F32 SHAFT RPM AT FAST CRUISE

DAVIDLOFLAND · Post by **DAVIDLOFLAND** » Thu May 17, 2012 12:06 pm

Before I start researching re-power options, I am figuring paramaters for new engines and tranny's. Forum consensus established that angle change at transmission should be about 10 degrees, for level install of engines.

I want to fast cruise at about 25 knots. Currently, to do that she's turning about 3600 rpm at engine, through 1.5:1 trannys, so assume shaft RPM to be about 2400. Current props are 16 x 18, (which are a little too steep for my current horsepower).

Will need to determine best cruise RPM of new engines, then apply the correct gear ratio to achieve the desired shaft RPM. Then, I can dial in the final package with prop pitch.

So, two questions for F32 owners...

1) Do you agree with 2400 RPM for a 25 knot fast cruise?

2) Do you find the boat seems to get a little 'squirrely' above about 27 knots?

rossjo · Post by **rossjo** » Thu May 17, 2012 12:31 pm

Join http://BOATDIESEL.COM for a great Prop Calculator that takes water-line length, hull-type, weight, gear-ratio & power into consideration (doesn't matter if its diesel or gas, it goes by HP & RPM). It is very accurate.

My F32 with twin Cummins 6BTA diesels runs 27-28 knots, and is only 'squirrely' in 2'+ following seas (which its 'squirrely' at any speed anyway):

http://rnr-marine.com/Trojan/Viva-Mahia ... 0mar08.MP4

DAVIDLOFLAND · Post by **DAVIDLOFLAND** » Thu May 17, 2012 12:37 pm

Rossjo,

Can you determine your shaft RPM at about 25 knots?

Stripermann2 · Post by **Stripermann2** » Thu May 17, 2012 1:38 pm

The boat diesel site is good. Can't remember their site fee though...$25.00 or something around there.

rossjo · Post by **rossjo** » Thu May 17, 2012 2:40 pm

DAVIDLOFLAND wrote:Rossjo,

Can you determine your shaft RPM at about 25 knots?

Totally different setup - diesels have a lot of torque, so I'm running a Velvet Drive 1:1. WOT about 2,400 RPM = 27-28knots., so 25 knots is about 2,200-2,300 RPM.

Running 17x20 props. This combo gets great fuel mileage at hull speed. I'm doing about 8 knots at 600RPM idle. Makes for fast docking.

$25/year for BoatDiesel.com - well worth it. The site owner (Tony) is a very knowledgeable diesel & boat mechanic and can answer anything if others don't already have an answer. The Power and Prop calculators alone are worth the $25.

Stripermann2 · Post by **Stripermann2** » Thu May 17, 2012 3:10 pm

Which brings to light... that this forum is also a great deal at only a $15.00 supportive contribution for all the knowledge, help and insight given each day!

Of course, it's voluntary.

prowlersfish · Post by **prowlersfish** » Thu May 17, 2012 8:21 pm

Boatdiesel is a great site and Tony A. is very sharp one of the best on Cummins and Yanmars around. However He is not the ownerof the site . The site is owned by Peter Compton .( Compton marine in St. Tomas VI)

Tony owns seaboard marine in Ca . I get a lot of Cummins parts from Him.

Commissionpoint · Post by **Commissionpoint** » Thu May 17, 2012 9:37 pm

According to my surveyor I am swinging 16x21's. Now, before the peanut gallery goes off its rocker, I don't beleive it either. Either way with the super quick handling around the docks at 650 in gear I feel that probably they aren't the 16x15's I am used to on these F32 hulls. (with gas in the 230-300hp range). Good news is its on the hard for paint, glass and some other BS I wanted to do, so I'll go take a pic or two of the running gear while its in the shop for posterity. Saturday should be good because they won't be spraying and the shop should be mostly idle. I heard this morning that it got moved inside, so it should be only a couple days or so until its done.

Nancy · Post by **Nancy** » Thu May 17, 2012 11:04 pm

Michael,

You made me go look at the paperwork. We had the props scanned and tuned in 2003 by St. Lawrence Propeller. They're 16x15. I have the printouts if you want them.

Nancy

Struts and Rudders · Post by **Struts and Rudders** » Fri May 18, 2012 9:08 am

I would think the investment in a flash tach would be a good idea.
You apply a small piece of reflective tape to the rotating mass and aim the light at the mass and the device will provide you with an accurate shaft speed.
Commonly used in marine business.
Not expensive

SRD
F36

DAVIDLOFLAND · Post by **DAVIDLOFLAND** » Fri May 18, 2012 12:05 pm

Rossjo,

I thin that puts us in the same ballpark at 25 kts. You're 2,200- 2,300 with 17 x 20's. I'm 2400 with a 16 x 18's.

I'm doing the computations for gear ratios, knowing the horsepower needed (approx 300 hp gas), and prop diameter (16"). I intend to check with the engine manufacturer for optimum cruise RPM, then apply gears to achieve a shaft RPM (at optimum engine RPM) in the 2400 range.

Seems to make sense, I think, unless there's something I'm missing.

rossjo · Post by **rossjo** » Fri May 18, 2012 12:32 pm

Yes, you're probably close, from a shaft RPM & HP perspective. The 1.5 ratio is getting you the torque multiplication on the 300HP to turn that 18p prop ...

BoatDiesel.com will be a great help.
It will actually recommend optimum gear-ratios and diameter for max efficiency/speed.

DAVIDLOFLAND · Post by **DAVIDLOFLAND** » Fri May 18, 2012 12:42 pm

Thanks Rossjo,

You have been very helpful. This forum is such a great resource.