This is a complex question whose answer obviously depends on multiple and interacting variables. I ask it for my engine's benefit. With my 160hp volvo diesel, top speed is between 2600 and 2700 rpm in my CD 28 FB. Brass panel at helm states that cruising speed should be 200 rpm less i.e., 2400-2500. Engine temp at that rpm is 175-180 and speed is in the 9-11 kts range. Fuel consumption is high. I usually cruise at 1800-2100 rpm. It is quieter, fuel consumption is significantly lower, engine temp is 165-170 and speed is 6.5-8kts. Am I underpowering and do I risk carbon build up? If yes, can I blow it off at the end of the day by running briefly at top speed?
stebbins@umich.edu
the optimal cruising speed for a CD 28 diesel trawler?
Moderator: Jim Walsh
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Walt Bilofsky
Re: the optimal cruising speed for a CD 28 diesel trawler?
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The first question is - what is the factory specified WOT range for that engine? (Ask your local Volvo-Penta dealer.) If you are not in the right range, you should re-pitch the prop.
Also, with that low a top-end speed, you should check the usual suspects for slowing down a boat. Is the bottom unusually fouled? Any other sources of drag (like are you running with full trim tabs)? Do you have extra weight on board - full water tanks, a quarter mile of anchor chain, anything like that?
Having looked at all that, let's talk about speed and fuel. I have a 30, which has a different hull shape and two engines, so you'll get better specific CD 28 answers from others. But in general, the most efficient speeds for a powerboat are (1) at or below hull speed, about 6 knots in a boat that size, and (2) somewhere around the slowest speed at which the boat is up on plane.
You can tell if you're on plane by increasing the speed until you see the wave from each corner of the stern crossing and making an X in the water a few yards behind the stern. This means you're outrunning the first crest of your bow wave, which is then able to cross behind the boat.
The planing speed for my boat is somewhere around 12 knots. Yours may differ a bit, but it sounds to me like you're not quite getting there.
Even so, the Cape Dory powerboats are not true planing hulls, but semi-displacement hulls. Again, I defer to CD 28 owners, but I'd be surprised if you could get more than a couple of nautical miles per gallon of diesel, except at slow speeds.
You mention noise at 2400-2500 rpm. Is there also significant vibration? That could indicate a problem. My shaft couplings were out of true, and when that was repaired, vibration was almost eliminated and I got another 200 rpm at the top end. You might take an experienced Volvo engine mechanic for a test run and see what he hears in that noise.
Hope this helps. Good luck.
- Walt Bilofsky
bilofsky@toolworks.com
That seems like a low WOT RPM, and definitely a low speed for 200 rpm under WOT.This is a complex question whose answer obviously depends on multiple and interacting variables. I ask it for my engine's benefit. With my 160hp volvo diesel, top speed is between 2600 and 2700 rpm in my CD 28 FB. (snip)
The first question is - what is the factory specified WOT range for that engine? (Ask your local Volvo-Penta dealer.) If you are not in the right range, you should re-pitch the prop.
Also, with that low a top-end speed, you should check the usual suspects for slowing down a boat. Is the bottom unusually fouled? Any other sources of drag (like are you running with full trim tabs)? Do you have extra weight on board - full water tanks, a quarter mile of anchor chain, anything like that?
Having looked at all that, let's talk about speed and fuel. I have a 30, which has a different hull shape and two engines, so you'll get better specific CD 28 answers from others. But in general, the most efficient speeds for a powerboat are (1) at or below hull speed, about 6 knots in a boat that size, and (2) somewhere around the slowest speed at which the boat is up on plane.
You can tell if you're on plane by increasing the speed until you see the wave from each corner of the stern crossing and making an X in the water a few yards behind the stern. This means you're outrunning the first crest of your bow wave, which is then able to cross behind the boat.
The planing speed for my boat is somewhere around 12 knots. Yours may differ a bit, but it sounds to me like you're not quite getting there.
Even so, the Cape Dory powerboats are not true planing hulls, but semi-displacement hulls. Again, I defer to CD 28 owners, but I'd be surprised if you could get more than a couple of nautical miles per gallon of diesel, except at slow speeds.
You mention noise at 2400-2500 rpm. Is there also significant vibration? That could indicate a problem. My shaft couplings were out of true, and when that was repaired, vibration was almost eliminated and I got another 200 rpm at the top end. You might take an experienced Volvo engine mechanic for a test run and see what he hears in that noise.
Hope this helps. Good luck.
- Walt Bilofsky
bilofsky@toolworks.com