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Sea trialed new boat and fuel burn was way off.

Discussion in 'Engines' started by Danvilletim, Apr 7, 2021.

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  1. Danvilletim

    Danvilletim Senior Member

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    We sea trialed a 90’ boat today and was concerned about the fuel burn performance. Boat has rebuilt c30s w platinum cat warranty through next feb.

    1840 rpm / 90 gallons per hour combined / 14.3
    1500 rpm / 50 gallons per hour combined / 12.6k
    1203 rpm / 25.5 gallons per hour / 10.4K

    I would this a 92 overall Hull would do 12 kts at or near 25gph especially w fresh rebuilds. ( 250 SMOH)

    so I recheck the listing and they report

    1400 rpm/ 18.5 gallons combined / no speed listed
    1900 rpm / 60 gallons combined / 14kts
    2100 rpm / 120 gallons / 18kt

    I have to assume if someone listed fuel burn ranges that most likely this is true or was at a time. This owner got a new 118’ 2 plus years ago and this boat has likely be sitting. Well maintained but sitting.

    What could cause fuel performance to go down so far? Boat is 2007. Hold 4500 gallons w only 800 on board.
  2. olderboater

    olderboater Senior Member

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    At WOT did it reach rated RPM?
  3. Danvilletim

    Danvilletim Senior Member

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    It was a pre survey test ride. The owner was gracious enough to allow this that and they only pushed it to 2200 rpm which was fine for test ride. I want going to demand full rpm until the actual survey. I guess it could be prop, could be bottom growth ( but noticeable growth).

    I don’t need another boat. Would be fun, but I don’t have the time or motivation to fix any of these issues.
  4. olderboater

    olderboater Senior Member

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    Well, they projected 0.15, 0.26 and probably something like 0.50 nmpg.

    You got 0.40, 0.26, and 0.16 nmpg.

    At 1200, 1500 and 1800 rpm, I would expect 0.55, 0.36 and 0.25 nmpg.

    So, I'd say it's definitely underperforming, but no idea why. Could be hull condition or many others things. Was there any vibration?
  5. Danvilletim

    Danvilletim Senior Member

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    Thanks for that data. There wasn’t a big vibration, but the water in the bottle on the table in the fly ridge was jiggling pretty good. Hard to tell if that was abnormal being as the first time I’ve run that boat.
  6. Pascal

    Pascal Senior Member

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    Hull speed is probably around 11kts on that 90 footer and you should burn about 22/25 GPH. Hard to tell without knowing what kind of boat it is.

    where did the sea trial take place ? How deep was the water ? In shallow water, under 12’ you re going to loose 1 to 1 1/2 knot

    if it takes 2100 rpm and 120 GPH to get it to a barely planning 18kts... either there is enough growth down there to feed a Haitian village or the boat is a trawler.

    Boat may have been sitting but it had to have gotten a monthly bottom cleaning...

    C30s are pretty small for a 90 footer. It’s a hull speed boat which can be pushed if needed
  7. mapism

    mapism Senior Member

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    My first thought for the possible culprit would be either hull or u/w gear fouled - or more likely both.
    Reaching 2200 rpm without trying hard, with engines rated for 2300 max, doesn't sound too bad, but there's still one hundred to go.
    And interestingly, if you take the two more comparable numbers (i.e. 1840 and 1900 rpm), you got not only a higher fuel burn, but also a slightly higher speed at the lower rpm, which is weird.
    I definitely would check that the bottom is clean, before doing another seatrial.
  8. Danvilletim

    Danvilletim Senior Member

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    Thanks. The boat is a 92 Paragon.
  9. CaptPKilbride

    CaptPKilbride Senior Member

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    My first thought is whomever wrote the listing got the numbers wrong.
  10. Danvilletim

    Danvilletim Senior Member

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    I don’t mind slowing down but I’d like to think you could cruise at 12kts at 25-28gph. It might be the bottom or running gear. Bottom was done Jan 2020 w prop speed. I kinda wish they’d prove out the numbers they advertise before plunking down 5-7k on full survey.
  11. Pascal

    Pascal Senior Member

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    I think 12 kts is going to be above hull speed. 11kts is more like it... that extra knot is going to cost you 10gph.
  12. olderboater

    olderboater Senior Member

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    That's a slow boat and may even be overrated in the ad. I'd say 21 knots is in a dream situation and perfect conditions. I'd guess under normal conditions 18 or 19 knots might be tops. I don't understand boats like that. It's not a trawler, not FD, but it's not got the power to plane decently. Whether the hull isn't capable of more or they're just all underpowered I don't know, but there's one in Seattle with MAN 1200's that does 18 knots and cruises at 14. The one in Halifax with CAT 1550's only claims 20 knots. There's a 2007 with CAT 1550's that only claims Max of 18 knots. That's the one that was at the Palm Beach Show. Is that the one you're looking at?

    I'm sticking to WOT at 18 knots and lousy performance due to design and hull conditions. It's a rather stereotypical Taiwan Tub. It is designed and built to perform poorly.
  13. Pascal

    Pascal Senior Member

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    14 kts is the worst cruising speed possible. You re not on plane and are just loading up the engines and burning a ton of fuel. Probably why the cats had to be rebuilt already. Unfortunately way too many boats are built that way.
  14. Capt J

    Capt J Senior Member

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    The listing fuel burn numbers are wrong. At 1200 rpms you'd be at about 45 GPH combined. 80% load is 112 gph both if propped to achieve WOT. Boat is a typical Taiwanese tub. If a glass was shaking on the FB of a 90' and it was relatively calm, you've got a problem. It's a typical Taiwanese Tub and an off brand one at that.....you truly get exactly what you pay for.
  15. Danvilletim

    Danvilletim Senior Member

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    Yes it’s the one at the Palm Beach show. I do look at it a trawler as the intent is to start traveling a bit more long distance. If Hull speed is 11 knots I could likely live with that unless it’s supposed to be better. Thus the stated numbers in the add are bothersome. I don’t think I’d cruise at 18 kts even if it only burned 100gph. Today I’m crossing to chub and we will cruise at 30-32kts ( at .5mpg) which is very efficient, but the ride is intense enough on a 55 that I would t want to do that day after day.

    The boat appears to be in above avg condition w a layout that really works for our family. Quality is good. Interrior is very good quality. All walls are wood vs the Hatteras 80 which has whisper wall falling down everywhere.
  16. Pascal

    Pascal Senior Member

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    Come on... you know better that trust the numbers in a listing, don’t you?
    CaptPKilbride likes this.
  17. olderboater

    olderboater Senior Member

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    Actually the Yachtworld listing shows a top speed of 18 knots.
  18. Capt Ralph

    Capt Ralph Senior Member

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    Sea trial data;
    90 gph combined could produce approx 450hp per engine.
    Listing;
    120 gph combines could produce approx 600 hp per engine

    What HP tune would these C30s be set up for?
  19. Capt J

    Capt J Senior Member

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    To answer both questions.

    I've always seen C30s rated at 1550 HP.

    I've never seen whisper wall falling down, especially on any Hatteras 80' as in ever. Hatteras 80' is a very high quality boat, compared to what you're looking at.

    Hull speed is determined mostly by length, and slightly by hull design.
  20. Capt Ralph

    Capt Ralph Senior Member

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    Then on the pins; 75 gph each, or 155 gph combined.

    Oh, If that what Hatt calls their wall treatment, Whisper Wall sux. When it is left to get hot (No A.Cs running) for a while, either the glue or material backing deteriorates.
    I can example a latter Hatt 72.