Engine rpm and speed

[Swanman]

Hi,

I have had my 350 for about 2 years now and am beginning to get to to know her.

The engine M35B is still a bit of a mystery to me, is mine a good one? Hard to know without seeing another one in action. I attach a picture as proof that it looks to be in good condition and I keep a close eye on it in the usual way.

I find it a little vibey and noisy but it starts 1st time every time and works, i just think that it should push the boat more quickly through the water.

So my question is around rpm and boat speed.

I know the speed varies depending on lots of factors, sea state, hull clean, prop type size etc.

However recently on a calm day I thought I would try and get some basic performance info. My boat has a featherstream prop, the bottom is pretty clean. I found that my maximum cruising rpm was 2500 to 2600 rpm before I started getting a little high temp chirping from the control panel next to the helm. In addition if I pushed it above that I could start to smell that the engine was getting warm from the cockpit, a coolant sort of smell! As soon as I brought it down to 2500 rpm all was fine again. Anyway the max speed I got through the water was 5.5kts. I find this quite low. How does this compare with other 350 owners is my question??

[MikeGovern]

2006 M35B with a 3 blade gori folding prop. I cruise at 2400 RPM, temp 160-170 and GPS reads 6 to 6.5 knots slack tide. Max RPM is 3000, but I don't leave it that high for more than a few minutes. I had the bottom painted three months ago, so pretty clean. If you are getting a temp alert at 2600 RPM, I'd check the heat exchanger for build-up.

If your max RPM can't reach 2900-3000 RPM, you prop may be too big/too much pitch which an cause a lot of problems. As you said, lots of variables...

[Swanman]

2006 M35B with a 3 blade gori folding prop. I cruise at 2400 RPM, temp 160-170 and GPS reads 6 to 6.5 knots slack tide. Max RPM is 3000, but I don't leave it that high for more than a few minutes. I had the bottom painted three months ago, so pretty clean. If you are getting a temp alert at 2600 RPM, I'd check the heat exchanger for build-up.

If your max RPM can't reach 2900-3000 RPM, you prop may be too big/too much pitch which an cause a lot of problems. As you said, lots of variables...

Interesting you are a knot higher with lower rpm.

[wolfe10]

I found that my maximum cruising rpm was 2500 to 2600 rpm before I started getting a little high temp chirping from the control panel next to the helm.

Anyway the max speed I got through the water was 5.5kts. I find this quite low. How does this compare with other 350 owners is my question??

Suspect you are "under-propped".

Easy to verify. With engine warmed up, briefly go to full throttle.

If the engine hits governed red line very quickly, you need more prop (more blades, more pitch, etc).

Ideally, full throttle will bring engine RPM up to within 150 or so RPM of governed RPM.

Yes, if max RPM at full throttle (with clean bottom, calm water, etc) is more than 200 RPM lower than governed RPM, you are over-propped. Many of us run slightly over-propped for more speed vs RPM, but you really need to be careful not to over fuel or over heat the engine. Black smoke from exhaust or engine coolant temperature rising much above thermostatic control means back off!