Boat Speed

[Deleted member 117556]

Was wondering what boat speed most people make in a Cat 310 while under power at a normal cruising RPM, ie: not at full throttle? If your normal cruising RPM is 2,400 then what boat speed will you typically make without wind and current? Am asking because I have a different engine than all other 310's and cannot get close to hull speed. My typical boat speed at the high end of cruising RPM (3,000 RPM for me) is ~ 5.5 knots. Have a 3 blade fixed 14" prop. Thanks.
bob

 

[c130king]

I routinely hit 6.3 - 6.5 knots at 2500 RPM. I ran at 2900 yesterday for about 3 minutes and hit 6.6 - 6.7.

I have M25 with 3-bladed prop. Shoal draft wing-keel.

Cheers,
Jim

 

[Long Story]

I'm Hull #52, and I see about 5.8 - 6.2 at 3000 RPM. Wing Keel, Fixed 3 Blade Prop, all linked to the standard M-25.

 

[Tom J]

I average over 6 knots at 2,500 RPMs; M25xpb, 3-bladed prop, wing keel. Which engine do you have? What is the pitch on your prop? Sounds like it might be a fine pitch, resulting in high RPMs and lower top speed.

 

[Deleted member 117556]

I average over 6 knots at 2,500 RPMs; M25xpb, 3-bladed prop, wing keel. Which engine do you have? What is the pitch on your prop? Sounds like it might be a fine pitch, resulting in high RPMs and lower top speed.

Tom,
I have an 18 hp Yanmar (2GM20F) and have determined that the prop & pitch is just about perfect, ie: 3,600 RPM max and in gear I can reach 3,525 RPM at WOT. Typically run at 2,900-3,000 RPM although maybe I should run harder?

 

[KZW]

Sail #307, M-25, wing keel, fixed three-blade. At 2600 rpm I see 5.5 to 5.8 knots. New ablative bottom paint in Jul 14.

 

[KZW]

The variation in replies makes me wonder how well the RPM guages are calibrated!

 

[Tom J]

Tom,
I have an 18 hp Yanmar (2GM20F) and have determined that the prop & pitch is just about perfect, ie: 3,600 RPM max and in gear I can reach 3,525 RPM at WOT. Typically run at 2,900-3,000 RPM although maybe I should run harder?

I'd say you are doing fine for 18 HP.

 

[LeoLambert310]

Normaly run at 2000rpm at 5 kts have 29hp yanmar 14" 3
blade

 

[Deleted member 117556]

Good point about tachometers. I measured mine with a digital instrument which is why I can read 2,530 RPM...

 

[quadrille38]

Good point about tachometers. I measured mine with a digital instrument which is why I can read 2,530 RPM...

Knot meter calibration can also be an issue.

 

[Long Story]

Knot meter calibration can also be an issue.

As can condition of the bottom, fresh or salt water, weight loaded of the boat, and how much canvas or enclosure you have. (wind resistance) Myself, I see very little change in boat speed between 2600 & 3000. Seems the boat "squats" down more at high RPM, getting the swim platform wet, but no real speed increase. So, I keep it backed off.

 

[tahomathundr]

I have a C309, same hull as a 310 with a fin keel and a 21 hp Yanmar 3-blade prop and I cruise at 2900-3000 rpm at 6.5 kts and max 7.0 kts in salt water at 3600 rpm. In freshwater I lose about .1 to.2 kts. My manual states that it should run with occasional max rpm periods to help keep soot deposits down.

 

[Deleted member 117556]

Great information, all over the map. I just assume these speed feedings are all via GPS. My paddlewheel shows top speed @ 3 knots..?. Catalina 309 is about 1,000 lb lighter..my story and am sticking to it.

 

[Tom J]

Great information, all over the map. I just assume these speed feedings are all via GPS. My paddlewheel shows top speed @ 3 knots..?. Catalina 309 is about 1,000 lb lighter..my story and am sticking to it.

I go exclusively with GPS. Pulled my paddlewheel out and never put it back in.

 

[KZW]

My paddle wheel turns freely, but reads about 0.5 knots less than the GPS.

 

[quadrille38]

My paddle wheel turns freely, but reads about 0.5 knots less than the GPS.

.5 knot is amazingly close. I think most people would be thrilled to get that close.

I find the variation differs over or under depending on a variety of conditions including current, direction into or with the wind, slime on the paddle wheel, .........

I mentioned calibration only because if you are trying to compare boat speed to hull speed to others speeds and you are using the know meter it might just be off calibration. GPS speed over ground is significantly more accurate than knot meter speed through the water.

 

[KZW]

the 0.5 knots is a more or less average taking into account current. The wheel is mounted just forward of and about 3 inches to the side of the leading edge of the keel.

 

[c130king]

I use my boat a lot and find with regular use my paddle wheel works well. Normally within .2 knots of GPS speed. Not too much current where I sail on the Mississippi Gulf Coast.

If I let the boat sit 5 days or so then the slime will build up and I will show lower boat speeds. Takes less than 2 minutes to clean. And if I let it sit more than about 10 days I get small barnacle growth and I have to use a pocket knife to scrape them off.

I did recalibrate maybe 6 months ago.

I need mine as the paddlewheel speed working is what makes the "True" / "Apparent" wind speeds work properly on my wind guage. If the boat speed on my speed display or on my chartplotter shows 0.0 then I can't show "True" wind speed.

Cheers,
Jim

 

[Deleted member 117556]

The original question was about boat speed under power at cruising RPM's and has been hijacked into paddlewheel discussions. Still interested in what boat speed people achieve under power - no current or wind and at cruising RPM?
bob