Low RPM at WOT?

TheNibbler · Joined: 11 Oct 2024 Posts: 3

I just took a 2008 C dory 22 cruiser for a sea trial yesterday. It has 2008 twin Honda 40s which I assume are original, with around 450 hours on them. At WOT rpms were around 3,500, speed was around 20 knots, flat conditions. Is this Ok for these motors?

DayBreak · Posted: Fri Oct 11, 2024 7:59 pm Post subject:

Those RPMs seem awfully low for those motors. Check the specs, but I would think those motors need to be running at about 5500 RPMs at WOT unless you had an incredible amount of weight on that vessel. Those RPMs will definitely be lugging the motors and will cause damage in the long term. Sounds as if you need lower pitch prop. Others will chime in, but those RPMs need to come up quite a bit.

clayhubler · Posted: Sat Oct 12, 2024 1:13 am Post subject:

Yes, those RPMs are too low. But something doesn’t quite add up. My top speed is less than 20kts at 5500 rpm’s and only if lightly loaded.

C-Wolfe · Posted: Sat Oct 12, 2024 1:29 am Post subject:

I will second Clay, without knowing all the detail, 20 knots doesn’t seems too bad, but not at 3500 rpm. with my twin 50s, I will be in the 4200-4500 for a 20K speed depending on conditions. What did the owner think of those number? It could be something wrong with the gauges.

tsturm · Posted: Sat Oct 12, 2024 5:04 pm Post subject: Re: Low RPM at WOT?

TheNibbler · Joined: 11 Oct 2024 Posts: 3

Thanks for the feedback! The sea trial was with the broker, I didn't get to talk to the owner. Im new to outboards, having come from a sailboat with diesel inboard. We were playing with the trim tabs and motor tilt at the time, maybe that could have been the issue? I'll delve into it some more.

thataway · Posted: Sat Oct 12, 2024 10:41 pm Post subject:

Yes, trim makes some difference. The broker should (not always the knowledgable, but should know how to properly trim. I start with motor and tabs all of the way down. (The C Dory 22 should have trim tabs (and Permatirm foil). Then as the boat comes up on a plane, the motor and tabs need to be brought to "neutral"
positions. Then play with the trim on the outboard--usually about neutral will be best speed.

Or the boat was over propped. What size props were run?
The boat should be close to 30 mph at full speed/WOT.

TheNibbler · Joined: 11 Oct 2024 Posts: 3

I will take a closer look at props next time I can see the boat. Its the size information stamped on the prop somewhere ?

T.R. Bauer · Posted: Sun Oct 13, 2024 6:38 pm Post subject:

I do not trust that tachometer at all. And yeah you should get close to 30 mph out of it running full throttle. I was a bit surprised with mine the other day when I hit 30 mph with my wife and two other very large and heavy passengers.

thataway · Posted: Mon Oct 14, 2024 3:29 pm Post subject:

There are several places where the prop brand type, diameter and pitch are located--on the hub between the blades, on the inner or outer face of the hub--if it is not visible on the hub or back surface, then pull the prop, and it should be on the inner face. Also the material of the prop is important. SS is most rigid. Then Aluminum and finally composite plastics. A certain amount of flex will have an effect on how the prop behaves and top speeds.

Also one needs to be sure the bottom is absolutely clean. It is amazing how much speed will be reduced with a minimal amount of marine growth on the hull. Also water trapped somewhere in the hull--for example under the cockpit floor the 22 or 25.