How much horsepower?

[BiminiJack]

The engines in the boats that I'm looking at have about 50/50 Atomic vs. Diesels. Now the Atomic 4 is rated between 25 to 30 hp. Most of the diesel engine are rated from 8 to 12 hp.
How well can a diesel push a 8500 to 9000# boat even in normal conditions?

 

[BarryL]

Hello,

My last boat was a Newport 28. It weighed about 7500 and had the Universal M18 diesel engine, rated at 14 hp. With a fixed 2 blade prop I had no problem motoring at 6 kts (with a clean bottom).

My current boat (O'day 35) weighs around 12000 and has the Universal M25 rated at 21 HP. I can cruise at 6.5 kts with a fixed 3 blade prop.

Based on my experiences, with a 8-9K lb boat I would want at least 14 HP.

Barry

 

[rajhnsn]

Barry,
I don't think I could say that I cruise at 6.5 kts. on my O'day 35. Of course it is hard to tell when considering different factors such as current and a clean bottom. My tachometer does not work consistently because of the high output alternator but I would say that I only make 5 to 5.5 kts. at 75% to 80% power which I consider cruising speed. I have a two bladed prop. Do you think that could account for the difference?

 

[DannyS]

I'd say I can 6.5 to 7 knots but that's pushing it pretty hard. My normal cruising speed is 5 to 5.5 comfortably and with less noise. I have the m25 (21 hp) with a fixed two blade prop.

 

[rajhnsn]

I have the same engine with a fixed 2 blade prop and it sounds like we get similar performance. I wonder if Barry has a little better performance with a three blade. A knot of speed could mean considerably less time on a long passage or more miles covered in a day. I wonder what the down side is?

 

[BarryL]

hi Guys,

According to my survey, here is the prop I have:
14" X 12" 3 blade bronze prop

With a clean bottom and prop I can motor at 6.5 kts at about 2300 rpm. FYI my engine has over 3700 hours on it, but it runs great. In flat water I can hit hull speed (a little over 7 kts) at max rpm. My engine will smoke at that speed, with the transom squatting down. When the bottom gets dirty the speeds slows down.

I believe the downside of the fixed 3 blade prop is drag under sail. For that reason I am thinking of moving to a 3 blade folding prop, most likely a FlexOfold

http://www.flexofold.com/Standard_2-Blade.php

I hope I don't lose too much motoring performance. My typical boat use is a simple day sail for a few hours. On that trip I won't motor unless it's because I need to get back to port. A few times a year (3-4) I go away for a weekend or longer. On those trips I want to make time, so I will travel at 6 kts, running the engine if I have to. Slowing down to 5 kts would be a bad thing.