Motoring speed

[Ross]

I have a 1987, 28.5 with an 18 horse, 2 cylinder Yanmar diesel and struggle getting much more that 4.5 miles per hour at 2800 rpms. Many other Hunter owners have the Yanmar engine and achieve greater that 6 miles per hour with their 2 cylinders. What am I missing? I currently have a 2 blade prop...would a 3 blade make up the difference?...possibly the pitch? Any thoughts would be appreciated.

 

[Jeff D.]

Not the prop

I believe that you have the stock prop which works very well on this model. Switching to a 3 blade is not the solution. First thing to look at is your knot meter. Is it calibrated properly? If you haven't checked then I suspect that is your problem. Second possibility is your engine itself. Are you able to reach 3400 rpm which is the max for your motor? If so, then your engine should be OK. If not, make sure that you are able to reach full throttle and your cable isn't slipping. Try accelerating your engine useing the lever directly on the engine. If it can reach 3400 then your cable is probably slipping. Tighten using the screw on the front of the pedestal. If you can't reach 3400 then you probably have an issue with your exhaust elbow which is well covered in this site's archives. Hope you get it figured out.

 

[Palmer King]

Prop will help, but

Going to a 3 blade prop should do the trick, but I had to wonder about running an '87 engine at 2800rpm. I know the later models Yanmar's can sustain 2800rpm cruising, but I didn't think the older ones liked more than 2400.Anyway, on my 290, I have a 3-blade, and a later model engine and I can sustain 6kn easily, but I find the difference between 2400 and 2800 to be no more than an hour over a ten hour day and the ride is a whole lot smoother.

 

[Terry]

Hi Ross, I think Jeff's suggestions come...

closer to things you need to look into. Our H28 had a 2GM 18 horse with the stock two blade prop, which could push the boat along at 6.5 to 7.5 knots running at 3,500 RPM. Something is keeping your engine from reaching the higher RPM's. A Yanmar technician told me to first warm up the engine, then in neutral open the throttle full. He told me this will not hurt the engine, but it should reach its maximum RPM somewhere around 3,800 to 4,200 RPM. Each model differs so check your engine manual.If your engine will not reach its maximum RPM in neutral, then something is holding it back. As Jeff suggested, it could be back pressure from a clogged mixing elbow, restricted fuel flow, etc. A three blade prop is not the answer.We were able to cruise all day at 3,200 to 3,300 RPM with our 2GM. If you have a hand held GPS, or can borrow one it will confirm whether your knot meter is correctly reading your boat speed through the water.Terry

 

[S. Sauer]

Performance of 2GM20F Yanmar

My experience is the same as Terry's. I have a 1986 28.5 with the 2GM20F Yanmar and usually run it at 2800-2900rpm as it 'red lines' at 3600 rpm. Crusing speed with a clean (folding) prop and clean bottom at 2800 rpm is typically 6.5 to 6.8 knots. Top speed at 32-3400 rpm is 7.4 knots and not something you should try to hold too long. The throttle cable is easily reached on the starboard forward side of the engine and has adjustment capabilities if you are not achieving full rpm. Barnacles on the prop or bad fuel could cause extreme loss of speed as well.

 

[J. Tesoriero]

Practical Sailor advice.

There was a similar question posed in the "Ask any Sailor" message board. In response, I saw a recent bit of advice in Practical Sailor (Aug 1, 2003): in flat water, at average displacement loading, an engine speed of about 70% of rated RPM's (see spec's in engine manual) should yield a boat speed in knots of between 1.1 and 1.15 times the square root of the LWL in feet, provided that your prop diameter and pitch are correctly matched to your boat. Must be the best of all possible worlds.