Repowering Your Diesel Inboard Boat

When your diesel inboard powered vessel is reaching the end of its service life, you must decide whether you want to rebuild or repower with similar inboard engines or look at what outboard powered options are on the marketplace.

Typically, inboard propulsion engines have provided power drive systems including: sterndrives, pod drives like the Volvo IPS System, waterjets, and traditional straight shaft installations. Historically, these different drive systems have been very successful in the midsize 30-45 foot boats for their ability to harness the variety of 300-700HP diesel inboard engines available in the marketplace. These engines were known for their reliability, long service life and ability to get more HP per gallon than any of the few gas options available.

But that was then, and now we have entered the age of reliable, large horsepower, outboard engines. The industry is trending toward outboard powered boats, and conversion to outboard motors is becoming more popular in vessels when doing a repower. When considering converting to outboards, it is important to think about who, when, where and how your boat will be used.

When to NOT Convert to Outboards
When the vessel configuration doesn’t lend itself to a conversion. Some hull forms might not lend themselves to a conversion to outboards. Plus, some vessels’ configuration of tanks, cabin or other features might make the conversion cost prohibitive.

When there is an operational requirement for diesel fuel. Although there is promise of some very exciting news regarding diesel outboards on the horizon, they aren’t available yet with a track record we can recommend to customers. So for now, if you need diesel, you should stay with your inboard.

When there is an operational requirement for a feature of the drive system not available with outboards. If you have a swimmer safety requirement that a waterjet fulfills, or if you have a shallow draft requirement that is fulfilled by a waterjet then you should keep your inboard.

When to Consider Outboards
In the commercial realm, there are a few scenarios in which converting from inboard to outboard power can be beneficial. Most of the time, if a customer is interested in updating its power source, it is more cost effective for them to install outboards than to build or repower with the same diesel power.

Silver Ships recommends outboards for vessels with missions in which gasoline is viable fuel and the vessel configuration lends itself to a conversion.

Outboard Pros
There are multiple pros to converting to outboards, including:

  • New outboards are much more reliable than the outboards of the past. We have customers with outboard engine hours in the thousands.
  • Outboards are easy to maintain because they offer easy access on the outside of the vessel.
  • Outboards have great power to weight ratios which translates into more speed for the same HP. Additionally, outboards ability to trim allows for fine tuning the vessel trim to optimize performance in many different loading and sea state conditions.
  • Installing outboards can open more cockpit space and/or floor space. When the engine is outside the boat, builders can instead offer other features such as storage and rear seating.
  • Outboards are less susceptible to freezing in the winter because their cooling system drains after every use.
  • Outboards can usually be trimmed up clear of the waterline, allowing for service to the propeller while the boat is in the water and keeping the lower unit free from marine growth.
  • There are a multitude of outboard brands to consider when converting, including Suzuki, Honda, Mercury, Yamaha and Evinrude. Your workboat builder will be able to recommend a brand that will best fit your vessel’s mission needs and maintenance requirements.

    Each conversion from inboard to outboard power is different, and each vessel should be looked at by a qualified Naval Architect to determine the impact of the change. Silver Ships has completed multiple highly successful repower conversions in the past few years.

    Our experience is that once the conversion is completed, a customer will notice their boat is often easier to maneuver, especially if fitted with twin or triple outboards.

    If you have questions about your inboard and outboard options, reach out to a member of our team.