Propulsion for a 36ft aluminum boat

Discussion in 'Propulsion' started by cmb1998, Apr 12, 2024.

  1. cmb1998
    Joined: Aug 2009
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    Location: Portland Oregon

    cmb1998 New Member

    Longtime lurker, first time poster...

    I've refitted / rebuilt a few aluminum boats over the last ~15 years. Current boat is a 30ft x 10ft aluminum pilothouse with 2x 300 hp Suzuki's. Somewhere near 11,000 LBS or so with a ~18 degree deadrise. Great boat, but looking for move up in size as kids are getting bigger and need one more birth. Also been a little disappointed with rough water handling.

    I'm looking at building a ~35ft aluminum boat with a 11ft beam (will still sit on a trailer) and ~20 degree deadrise. Would be used for weekend trips and summer trips in the inside passage. Estimated weight of new boat would be ~13,000 lbs. Goal would be to cruise around 25-27knt's. Rough math puts this around 600 hp required. Diesel would be preferred due to safety and availability in rural Alaska / Canada.

    I've owned a few boats with OB's, although current boat is the largest, and always felt they didn't handle chop / swells as well as IB boats. I've attributed this to having all the weight up high and in the very back of the boat.

    Here are my questions:
    1) From an engineering point of view, is there validity to my thought process here?

    2) Given this will need to fit on a trailer (oversized), if I want an IB I think my only options are stern drives (likely Konrad's) or Jets. I've owned several Jet's and really like them. I've done a ton of reading on here and am thoroughly confused about the efficiency of jets (beyond Hamilton's marketing info). Some of the posts have an efficiency curve that looks to be better than most prop's in the 25-40knt range. Is this real? I've always found jets to be less efficient. (I know surface drives are an option too, but always found them to be loud and problematic, not considering here).

    3) Other advantage of a jet would be I can go with a single larger diesel and keep the bulk of the weight centerline for stability. I'd have a kicker (~30-50hp) for trolling and get back in an emergency.

    Any help is greatly appreciated, this site is a wealth of info and has helped greatly in prior projects. Thank you.
  2. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Guessing the ob to all other propulsion methods us 90:1 for what we'd call a pacific northwest special. Haven't seen a surface drive on one for years, and the jet and outdrive ones are mostly older boats that predate the explosion in big outboards.

    Have been toying with the idea of building a pws drift boat, it's less dead rise but similar size and weight. Overwhelming majority are twin jets with a cummins based main. Seems like alamarin has largely taken market share from ultrajet with Hamilton. Several guys tried 700 hp singles instead of twin 400ish hp engines. Most I talked to said they would go twins as the singles never really got the speed. Probably due to having to step up to the bigger engines to get a reliable 600+ hp. The euro cat and the qsc can go 600hp but it's a lot of power out of those blocks. Most had the next step up, 13 liter John deere or scania power with a couple qsm 11s. Downside is weight and size. The big jets are really long and the 11-13 liter engines are all pretty hard to stuff in small engine bays or under the deck..

    Much easier to use smaller jets sized for something 5.9 or 6.7 cummins based. (Several companies aside from cummins marinize them, while other companies make engines in this space but the cummins options dominate the space.) Easy to fit under decks and much more common and compact jet units.

    It's worth putting out there the cost. Twin suzuki 300 dual props is going to be in the low 50s rigged in the pacific northwest. Twin big bore 350 Hondas probably the upper 50s to low 60s. A single 11-13 liter diesel is going to be 68-90k wire another 13k for a gear and 35-50k for a jet. Twin jets easily run 150-180k in current pricing.

    Can buy an inordinate amount of gasoline for the price difference. Diesel boat is likely more efficient per knot, but initial cost being so much more it's hard to make it pencil. If it were my money, I'd probably add a little extra tankage and go with some proven big outboards. We have charterman up here routinely getting 4500+ hours from big outboards, the $ per horse power hour is hard to beat.
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  3. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Jets in testing show less range and speed than a comparable prop setup, assuming it is properly designed and executed.

    I actually suggested surface drives, then deleted it, because you don't like them. The local (Port Hardy-Campbell River) large water taxi uses them and the owner operator loves them. He's about to sell the boat after more than a decade of operation.

    You actually get better roll stability with weight farther from the centerline because it increases the roll moment of inertia.

    Edit: just read @comfisherman 's post. Hard to follow that. Ignore me. Read him.
    Last edited: Apr 12, 2024
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  4. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    Correct per se, but it also means that the rolling will be resonating with longer waves, having more energy input to increase the roll amplitude.
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  5. DogCavalry
    Joined: Sep 2019
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    DogCavalry Senior Member

    Of course. Natural roll frequency corresponds to a particular sea state. I really need to be more precise. Like when I said that men are rheopectic, while women are thixotropic, but I failed to provide context, thus spoiling the joke.
  6. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    How many berths will you need now?

    Have a look at this thread - in it I suggested Tad Roberts' Mollymawk, which can cruise happily with half the power of your current boat.
    36' x 9' hull design

    Mollymawk is 32' - you could maybe ask Tad to scale her up a bit to 36' if you like the concept?
    Or use this vessel as a basis for your own design?

    Here is a video of her underway -

  7. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Not withstanding the cost issue issue, 2 x smaller units are the best way to go.
    Ostensibly for redundancy and better fuel economy too. As you can always shut one engine down and loiter on a smaller engine.

    Pod drives, are excellent, but is shallow water, fouling or floating debris an issues in the waters you ply?

    Having an o/b does increase the KG a tad more compared to an i/b engine - which could have issues on stability, - statical - as well as roll period.

    As for your hull, you don't need any more than 15 degrees at the midships...and i would taper it down to 5 degrees at the transom.
    You're not going for ultra high speeds (Fn) and so lesser deadrise aids in more volume aft too - which is always a good thing for fitting engines/jets and buoyancy too.

    A basic sketch of what you are proposing would be good too.
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  8. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I can’t compete on the engine dialogue, but can offer. The only big difference for me between inboard and outboard not mentioned, notwithstanding the ones mentioned, of course, is the risk to engines drowning. That part of the decision is most important for me.

    If your use case is a pleasure cruiser not crossing surf (much), then ob is at least worthy of consideration.
  9. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    Pod drives may be hard to trailer, and might be the only form of propulsion with a more questionable track record than outdrives. While awesome in theory and operation, they haven't proven themselves in the consumer market. Can think of one boat with them that likely has less than 100 hrs on it, hard to tell its long term viability. The previous two were from the earlier 2000s when they first came out, both have received stern extensions and been repowered with v shaft drives.

    Jets aren't exactly straight shaft levels of low maintence but the common sized Hamilton, alamarin, and ultra jets have fairly well known service intervals. They would certainly make for an easier to trailer boat.

    Outboards won the propulsion preference war. With the exception of ski boats and the legacy repower market outboards are taking over much of the market. Heck these big bore monsters are taking a crack at most diesels up to about 12 liters.

    Just hard to beat the economy of scale, I'd guess suzuki sells more 300 hp outboards in one zip code of Florida than Hamilton sold 3-500 hp jets nationwide last year.
  10. Ad Hoc
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    Ad Hoc Naval Architect

    For ref:
    They are used extensively in the European pleasure boat market and now, becoming more popular with CTVs at windfarms in the North Sea.

    Just different market sectors, as it is on the cross over aspects of o/b v i.b.
    So it Becomes a simple cost issue, unless shallow water is the driver.
  11. Barry
    Joined: Mar 2002
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    Barry Senior Member

    I would be concerned with kelp and weed injestion with a jet. Perhaps with sea strainers and and a freshwater cooled system you may keep the engine block free of weed and crud build up.
    Kelp with jets can cause issues.
    What is your experience with say 212 Hamiltons in the Alaskan waters? Certainly the inside passage sees more kelp than I would think that a jet could handle. Especially past July

    To the OP
    Gas and diesel is readily available from Seattle north to Anchorage. I would not worry about this.
    13,000 form a 36 foot boat, mmm? I think that this is optimistic. I am pretty sure our last aluminum a 30 pilothouse Thunderjet with twin 250s weighed over 10,500 pounds without
    people, grub, fuel and other carry ons.
    Your own 30 x 10 was 11,000 pounds. Not sure how you will get to 36 by 11 feet at the same target weight. (especially if you go with diesels) (If you purchase diesels and decide on jets,
    I believe that Hamilton has specific models for the jets.)

  12. BlueBell
    Joined: May 2017
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    BlueBell . . . _ _ _ . . . _ _ _

    Hamilton 42's and bull kelp don't play well together in my experience.
    But, if you stay away from the stuff, it's all good.
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