Cruising RPM range... just to be sure...-gctid377560

[Guest]

I'm picking up my "new to me" boat on Saturday and will be traveling over 90 miles.

I've got all the extras I can think of: belts, fuel separator, gps, vhf, impeller, etc. and while I plan on taking it easy I'd like a quick reminder on just how high I should be able to run the RPM at cruise.

At the sea trial the engine hit 4400 easily, with a little room to spare (like, maybe, it could have hit 4500). So what is max cruising RPM with this new set up, assuming I would sustain it for 5 hours?

thanks.

[MidnightSun]

The lower the better but I would say you will most likely want her around 3500.

[Guest]

What he said. About 70% of WOT is pretty comfortable for an engine.

[Guest]

telebob wrote:
What he said. About 70% of WOT is pretty comfortable for an engine.

Are you sure?... that would put me at under 3100 RPM. I've always heard that on the SB 350 with a WOT of 4400 you could cruise 3600-3800 all day long. I wasn't sure if it would be the same for the big block, but I can't imagine that 70% is correct.

[JimMc]

Get up on a comfortable plane. Find the minumum RPM you can stay on a nice plane and then go up about 10%. See where that puts you.

You don't want to be pushing water and not pushing the motors.

--

On our 305 we plane at 3100. WOT is 4400.

Between 3400 and 3800 my fuel efficiency MPG is pretty constant and the cruise is very comfortable.

[Guest]

JimMc wrote:
Get up on a comfortable plane. Find the minumum RPM you can stay on a nice plane and then go up about 10%. See where that puts you.

You don't want to be pushing water and not pushing the motors.

--

On our 305 we plane at 3100. WOT is 4400.

Between 3400 and 3800 my fuel efficiency MPG is pretty constant and the cruise is very comfortable.

So I guess it doesn't matter, big block or not? This motor can hold plane at 3200 so I'm probably in the same range as you. It's also go the the flow meter so I'll keep an eye on that also.

[Guest]

My 32 with twin 305s has a top end of 4200 rpm. We run at 3500-3600 and get 16-17 kts at that rpm. Anything less and it starts to fall off plane and push water. I can tell by listening to the engines that they are running at their sweet spot...... not straining (low pitch) to push the boat and not winding out (high pitch). It shouldn't take you long to get a feel for the new boat.

Good luck and safe travels.

[2850Bounty]

JimMc wrote:
Get up on a comfortable plane. Find the minumum RPM you can stay on a nice plane and then go up about 10%. See where that puts you.

I think that this is your most reasonable answer. On plane, and a tad bit more.

You'll reduce hull drag and engine load.

3,800 rpm and a light load, is far better than 3,200 rpm and a heavy engine load caused by hull drag from not being quite fully up on step.

Like Jim said...., see where that puts you.

You may want to consider installing that new impeller prior to this trip.

I see that you made a well deserved change to you BOC signature! :coo-

.

[MidnightSun]

You have a single on a Contessa = big boat. Some have posted 3200 rpm but this is with twins. You will be looking at 3500 or a couple hundred more to get a nice economical and safe planing speed.

[Download_Complete]

I am VERY curious to hear what she will top out with a 7.4 and a DP.

[Guest]

Not to deviate from your question, but a second thought would be: be sure of the height of your boat. When I was taking my new one to the slip I cleared a bridge by about an inch. That would have sucked.....

[itsabowtime2]

I think some of this will be trial and error.

I would first determine the minumum RPM's you can hold plane at. From there bump up in 100 RPM increments until you find the sweet spot for speed and efficiency.

[Guest]

Through hull exhaust with fiberglass mufflers and through hull raw water intake

Earplugs should be on your list......

[2850Bounty]

MidnightSun wrote:
You have a single on a Contessa = big boat. Some have posted 3200 rpm but this is with twins. You will be looking at 3500 or a couple hundred more to get a nice economical and safe planing speed.

I don't know, Hans! This new boat of Mike's has both the BBC and the Duo Prop drive. We don't see this combination very often here on the BOC.

I'd have to say that most here would be SBC and single prop drives... and after 19??, BBF w/ KC single prop.

Hard to say... it will be interesting to see what he finds out.

Pau Hana wrote:
Earplugs should be on your list......

Those, and a selection of neck braces, Pete! :kidding

.

[Guest]

Pau Hana wrote:
Earplugs should be on your list......

If the thru hull exhaust exists below the water line, additional noise from the OEM configuration shouldn't be a problem. I'm taking this route, and my exhaust exit will be located below the water line. It will be a foot or so above the outdrive's exhaust port, but it shouldn't make too big of a difference in the amount of noise. The big difference will be the gain in horsepower, since the engine isn't required to push through the backpressure created by the water.