Newport 28 mk2 cruising speed

[adamv]

I get the impression my boat cruises slower than those around me so I'd be keen to hear from other N28 owners who have the M18 what sort of speed they get at upper cruising revs. I'm generally around 4.2kts. (My boat doesnt have a tach but the throttle is at around 80%) Perhaps I'm being too cautious with the revs but of course the gains diminish as you increase revs anyway, so would be keen to know whether my boat has something that is slowing it down or not
Thanks

 

[sail sfbay]

Theorectical hull speed = 1.34 x sqrt (waterline length of 25 ft) = 7.6 knots. Hopefully, N28 owners on this forum will confirm their cruising speed.

What engine (mfg and model) and prop type and size (diameter and pitch) is installed? It is possible the engine is underpowered and/or prop is not sized to achieve top end speed.

 

[jssailem]

Hi Adam.
Without a Tach you need to rely on sound and feel. The speed of your boat under auxiliary power is dependent on the condition of your hull/keel/rudder, the prop size/pitch, engine size/performance and how much fuel you feed the engine.

It is normal for newish owners to go lightly on the throttle of their new to them boat. The thinking is that if I treat my diesel with light use it will give me a long and enjoyable experience.

This is not the case with a diesel. They need to run at 80-85% of WOT (wide open throttle). Running them an 45-50% will foul the cylinders and shorten the engine life.

Tach’s are not hard to install. In absence try to go out on s calm day and experiment pushing the engine/boat at various speeds. Monitor a GPS speed and the sounds/vibration of the engine. While not perfect you can find a sweet spot in the upper speed range. You are striving to reach about 6 knots in calm no current conditions

if not possible it may be time to examine your prop. There is a great maker of propels near Comox, BC. West by North. I found them very helpful.

 

[jon hansen]

sfbay, you are always so helpful. and nice.

that said. 1.34 x 5 = 6.7. not 7.6. so if adam's hull is like new condition he could see 6.7 for hull speed.
many boats do not have the power to achieve hull speed when motoring. so prolly something less.

just saying

 

[adamv]

Thanks all, the hull and prop are in decent shape. It's been painted a few times and could do with being brought back but certainly no fouling. Didn't even need a spray last time we took it out the water a couple months back so I'd be surprised if the hull itself is dragging
I do notice a subtle surging - a kind of rolling to the revs where the revs sound 10% higher and then lower, sinusoidally, every 5 seconds. Is this common or could be a factor? EDIT - I caught a post about something similar sounding - could be the alternator kicking in. The amp guage doesn't work so perhaps there is something in this- would explain being down a few HP