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Old 07-18-2015, 05:50 PM   #1
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Here we go again !!

We did a survey on a 41 President with Ford Lehmann. It didn't go so well. Now looking at a 37 with cummins 6bt 300hp 1987. What can a person expect on fuel burn and what is recommended cruise speed? If any body has had any experience with this set up it would sure help.Me and wife are not in no hurry to travel in our first big boat. Any help would be appreciated ..
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Old 07-18-2015, 10:25 PM   #2
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It depends entirely upon how fast you push the boat. A high hp engine has the ability to push the boat faster but if the boat is pushed at a bit less than theoretical hull speed then the high hp engine and the low hp engine will use approx. the same fuel. Yes, the high hp engine will use a bit more but likely only a few %.

Keep this in mind; for a given hull run at the same speed it takes the same hp regardless of the engine as long as either engine is adequate to reach the same speed.

Your 37 hull likely has a waterline length of 34'. Using the formula of
WL length square root x 1.34 yields a hull speed of ~ 7.8 knots.

sq.rt 34 = 5.83 then 5.83 x 1.34 = 7.8.

At that point [7.8] the hull will start trying to climb the bow wave which then becomes fuel use heavy.

In fact that speed will not result in the best economy. Rather somewhat less, maybe 7- 7.25 knots.


Use that formula for ANY boat.

There are all kinds of variations of hull shapes which will change the formula somewhat but it works to get an estimate. Some boats will travel a bit faster, some slower but in that range predicted. It's a guidline only.

I will guess that if you follow a guidline like this the engine will use ~ 2-3 gph at 7 knots. On a personal thought I think at 7K this hull would be in the area of 2.5 - 3 gph. Only trials will zero in better than that.

At these speeds the engines do not use a lot of fuel even though the capable power is much higher.

This would have applied to the 41 also, just the hull speed would be somewhat higher and maybe a bit more fuel.

However, note, push that hull speed hard or past it and the fuel use WILL climb, usually sharply.

With any of these boats, when running, if you detect a noticeable bow rise and see a good size rolling wave following you are pushing into higher fuel use.

Of course substantially larger vessels will use more fuel than I am talking about here but this should stand for most vessels in up to the 42 or 45 ranges adding a bit more fuel to account for the larger hull and weight.

There will be other explanations, better than mine, but this should get you started.

Let the arrows fly.
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Old 07-19-2015, 02:16 AM   #3
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C - Here's the first "arrow"...

You hit Bulls-eye on your figures and comments!

IMHO - Art
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Old 07-19-2015, 07:31 AM   #4
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A multiplier (SL) of between .9 and 1,15 will be best for speed efficiency guesstimating .

Motor boats seldom will travel at hull speed as the fuel burn is high and the wakes are huge.

Usually only sail boats with free power will get to hull speed , under engine power most will do SL x 1.1 like the rest of the motoring folks..
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Old 07-19-2015, 09:47 AM   #5
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A multiplier (SL) of between .9 and 1,15 will be best for speed efficiency guesstimating .
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Motor boats seldom will travel at hull speed as the fuel burn is high and the wakes are huge.

Usually only sail boats with free power will get to hull speed , under engine power most will do SL x 1.1 like the rest of the motoring folks..
Very true, Fred

For best speed to fuel-use efficiency... actual "hull speed" when calced by the infamous 1.34 x square of WLL is not necessarily the most economical.

In our boat's case 7.58 is its “calced - hull speed". With twins running us at 7 to 7.5 knts (through-water-speed - not necessarily OL speed, which is dependent on current’s direction) we get right at 2 nmpg for OL speed in a slack tide. With twins running, if we drop it back to between 5.5 and 6 knts we get around 2.5 nmpg. With one engine shut down and a single pushing us slowly at 4.5 to 5 knts we get near 3 nmpg.

Of course... with the twins and boat’s well designed planning hull we can also easily cruise-out at 16 to 17 knts, then averaging only 1 nmpg… which is what we do if/when we need/want to make time, for one reason or another. Otherwise… slow going for economical, laxidasical enjoyment on our bridge while cruising along deep in the SF Delta’s sloughs enjoying sights and stopping for a quick swim or at a marina for dining is just fine with us!

“Ride the Tide” (at any speed you may want to mechanically push a boat) is my best suggestion for economy of operation. Tidal current moving for or against direction traveled at any speed through water will increase or decrease OL nmpg (in direct proportion to current' speed). So, in the limited tidal change and relatively slow currents inside SF Delta – our 5 +/- knts through water speed using one engine for best economy can easily result in 6 to 7 + knts OL speed by utilizing correct tide-change choice. And, if wrong choice is made… we can fight the current which results in a mere 3 to 4 knts OL speed.

Timing and speed-opportunities to cruise becomes just like most other things about boating… simply an interactive group of “Trade Offs”! – LOL

Happy Econo-Cruise Daze - Art
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Old 07-19-2015, 09:44 PM   #6
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That does not sound bad. We just looked at a Californian 42 with twin 3208 320 hp and the guy says it burns 25 GPH at 10 knots. That is just unacceptable. Other than that the boat is beautiful and priced right.
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Old 07-19-2015, 11:50 PM   #7
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That does not sound bad. We just looked at a Californian 42 with twin 3208 320 hp and the guy says it burns 25 GPH at 10 knots. That is just unacceptable. Other than that the boat is beautiful and priced right.
12.5 gal per hr at 10 knots for each engine sounds steep. The boat wheeled correct dia and pitch? Wonder what she is turning for rpm at 10 knots... at that speed she may be pushing a fair wall of water at her bow; thus high fuel burn. And, I wonder what gph fuel burn she gets at 7 to 8 knots?
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Old 07-20-2015, 01:38 AM   #8
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He is pushing the boat past the hull speed but not getting it planing which is an expensive way to run. Not just fuel use but also engine strain as "climbing out of the hole" is tough.

If he dropped to a bit less than 8K the fuel use would drop a LOT. No predictions.

I'll bet the boat is running bow way up, big bow wave, stern way down in a hole, a large rolling wave just aft of the stern, and a LARGE wake.

He may actually do better on a full plane, not in outright GPH but in MPG.

He also sounds like he lost a potential sale because he hasn't explored slower speed economy. But that may not be acceptable to him. I see this on many boats. The owners have decided on an acceptable speed to them without understanding fully what they are doing.

Oh well.

FF You are right about the lower 0.9 - 1.15 for more likely cruise speed estimates.
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Old 07-20-2015, 03:36 AM   #9
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FF is right for displacement hulls, and probably for planing hulls running is displacement mode.

But with a semi-displacement hull you can run a bit higher. Using Gerr's Speed-Length ratio formula from his Propeller Handbook I get an SLR of 1.45 for my boat. I can cruise reasonably economically at what would seem a bit high using the 'old 1.34' figure. With clean bottom and 1700 rpm it is 8.8 kn and 5.25 gph total. Slowing down to 8 kn uses 4 gph.

The guy running a Californian 42 at 10 kn has picked a very inefficient speed to run at, or answered a question truthfully of how much it will use at 10 kn. Fact is, 11 kn +/- 2 kn is not a speed you would use in practice in such a boat. Better to say it cruises very economically at 7 kn or you can crank it up to mid teens or better and still have reasonable mpg.
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Old 07-20-2015, 06:26 AM   #10
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The moral of the concept of displacement speed cruising is to install a good auto pilot and enjoy the ride.
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Old 07-20-2015, 07:25 AM   #11
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That does not sound bad. We just looked at a Californian 42 with twin 3208 320 hp and the guy says it burns 25 GPH at 10 knots. That is just unacceptable. Other than that the boat is beautiful and priced right.
From my experience running at 10 kts is above hull speed and below planing speed, a speed I would avoid. You are pushing a lot of water and your fuel flow will prove it. That said each boat is different, and this is my experience.

My twin Cummins 330's burn about a gallon every 2 miles (both engines) just below hull speed which is about 7.5 kts and a gallon per mile at planning speed which is about 16 kts.
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Old 07-20-2015, 08:17 AM   #12
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Like many others the California guy doesn't understand boats. He picked a very poor speed from a fuel use and engine load standpoint.
The above explanations are getting to the heart of the matter. 40'+_ boats will do about 2 nmpg if you keep the bow down. Probably approaching 3nmpg at 6 kits pr so.
I didn't mention engine brand or size because it won't matter.
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Old 07-20-2015, 08:32 AM   #13
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If you plan a 40' boat at mostly slow operation~ 7 knots don't obsess about fuel use. It won't vary enough to make any real difference from boat to boat or engine to engine
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Old 07-20-2015, 09:28 AM   #14
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25 gal per hr at ten knots that owner mentioned has got to have some sort of grand error of some manor for his reaching that number... as can be seen by most posts on this thread.

Setting aside the owner's 25 gal per hr remark... and... taking into play more usual fuel burn results mentioned by many experienced boat owners who posted in this thread:

- For every 100 hours you operate at economical speed (let's say 7 knots) which should burn fuel slowly (letís say 4 gph Ė at 2 gals per each engine) = 400 gal fuel for 700 n-miles traveled.
- 400 gals fuel x fuel cost (letís say $4 per gal) = $1,600 for entire 100 hr use - which equivalates (is that a word? Ė lol) to $16 per hr fuel cost (if you canít afford that Ė donít go boating).
- Therefore on an 8 hour cruise youíd spend about $128 in fuel that day while covering 56 n-miles.

All ní all there are many ways to dissect the fuel expense of a boat as compared to time running, speed attained on a continual basis and resulting miles traveled.

Suffice it to say: Boating tait too inexpensive as an overall factor, but, in the long run of affordable enjoyment opportunities available in life Ė ITíS SIMPLY PRICELESS!!
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Old 07-20-2015, 10:55 AM   #15
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3208 Ts on my last boat...a 37 sportfish...burned 10 gallons per and gave me 20 knots.

Lighter, smaller boat..so I could very easily see a 42 foot heavier boat not getting up on plane and burning 25 gallons an hour at 10 knots.

But everyone who said either faster or slower would probably improve the NMPG number.
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Old 07-20-2015, 11:22 AM   #16
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I bet the. 25 was at high cruise speed
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Old 07-20-2015, 12:10 PM   #17
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What type and size boat you running, Timjet ?
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Old 07-20-2015, 12:24 PM   #18
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Cat 3208 fuel burn info


3208TA 375hp


At 2600 rpm (200 under max)...they burn about 15.9 gal per hour per engine....


at 2400rpm.... 12 gph


https://marine.cat.com/cda/files/101...Propulsion.pdf
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Old 07-20-2015, 01:13 PM   #19
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Then 2400 would be a nice conservative high cruise assuming it hits rated WOT +.
Maybe the op should get clarification.
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Old 07-20-2015, 10:43 PM   #20
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What type and size boat you running, Timjet ?
Carver 355 , 39' 25,000 lbs planning hull.
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