Expected Speed With Larger Engines

[REBrueckner]

Somebody asked about approximate speed in going from1,000 to 1800 HP engines....this discussion with a formula previously posted by SCROD may provide some rough insight....several other examples are posted by other foprum members in the discussion started 12-28-2005...

While I still think this approach is a good first cut it does not fit the results posted here more recently:
http://www.samsmarine.com/forums/showthread.php?t=16952

where 485HP DD were replaced with 480 HP Yanmars and cruise speed reportedly increased from 17 to 27 knots...so there is clearly an inconsistency.


From an old thread:
Expected Speed With Larger Engines
http://www.samsmarine.com/forums/showthread.php?t=2555&highlight=expected+speed+larger+engines


What speed can you expect with larger engines? I found the following really interesting:
In June, 2005 SCROD posted: (And thank you for the post!!)

Here's a rough guide for horsepower to speed ratios for planing hulls with moderate deadrise 12-18 deg. 180 divided by the square root of (displacement divided by total horsepower.) For a warped hull like a Hatteras reduce top speed by 2-3 kts.

For example, my 36 has 520 total HP and presently tops out at 19 kts. If I use the formula at 36K lbs it comes out to 21.6 kts, I subtract 2-3 and come up with 18.6 to 19.6 kts. Pretty good for a rough guide..."

Comment: For my 48ft Yachtfish with 435 HP 8V71TI's, 45,000 lbs (that may be a bit light, but its what the brochure says), the formula suggests 180/(rt(45,000/870) or about 25.3kts top, a bit high. Actual speed when new was 22.2, so let's subtract 3 from the formula as a baseline. If I put in 625HP 8V92's, the formula gives 30kts and subtracting three suggests about 27 knots top...boy it takes a lot of HP for that extra speed!!!

As a note of interest, in 1999 after having the engines gone over, we clocked the old girl at just over 21 knots for a one hour trial at WOT with the mechanics aboard. I later found out the throttle control linkage keeps her at about 150 RPM short of 2300, so she was darn close to original speed after 27 years!!! (I was so excited during the trial I guess I did not look closely at the tach readings.)

Since 625/435 (HP) is about 1.44, it looks like 27/22 (kts) or a 23% increase in speed requires about a 44 % increase in HP; that's also about a 44% increase in hourly fuel consumption, about a 17% loss in NMPG.


How does the speed formula work out for others?

 

[REBrueckner]

If you want to locate the original discussion for examples other members posted about speed vs HP, use the exact title above
EXPECTED SPEED WITH LARGER ENGINES....

I tried a lot of combinations, some close, and for some reason the search function did not identify this thread in the first 50 or so search results.......

Also, I more recently found out, from Lumina, I think, that our 1970's YF weigh more like 49,000 lbs than the 45,000 I used above....so my calculation above would be a bit closer...24.6 kts instead of the 25.3 above.....In this theory, if I used Yanmars at about 480 HP each and say 3,000 lbs less weight I'd get about 26 knots, less about 3, so maybe a bit over 23 knots WOT. That seems about right.....just an intuitive feel, nothing more...but maybe I'm still missing something.

 

[bostonhatteras]

I agree with Rob. hard to beat Physics at its own game! maybe yanmar will introduce the marine version of the perpetual motion engine in 2012!

 

[madhatter1]

When I run the numbers on my 66 34C with 270 (260) Cummins this is what I got. I used 20k lbs as a guess since I have seen 16-19K but I think the higher number is closer. 180/rt(20000/520)= 29.03 less 3 is 26.03 kts. Since my GPS is set to statute mph I multiply by 26.03 X 1.15 = 29.93. That is exactly my clean bottom top speed. I will say that light on fuel my speed stays the same. Just for fun I plugged in some numbers on the new 5.9 QSB's. Dont think 41 mph would be a good idea.

I think this is a good formula for Diesel but not for gas in a Hatteras. A 260hp gas engine will not match my 270's which are better performers than the 440's or the 454's that were there first & second. My peak torque is in the high 600's and the small block would need gear ratios to get it on plane but then could not reach a high speed. Thanks for the info.

Thought of this before I hit submit. Did the math on my friends 36C using 26K lbs and his 370 Yanmars. Formula close to his 33mph max speed. Good guage to go by. Thanks again.

 

[rsmith]

Somebody asked about approximate speed in going from1,000 to 1800 HP engines....this discussion with a formula from SCROD may provide some rough insight....several other examples are posted in the discussion started 12-28-2005...

While I still think this approach is a good first cut it does not fit the results posted here more recently:
http://www.samsmarine.com/forums/showthread.php?t=16952

where 485HP DD were replaced with 480 HP Yanmars and cruise speed reportedly increased from 17 to 27 knots...so there is clearly an inconsistency.


From an old thread:
Expected Speed With Larger Engines
http://www.samsmarine.com/forums/showthread.php?t=2555&highlight=expected+speed+larger+engines


What speed can you expect with larger engines? I found the following really interesting:
In June, 2005 SCROD posted: (And thank you for the post!!)

Here's a rough guide for horsepower to speed ratios for planing hulls with moderate deadrise 12-18 deg. 180 divided by the square root of (displacement divided by total horsepower.) For a warped hull like a Hatteras reduce top speed by 2-3 kts.

For example, my 36 has 520 total HP and presently tops out at 19 kts. If I use the formula at 36K lbs it comes out to 21.6 kts, I subtract 2-3 and come up with 18.6 to 19.6 kts. Pretty good for a rough guide..."

Comment: For my 48ft Yachtfish with 435 HP 8V71TI's, 45,000 lbs (that may be a bit light, but its what the brochure says), the formula suggests 180/(rt(45,000/870) or about 25.3kts top, a bit high. Actual speed when new was 22.2, so let's subtract 3 from the formula as a baseline. If I put in 625HP 8V92's, the formula gives 30kts and subtracting three suggests about 27 knots top...boy it takes a lot of HP for that extra speed!!!

As a note of interest, in 1999 after having the engines gone over, we clocked the old girl at just over 21 knots for a one hour trial at WOT with the mechanics aboard. I later found out the throttle control linkage keeps her at about 150 RPM short of 2300, so she was darn close to original speed after 27 years!!! (I was so excited during the trial I guess I did not look closely at the tach readings.)

Since 625/435 (HP) is about 1.44, it looks like 27/22 (kts) or a 23% increase in speed requires about a 44 % increase in HP; that's also about a 44% increase in hourly fuel consumption, about a 17% loss in NMPG.


How does the speed formula work out for others?

Sorry rob but there is no way that old 48 could do 22.2kts with 8ti's unless you ran it off the edge of the Niagra falls and even then all the windage from the boxy superstructure would slow you down. 16kts maybe down wind. At least lucky dave is honest when he says "the older I get the faster I was"

 

[J's Dream]

I don't recall my exact weight...its right around 50k, but on my way home from a haulout, I clocked 24.1kts on my GPS. 45C, 8v92N's, 425hp each. What does the formula say for me? Anyone?

 

[madhatter1]

I don't recall my exact weight...its right around 50k, but on my way home from a haulout, I clocked 24.1kts on my GPS. 45C, 8v92N's, 425hp each. What does the formula say for me? Anyone?

21.82, close enough to use this formula as a good ballpark figure.

 

[REBrueckner]

rsmith:" Sorry rob but there is no way that old 48 could do 22.2kts.."

SURPRISE!!!!!!

The 22.2 knots top speed was when new as provided by Hatteras to the owner from whom I purchased the boat. He also recorded fuel and water capacity, and some other stuff, all accurate, so I never verified the original speed figure.

Just after engine overhaul when I first owned the boat, I clocked speed at about 20.2 kts at 2150 RPM during the sea trial for the engine work...which was surprisingly close the the Hatteras figure at 2300 RPM (I assume) ....I may have been light on fuel, don't recall.

I had two mechanics on the boat for the one hour sea trial, one on each engine, plus the owner of Precision Marine, New Rochelle,....[and my Yorkie of course].....he was a bit surprised by the speed, too, but did not dispute the GPS readings.


I've previously asked other owners of the 1970's 48 YF with 8V71TI's to post their known WOT speeds for comparison but have never seen any here.


MikeP: What's the WOT speed of your 53 MY with 8V71TI's???? Is it about 18 knots or so???

 

[TopHattandTails]

53my with 871's -I cruise at 1950 rpm at 13 knots and WOT at 2350 rpm and 15 knots. Only time you'll see 18 knots is down bound under the bluewater bridge from lake huron to st clair river! Now i also have granite everywhere and a lot of chain aboard.

 

[REBrueckner]

Top Hatt...are those naturals, 350 HP or turbos at 435 HP???

sounds like naturals??? owners I talked with in the past said they cruised 12-13 kts with
8V71 naturals.....

 

[rsmith]

rsmith:" Sorry rob but there is no way that old 48 could do 22.2kts.."

SURPRISE!!!!!!

The 22.2 knots top speed was when new as provided by Hatteras to the owner from whom I purchased the boat. He also recorded fuel and water capacity, and some other stuff, all accurate, so I never verified the original speed figure.

Just after engine overhaul when I first owned the boat, I clocked speed at about 20.2 kts at 2150 RPM during the sea trial for the engine work...which was surprisingly close the the Hatteras figure at 2300 RPM (I assume) ....I may have been light on fuel, don't recall.

I had two mechanics on the boat for the one hour sea trial, one on each engine, plus the owner of Precision Marine, New Rochelle,....[and my Yorkie of course].....he was a bit surprised by the speed, too, but did not dispute the GPS readings.


I've previously asked other owners of the 1970's 48 YF with 8V71TI's to post their known WOT speeds for comparison but have never seen any here.


MikeP: What's the WOT speed of your 53 MY with 8V71TI's???? Is it about 18 knots or so???

Was the Yorkie pushing or pulling?

 

[saltshaker]

53my with 871's -I cruise at 1950 rpm at 13 knots and WOT at 2350 rpm and 15 knots. Only time you'll see 18 knots is down bound under the bluewater bridge from lake huron to st clair river! Now i also have granite everywhere and a lot of chain aboard.

That does seem a bit off. You would expect more than a 2KT spread between 1950 and 2350. As a point of reference here, your boat will run a bit slower in fresh water compared to salt water.