat that will do 54 mph.You'll have to tell us about An H203. When your over 50 and you appear to be propped right it will be difficult to gain much.Other than being sure in excellent tune,excellent prop condition,Throttle opening all the way,I'll leave prop suggestions to the more expert members.

 

[barman84]

Re: Which way for a little more top speed?

Step up to 4-blade = little bit less top speed

If you stick to three blades and step down one level in pitch, you are gonna be at max operating RPM at WOT and a couple of MPH faster if it works out just right (no guarantees on top speed). You could have the current prop tweaked by a prop shop and they can add a couple hundred RPM to your WOT runs, so that you are in the middle of the max operating range (so that when you are loaded up, you aren't lugging the engine).

Umm... important clarification: You're taking SS, right?

By SS I guess you mean the 200 ss by four winns. Nope just a simple 203 they no longer make them stopped in 2009. Same haul as a 200 though. I might try the 20p and see what happens.

 

[QC]

Re: Which way for a little more top speed?

step down one level in pitch, you are gonna be at max operating RPM at WOT and a couple of MPH faster if it works out just right (no guarantees on top speed)

Most data shows higher speeds propped towards the bottom of the RPM range. i.e. 21 at 4800 faster than 19 at 5000 . . . If it were me, I'd spend time maximizing trim capabilities by shifting weight aft. If she porpoises I'd add trim tabs, but if you cannot make her porpoise now at WOT and full trim then I would try to get there. Not a good thing, but faster just as she begins to Porpoise. Also, If you don't have a spare, you need one

 

[45Auto]

Re: Which way for a little more top speed?

I know you know it QC, but for everyone else's benefit, the reason that they're usually faster at 4800 RPM is because that's where the maximum POWER is.

For example, notice on the GM dyno chart below that the 5.7L makes maximum power (292 HP) at 4800 RPM. Doesn't do you any good to prop it to a zillion RPM, power is dropping off above 4800.

The 260 HP 5.0L shows a max power (253 HP) at 4800 RPM. It looks flat between 4800 and 5000, so hopefully your speed will at least stay the same with the smaller prop. You can look at the dyno chart here, I didn't feel going to the trouble of copying it:

http://www.gm.com/vehicles/innovati...specialized/marine/REV_F_2011_5000_Marine.pdf

 

[Philster]

Re: Which way for a little more top speed?

SS was about stainless steel. If you are talking about a SS prop, all good.

I suggested tweaking the current prop because there are no guarantees or real benefits to jumping way up in RPM by a whole step down in pitch. As the prop nazis have indicated, the jump up in RPM doesn't necessarily mean more MPH. Might mean less.

Is your current prop cupped? Need to know more about the current prop. If she is basic, non-cupped, you might get better all around performance from going down 1 step in pitch, but getting a prop that is cupped. You might see 100-200 more RPM and save/gain speed/bite and ability to trim out more and avoid some ventilation.

The theme that is developing here is that there are no magical/drastic things to expect.

 

[QC]

Re: Which way for a little more top speed?

I prefer Engine Nazi . . . WTF??

With that noted . . . Propellers are 37% Hydrodynamics, 27% Art, 19% Luck, 24% Mathematics, and the rest is Voodoo . . .

 

[Tail_Gunner]

Re: Which way for a little more top speed?

I know you know it QC, but for everyone else's benefit, the reason that they're usually faster at 4800 RPM is because that's where the maximum POWER is.

For example, notice on the GM dyno chart below that the 5.7L makes maximum power (292 HP) at 4800 RPM. Doesn't do you any good to prop it to a zillion RPM, power is dropping off above 4800.

The 260 HP 5.0L shows a max power (253 HP) at 4800 RPM. It looks flat between 4800 and 5000, so hopefully your speed will at least stay the same with the smaller prop. You can look at the dyno chart here, I didn't feel going to the trouble of copying it:

http://www.gm.com/vehicles/innovati...specialized/marine/REV_F_2011_5000_Marine.pdf

above is a good example of marine motors and there cam shafts..very weak at the top and unpredictable sometimes.... combine that with " Propellers are 37% Hydrodynamics, 27% Art, 19% Luck, 24% Mathematics, and the rest is Voodoo"

It could be quite possible that 21 is just floating at 4800 Your getting good performance now It could be very possible that a 21p ss 3 blade would get the same rpm create some lift and gain the speed your looking for

 

[Philster]

Re: Which way for a little more top speed?

I prefer Engine Nazi . . . WTF??

With that noted . . . Propellers are 37% Hydrodynamics, 27% Art, 19% Luck, 24% Mathematics, and the rest is Voodoo . . .

I love the prop forum. 45Atuo. Tailgunner. QC. Love getting them into it and providing the hard details.

Props are like women, too. Get the right one and she can lift your bow. Spin her too fast and you'll ventilate her. Get one too big and she'll slow you down. And no matter how perfect she is, you'll always believe you can find one better. Some of us will even swap out a perfectly good one for an untested one. And, many of us keep backups.