8mph 8mph 8mph

My new to me 1982 Marine trader cruises at 8MPH. My friends Prairie 29 cruises at 8MPH. Several of the Trawlers I looked at in my search cruised at 8MPH. Many larger Trawlers I saw advertised said 8MPH--- these ranged from 36ft to 46ft. A K-K42 said 8MPH. I come from the sailing world where longer waterline means higher cruising speed. This does not seem to be true in the Trawler world. The mysterious 8MPH.

I cruise at 7 mph, 34 foot wayerline on a 40 foot trawler...

Don't worry.... a lot more experience and you will figure it out.

I don't cruise at 8 mph, I cruise at 7 knots, but then I'm not land locked.

You can certainly go faster if you're prepared for the fuel burn. I burn:

6 knots 1.3 GPH
7 knots 2.0 GPH
8 knots 3.7 GPH

Burning 85% more fuel to go from 7 to 8 knots isn't worth it to me.

Ted

Many people, me included, say 8 knots because it is easier than saying 7.2 or 7.6 or 8.3.

In most cases 8 knot is good enough unless there is a specific reason for being picky such as fuel use comparsions which I have also done and apparently so has O.C. Diver.

The rule about w/l length does stand although modified somewhat sometimes for power boats depending upon planing, semi planing, full displacement..

C lectric said:

Many people, me included, say 8 knots because it is easier than saying 7.2 or 7.6 or 8.3.

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+1. Saying 8 kts is usually implying a range close to that speed, in my experience anyway.

I travel at 8 knots = 1800 rpms. It’s Southerlys sweet spot! 36ft. Waterline 42 ft. Overall.

I think length may play into it .mine seems to be like oc's but 1/2 to 3/4 knot slower at each burn rate.

I usually speak in knots when discussing vessel speed, but find I have to give a commensurate mph number when speaking to non-boaters.

If you cruise at 7 knots it’s 8 mph. At about 8 knots (7.82) it works out to 9 mph. Easy math to remember.

7.8 mph or 6.8 kts is where IL likes to run. You get used to it or not.

Moonfish said:

I usually speak in knots when discussing vessel speed, but find I have to give a commensurate mph number when speaking to non-boaters.

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ASD normally cruises a 8.6kts (10mph) unless I am making the trip from Neah Bay To Astoria (or the other way) at 15kts (17mph). You tend to see more things at 8kts.......

My maximum speed is eight statute miles an hour (7 knots). A wasteful speed, so cruise at a knot less.

7.5 to 8 knots and no wash for us at 1150rpm on 60ft.
Faster with tide and slower pushing it.

8 knots on shorter boats will be pushing water and burning extra fuel.

Donna said:

I travel at 8 knots = 1800 rpms. It’s Southerlys sweet spot! 36ft. Waterline 42 ft. Overall.

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What indicates it as the sweet spot?

Moonfish said:

I usually speak in knots when discussing vessel speed, but find I have to give a commensurate mph number when speaking to non-boaters.

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You speak to non boat people?

Let’s be honest for a moment. If somebody seems in a hurry, I’ll tell them 9knots, maybe even ten if I want to appear as a speed demon. Someone taking it easy I’ll say 8. If they ask whether I mean miles per hour, I’ll say yes. It’s so much more polite than appearing smug and I say I don’t care. Truth is that not only do I not really care, I often don’t even know.

Reality is that I push the throttles forward until the engines make a sound that matches my mood. Which usually translates to about 1400 rpm, give or take. If I do give or take, I give or take from the port engine because it’s throttle lever is a bit smoother.

No, but really, we get there when we get there. Only days I pay closer attention is when I need to make slack. In which case I go early to not miss it, get there early and usually run it early anyway, cause who wants to wait.

So yeah, I go 8 too.

Its really funny after awhile how little the exact speed seems to matter. It’s like asking what time it is. I don’t know that either. I have a vague sense of what meal we are closest to and that also becomes the exact time, such as “a little before our mid morning snack”.

Simi 60 said:

You speak to non boat people?

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Dirt Dwellers.....

We were dirt hotelers recently in Astoria. I pulled up ship finder and saw some boat named Alaskan Sea Duction on AIS just up river. You should reach out to them having a similar boat name and all.

Inshore charts are frequently in statute miles , so planning in statute miles is less work.

14.816 kph here

We cruise often at 8 MPH.



Knot really; usually somewhere between 7-8.5 kts. Waterline length is 43' so our economical speeds would theoretically be between about 6.5 kts and 8.8 kts... and at the lower end of that range would use less fuel...

But at the same time, we're usually balancing speed versus sea states versus backwash/froth rocking our aft-mounted dinghy too much if we go too fast but not on plane versus engine temps.

For the latter, we usually need around 1200 RPMs to get temps over the thermostat opening and steady within the (lower end of the) recommended operating range. After we reach that point, we can eventually move to lower RPMs without losing temps...

But it usually happens that about 1200 RPMs (plus/minus 25 wherever I can get the engines synced on a given trip) usually gives us about 8 or 8+ kts with no current to speak of, or about 7 kts into a stiffer current.

But then sometimes we run at 20 knots.

All good.

-Chris

dhmeissner said:

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aren't you dudes something real Admirals of the ocean sea!

That's funny, The ICW and the Okeechobee waterways have mile markers. Guess all those people in boats on them are not "real boaters" (maybe robots?) lol And the amateur Previous owner I bought my trawler from had all his instruments set to miles ---- he had only traveled 3600 miles every year for the past 15 years round trip from Maine to Stuart Florida using miles-he evidently was not a "real boater" I wish I was a real boater . I have only traveled in my own sailboat s over 26000 nautical miles offshore in the past 47 years.

When I am in an area that uses regular miles I set my GPS that way. When I am in an area that uses nautical miles I change again.
No big deal.

Simi 60 said:

What indicates it as the sweet spot?

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She is quiet, nice smooth ride, gauges look perfect, handles well.

Speed consideration in a trawler is a bit of an oxymoron, kind of like "sailboat. racing". If the excitement of slicing through the waves with your combover flailing in the wind is your idea of proper boating, perhaps a trawler isn't the best choice.

I do 1750 RPM. It seems way faster to say.

Whenever we were happy to go slow we did 6-6.5 knots (45-47' boats) because that was a very quiet and efficient speed. There would appear no reason to exceed that unless we were wanting to go fast which would be sacrifice the efficiency.
We often went fast but it was not a matter of stepping up 25% in speed for a 75% increase of fuel use per mile. ( 8 vs 6 knots)

To the original question, it’s not just length, but also beam. Most trawlers increase the beam along with the length, keeping the same basic ratio intact, meaning the same basic max speed, give or take a knot.