WOT, efficient RPM's ?

Wood Zeppelin · Posted: Sat Aug 29, 2020 12:49 am Post subject: WOT, efficient RPM's ?

I apologize if this is a "dead horse" question. But what is the best cruising speed? When I'm WOT, ~5000 RPM's the boat seems to move with the least resistance. I saw ~8mph @ 3500rpm, and ~20mph at 5000rpm. But is running the motor at full speed (WOT) for several hours potentially bad for the motor?

Around 4500 rpm gave me ~16mph.

What is the most efficient speed/rpm? And if it's WOT, is that ok for the motor for several hours?

DayBreak · Posted: Sat Aug 29, 2020 11:52 am Post subject:

If you wanted to go the most distance for a tank of gas, it would be about 1000 rpm's and you would only travel at about 4 mph. With your present set-up, I would think that between 4000 rpm's to about 4500 rpm's would give you the most efficient cruise speed for travel and you should be well on plane. To run an outboard beyond 80% for long periods of time or beyond a few minutes puts additional stress and wear on your motor.

thataway · Posted: Sat Aug 29, 2020 12:02 pm Post subject:

to fully answer you question we would have t know what motor and prop you have.

5,000 RPM, is a little low for WOT on most outboards. It appears That you have a 45 hp Honda. The WOT should be 5000 to 6000 RPM. If that is all you are getting it could be lugging the motor. I would consider dropping the pitch 2" and going with SS prop. I agree thot you don't want to run WOT all of the time. You might change the prop--even different cup and rake would make some difference as well as diameter and pitch. (Also SS over Aluminum would make a difference).

Boat motor trim, & trim tabs make can make a world of difference in ride and top speed in a C Dory--so we have to know what the trim is at your WOT.

DayBreak is correct in his assessment.

What are others getting with speed and WOT with 40 to 50 Hondas? Also is this on the Cruiser or Angler?

T.R. Bauer · Posted: Sat Aug 29, 2020 1:53 pm Post subject:

I don't like running anything at 100% capacity for long durations. Everyone has given you good advice and your BF 45 has no issues with running 5000 rpms, but full throtle should be more like 6k, or even a tad bit more. At 5k RPM a BF 45 will only make around 37 hp. It has to reach full RPM to reach full HP. My twin 45s will run 6200 rpms on my CD 22 with a light load, but I always run them at 4-5.2k as they are very efficient there with around 2-3 GPH being consumed per engine.

The problem with full throttle, especially on carbed motors, is they start to dump too much gas into the cylinders and some of that gas get by the rings and into the engine oil. It won't wreck the engine as long as oil changes are regular, but it does probably increase wear. But, I'm not sure that is a huge concern either, because your BF 45s will probably corrode away before they actually have a mechanical failure if you take care of them. They are very good and simple little motors.

Anyway, I think you need a less agressive prop to attain around 6k rpm at full throttle with a light load. A good prop shop will steer you in a good direction on this and I will suspect they will tell you an SS prop will make very little difference on motors of this class, since it is a percentage thing. Then, after you have it propped for max RPM, 5k rpms will be about 80% load and you should only burn about 3 GPH there. At 6k you will be burning 4.5-5 GPH and that is all the BF 45 can burn. Good luck and I hope this helps. Lots of good advice by other posters too.

Wood Zeppelin · Posted: Sat Aug 29, 2020 7:33 pm Post subject:

It’s a BF40. 3 blade prop. New permatrim.

Yes, actually I was running closer to 5500 RPM at WOT. Up to 24 mph.

At 4500 rpm I was doing 17-19mph I think.

On a 16 cruiser, me and my camping and fishing gear. Also currently a Honda 9.9 as trolling motor (I know it’s a little big but that’s what I’ve got to work with right now)

So I guess I want to cruise along at 4500 RPM to be kinder to the motor.so I guess I want to cruise along at 4500 RPM to be kinder to the motor. It seems if I go to full throttle to get her on a plane and then slow it back down is the best way to go!?

thataway · Posted: Sat Aug 29, 2020 8:47 pm Post subject:

8 mph is a very inefficient speed for a 16. LWL about 13', Sq rt x 1.34= a little less than 5 knots is your max displacement speed.

We were asking about the diameter and pitch of our prop. If you get 5500 at WPT that is OK, you could drop the pitch down 2", but fine running at 4500. Be happy with that. The C Dory 16 Cruiser is a lot of boat for 16 feet and a 40 HP outboard!