What is the best Cruising speed for my twin 4.3l

[Billhaskin]

I just bought a 1988 sea ray 270 sundancer
and was wandering about fuel Consumption.
What r.p.m. I should cruise at to get the best g.p.h

 

[rcon]

As low as possible without falling off the plane.

If you're looking for best mileage, it's probably a touch fast just before you're "leveled out" on the plane.

 

[kaz911]

While it might be true the best speed for low GPH is the lowest before falling of the plane - it might not be true for "Best Miles pr. Gallon" figure - so not the most efficient way of "getting there" as rcon writes. But...

Most energy efficient speed is < hull speed. So for your boat a speed estimate below 7-7.5 mph.

But for on the plane - My boat MPG figure is just about flat from 3300-4200 RPM - So that is what I travel with. But get a fuel flow gauge - that can show you MPG figures. Then you know for certain. Each boat is different - and fuel consumption is dependent on mostly RPM - where boat speed is dependent on hull and load.

 

[KC_Sundancer]

While it might be true the best speed for low GPH is the lowest before falling of the plane - it might not be true for "Best Miles pr. Gallon" figure - so not the most efficient way of "getting there" as rcon writes. But...

Most energy efficient speed is < hull speed. So for your boat a speed estimate below 7-7.5 mph.

But for on the plane - My boat MPG figure is just about flat from 3300-4200 RPM - So that is what I travel with. But get a fuel flow gauge - that can show you MPG figures. Then you know for certain. Each boat is different - and fuel consumption is dependent on mostly RPM - where boat speed is dependent on hull and load.

I agree, just above falling off plane is not the most efficient for us in our 270DA with V6's. It may seem like I am using less fuel at GPH but the decreased speed has us running much longer to get to the destination so fuel use ends up more. I try to push 30 to 32 mph on the GPS which has the engines running just below where the secondaries on the carbs are opening. Our overall fuel use is much lower like that. I do not have actual figures but based on the pocketbook it works. Fuel flow gauges are going in this summer so exact numbers can be followed to prove this out and or improve. Another thing that I find helps is using the trim tabs to help shorten planing time when getting up to speed and heading out.

 

[boater4life]

This may help: http://www.hurleymarine.com/CruiseFormulaWeb.pdf

 

[rcon]

I guess I should have asked him to define cruising first, I assumed something faster than hull speed

 

[joalche]

I agree, just above falling off plane is not the most efficient for us in our 270DA with V6's. It may seem like I am using less fuel at GPH but the decreased speed has us running much longer to get to the destination so fuel use ends up more. I try to push 30 to 32 mph on the GPS which has the engines running just below where the secondaries on the carbs are opening. Our overall fuel use is much lower like that. I do not have actual figures but based on the pocketbook it works. Fuel flow gauges are going in this summer so exact numbers can be followed to prove this out and or improve. Another thing that I find helps is using the trim tabs to help shorten planing time when getting up to speed and heading out.

What size props are you using? I have similar engines and my top speed on a calm day is about 30.

 

[kaz911]

This may help: http://www.hurleymarine.com/CruiseFormulaWeb.pdf

Their sheet is good but the instructions are wrong for a boat that planes at around 2500-3500 RPM. Everything in that range will be slightly distorted since it requires slightly more RPMs to get the boat ON the plane than keeping it there. I can keep my boat on the plane at 3000 RPM - but I can't get on the plane at 3000 RPM

Start in REVERSE - so @ 5000 RPM and work your way down... It is MUCH simpler and requires less "work" to keep it on the RPM - just remember to let the boat stabilize before recording numbers.

But if it matters for MPG/RPM calculations - I dont know

 

[JimFromMD]

I just bought a 1988 sea ray 270 sundancer
and was wandering about fuel Consumption.
What r.p.m. I should cruise at to get the best g.p.h

My Dad has a 1987 270. She holds plane at 3100-3200 RPMs and 22kts, but runs best around 3500rpms which is aroudn 25kts. She has new (10 yrs old) 4.3L mercuriser v6s with alpha drives.

 

[JimFromMD]

I agree, just above falling off plane is not the most efficient for us in our 270DA with V6's. It may seem like I am using less fuel at GPH but the decreased speed has us running much longer to get to the destination so fuel use ends up more. I try to push 30 to 32 mph on the GPS which has the engines running just below where the secondaries on the carbs are opening. Our overall fuel use is much lower like that. I do not have actual figures but based on the pocketbook it works. Fuel flow gauges are going in this summer so exact numbers can be followed to prove this out and or improve. Another thing that I find helps is using the trim tabs to help shorten planing time when getting up to speed and heading out.

KC what RPMS are you running to get 30-32 MPH?

 

[Billhaskin]

Thanks everyone for feedback. I checked the boat and it has a fuel flow
gauge in it. This weekend will be the second time I've taken it out.
Im every excited. Thanks again

 

[KC_Sundancer]

What size props are you using? I have similar engines and my top speed on a calm day is about 30.

I am running a pair of stainless, Mercury Marine High Five propellers.

Five Blade 19" pitch 13 1/4" diameter

KC what RPMS are you running to get 30-32 MPH?

3700 to 3900 RPM range. With the high profile the it changes with head/tail winds.