Beneteau 36cc motor cruising speed

DougM · Jul 30, 2014

To start with, the maximum theoretical hull speed is calculated using the value of 1.34 x square root of the waterline length.
I know the waterline length of a 36 foot boat isn't 36 feet, but it's an easy number to illustrate. So the max hull speed would be 1.34 x 6 = 8.04 knots.
That would be under perfect conditions, no current, no friction, etc. Cruising speed would certainly be less.

For argument's sake assume the max hull speed on your boat would be somewhere around seven knots assuming a 32 foot waterline length and a fairly clean bottom To get there, even at maximum RPM, the first thing to consider is propeller pitch. If you are cruising at 2800 RPM an under pitched prop may not get enough bite to go any faster than your indicated speed. However, the same prop may be over pitched for the recommended RPM for your engine. It's a crap shoot. in my opinion 2800 is not a bad cruising RPM. I would suggest that around six knots would be about all you could expect. Others may differ.

Other considerations:
Is your tach reading correctly, or are you actually making less than the indicated RPM?
Is your knot meter, and/or your GPS speed over ground reading correctly?

MSN-Travelers · Jul 30, 2014

How are you measuring your speed?

In theory, hull speed for your boat just under 7.5 knots at full throttle (I believe 3,200 RPM). This assumes flat water and clean hull.

I have a 352 built at roughly the same time as yours and I also have a 3GM30f with a factory fixed 3-blade. We cruise regularly at the same throttle setting (2,750-2,800). I'm going to assume your tach is pretty accurate.

Here is where the same question carlager asked comes into play. Is your referenced speed from the boats knot meter (speed thru the water) or GPS (speed over the water). There is a good chance that boat's knot meter reading is suspect. Crud on the paddle wheel, weak magnet, currents all affect that meter reading. GPS is a better measure of actual speed made good.

Having said this, my knot meter always reads less than GPS. My boat's waterline is 1" less than your and about 1,200 pounds lighter. My 352's hull speed is about the same as yours. We regularly do 6.25 knots (GPS) at 80% throttle motoring in favorable conditions. Motor sailing we gets us an extra .2-.4 kts. Following seas often let us surf well above hull speed. Last Monday we were motoring in 1-2' waves with follow seas on the quarter and were moving between 7.2 & 7.8 kts.

Your speed, if measured by the boat's knot meter, may be correct. If by GPS, it does seem to be a little low.

marian · Jul 30, 2014

Re: How are you measuring your speed?

My 351 with 3gm30f engine, at 2800 rpm, boat speed is 7 knots in good calm condition, by gps.

Marian

donvanv · Jul 31, 2014

I have a 36cc and seem to get about 6.5 kts at about 2500 rpm. Max rpm on my engine seems to be around 2800. However backing it off to 2000 still gets me about 6 kts, which is what I cruise at.

woodster · Aug 1, 2014

carlager said:
The speed I am referring to is speed in the water and NOT speed over Ground (GPS speed) . I have calibrated the knot meter and am pretty sure it shows the correct speed

We do not have any other 36ccs to compare with. I would however assume that the boat should be able to do at least 1 knot more ( i,e 6,5 knots) at 2.800 RPM.

could you have been running with the tide or current when you took this reading

or against it

donvanv · Aug 1, 2014

I have the 3GM30F. I just purchased in March. Survey says HP 27 @3400 RPM. I am still trying to work out some issues like shaft vibration and hard starting. Previous owner had a Raycor 2 micron filter on it which I mistakenly replaced with another 2 micron. I think that may be restricting fuel flow causing hard start and perhaps limiting top RPM. I have a 10 micron but Just haven't changed it yet.

Ron20324 · Aug 1, 2014

donvanv said:
...Previous owner had a Raycor 2 micron filter on it which I mistakenly replaced with another 2 micron. I think that may be restricting fuel flow causing hard start and perhaps limiting top RPM. I have a 10 micron but Just haven't changed it yet.

That would be interesting if you'd note, please, the before and after idle and/or max RsPM to share with us.

Kevin13745 · Aug 4, 2014

I have had a 36 CC for the past 13 years. When launched, the speed is far better than mid season, I realize that in the Chesapeake with water temps near 80 deg the marine growth can be quite somethjing, knowing than, I usually use a Dri-dive to clean the hull, but can't get to the keel. Also have the 27 hp and usually cruise at 2750 which will yield about 6.3 knots when launched, 5.7 to 5.9 at mid season (knot meter, not GPS) due to the growth on the keel. Given a head wind and 1 to 2 ft waves, speed will ofter drop to near 5.2. We do carry a RIB on davits, when towing we generally will lose also a knot in boat speed, regaining the lost speed when the RIB is carried. Can't say I have ever seen 7 knots under power, but have surfed down some seas at over 9 knots (again knot meter and not GPS0, although the latter has given readings of 10 + knots during that time). I suspect that your speed is a tad slow but not unreasonable based on my experience on the Chesapeake. One possible factor which might be relevant would be the calibration of your meter. Have you run a trial in flat water to verify the setting?

Regards,

Kevin
Esprit Irlandais 36 cc

donvanv · Aug 5, 2014

Just put on a soft coupling between the transmission and drive couplings. Had to test it at max rpm and noted that I got a good 7 kts on smooth water at 2900 RPM. Regarding the soft coupling; I do get significantly less shaft vibration than before, much smoother running.

zzmeyer · Aug 11, 2014

Knot meters are notoriously wrong (calibration etc). Use GPS going both directions for speed. Next, run your engine at full throttle. It should reach rated rpm which I believe is 3400 rpm (if it doesn't either the tach is wrong or you are over or under propped. At 13,200pound displacement you are slightly underpowered. Your waterline length is 31' 2". In fairly flat water you should be seeing about 6.5 at 2800 rpm.