Hull question

[mstailey]

Good day to all,
Can someone explain to me what hull speed is and its application? I see this term used a lot here on the forum. When I google for hull speed I basically get calculators to tell me what my maximum hull speed based on LWL but no explaination on what hull speed is.


Thanks.

 

[Nonchalant1]

Mike,

It is sort of a "natural rythm" speed at which the hull of a boat moves through the water without requiring excess power just to push more water around. Going any faster causes the stern to drop innto the trough of the wave being created and the bow rides up, so you're climbing a wall of water. The main factor is the length of the hull at the waterline (LWL). For big, full displacement boats, it is effectively their maximum speed because any speed above that requires huge amounts of additional power.

The formula for hull speed, in knots, is 1.34 times the square root of LWL. So a boat with a 49 ft. waterline would have a hull speed of 7 X 1.34 = 9.38 knots.

Doug

 

[Freebird]

Mike,

The formula for hull speed, in knots, is 1.34 times the square root of LWL. So a boat with a 49 ft. waterline would have a hull speed of 7 X 1.34 = 9.38 knots.

Doug

Hey Doug,

How bout figurin' hull speed for my 58TC! I ain't to good with them square roots and stuff anymore. I done forgot all that high end math I took in college. You know, it might be true what they say about partyin' and killin' brain cells and all. Hell far, I think I might have dain bramage or sumthin'.

 

[ThirdHatt]

For the 58' Hatts hull speed is about 9.6-9.7kts. Now, if we could just get someone with FlowScans to CONFIRM with hard numbers that hull speed is indeed the most efficient cruising speed (max speed/lowest fuel cunsumption) or "sweet spot" in the rpm curve we would really be getting somewhere!

 

[Freebird]

For the 58' Hatts hull speed is about 9.6-9.7kts. Now, if we could just get someone with FlowScans to CONFIRM with hard numbers that hull speed is indeed the most efficient cruising speed (max speed/lowest fuel cunsumption) or "sweet spot" in the rpm curve we would really be getting somewhere!

Thanks for figurin' that out for me there Bayou Boy. Yeah, maybe somebody out there does have Flo-Scans and will share their "sweet spot" with us poor boys who are still tryin' to read fuel gauges.

 

[mstailey]

Doug,
Thanks. Makes sense explained that way.

 

[Nonchalant1]

Hey Doug,

How bout figurin' hull speed for my 58TC! I ain't to good with them square roots and stuff anymore. I done forgot all that high end math I took in college. You know, it might be true what they say about partyin' and killin' brain cells and all. Hell far, I think I might have dain bramage or sumthin'.

I agree.

Doug

 

[luckydave215]

Would it bother any other forum members if I start typing in ebonics (or Phoenician)? It wouldn't be any more annoying than the fake redneck stuff.
On second thought forget it, I'm just here to talk and learn about Hatts.

 

[Nonchalant1]

For the 58' Hatts hull speed is about 9.6-9.7kts. Now, if we could just get someone with FlowScans to CONFIRM with hard numbers that hull speed is indeed the most efficient cruising speed (max speed/lowest fuel cunsumption) or "sweet spot" in the rpm curve we would really be getting somewhere!

Do an advanced search for "RPM / Speed / GPH for 53MY" and you'll see the curve for my 53MY which was obtained via floscans and 2-way measured runs on calm water. A 53MY hull speed is 9.1 knots....where it burns 1.0 gal/NM. To run at 10.0 knots, she burns 1.6 gal/NM. There's your answer.

You have a higher hull speed, but the same principles will apply above your hull speed.

Doug

 

[bobk]

Actually if you run below 'hull speed' you will get even better economy. the boat starts to climb the bow wave at hull speed, so this is where economy goes down hill fast. Six knots would be very efficient if you can and want to run that slow.

Bob

 

[Nonchalant1]

Roger that........as shown in the above mentioned chart, my boat gets 3 NM per gal. at 6.3 knots (effectively idle speed at 640 RPM). Bad for your engines though.

Doug

 

[Genesis]

Yeah, the problem with that is that your engines will gunk up and die.

Thus my interested in a D/E hybrid system; run on the GENERATOR at 6-7kts, and only run the mains when you need the speed......

That would be incredibly efficient..... and real expensive to design and install!

 

[Boss Lady]

Maybe Hatteras was on to something when they made the Motorsailer! Free windpower, and real Detroit Diesel power when you need it, and the best part is it is still a Hatteras! Downside is they only made a handful of them. I have never been on one but I think it would a great project.

 

[MikeP]

Doug,

I downloaded your fuel burn "table" when you first posted it a while back. I was wondering - is your speed/RPM data based on oem 28 x 31s props for your 53? The previous owner repropped our 53 to 28 x 29s. My speed per RPM show a bit lower than yours which would make sense given a "smaller" prop.

I have a brand new oem set of 28 x 31s still wrapped as Hatt supplied them as spares when the boat was new. I have occasionally thought about putting them on but the boat seems happy enough with the 29s.

 

[SKYCHENEY]

Mike,
Now that you have brought it up, the PO had 28x32's on my boat and it was overpropped. OEM for mine was 28x31's but I went down to 28x30's. But when Michigan Wheel made the calculations for me they came up with 28x29's just like you are running. So, I think that Hatt may have either overpropped them to begin with or they were just not anticipating the extra weight that most of our boats have gained over time.

 

[Paul45c]

Mike,
Now that you have brought it up, the PO had 28x32's on my boat and it was overpropped. OEM for mine was 28x31's but I went down to 28x30's. But when Michigan Wheel made the calculations for me they came up with 28x29's just like you are running. So, I think that Hatt may have either overpropped them to begin with or they were just not anticipating the extra weight that most of our boats have gained over time.

FWIW, I've had several diesel mechanics tell me that Hatteras (and several other brands) were famous for overpropping, especially since speed became the rage.

 

[Nonchalant1]

Yes....I have the 28X31s that were original spec. for the boat. They have been tuned by PropScan. They work great but are probably are a bit overproped for the boat. I believe it will only go up to 2350 RPM or so.