What should my hull speed be?

[Casdeop]

Hello and good day to all.

I have a 36’ Albin Trawler. She’s a heavy boat 31000 Lbs on the scale last time she was on the dry.

The boat has a Cummins 6BT 210HP with low time.

What should the hull speed be at 2000 RPM? I’m only getting 7 knots. Should I look into a different prop?

Let me know if you have any ideas.

Kind regards,

Carlos

 

[rslifkin]

Assuming a waterline length of 32 - 33 feet on that boat, hull speed is about 7.6 kts. You'll start to feel the drag rise by 7 kts or a little less, where it takes significantly more power to get more speed.



What does the current prop give you for speed and RPM at full throttle? A 6BT 210 should be rated at 2600 RPM, so you'd want it propped to turn 2650 - 2700.

 

[rgano]

Are you seeing any signs of engine overloading like hull sooting or dark smoke on the current prop?

 

[Casdeop]

Hello,

Thank you for the prompt reply.

I’m getting 8.5 knots at 2600 RPM

 

[Casdeop]

The engine will give me much more than the 2600 RPM it’s rated for. I can probably get over 3000 RPM But I have only run it up to 2800

No sooting at all.

 

[rslifkin]

The engine will give me much more than the 2600 RPM it’s rated for. I can probably get over 3000 RPM But I have only run it up to 2800

No sooting at all.

In that case, it sounds like you're underpropped.

 

[rgano]

Yup. Time to consult the prop specialist.

 

[Casdeop]

That sounds expensive

 

[Delta Riverat]

You can probably get it re-pitched 2 inches easy. Still expensive but less than a new screw.

 

[backinblue]

Sounds like you might be confusing the term "hull speed". Hull speed is related to the LWL. The RPM required to get to that speed depends on a number of things. But it's not clear what problem you are trying to solve. Not sure if you are looking to get a higher top speed. If you normally cruise at 7 knts maybe it's fine the way it is. It may not be ideally propped but depending on how you normally run the boat it may not matter. Also, probably better to be a little under propped than over propped if you intend to run on plane normally. A friend with a Cape Dory can't run over 10 knots w/o the motors getting pretty smoky even though not at max RPM (i.e. likely overpropped).

 

[Jeff F]

The engine will give me much more than the 2600 RPM it’s rated for. I can probably get over 3000 RPM But I have only run it up to 2800

No sooting at all.

Check your tach first. Getting to 2800+ rpm doesn't sound right. That engine should be governed at 2750 or so.

An easy check is to go to WOT in neutral and note the rpm. Then do the same in gear.

There's no real harm in being under propped, but if you see 3000 rpm the tach is wrong.

 

[rslifkin]

Check your tach first. Getting to 2800+ rpm doesn't sound right. That engine should be governed at 2750 or so.

An easy check is to go to WOT in neutral and note the rpm. Then do the same in gear.

There's no real harm in being under propped, but if you see 3000 rpm the tach is wrong.

Good point. A photo tach is fairly cheap to buy and will allow you to calibrate the tach and make sure it's accurate.

 

[Comodave]

Absolutely get a photo tach. Then run the boat with it’s normal load at WOT and see what RPMs you are actually getting. You want WOT to be about 2650 RPM. If you are getting more RPMs then you need more prop, more pitch. If you are not getting 2650 then you want less prop, less pitch. A good prop shop can probably get up to 2” in pitch by reworking he prop depending on what, if anything, has already been done to the prop. As to what speed you should get at a certain RPM, you get what you get. The big thing is to get the proper RPMs at WOT. Then you can run the boat up to about 75 to 80% of WOT if you are willing to pay for the fuel. The slower you go the better fuel economy usually. Running faster than the theoretical hull speed will cost in fuel economy.

The formula for calculating hull speed is here.

1.34 x square root of waterline length (not boat lenght)

This will give you the theoretical hull speed in knots.

 

[psneeld]

Albins are semi displacement.

The groove I found for my Albin 40 was 6.3 knots. I got 3.3 NMPG at that speed whoch equater to 1650-1700 rpm on a Ford Lehman120hp. I belueve at that rpm I was down around 45hp swinging a 4 bladed prop that I now forget the othe measurements.

The waterline on the 40 Albin wasnt a whole lot more than the 36, I royghed a waterline measurement I think at about 34 on the 40 Albin.

 

[rslifkin]

Albins are semi displacement.

The groove I found for my Albin 40 was 6.3 knots. I got 3.3 NMPG at that speed whoch equater to 1650-1700 rpm on a Ford Lehman120hp. I belueve at that rpm I was down around 45hp swinging a 4 bladed prop that I now forget the othe measurements.

The waterline on the 40 Albin wasnt a whole lot more than the 36, I royghed a waterline measurement I think at about 34 on the 40 Albin.

That seems about right to me. We've got a 33.5 foot waterline and I consider our optimal slow cruise to range from about 6.4 - 6.8 kts depending on fuel load (stern rises as we burn off fuel and the boat gets easier to push).

 

[psneeld]

My numbers were pretty accurate after 20,000 miles of snowbirding NJ-FL.

 

[High Wire]

31000 lbs is very heavy for a 36 ft boat. My 36 Monk is very similar and is about 24,000 wet with full tanks in cruising mode. Travel lift load cells take a heck of a beating and almost never get calibrated.
Standard analog tachs driven by alternators are notoriously inaccurate for several reasons.

Agree that 6.4 to 6.9 KNOTS (7.4 - 8.0 MPH) would be an efficient cruise speed IF your Cummins likes to live there.

 

[rslifkin]

My numbers were pretty accurate after 20,000 miles of snowbirding NJ-FL.

I'd expect so. It's always good to know when accurate numbers line up with expectations.

 

[Flatswing]

What should the hull speed be at 2000 RPM? I’m only getting 7 knots. Should I look into a different prop? /QUOTE]

Others have already defined “theoretical hull speed”based on waterline length. Most of us quickly find that (especially for semi-displacement boats) there is a zone where stern squat is minimized, the engine(s) seem quieter & fuel consumption is reduced. Often 1.0-1.5 knots below theoretical hull speed and that’s where we cruise most often. For my 49 DeFever, 7-7.5 knots is best in flat water. A few designed full displacement hulls can do better, emphasis on “a few”.
Agree: buy a photo tach and adjust your prop only if necessary to meet WOT per Cummins specs with clean bottom & running gear.

 

[markpierce]

You will increase efficiency (reducing fuel consumption by 50% or more) moving one knot below hull speed. At hull speed, my fuel consumption goes from 4 gallons an hour to 1.7 at one knot below hull speed.

 

[NWboater2]

Good advice so far. I suspect you really meant how fast should I be going with a 210 Cummins at 2000 rpm?
Hull speed is a number not dependent on engine or prop or rpm, as you now know. If it’s 7 knots you should be able to get there a good bit lower than 2000 RPM with that motor.

I think the answer to your real question is, “more than that.” Sounds like you are way under propped (the “threads” on your screw are too flat- if you had no pitch you could go 5000 rpm and still not move) … and/or your tach is wrong, and you’re really getting 7 knots at a more plausible number like 1700 rpm. Easier and cheaper to check the tach, first.

Good luck!

 

[markpierce]

Casdeop, your engine is much too powerful for your boat. You would probably do ok with only 80 hp. But too late to change.

 

[rslifkin]

Casdeop, your engine is much too powerful for your boat. You would probably do ok with only 80 hp. But too late to change.

He doesn't have a full displacement hull, so no, it's not overpowered. It's just capable of going far than someone concerned with fuel economy would go most of the time.

 

[fgarriso]

This will give you the real numbers.

https://vicprop.com/displacement_size_new.php

 

[Pete Meisinger]

Your speed is about right. I don't think your boat weighs 31,000 pounds though, probably more like 21,000.

Re-proping might give you a little more speed but why bother for a half a mile per hour?

pete

 

[MYTraveler]

I’m getting 8.5 knots at 2600 RPM

Then (assuming the assumptions above about your length at waterline are correct) you are exceeding the maximum displacement speed of your boat and it is starting to plane. The bottom configuration of true displacement hull boats are incapable of planning, no matter how much power is applied to the props. Since you are capable of exceeding displacement speeds, you probably have a semi-displacement hull. Regardless, efficiency drops off as you reach and exceed max displacement speeds, at least until you are planning cleanly (and 8.5 knots isn't nearly fast enough to produce a clean plane on a semi-displacement hull). Reducing speed to about 80% of max displacement speed is much more efficient than traveling at faster speeds (at least below a clean plane). In your case, that probably means limiting your speed to 7 knots.

For comparison, the theoretical max displacement speed of my hull (LWL is 59') is 10.3 knots. I usually travel at 8.5 knots, and very rarely at or above 10 knots. My hull is a semi-displacement design, and I have the power to go 18 knots. 18 knots isn't fast enough to get a good clean plane and my fuel efficiency is horrible at that speed, though maybe a little better than say 14 knots.

 

[SteveK]

Sling weight and displacement weight are not the same. My GB 36 was rated 26K displacement and weighed 34K in the sling

 

[rgano]

Sling weight and displacement weight are not the same. My GB 36 was rated 26K displacement and weighed 34K in the sling

Which physics book are you using?

 

[SteveK]

Which physics book are you using?

The scale on the lift and the book that says the displacement weight.

 

[LovetoBoat]

Two points I would add.
First - I would never run WOT in neutral. Never ever.
Second - Assuming your bottom and prop are clean you need to run your engine WOT at least twice a year. If you notice a drop in speed from your additional runs at WOT you may have engine issues - at that point you may want to engage a qualified technician.