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Old 21-11-2018, 10:50   #1
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Adding supplemental gas tanks

So my boat is an older 28 foot, 10 foot beam, with a displacement of approximately 14,000 lbs, semi displacement hull. The motor a Chevy block 350 with 270 hp I/O and two 50 gallon tanks (total 380 liters). This boat doesn't get on a plane and cruises at 7.5 knots burning 15 litres of fuel. This statistic is about to change as I recently bought this boat at the beginning of August 2018 only to discover it was "over propped." I am changing out the propeller to conform to the original type and size and stainless steel which means the engine will rev higher and the cruising speed might go up slightly with a concurrent change (higher) use of fuel.

Based on my calculations I'd feel a lot safer with a bit more cruising distance as my local area (Jarvis Inlet, Desolation Sound and The Broughtons) can have substantial distance between fill ups - The Broughtons more so.

I'd like to add something like 20 gallons on either side however my guy in refit isn't crazy about the idea as he said it will change the "characteristics" of how the boat will behave. Looking for opinions, for or against, of my suggestion to up the gallons.
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Old 21-11-2018, 18:22   #2
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Re: Adding supplemental gas tanks

Maybe your post would be more appropriate on the
  1. Powered Boats
  2. Recreation, Entertainment and Fun
  3. Engines and Propulsion.
If you click on "Forums" at the top of the page and go from there.


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Old 21-11-2018, 18:53   #3
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Re: Adding supplemental gas tanks

I'll post to Powered Boats, thought the more fixer upper types would hang here.
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Old 22-11-2018, 07:33   #4
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Adding supplemental gas tanks

His concern is weight, and he may be correct, or maybe not.
20 gls is about the weight of the average Woman, so itís not like itís a lot of weight.
However very likely the best way to extend range is to slow down, you would be amazed how much that extends range.

If your waterline length is 23í, then your hull speed is about 6.4 kts.
Slowing from 7.5 kts to 6 kts would very likely reduce your fuel consumption by at least 1/2 and very significantly increase range.
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Old 22-11-2018, 10:47   #5
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Re: Adding supplemental gas tanks

As per 64 Pilot- reduce your speed to 6 kn and go twice as far on the same gas.
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Old 22-11-2018, 10:52   #6
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Re: Adding supplemental gas tanks

I push a 21' foot waterline, 5000 lbs sailboat with a 6 HP outboard at 3/4 throttle (maybe 4 HP developed). Your boat needs only about 20 HP to go 6 knots. And about 60 HP to go 7.
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Old 22-11-2018, 12:06   #7
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Re: Adding supplemental gas tanks

Here's what I was thinking, talking to one of the workers at the refit. I can bring on extra passengers without to much change in boat performance, yet adding the same weight lower down, below water line, should provide more stability than one more person walking around on the deck, etc. and affect performance less than added passengers.

When googling, you can find a million threads on anchors, rodes, horsepower, fuel cells, etc. When I googled "adding extra fuel tanks affecting boat performance" or the equivalent, I come up with almost nothing. Why I posted here.
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Old 22-11-2018, 17:37   #8
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Re: Adding supplemental gas tanks

Quote:
Originally Posted by rsn48 View Post
When googling, you can find a million threads on anchors, rodes, horsepower, fuel cells, etc. When I googled "adding extra fuel tanks affecting boat performance" or the equivalent, I come up with almost nothing. Why I posted here.

In your google search you added extra words that affected your result. "Extra Fuel tanks" makes your search too specific and is irrelevant. What you want to know is how does extra weight affect performance?

Try doing a search on how does extra weight affect boat speed

https://www.google.com.au/search?ei=...39.qz9fyEjKtjw

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Old 23-11-2018, 20:10   #9
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Re: Adding supplemental gas tanks

I hope I don't come across as patronizing but some people may not be aware that if you want to get articles comparing two (or more) "things" always use Vs (versus) in your search string eg

  • advantages ketch vs sloop
  • boat power 12V vs 24V advantages
  • Aluminum vs stainless fuel tanks corrosion
  • Beneteau vs Bavaria yacht seaworthiness
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Old 24-11-2018, 15:49   #10
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Re: Adding supplemental gas tanks

You may find that putting a more easily turned propeller will actually LOWER your use of fuel rather than increasing it, thereby making the exercise unnecessary.

Petrol vessels scare the bejeezus outa me anyway.
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Old 02-12-2018, 12:07   #11
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Re: Adding supplemental gas tanks

I've talked to my refit guy and I think it was more the addition of two extra tanks. What he said was that to add gas gauges (didn't come equipped with one) and to add extra tanks, the original fuel has to be drained and stored (about 50 gallons worth), the tanks filled with water so that holes can be drilled into the tanks without explosion, the water drained, collected and properly disposed of so as to reduce pollution.

The boat is built like a tank and its weight, at 28 feet, is more like a 32 - 34 foot contemporary power boat. I said I wanted more fuel capacity as my cruising distances would be much further than typical local power boat cruisers. So he has said, although more expensive, than adding to new smaller tanks and adding gauges, it wouldn't be that far off with all the labour and time required to modify the old versus adding two new 80 gallon tanks and removing the old 50 gallon tanks I currently have.

Assuming I drained the tanks (of course I don't want to), my cruising time would be 40 hours instead of 24 hours at a cruise of 6 - 7 knots. This effectively allows me to puddle jump up and down the BC coast and into Alaska waters more effectively. Especially heading north of Campbell River BC, fueling stations become less frequent and more expensive.
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Old 02-12-2018, 12:15   #12
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Re: Adding supplemental gas tanks

Just a thought - my friend has a big, comfortable, fun motor yacht that he lives on in CR. The only real problem is the two 454ci engines, and the attendant fuel consumption.

Have you looked into re-powering the boat with a more fuel efficient engine, rather than adding or swapping tankage? Over the long haul, the money may be better spent?

Apologies that it doesn't address your question, but perhaps it's a viable option which would solve the issue of range.
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Old 02-12-2018, 12:45   #13
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Re: Adding supplemental gas tanks

Quote:
Originally Posted by rsn48 View Post
So my boat is an older 28 foot, 10 foot beam, with a displacement of approximately 14,000 lbs, semi displacement hull. The motor a Chevy block 350 with 270 hp I/O and two 50 gallon tanks (total 380 liters). This boat doesn't get on a plane and cruises at 7.5 knots burning 15 litres of fuel. This statistic is about to change as I recently bought this boat at the beginning of August 2018 only to discover it was "over propped." I am changing out the propeller to conform to the original type and size and stainless steel which means the engine will rev higher and the cruising speed might go up slightly with a concurrent change (higher) use of fuel.

Based on my calculations I'd feel a lot safer with a bit more cruising distance as my local area (Jarvis Inlet, Desolation Sound and The Broughtons) can have substantial distance between fill ups - The Broughtons more so.

I'd like to add something like 20 gallons on either side however my guy in refit isn't crazy about the idea as he said it will change the "characteristics" of how the boat will behave. Looking for opinions, for or against, of my suggestion to up the gallons.
Have done something similar with an old 25 footer.

Main concerns are:

1. Additional complexity.
2. Center of gravity in ALL dimensions
3. Dynamic stability.

1. Additional tanks, as you have noted, might require additional gauges (not essential in my books) but will CERTAINLY require additional pipe work, and that's more points of failure right there. Serious stuff with gasoline powered engines.

2. Putting the tanks amidships might keep the fore and aft trim ok, but unless you draw from them at an equal rate you'll get port/starboard trim issues. Maybe minor maybe not. BUT, you also risk compromising stability or at least ride quality if the tanks are higher than the waterline. Maybe minor, maybe not.

3. Finally, all the extra fuel sloshing around could create some odd behaviour, so if you go this path, tank baffles are a MUST.


So, I'm with your guy recommending increasing the size of the existing tanks. I think he is giving you good advice. The only better advice is coming from the go-slower crowd. But then I sail an old heavy cruiser so I think I am flying if I am doing 6 knots. (I also burn around 10 gallons of diesel PER YEAR, so the whole fuel capacity thing is a bit of a non-event to me.)

P.S. As noted, the prop spinning at higher revs does not necessarily translate to higher fuel consumption. If the engine is lugging, it will benefit from running at the correct load.
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Old 02-12-2018, 13:18   #14
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Re: Adding supplemental gas tanks

"This boat doesn't get on a plane and "
Some hull designs just never will, even if you strap on JATO bottles and try to make it fly. I don't know anything about Grenfell, little about power boat brands, and no idea what was common up in BC, but my first suggestion would be to find out what kind of boat Grenfell built. Are they known to be fast? To get up on a plane at all? Or, were they just cheap boats?
Is the engine original or normal to the boat? Re-propped usually means someone was trying to change a similar problem in the past. So the problem of speed and range, may be intrinsic to the boat. (There's often a reason why some very expensive brand names could charge what they did and do, especially when hull design was still a black art and computers had no say in it.)

Putting a woman on each side rail (nice idea, a64!) would make the boat more tippy, make it roll more all the time. That's the fuel weight distribution problem. If you want to minimize that, you want to add the fuel as low down and as centered as you can. Perhaps more toward the rear than really centered, in order to push for that planing. Or, perhaps trim tabs on the stern would help you get it up on a plane. Trying to add a "false floor" in order to tuck 40 more gallons of fuel in the boat may be feasible, depending on how the boat is built, but it certainly wouldn't be cheap.
Dedicating in a locker or space for some jerry cans "just in case" on those long runs, or adding in a fuel bladder and transfer pump, and again just using it on those long runs, might be worth considering if building in permanent tankage for what will be occasional use becomes unfeasible.
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Old 02-12-2018, 19:01   #15
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Re: Adding supplemental gas tanks

Thorton Grenfell was a BC guy who built custom boats locally, before stamped or whatever you want to call it, fiberglass hulls. His hulls were clinker (lapstrake) built and always blue. Here is a link to one of his latter boats, I think this very one is at the marina I moor at.

https://www.google.ca/search?q=thorn...qFjJjRHnmkyqM:

My boat has something so stupid, I don't believe Grenfell built it, just designed it which he often did for others. My boat was also built in Vancouver BC in 69.

I actually don't mind if the boat doesn't get up on plane as I know my wallet would also get up on a plane as well. Averaging 6 or 7 knots coming from a sailing background is fine by me, no tacking or jibing so faster than a sail boat even if the speeds are identical.
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