Minimum Cruising RPM for a Carver 3607 Aft Cabin (Lake Crawler)

[Skydancer]

Looking for advice on minimum cruising RPM for a Carver 3607 Aft Cabin - looking for "Stately Lake Crawler Speed"

[km1125]

I have a 3607 with 350HP Crusaders.

Cruising speed depends on what we're doing. A lot of times I'll just get up on plane then back down a bit to where it just barely stays on plane. We can kill an hour or so for a nice cruise and not really notice the fuel gauges losing much of their readings. We'll be 12-14MPH around 2500 RPM, iirc.

If we're not in a hurry to get somewhere but are headed to a destination, then I'll push up around 3200 RPM which is around 18MPH

I've not done it for a while, but the fastest I've ever had the boat was about 28MPH at over 4200 RPM.

[Skydancer]

KM - Thanks for the response.
I'm not particularly interested in bringing the yacht on plane. Mostly what I want to do with it is something akin to "House Boat" speed (9 or 10 miles/hr). My concern was keeping the 454s at a low RPM for extended periods of "Stately Cruising"; and what -if any- concern there might be for running them that slow?

[tomschauer]

There is no problem running them slower. However, I think you will find at around 9 miles per hour or so the tail will start to "squat" and you will begin to plow water, in this case you will burn more fuel than if you were on plane.
So slowly creep up the throttles until it starts to squat, then back off a few RPM's until it comes back up. That should be your best "hull speed".

[Skydancer]

tomschauer - Thanks for the response.

I will experiment using your suggestions.
Is there a formula (rule of thumb) for calculating MPH from the engine RPM?

[Cooler]

Your engines are probably running at 800 RPM, when warm, at idle. There is no real problem with running them at 1000RPM, other than you may need to change spark plugs more often. 454's typically burn a disproportionate rate of fuel at low RPM's, so fuel economy, if it is a concern, would be better at a higher RPM. Had a friend that replaced his 350's with 4 cyl engines because he does not cruise at more than 8 - 10 mph. That is on a 34 Santego. He will have a challenge when he sells, but saving gas for now. er

[Skydancer]

Cooler - Thanks for the response

Good information!
Based on what km1125 said above, I guess I'll be running at something above 2k

Do you have any thoughts on where I might find replacement Rub Rails for my 1983 Carver 3607?

[bud37]

Welcome to the forum......my opinion is you can run all day at 1500 with no problems.

With these boats with almost a planing hull there are two spots to efficiently run, one on plane, the other with the hull in the water below max hull speed for a planing hull. That is something that can be calculated, check online. As Tom said around 6-7 knots would be about right. These basic rules are what I have always followed......

Just my two cents, good luck with your new boat..

[buster53]

Ok, hull speed is going to be around 7-8 knots. Anything above hull speed and below full plane is wasting gas. I'm willing to bet that I get better MPG's at 17-19 knots than I would at 13-15 knots.

FYI...hull speed is the square root of the water line X 1.333. So, a 40' boat probably has a 36' water line. Square root of 36' is 6. 6 X 1.333 is 8 knots. Your 3607 is a little less.

[Cooler]

Skydancer - for my Mariner, 2000RPM is kinda no man's land - plowing water and not enough for trim tabs to do their thing. I run at 3000 rpm most of the time. 18 - 19 MPH, and 15 - 17 GPH. You are heavier though, so maybe that RPM works best for you. Just experiment and you will find that sweet spot. You should be able to order rub rail from any decent marine store. They should have a big sample chain of 30 different types to pick from. Otherwise, if there is a boat salvage yard in your area, you could check that out. Trouble is, most salvage yards don't really save much in terms of value. Lower price yes, but bent up rub rails also. er