Downsize engines possible?

Discussion in 'Powerboats' started by naffgap, Jan 30, 2025.

  1. naffgap
    Joined: Jan 2025
    Posts: 6
    Likes: 0, Points: 1
    Location: London

    naffgap Junior Member

    Hello!

    I'm looking at a 1990 35' powerboat to replace my old boat. It has two big thirsty 220hp diesel engines, one of which isn't running. My question is: would it be possible to remove these engines and install two smaller inboards of ~100hp?

    My reasoning is that I never really push my boats to their speed limits, preferring a gentler cruise, and fuel costs are becoming unsustainable. I'm basically happy to sacrifice speed for fuel efficiency.

    Thanks!
     
  2. Will Gilmore
    Joined: Aug 2017
    Posts: 1,040
    Likes: 497, Points: 83
    Location: Littleton, nh

    Will Gilmore Senior Member

    Can you be more specific about how you use your boat? Do you cruise over-night, short distances to take a turn around the harbor, or troll for fish in the ocean, or maybe only putter up and down the Thames for a couple of hours with friends.

    What type of boat? Are you a planing high-speed sport boat or a heavy displacement hull? It sounds like a planing hull, but maybe it takes 400 hp to get you to hull speed.

    -Will
     
  3. naffgap
    Joined: Jan 2025
    Posts: 6
    Likes: 0, Points: 1
    Location: London

    naffgap Junior Member

    Hi Will, thanks for your reply.

    The boat is a Sunseeker with GRP planing hull. It's possible to push it to 40+knots, and cruise at 20+ with the original dual engine setup.

    I use my boat as a bit of a tourer for extended trips. Last year was coastal hopping the the south coast for a few months, before that I've done summers on the Thames and some canals. This Spring I want to cross the channel and head through France. Not really a night cruiser.

    I appreciate there is a whole world of cruiser boats out there (I've owned one!) but this particular boat is going for a very good price, it fits through locks and bridges, and I like it!

    Basically I want to know whether downsizing the engines (seeing as how I need to replace one of the current ones anyway) will keep the boat usable and improve fuel efficiency and thus lower fuel costs?
     
  4. Rumars
    Joined: Mar 2013
    Posts: 1,940
    Likes: 1,226, Points: 113, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    Of course it's possible but why bother? Two 100hp engines are going to have worse fuel economy then one 220hp engine so the obvious answer is to just ignore the non running one. If the boat has shaft drives and exhibits poor maneuverability at slow speeds on a single engine either repair the non running one or replace it.
     
  5. naffgap
    Joined: Jan 2025
    Posts: 6
    Likes: 0, Points: 1
    Location: London

    naffgap Junior Member

    Hi Rumars, thanks for your response.

    Yes just running one engine was my first thought. But there are two reasons not to do it.
    1. As you allude to, the boat has shaft drives and only running one will probably affect manoeuvrability at slow speeds, something I will definitely need.
    2. Redundancy. When out at sea it is nice to have two engines for peace of mind.
     
  6. Rumars
    Joined: Mar 2013
    Posts: 1,940
    Likes: 1,226, Points: 113, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    Repair the non runner or replace it with a used one of the same make and model. This way you have redundancy and maneuverability, and more importantly you retain the boats value. It will also cost less overall.
    Once you fit lower power engines the boats value will plummet to no engine levels. Fitting new engines is expensive even if you do the work yourself, there's a lot of ancillary things to do, new bedlogs, new prop, new exhaust, all piping usually needs rerouting, etc. You can buy a lot of fuel for that money.

    Take the boat out for a spin as is to see how it behaves on one engine. What model Sunseeker is it, and what engines are currently fitted?
     
    Will Gilmore likes this.
  7. naffgap
    Joined: Jan 2025
    Posts: 6
    Likes: 0, Points: 1
    Location: London

    naffgap Junior Member

    Thanks for your response. Yes, I've thought about all of this.

    To be honest, this question was more about the theoretical possibility of downsizing engines and how that will affect the boat's operation. For example, can I install two smaller engines and the boat will be absolutely fine (considering I'm not in it for the speed) or is the boat designed for very specific engine sizes and smaller engines will affect the weight, weight distribution, etc and will make the boat completely useless? This is what I'm interested in at this point -we can leave financial considerations out of it for the mo.
     
  8. Will Gilmore
    Joined: Aug 2017
    Posts: 1,040
    Likes: 497, Points: 83
    Location: Littleton, nh

    Will Gilmore Senior Member

    Yes.

    The boat is designed for general size, weight, power, engine(s). Whether changing these specs. has a significant effect is hard to say without knowing more about the design.

    -Will
     
    Rumars likes this.
  9. naffgap
    Joined: Jan 2025
    Posts: 6
    Likes: 0, Points: 1
    Location: London

    naffgap Junior Member

    Short and sweet, thanks Will. :)

    I agree, but I wonder to what extent. Obviously very difficult to answer - I think I was just wondering if it was catastrophic! I've messaged the Broker for more info about the current non-running engine to see how non-running it is. We shall see!
     
  10. Rumars
    Joined: Mar 2013
    Posts: 1,940
    Likes: 1,226, Points: 113, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    We will join your wondering because getting details is like pulling teeth.
    Nevertheless we can offer scenarios:

    1. Boat has period 200hp Volvo Pentas at roughly 450kg/engine, you replace them with new 100hp Beta at 425kg/engine. Total weight reduction 50kg, zero perceived effect on trim or handling.

    2. Boat has Ford Sabre at 500kg/engine you replace them with new Yanmar at 229kg/engine. Total weight reduction 542 kg. Hull shape, tank placement and passenger capacity will determine if the effects are positive or not.
     
    Will Gilmore likes this.
  11. comfisherman
    Joined: Apr 2009
    Posts: 830
    Likes: 420, Points: 63, Legacy Rep: 10
    Location: Alaska

    comfisherman Senior Member

    We have a weird situation up here where our boats have long outlived their expected utilization. Depending on what was the valuable species at the time, it dictated whether or not they came with massive horsepower or not. As many of them were made from the same molds that kicked out tens to hundreds of boats we have an odd group of boats that did just what your asking. A boat from a mold in 84' would likely have a 10 liter main a bit over 300 hp, same hull in 89' or 90' came with a 17 liter lugger pushing 700 hp to chase the gold rush of herring.

    Market changed, herring went bust and in the early 2000s when all those 15 to 20 year old boats needed new iron a lot of guys repowered with much smaller mains. Usually the operational speed of 95% of the boat was hull speed so pulling a 3408 out and dropping in a 375 hp 8 liter john deere had little impact on day to day boat performance. It just cut the top end speed in half and made the hydraulics at idle a bit slower.

    Saw a long list of 1000 hp boats end up with 400 hp over a 10 or so year period.

    Usually it's a swap of a gear to a different ratio plus the smaller main, the best transitions have also had either a re tuned or new prop. Saw a boat with a 1000 hp man swap in a 375 hp John deere. Went from a 1.7:1 gear to a 2.8:1 and still took a few inches of pitch off the prop.

    Seems like the exhaust is always a little noisy when oversized and you end up dragging around a heavier shaft and cooler for the little engine. But it's been successfully done for a lot of guys, we're currently in a second cycle of repowering with less.


    Know a boat built with pumped up twin turbo 8v92s in 1991. In 2005 one blew up and the pair was replaced with some 330 hp 6.8 liter john deeres. In 2013 the fish price was up and so was the competition. They injested some water and the owner decided to re-power with new 13 liter 700 hp mains new gears/props/exhaust. They are now over a decade old and need a full rebuild. Owner is considering swapping in a couple small cummins in place as he still has the smaller gears and props from the John deeres. It's cheaper to put in new small cummins than to rebuild the big mains, and 3 of the last 4 years have had fuel over 5$ and fish prices collapsed again.
     
  12. montero
    Joined: Nov 2024
    Posts: 182
    Likes: 20, Points: 18
    Location: Poland

    montero Senior Member

    What problems with diesel fuel do you have there ?
     
  13. kapnD
    Joined: Jan 2003
    Posts: 1,393
    Likes: 470, Points: 83, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    Have you costed out repairs to the ailing motor? Chances are it will cost less to repair than to replace it.
    I would repair it and downsize the horsepower by limiting the travel of the throttle.
     
  14. naffgap
    Joined: Jan 2025
    Posts: 6
    Likes: 0, Points: 1
    Location: London

    naffgap Junior Member

    Thanks Rumars, comfisherman, montero, kapnD for your replies.

    The current engines are Volvo Penta 220hp.

    I've costed repair v replace and (unsurprisingly) repair is cheaper. But cost wasn't everything in my motivation for doing this: the possibility of better fuel efficiency, replacing 35-year-old engines, reduced emissions, better reliability all matter too.

    I'll ask to take the boat out for a run on the one working engine and see how it handles. If it's fine then I'll repair the second engine anyway for safety's sake, but just run the boat on a single engine.
     

  15. gonzo
    Joined: Aug 2002
    Posts: 17,480
    Likes: 2,023, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I would keep the good engine and set it in the middle. Add a bow thruster for a lot less money than two smaller engines and the boat will be easy to maneuver.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.