Question about boat speed under power (Tartan 31

[Ralph Johnstone]

2800 is way, way off, no?

No.

2800 RPM (78% of max) is the recommended cruising speed for a 2GM20 rated for a max of 3600 RPM. Diesels are rated at approx. 80% max for cruising RPM. This engine is more likely the 2GM20 which was introduced in 1983 making a 1989 boat unlikely to have the 2GM.

If the thing is supposed to be in the 34-3600 range,

This is not the operating range of the 2GM20. Operating the engine at this sped MAY lead to premature wear. 80% is optimum RPM.

 

[Mikem]

My son had a H306 with the 2gm. It did 6kts all day long. My daughter had a Catalina 30 with the 11 hp universal and a Martec 2 blade folding prop and it would do 4.8 kts all day long as well…both boats slack water by GPS. Get a GPS reading in slack water to establish your baseline.

 

[Tally Ho]

I have an O’Day 322 with a 2GM20F. I am on the Great Lakes, so no currents to deal with.

I run the engine around 2,800-3,000 RPMs that pushes the boat around 6+ knots.

Greg

 

[jaysweet]

Thanks to all for the quick and informative responses. Let me answer a few of the questions I should have included in my original post.

The bottom and prop are clean as the boat was just launched 3 weeks ago after new bottom paint
The speed of 4.1 knots I referred to is boat speed from a mechanical knot meter and not SOG from the GPS
The engine is 30 years old with 1400 hours
RPM at wide-open throttle is 3100 RPM
The prop is two-blade and folding

I have a 30-ft Hunter (306) with a 2GM20F with a fixed 2-blade prop, and our RPMs are exactly as you say: I tend to cruise at 2800, sometimes 3000, and rarely, 3200 (but not for too long). With a clean bottom, in slightly choppy water, I'm doing 5.4-5.5 knots, GPS-measured at 2800 RPM. My guess would be (given the age of your Tartan) that our boat is less streamlined than yours, with a broad stern. So I think you should be doing better than we are, or at least the same, not worse.

The folding prop may be part of the issue: I'm not qualified to have an opinion.

Just to remove the engine/prop from the picture, last Saturday I was doing 6.1-6.3 knots under sail in 15-17 knots apparent wind. So that tells you something about our actual hull speed, which is probably 6.6 or so, I'd guess.